Dr Julien Pinaud

Dr Patrick Lacroix-Desmazes

Dr Jean-Pierre Malval

Dr Séverinne Rigolet

Mr. Gautier Mr Schrodj

Mr Aissam

Stephan Knopf

Dr Didier Le Nouen

Cécile Joyeux

Dr Rémi Métivier

PPSM, Mr Arnaud Brosseau

Dr Emeline Placet

Ms Corine Reibel

de Bordeaux) for their extensive help with experiment and commitment towards many concepts presented in this thesis. Also, I thank my

GENERAL INTRODUCTION

In the field of polymer chemistry, an increasing number of studies have involved the use of N-heterocyclic carbenes (NHCs) either as organocatalyst or ancillary ligand. [1][2][3] Despite their high utility, there are practical issues when handling highly reactive free carbenes including premature degradation or dimerization reactions due to their high Brønsted basicity and nucleophilic character. 4,5 To overcome these challenges, a series of thermally triggerable latent NHC precursors have been developed where NHC reactivity can be tamed by the presence of different protecting groups such as CO 2 or appropriate metal ions. 6 In this case, the free NHC species can be released on demand by application of heat, enabling a temporal control of polymerization and the use of a storable and ready-to-use formulation containing both the latent catalyst and the monomer(s).

Recently, radiation has emerged as a versatile stimulus offering an even more accurate activation of polymerization than heat. 7 A distinctive advantage of photolatent generator is to make possible a spatially-controlled polymerization, opening avenue for the development of patterned materials. Surprisingly, there is hardly any research activity in the field of NHC photogenerator. Herein, the main goal of my thesis is the development, for the first time, an air-stable photolatent NHC and its exploitation as photoligand in photoactivated ring-opening metathesis polymerization (ROMP). This PhD thesis is part of the ANR collaborative project -Photolatent N-heterocyclic carbenes for delayed ring-opening polymerizations‖ (PHOTO DROP) in which a multidisciplinary approach is undergone to harness the potential of new NHC photogenerators in a range of photochemically initiated polymerizations such as ROMP but also ring-opening anionic polymerization (ROAP) in solution and in dispersed media. The project partners are ICGM, Université de Montpellier (Dr. Julien Pinaud, Dr. Patrick Lacroix-Desmaze, Dr. Emeline Placet and Dr. Shashikala Indudhara) and at LCPO, Université de Bordeaux (Dr. Valérie Héroguez and Dr. Loïc Pichavant).

Returning to my PhD subject, the key idea is that upon exposure to radiation a photolabile NHC precursor can generate a free NHC, which subsequently reacts with an inactive Ru precatalyst to drive the in situ formation of an active metathesis NHCcoordinated ruthenium-arene complex. The use of this methodology is desirable because it gives access to a truly photolatent ROMP initiator easy to prepare and exhibiting a high catalytic activity. ROMP has been targeted as the main application field for our new photolatent NHC because ROMP polymers have an excellent record of mechanical properties and chemical resistance, and there is currently no UV-curable ROMP polymer coatings. 8 Polymers obtained by ROMP have been widely used as an alternative to metals in commercial and industrial materials (Figure 0.1). With respect to the NHC photogenerator, a complete photochemical study was carried out to elucidate the photochemical process associated with the free NHC liberation. On the other hand, the use of photolatent NHC has been also broadened to assess their utility as organocatalyst in other polymerization reactions. -A literature review in chapter I gives an advanced update on emerging trends in latent NHC progenitors. In particular, the various synthetic pathways to produce in situ NHC and their application in the polymerization are discussed. On the other hand, the different techniques and strategies for photochemically activated ROMP (photoROMP) are commented in details. A critical approach has been adopted to highlight advantages and drawbacks of each study.

-Chapter II describes the synthesis and characterization of a two-component air-stable NHC photogenerating system based on a mixture of 2-isopropylthioxanthone (ITX) and 1,3-bis(mesityl)imidazoli(ni)um tetraphenylborate (NHCH + BPh 4 -). The photoinduced liberation of NHC at 365 nm is evidenced by reaction with a mesitoyl radical to form a NHC-radical adduct detectable by electron paramagnetic resonance (EPR) spectroscopy. The NHC yield can be determined by 1 H NMR through the formation of a soluble and stable NHCcarbodiimide adduct. Three different photoNHC-mediated polymerizations are described:

synthesis of polyurethane and polyester by organocatalyzed step-growth polymerization and ring-opening copolymerization, respectively, and generation of polynorbornene by ringopening metathesis polymerization using a NHC-coordinated Ru catalyst formed in situ.

-Chapter III presents the investigation on the photochemical mechanism that drives the formation of NHC at 365 nm from the mixture of IMesH + BPh 4 -/ITX. The reaction intermediates and final photo-products are identified via a combination of laser flash photolysis, 1 H, 13 C and 11 B NMR spectroscopy, EPR, acid/base titration and gas chromatography-mass spectrometry (GC-MS).

-In chapter IV, photoactivated ROMP of norbornene (NB), 5-ethyliden-2norbornene (ENB) and dicyclopentadiene (DCPD) at different wavelengths (365 and 405 nm)

were carried out using IMesH + BPh 4 -/ITX as NHC photogenerator and a commercially available dimeric Ru complex as metathesis inactive catalyst. The main aim is to demonstrate the utility of this tandem approach for the photoinduced preparation of cross-linked ROMP films -An Appendix contains additional figures, experimental procedures, emission spectra of lamps as well as the set-up for photoROMP reactions.

CHAPTER I. STATE OF THE ART

The text in this chapter is reproduced in part with permission from:

E. Cao, L. Pichavant, T. K. H.

A LIBERATION OF N-HETEROCYCLIC CARBENES FROM LATENT

PROGENITORS AND THEIR USE IN POLYMER SYNTHESIS

Over the last three decades, N-Heterocyclics Carbenes (NHCs) have become essentials tools for chemists, either employed as ligands for organometallic complexes, thus providing better catalytic activity and stability, or as organocatalysts where they allowed expanding the scope of chemical reactions by providing unexpected chemical mechanisms.

Nevertheless, their strong reactivity associated with their relative stability has humped back their use at industrial level and to some extent at academic level. Consequently, several strategies have been developed to generate them in situ and to facilitate their handling. While most of them relied on the thermal liberation of NHCs in the reaction media from a variety of progenitors, no report was made, at the start of this PhD thesis, which described the liberation of NHCs by a photochemical pathway.

In the frame of the PHOTON DROP project, one of the objectives of this PhD thesis was the determination of the photochemical mechanisms underlying the generation of NHCs under UV irradiation from various progenitors. The photogenerated NHCs could then be employed for the catalysis of different polymerization reactions thus bringing temporal control and potentially spatial control. Consequently, after a brief description of the main features of NHCs, this bibliographic chapter will provide a comprehensive overview of the different strategies that have been employed to generate NHCs in a reaction media through the trigger of an external stimulus. Furthermore, the success in polymer chemistry of previous latent systems is also discussed and has been a source of inspiration to expand the scope and potential applications of our well-designed photolatent NHCs.

Essential features of N-Heterocyclic carbenes

Carbenes are defined as neutral compounds featuring a divalent carbon atom and a six-electron valence shell (Figure 1.1). 1 The best-known and simplest carbene sought to date is methylene, :CH 2 . Despite pioneered as early as 1835, 2 attempts to isolate carbenes remained fruitless, due to their elusive nature such as high reactivities and short life-times, until the ground-breaking works of Bertrand et. al. 3 and Arduengo et al.

Electronic structures of N-Heterocyclic carbenes

According to Heinemann et al. 5 and Carter et al., 6 the usual reactivity of free and uncoordinated carbenes depends primarily on the electronic configuration of their nonbonding electrons. 1,7 Within this criterion falls two classes of carbenes, conventionally termed as singlet and triplet carbenes. In the singlet ground state, the two unpaired electrons only occupy a sp 2 hybridized orbital of the sp 2 hydridized carbon center (HOMO) (Figure 1.2). Whereas, in the case of triplet carbene, each of the non-bonding electrons occupy either a sp 2 or a p π orbital, bearing a parallel or an antiparallel spin orientation (SOMO). 1,7,8 Consequently, these ground-state electronic configurations drive towards carbene's reactivity.

While the singlet carbenes possess an ambiphilic behavior, their triplet cousins are regarded as diradicals. 1,9 However, it is known that singlet carbenes are much more stable than the triplet species. 10 N-heterocyclic carbenes are a specific form of the singlet carbenes; therefore, they bear both a filled sp 2 orbital and an empty p π orbital. 11 Consequently, the first isolation of a NHC was made possible through the incorporation of a bulky adamantane on α-amino substituents of the carbene center. Since this outstanding achievement, it brought an explosion of investigations on properties and applications of divergent NHCs. 13 Among the different structural motifs (Figure 1.4), the five-membered rings, imidazolinylidene (1) and imidazolylidene (2), have been widely used,

for instance in organometallic chemistry and in the field of organocatalysis. When serving as ligands, the NHCs are able to enhance stability and reactivity of transition metal complexes for two reasons. Firstly, the great ζ-donor ability of C 2 carbon brings strong binding to the metal centers. Secondly, simple modulation of N-substituent architectures eases to turn and optimize the reactivity and hindrance of the carbene center. 14,15 Thanks to their high nucleophilicity and Brønsted basicity, NHCs have also proven to be powerful tools to mediate organic transformations 16,17 and polymerizations, 18,19 potentially reducing cost and toxicity in comparison to organometallic catalysts.

Synthesis of N-Heterocyclic carbenes

Because of their strong nucleophilicity and basicity, NHCs are highly sensitive to moisture and to substrates, which limits their use as bare compounds. 13,20,21 Thus, several strategies to generate NHC in solution have been reported so far. The most common method is the deprotonaton in aprotic solvent of the corresponding azolium salt precursors upon contact with a suitable strong base (Scheme 1.1a). 11,13,22 A wide range of bases with high pK a (pK a 14), such as KOt-Bu, 23 potassium hydride 24 and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) 25,26 have been used to accelerate the proton transfer at C 2 position. Another option to produce free carbenes is the electrochemically or chemically driven reduction of azolium salts (Scheme 1.1b). 20,28 In both of these synthetic methods, NHCs are revealed via the cleavage of a hydrogen radical from a transiently formed radical [NHC-H] • .

Generally, the latter is generated by activating the azolium salt either electronically or in the presence of an excess of potassium. Other alternative NHC precursors are thione compounds, undergoing thermal-induced desulfurization in molten potassium (Scheme 1.1c). 29 Here, the insoluble potassium sulfide can be filtered off and thereby allows obtaining the free NHC in high yield. In all synthetic routes, the NHC precursors, which are commercially available or easy to prepare, are very stable. Thus, they can be stored for long periods prior to use.

Scheme 1.1.General approaches to produce NHCs. 13,20,28,29 Finally, when NHCs have to be employed in the presence of substrates that are sensitive to strong bases, NHCs can be also generated in the reaction media from air-stable stimuli-responsive progenitors. Their synthesis and use, in particular for polymerization reactions, have been the subject of an intense research over the past 10 years and will be the subject of part 1.2 and 1.3 of this bibliographic report.

Principal applications of N-Heterocyclic carbenes

As ancillary ligands, NHCs have been found to be stronger ζ-donating and less πaccepting as compared to their neighbor analogues such as phosphine. These two features allow for both an improved stability and catalyst efficiency of NHC-metal analogues, making them nowadays at the forefront of organometallic chemistry. 15,30 To date, the most prestigious utility of NHCs ligands is perhaps in the well-defined Grubbs' family catalysts, where the labile phosphine ligands were substituted by the persistent NHCs 1a or 2a. Displaying airstability and outstanding efficiency, these catalysts have become crucial synthetic tools for metathesis reactions. 1,13,31,32 More recently, Pd complexes with mono coordination of 2a or 2c

have shown remarkable catalytic activity in various C-C and C-N cross-coupling reactions. 31,33 On the other hand, NHCs have also contributed to surface functionalization of nanoparticles with respect to stabilization and control of morphologies. [34][35][36][37] Besides being employed as ancillary ligands, NHCs have emerged as powerful organocatalysts in a variety of Umpolung and coupling reactions because of their strong nucleophilic character. 16,22 In particular, they are well suited to drive the polymerization of a number of common monomers, including lactones, 38 epoxides, 39 siloxanes, 40 cyclic anhydrides, 41 N-carboxyanhydrides 42 (via ring-opening polymerization (ROP)), (meth)acrylic monomers 43 (via group transfer polymerization (GTP)), and isocyanates and polyols 44 (via step-growth polymerization). By simply washing or using trapping agents, the NHCs are easily separated from resultant polymers, thus leading to reduce costs and toxicity for use in the polymer industry.

Liberation of NHCs from stimuli-responsive latent progenitors

Despite being known as one of the most popular methods to produce NHCs in situ, use of strong bases to deprotonate azolium cations retains some problems when regarding organic or macromolecular reactions. The key issues have been identified as (a) the need of harsh experimental conditions (anhydrous aprotic solvent, inert atmosphere), (b) the presence of bases/ protonated bases residues, which can be either reactive to some sensitive substrates or competitive to generated NHCs, and (c) the difficulties in final product purification. To overcome these drawbacks, many chemists have turned their scope of research towards the development of latent NHC progenitors that can generate the free NHCs upon application of a stimulus (temperature, ultra-sound, light). Among them, thermo-latent NHC progenitors (Scheme 1.2) that can regenerate the free NHCs by heating have been certainly the most studied.

By thermal-trigger

One of the main advantages of utilizing thermo-latent NHC progenitors is the temporal control brought to the polymerization reaction. By simply alternating temperature, the reaction can successively undergo on/off state -on demand‖. Furthermore, when using these compounds it does not only avoid drawbacks resulting from incomplete mixing, but also allows reactions to be performed in a one-pot process. According to the -blocking groups‖ used, protected NHCs can be divided into 5 typical categories: (a) betaine adducts, (b) azolium hydrogen carbonate, (c) azolium carboxylate salts, (d) neutral NHC adducts and (e) NHC-metal adducts (Scheme 1.2). 45 Eventually, the thermolysis of these adducts leads to the formation of by-products that can strongly affect the activation and growth of polymer chains (in a positive or negative manner depending on the by-product). Like typical NHC betaine adducts mentioned above, the lability of NHCsisothiocyanate zwitterionic compounds (Figure 1.5d) decreases with the electron-donating properties of moieties (R' = aryl or alkyl) present onto the isothiocyanates. 45,48 On the other side, when protected by the same heteroallene molecule like CO 2 , the adduct degradation is decisively influenced by both solvent polarity and steric hindrance of N-substitutents of the azolium part. [49][50][51] Besides, large and bulky substitutents on N atoms flanking the Thanks to the innocuity and abundance of carbon dioxide, NHC-CO 2 complexes are the most employed compounds to generate NHCs in a reaction media, either to be used in the formation of organometallic complexes, or as organic catalysts. Nevertheless, they are prone to hydration upon storage under air for a long time, resulting in the formation of azolium hydrogene carbonate salts. As discussed below, the latter are also able to generate NHC upon thermolysis, although much higher temperatures are required than compared to their NHC-CO 2 counterparts.

b. Azolium hydrogen carbonates

Azolium hydrogen carbonates, denoted as [NHC(H)][HCO 3 ], are special azolium salts enabling access to free NHCs upon heating that have been reported recently. 43,53 These adducts are very robust and can be stored under ambient conditions without special precautions. Based on TGA experiments and DFT calculations, Taton et al. 43,53,54 have demonstrated that the temperature activation of these NHC-precursors (generally higher than 100 o C) causes a formal loss of H 2 O and CO 2 . Similarly to NHC-CO 2 adducts, the temperature degradation can be altered by varying N-substituents of azolium cations.

Eventually, a challenge coming from the use of these NHC salts is the elimination of H 2 O during thermolysis, otherwise causing the hydrolysis of free NHC or the termination of several reactions. To avoid such side reactions, [NHC(H)][HCO 3 ] have often been employed in combination with molecular sieves to trap the released water and eventually drive the release of the NHCs.

c. Azolium carboxylate salts

Alternatively, azolium carboxylate salts, referred to ([NHC(H)][R'OO])s, have been widely used as a reservoir of NHCs in many fields of chemistry. [55][56][57] According to Taton et al. [56][57][58] and Sander et al., 59 the carboxylate counter-anions with a slightly basic character such as acetate and bis-carboxylate anions are able to deprotonate azolium cation as activated by simple heating. It drives to the spontaneous formation of NHCs and carboxylate acids on demand. Besides, recent studies ruled out the influence of solvent and suggested that the deprotonation is favored in a non-polar or weakly polar environment. 60,61 For the preparation of [NHC(H)][R'OO], the carboxylate anion is easily incorporated into the ionic liquid motifs after an anion metathesis process 56 or a simple anion-exchange reaction from the corresponding azolium halide salts. 57

d. Neutral adducts

Due to inherent electron-donating properties, the nucleophilic center of NHCs can insert into a labile R'-H bond of organic species such as chloroform, pentafluorobenzene and alcohols, allowing obtaining neutral adducts (Figure 1.6). 45,62 Such compounds are capable of providing a facile entry to the preparation of NHC-metal complexes because it avoids the need of working under air-free conditions. However, some downsides when using these adducts have also been found. As a matter of fact, full latency of these adducts was only obtained in rare cases with weak Brønsted-basic NHCs or high-pK a alcohols, where the NHC/alcohol interaction stands in the direction of adduct formation. Most of them were less stable in solution, gradually releasing free NHC and protecting species even at low temperatures. For example, 1a.MeOH adduct dissociates entirely after 10 min at 25 o C, while 4a.MeOH requires higher temperature to fully release NHCs 4a (80 o C, 16h). 63 Another disadvantage is the production of alcohol during thermolysis of NHC-alcohol adducts which can be restrictive for using with low-oxophilic metal or in anionic polymerization.

e. NHC-metal adducts

In recent studies, halogenated metal complexes have been used to mask NHCs resulting in the formation of NHC-metal adducts (Figure 1.7), which are highly air-and moisture-stable and inactive for catalysis at room temperature. 45,64 Figure 1.7. General air stable NHC-metal precursors. 45,64 Despite the drawback of introducing metal residues in the reaction media, which can be detrimental for biomedical and microelectronic applications, there are two attractive features driving the development of such NHC-metal species. First, there is a variety of inexpensive and non-toxic halogenated metal compounds that can serve as alternative protecting sources of NHCs. Second, at elevated temperatures, such complexes with low binding energy, can equally dissociate into free NHCs (Lewis bases) and metal ions (Lewis acids), providing respectively a nucleophilic and an electrophilic activation of the substrate which results in enhanced catalytic activities. In addition, proper recombination of these Lewis acids/bases at low temperatures provides an effective control of catalytic activity (Scheme 1.4).

Scheme 1.4. Thermal-triggering liberation of NHC-supported metal ions. 45 These coinage NHC-metal complexes are very facile to synthesize outside a glovebox via one-step reaction between metal precursors and two major latent NHC progenitors, azolium salts and NHC.CO 2 . 31,45 Among the various NHC-metal compounds, NHC-Ag complexes have attracted much interest in a variety of applications such as transmetalation agents in organometallic chemistry, luminescent materials and homogeneous catalysis. 15,35,45

By mechanochemistry

The -mechanochemistry‖ is recognized as a green method that can activate transformation of covalent bonds based on the use of mechanical energy such as compression, shear, vibration or friction. [START_REF] Ohm | A New Synthetic Rubber Norsorex® Polynorbornene[END_REF] Recently, Sijbesma et al.

By photochemistry

Though not practical from a synthetic chemist point of view, Endres et al.

Latent NHC progenitors for polymerization reactions

NHCs have been figured as exceptionally versatile tools for the preparation of macromolecular architectures. 22,74,75 In recent years, the use of latent NHC progenitors has significantly increased, mainly due to its ability in switching on NHCs' catalysis in situ when required. Thereby, the following discussion emphasizes on valuable achievements and problematic findings when these latent systems are employed in polymer syntheses. It provides also a desirable source of inspiration for perspective applications of our NHCs photogenerators.

Ring-opening polymerization a. ROP of cyclic esters and cyclic amides

Among the prolific research studies that have been conducted on organic catalysis for ring-opening polymerization (ROP) of cyclic esters, employing NHCs as organocatalysts has proven to be a powerful strategy to obtain polyesters with various architectures. With the aim at liberating NHC at lower temperatures, a new series of thermo-latent NHCs in the form of alcohol adducts of 1a was then prepared by the same group. 76 While being air-stable solids at ambient conditions, these compounds rapidly decompose at 25 o C in solution to release 1a as well as different alcohols in a reversible manner. Accordingly, when employed for the ROP of lactide, they generated effective single-component catalysts/initiators, resulting in a complete consumption of the monomer within minutes. In following studies, Hedrick et al. 63 used 4a.MeOH adduct to provide a facile entry to ROP of lactide and β-butyrolactone -on demand‖ by controlling the temperature. However, due to the remarkable stability of the 4a.MeOH species, the polymerization process required long periods of time to obtain quantitative monomer consumption at 90 o C. This certainly reveals the difficulty to achieve latent catalysts exhibiting pronounced latency at ambient conditions and high activity when triggered.

To improve this situation, a wide range of NHC.CO 2 adducts acting as thermally latent precatalysts were applied in bulk polymerization of lactide, 77 lactones (ε-caprolactone and β-butyrolactone) 64,77 and lactams (ε-caprolactam and laurylactam). 78 As desired, when mixed with initiators and monomers, some of the NHC.CO 2 species (1f.CO 2 , 2f.CO 2 , 3c.CO 2 and 3e.CO 2 ) acted as truly latent catalysts at low temperature, while by simple heating, polymerization occurred swiftly and in a controlled manner. According to Hedrick et al., the in situ generated NHCs behave as nucleophilic agents, thus driving the ROP in two different mechanisms: (i) a monomer activated mechanism (zwitterionic or anionic) when alcohol initiators are absent from the reaction media (Scheme 1.9) and (ii) chain-end activated mechanism in the presence of protic species (alcohol) (only for lactones and lactide) (Scheme 1.10). 63,64,77,78 Success in ROP of these monomers highly depends on the ring size and Nsubstituents of the initiating NHC. For example, in the synthesis of polyamides, when using six-membered NHCs behaving as stronger Brønsted bases than their five-membered ring counter-parts, high yield (85%) production of polycaprolactam was accessible within relative short reaction times of 45 min at 180 o C. In contrast, only 62% yield were achieved in the same conditions upon employing their five-membered counterparts. 78,79 Scheme 1.9. Schematic representation of monomer-activated ROP of lactam and lactone mediated by NHC.CO 2 precursors.

On the other hand, Kuckling et al. focused on using, for the first time, the NHC.CO 2 progenitors for the ROP of trimethylene carbonate (TMC) in bulk and in different solvents. 80 As a result of the solvation effect, active NHCs are readily available in TMC/solvent mixture even at ambient conditions. Thus, the ROP can proceed at relative low temperature. When Naumann et al. also got interested in the ROP of lactone using thermolatent NHCs progenitors. In their study, a homologous series of different NHC-metal complexes was designed, using water-free metal salts such as SnCl 2 , ZnCl 2 and MgCl 2 as blocking groups. 45,64 Quite surprisingly, polymerizations of ε-caprolactone in the presence of such type of adducts were found to be much faster than those obtained when using NHC. Recently, Naumann et al. 41,84 described the thermo-latent curing of epoxide and anhydride using CO 2 -or metal-protected NHC species as sources of base/catalyst. Herein, the in situ generated NHCs were found to be well suited to activate the curing of anhydrides and nucleophilic attack of the NHC to the alcohol followed by its addition onto the isocyanate moiety (Scheme 1.14). 44 In term of efficacy, NHCs have been demonstrated to be more active than conventional organic bases including tertiary amines. Regarding latent NHC catalysts, Buchmeiser et al. 85,86 prepared a number of thermally labile NHC progenitors serving as delayed-action catalysts for the synthesis of polyurethanes (PUs). The catalytic activity of these latent precatalysts was probed and then compared to phenylmercury neodecanoate (PMND) and dibutyltin dilaurate (DBTDL) as references. As expected, DBTDL revealed no latency at all, while PMND and the protected NHCs showed no activity at room temperature. More interestingly, the performances of the protected NHCs were in some cases even better than those exhibited by PMND upon thermal activation. Additionally, among the latent NHC precursors, all CO 2 -, Sn II -, and Zn II -protected NHCs displayed pronounced activity, while slower activity was observed using Mg(II), and Al(III) as protecting agents. 45,85,86 Scheme 1.14. Proposed mechanism for thermal liberation of NHCs from their inactive precursors applied in PU syntheses. 45,85,86 Regarding environmental aspects, NHC.CO 2 are of great interest owing to their ability to provide favorable surrogates to metal-free PU formation. Nevertheless, metal-protected NHCs thermally liberating metal salts displaying Lewis acidity were found to enhance efficiency in the syntheses of cross-linked PUs because of a dual activation effect (alcohol/isocyanate-activation pathways). 86

b. Polycarbonates

Extent application of 7a.CO 2 precursors for the bulk synthesis of polycarbonates (PCs) has been explored by Plasseraud et al. 87 . In their study, the polycondensation of dimethylcarbonate (DMC) and a number of diols proceeded in two subsequent steps (Scheme 1.15). First, the reaction mixture was heated at 100 o C for 15 min to accelerate decarboxylation of the NHC-CO 2 adduct, thereby releasing 7a that was immediately available to promote the transesterification of monomers. Then, the reaction temperature was increased to 150 o C while applying vacuum to remove by-product (MeOH). The PCs obtained after 1 h showed relatively low molecular weights (1700 -6700 g.mol -1 ) and broad polydispersities (Đ= 1.4 -2.8).

Scheme 1.15. General synthetic process of aliphatic polycarbonates employing 7a.CO 2 as precatalyst.

Ring-opening metathesis polymerization

Ring-opening metathesis polymerization (ROMP) 88 is a chain growth polymerization where ring-strain olefins insert into active sites of metal centers affording unsaturated polymeric materials (Scheme 1.16). . In all cases, for polymerizations carried out at 60 o C, the introduction of five-membered NHCs such as 1a, 1b, 2a and 2b significantly enhanced the catalytic behavior in comparison to 1c and 2c. Additionally, at this temperature, similar catalytic performances and physico-chemical parameters of polyoctenamers were observed for active species derived from azolium salts and from NHC.CO 2 adducts. However, at room temperature, superior catalytic efficiencies were only achieved in the carboxylate systems rather than in the corresponding azolium salts. 92 Solubility limit of azolium salts at ambient temperature was invoked to consider on this lack of evolution in ROMP.

Scheme 1.17. Highly active NHC-ruthenium complex obtained via reaction of ruthenium dimer and NHC.CO 2 adducts.

Conclusions

NHCs with extreme versatility continue to be valuable tools for polymer synthesis.

The developments of a variety of inert and air stable protected NHCs have overcome the challenges occurring when applying directly the free carbenes. The use of these protected forms enables an ease in handling and potentially latent polymerization systems, paving the way for the use of NHCs at industrial-scale. Among the approaches to generate NHCs in situ, less attention has been paid to photolatent NHCs, even if they can provide a precise temporal and potential spatial control of the polymerization reactions, which would be well suited for phororesist technology. Thus, it encouraged us to drive the scope of this thesis towards the further development and application of NHCs photogenerators.

B RECENT DEVELOPMENTS IN PHOTOCHEMICALLY ACTIVATED RING-OPENING METATHESIS POLYMERIZATION

Progress in olefin metathesis has been driven by a wide variety of increasingly advanced and well-defined catalysts. 93 However, these developments have generally met the needs of synthetic organic chemists, and are less suited to polymer applications. As a result, the use of ROMP polymers has so far been restricted to the manufacturing of specific structural and engineering materials including heavy-vehicle exterior components, sport products and day-to-day items. 94 The weak development at industrial level of ROMP polymers does not stem from limited intrinsic properties but rather from restrictive conditions imposed by the manufacturing process. -Smarter‖ catalysts able to release active species in situ and -on demand‖ are currently the main lever to ease polymerization processing. 95 Of high interest are ROMP photolatent catalysts, for which activity can be triggered under visible or ultraviolet irradiation. 96 Firstly, they enable the use of storable, ready-to-use, and cost-effective formulations without pot-life issue. This decreases risk potential of misuse and hazard due to incomplete mixing or early polymerization. Secondly, solvent-free, fast and energy-saving cross-linking procedures may be implemented for the synthesis of new UVcurable coatings and inks for example, as has already been described with radical and cationic radiation curing technology. 97 Last but not least, phototriggered catalysts allow spatially-controlled polymerizations and surface modification.

Unlike recent reviews devoted to photochemically induced olefin metathesis in general, [START_REF] Eivgi | Turning the Light On: Recent Developments in Photoinduced Olefin Metathesis[END_REF][START_REF] Reany | Light guided chemoselective olefin metathesis reactions[END_REF] our description shall be confined to photochemically activated ROMP (photoROMP). Other means for externally regulated ROMP such as heat, mechanical stimulus or ultrasound shall not be covered. 100 For sake of clarity, the studies are not classified according to the type of catalyst as is usually the case, but rather on the mechanism.

Following this idea, four main strategies have been built in the literature, as depicted in respectively, led to an active ROMP catalyst. With the WCl 6 /pentachlorophenol mixture, only moderate catalytic activity was observed in the ROMP of cyclooctadiene (COD), cyclopentene (CP) or dicyclopentadiene (DCPD). In contrast, the second system W(CO) 6 /CCl 4 proved to be much more active, thus reaching 100 % conversion with norbornene (NB) and CP as monomers.

The high catalytic activity of this latter system then encouraged many research groups to gain deeper insight into the photochemical pathway. [103][104][105] It is now well established that the primary step is CO ligand photodissociation (360 nm) and finally leads to a catalyst containing carbonyl and chlorine ligands around the tungsten center, in addition to the metal carbene unit. 104 However, the intermediate reaction steps as well as the precise structure of the final catalyst are still not clearly defined. Interestingly, the system is also active for the polymerization of phenylacetylene (PA), 105 and while investigating the presence of the metal carbene bond in this active species, Sudarajan et al. performed the synthesis of a PNB-b-PPA block copolymer. The reaction was possible due to the living nature of the polymer chains that could be coupled by end-capping with terephtaldehyde.

Photolatency in ROMP could also be accessed from well-defined tungsten catalysts . 107 Due to their cationic character, the 27 complexes studied were only soluble in polar solvent and photoROMP reactions were thus mostly performed in ethanol. Although these systems displayed moderate activities after UV-irradiation at 364 nm, they provided good photolatency. In particular, no polymerization was observed when heating the sandwich complexes with the monomers for 24 h at 50 °C. However, addition of the monomer to the irradiated complex and rise of the temperature to 60 °C produced polymers with high molecular weights (M w > 100 kg.mol -1 ) and high dispersities (Ð > 2.0). Sandwich and halfsandwich complexes were proposed to proceed through arene ligand dissociation to generate the active [Ru(solvent) 6 ] 2+ complex. With nitrile complexes, gradual photodissociation of the nitrile ligands might account for the formation of the active species Ru(NC-R'

) 6-x (H 2 O) (R' = CH 3 , CH 2 CH 3 or Ph).
In an effort to access organosoluble and active photocatalysts, Mühlebach et al. then introduced phosphine ligand to ruthenium and osmium arene complexes (Ru4 and Os1, Figure 1.11). [START_REF] Hafner | One-Component Catalysts for Thermal and Photoinduced Ring Opening Metathesis Polymerization[END_REF] Although no thermal latency was observed with NB, Os1 proved to be inactive after 6 h at 80 °C with the same monomer. By contrast, the later complex displayed high activity under UV-irradiation, complete monomer consumption being achieved after 10 min at 30 °C. Displacement of the p-cymene ligand under UV irradiation was proposed as first step to generate the active catalyst. In addition, activity of the osmium complex Os1 was found to increase with steric hindrance of the phosphine ligand. dispersities between 1.8 and 4.9 were usually obtained in yields of 85 -100 %. Catalysts Ru8a and Ru8b were also found to be active at 308 nm, although lower yields were obtained in the same reaction time. It was hypothesized that these catalysts (Ru8-10) became active upon loss of two tert-butylnitrile ligands under UV irradiation. Other precatalyst base on Ru center was disclosed by Lemcoff et al.. 113 The commercially available ruthenium phosphite complex Ru11 was found able to liberate the phosphite ligand under 350 nm irradiation, thus allowing quantitative polymerization of COE and COD at room temperature in 1 h and 2 h respectively. Polymers with molecular weights close to theoretical molar masses and with relatively low dispersities (Ð < 1.8) were obtained, which suggests that a high proportion of the photocatalyst was active under UV irradiation. This is in direct contrast with previous photocatalysts that usually displayed 10-20 % active species under UV-irradiation.

A last strategy for Ru proceeding through ligand dissociation was recently demonstrated by Rovis et al. [START_REF] Keitz | A Tandem Approach to Photoactivated Olefin Metathesis: Combining a Photoacid Generator with an Acid Activated Catalyst[END_REF] In their study, an inactive bis-NHC ligated ruthenium (R11') was merged with 2,4,6-triphenylpyrylium tetrafluoroborate as a highly oxidizing photocatalyst. Upon exposure to visible light (450 nm), a photoredox process occurred promptly that leaded to the dissociation of one NHC. Subsequently, 14-electron active Ru complex was formed in situ and thus readily initiating the ROMP of norbornene and its derivatives with high conversion (> 95 %) within 1 h.

Catalyst activation through in situ photogeneration of ligands

Active ROMP catalysts can also be generated from an inactive and non-absorbing precatalyst after reaction with a ligand liberated by an exogenous photosensitive species ). [START_REF] Keitz | A Tandem Approach to Photoactivated Olefin Metathesis: Combining a Photoacid Generator with an Acid Activated Catalyst[END_REF] Upon irradiation at sub-300 nm, HCl was released which allowed protonation and subsequent displacement of the acac ligands of Ru12a or Ru12b. Subsequent formation of the active 14 electron complex was proposed that featured the chloride anion as ligand. The system was found to be inactive in the absence of radiation or without the PAG. Also noteworthy is that the replacement of the chlorine anion in the PAG by a non-nucleophilic nonaflate anion resulted in a complete loss of activity, thus demonstrating the critical role of Cl -in the formation of the active catalyst. The tandem system Ru12/PAG provided excellent conversion in the photoROMP of COE, DCPD and NB derivatives at room temperature, although incomplete catalyst activation was observed. A PAG based on triphenylsulfonium was also employed by Piers et al. to activate the ruthenium carbide complex Ru13 (Scheme 1.20). 115 Triflate anion was employed in place of the chloride anion. The approach proved effective since under 254 nm irradiation, ROMP of NB, DCPD, COE or COD could be achieved in 5 minutes. Nevertheless, incomplete catalyst activation was observed as attested by the presence of peaks attributed to Ru13 in the 1 H NMR spectrum of the reaction medium. Latency of the system was confirmed at room temperature, since without UV irradiation no polymerization was observed. Eventually, presence of the PAG was found to be not necessary for ROMP to proceed, even though lower activity was observed.

Ru15-trans

Ru14-cis Ru15-cis Ru16-cis

OSi(SiMe 3 ) 3

Ru16-trans Ru14-trans hν hν

OSi(SiMe 3 ) 3 OSi(SiMe -Photoisomerisation into an active trans isomer. The first complex that exhibited cistrans-dichloro isomerization was developed in 2004 by Grubbs and Schrodi using a pyridine alkylidene chelated ruthenium catalyst (Ru15, Figure 1.14). 123 However, the reaction was induced by heat and resulted only in a reactivity jump; Ru15-cis was not originally latent. Inspired by this work, Lemcoff et al. first examined in 2008 the sulfurchelated complex Ru14-cis as a thermolatent catalyst for ROMP. 124 Polymerization of exo substituted norbornene was successfully triggered upon heating at 100 °C. Only a year later, they developed the idea of using the same precatalyst in photopolymerization starting from the assumption that many isomerizable groups can be interconverted not only thermally but also photochemically. 118 To their great delight, Ru14-cis precatalyst was partially (30 %) converted into Ru14-trans in dichloromethane solution at 365 nm, driving the ROMP at room temperature of three conventional cyclic olefins: exodimethyl-5-norbornene-2,3-dicarboxylate, COE and COD (40 -90 % after 24 h, Table 1.1 (entry 1 -3)). 120 Visible irradiation yielded smaller conversions because of poorer photoisomerization yield. b The mixture of monomer/ Ru-14-cis was kept at 30 o C for 3 days.

c A medium pressure 100 W Black Ray B100AP UVP was used as an irradiation source.

In photoROMP, Lemcoff proved that replacing the phenyl substituent of the sulfur atom by β-naphtyl 120 and trifluoromethyl [START_REF] Eivgi | Turning the Light On: Recent Developments in Photoinduced Olefin Metathesis[END_REF] groups resulted in a higher activity. In addition, they showed that supersilyl groups attached to the NHC (see Ru16 in Figure 1.14) could improve activity [START_REF] Reany | Light guided chemoselective olefin metathesis reactions[END_REF]125 leading to full conversion of COE and functional NB within 4 h when irradiated with a fluorescent lamp (λ max = 350 nm) inside a Rayonet photoreactor.

Interestingly, irradiation at 254 nm caused the destruction of the catalyst by photocleavage of the silyl protecting group. Lemcoff et al. also engaged a complete structural investigation of S-chelated NHC benzylidene structures with the aim of increasing the stability of the cis form and enhancing the activity of the trans form in heat or irradiation-activated metathesis reaction. In photochemistry, unfortunately, most results concerned small-molecule transformations using photoinduced Ring-Closing Metathesis (RCM) of dienes, 113,[START_REF] Keitz | A Tandem Approach to Photoactivated Olefin Metathesis: Combining a Photoacid Generator with an Acid Activated Catalyst[END_REF] and in less extent photoROMP. [START_REF] Eivgi | Turning the Light On: Recent Developments in Photoinduced Olefin Metathesis[END_REF][START_REF] Reany | Light guided chemoselective olefin metathesis reactions[END_REF]120,125 Regardless of the metathesis reaction, they elegantly evidenced that the thermodynamically stable form was the cis-dichloro configuration, while the trans-dichloro isomer was the kinetic product. [START_REF] Reany | Light guided chemoselective olefin metathesis reactions[END_REF] The catalytic activity of the trans isomer may be attributed to the fact that the benzylidene ligand is in trans to the NHC, and therefore dissociate more easily (than its cis analogue) by Ru-S bond cleavage, to yield the 14 electrons catalytically active species. Therefore, a dissociative pathway takes place, but in contrast to route 1, irradiation is not responsible for the dissociation of the chelating ligand.

Chelation by nitrogen atom. The well-known azobenzene cis-trans photoisomerization has been used as leverage to tune the activity of N-chelated Ru complexes. Without having a clear vision of the effect of the photoisomerization on catalyst activity, the presumed idea was that such conformational change may entail a modification of the coordination sphere of the complex, driving the shift from a latent species to an active species. In 2010, Lemcoff was the first to propose an azobenzene-based complex Ru17

(Figure 1.15) with a nitro group whose strongly electron withdrawing properties might be enhanced in the trans isomer. Unfortunately, the complex decomposed under UV irradiation. 126 In 2013, Buchmeiser and coworkers synthesized another N chelated complex displaying a triazene group (Ru18). 127 Unlike azobenzene groups, this latter is not photoisomerizable, and the activity under irradiation was attributed to a photocleavage liberating N 2 , then causing a decoordination of the chelated ligand. This complex showed significant photo-ROMP activity for both COE and DCPD (Table 1.2, entry 1 -2). In 2016, Sashuk et al. synthesized the N-chelating complex Ru19 existing as two isomers: a 5membered ring (Ru19-1) as active kinetic product and a 6-membered ring (Ru19-2) as stable thermodynamic product. 128 Starting from the latent form Ru19-2, ROMP of diethyl norborn-5-ene-2,3-dicarboxylate was only possible by irradiation at 254 nm and not at 320 nm, the established excitation wavelength of azobenzene (Table 1.2, entry 3). Obviously, this result questions the photoisomerization as the main pathway leading to the formation of an active form. As a final comment, Bielawski and coworkers reported the reversible photocyclization of diethylene-functionalized NHC Ru benzylidene complex. 129 A photoisomerization governs the formation (UV) or the ring-opening (Vis) of a 6-membered cycle depending on the excitation wavelength. However, this reaction does not allow to switch from a dormant to an active substance, but only to modulate the rate of ROMP. a Reaction conditions: CHCl 3 , ratio monomer/ Ru18 200:1. The photolatency was performed at room temperature for 24 h, while a 9W Hg lamp was used as an irradiation source.

b Reaction conditions: toluene-d 8 , ratio monomer/ Ru19 300:1. The photolatency was performed at room temperature for 2 weeks, while a Benda UV lamp (254 nm, 4W) was used as an irradiation source. Starting from the same electrochemical set-up, Boydston used an electron-rich vinyl ether as initiator and NB as monomer (unactivated olefin). 130 Remarkably, ROMP polynorbornene (pNB) was produced with a molecular weight of 11.8 × 10 3 g•mol -1 , but only in low yield (3 %). Thus, they envisaged that a cyclobutane species might form (Figure

1.16B).

Because the cyclic species was not obtained as final product (olefin cross-coupling), they hypothesized that reduction was likely to be out-competed by ring-opening reaction driven by the alleviation of the NB ring strain. This would maintain the radical cation species, and ensure chain propagation by iterative ring-opening of NB. They later envisaged also a reversible deactivation polymerization mechanism between a propagating radical cation species and a dormant neutral species through a reversible electron transfer reaction. 134

Photoredox ROMP.

From an electrochemical electron transfer to a photoinduced electron transfer (PET), the shift was easily made. For this, a broad list of photocatalysts such as ketones, pyrylium salts, organic dyes, metal-polypyridyl complexes was available, and redox catalysis was already in 2015 an intensively investigated field. [START_REF] Romero | Organic Photoredox Catalysis[END_REF] In a model experiment, Boydston used a tri(4-methoxyphenyl)pyrylium tetrafluoroborate P1 as photocatalyst (λ excitation = 450 -480 nm) (Figure 1.17 1.3). The polymers obtained were mostly atactic, and the pseudo-living character was further exploited to synthesize block copolymers. [START_REF] Pascual | Investigation of Tacticity and Living Characteristics of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization[END_REF] NB polymerization was followed by chain extension using exo-dihydroDCPD. Light exposure not exceeding 1 h was necessary to minimize the fraction of dead chains by irreversible end chain coupling. Subsequently, they optimized protocols to enable copolymerization of NB with DCPD [START_REF] Goetz | Metal-Free Preparation of Linear and Cross-Linked Polydicyclopentadiene[END_REF] or with functionalized NBs. [START_REF] Goetz | Expanded Functionality of Polymers Prepared Using Metal-Free Ring-Opening Metathesis Polymerization[END_REF] Use of divinyl ether initiators enabled to produce ditopic pNB macroinitiator via bidirectional chain growth. 140 Thiopyrilium P2 performed better than pyrilium P1 (Figure 1.17), but the authors were not able to correlate the photophysical properties of the photocatalyst (oxidation potential) with final conversions. 141 b Initial concentration of NB.

c Reaction mixture was exposured under the light from fume hood.

For the moment, metal-free photoROMP has been exclusively explored by the group of Boydston, a situation that is expected to evolve in the future. We envision that the advantage of this process is not only related to its photochemical control and use of visible light, but to the absence of metal. The advantages of organocatalyzed ROMP are well recognized in chemistry. [START_REF] Ottou | Update and challenges in organomediated polymerization reactions[END_REF]

Conclusions

Over the past 40 years, photoROMP has gain-increased attention, leading to a variety of precatalysts and/or photochemical ROMP approaches (see the 4 routes presented in Figure 1.9). Nevertheless, very few photoROMP systems can boast full thermal stability and high activity at room temperature. Additionally, most catalysts proved effective only in solution and with a limited range of monomers, if not with a specific monomer. It is striking how little attention has been paid to innovative applications made possible by spatiotemporal control of the photopolymerization such as thermoset preparation 111 or photolithography. [START_REF] Weitekamp | Photolithographic Olefin Metathesis Polymerization[END_REF] With the advent of LED and the recent developments of stereolithography, it is unquestionable that photoROMP will be the subject of numerous research efforts in a near future.

CHAPTER II. MIXTURE OF AZOLIUM TETRAPHENYLBORATE WITH ISOPROPYLTHIOXANTHONE: A NEW CLASS OF N-HETEROCYCLIC CARBENE (NHC) PHOTOGENERATOR FOR POLYURETHANE, POLYESTER AND ROMP POLYMERS SYNTHESIS

The text in this chapter is reproduced in part with permission from:

INTRODUCTION

In a ground-breaking work, Arduengo et al. presented in 1991 the first successful isolation of a stable N-heterocylic carbene (NHC). 1 Since this achievement, NHCs have been at the forefront of organometallic chemistry, 2,3 catalysis, 4 and more recently polymerization. 5 To date, the most frequently encountered NHCs are 1,3-bis(mesityl)imidazol-2-ylidene (IMes) and its saturated congener 1,3-bis(mesityl)imidazolidin-2-ylidene (SIMes) (Figure Thanks to their excellent ζ-donor ability, the NHC ligands have gradually replaced phosphines, that are not environmentally benign. [6][7][8] After reaction with transition metals, they can form well-defined NHC-metal complexes displaying outstanding catalytic activity. The most prominent examples are the ruthenium alkylidene complexes coordinated with NHC ligand used in olefin metathesis, including the so-called Hoveyda-Grubbs second-generation catalyst and its multiple derivatives. 9 In addition, their strong nucleophilicity and Brønsted basicity have contributed to their use as organocatalysts. In polymerization, they can replace organometallic compounds with advantages of reducing cost and toxicity. The range of NHCmediated polymerizations 5,10,11 already covers ring-opening polymerization (lactone, 12 lactam, 13 epoxide 14 ) as well as step-growth polymerization for the synthesis of polybenzoin 15 or polyurethane. 16 Despite the wealth of research conducted to date, the practical use of NHCs presents a number of challenges: NHCs are highly sensitive to moisture, and must be handled under dry atmosphere. Moreover, they react with acids due to their high pK a , and are also prone to dimerize. 17,18 Because of these constraints, NHCs are usually put in contact with substrates or monomers at the last moment. Considering the central role of NHCs in chemistry, it is essential to control and optimize their formation conditions. The issues are to facilitate storage, handling, processing, and more broadly, to promote scale-up from laboratory to production. Currently, in situ deprotonation of an azolium salt is by far the most commonly used methodology to access a free or ligated NHC. [19][20][21] However, this methodology requires a strong base, that limits the scope of substrates and may interfere in the polymerization process as adventitious initiator. 22 In the search of smarter routes for in situ and -on demand‖ release of NHC, -masked‖ or -protected‖ NHCs have been investigated recently. 22,23 Of high interest are thermally-labile NHC progenitors based on air-stable NHC-CO 2 adduct, 24 NHCisothiocyanate adduct 25 or NHC-metal complex. 26 Upon heating, the adduct/complex bond may be broken and the carbene transferred. Despite promising results, such approach has important limitations related to reversible cleavage, poor solubility of many NHC adducts, incomplete latency at ambient conditions resulting in early reaction, byproducts' formation, and additional energy expense due to heating. 22 Despite the many developments in the field of photoinitiators, 27,28 very little attention has been given to photolatent NHC precursors. In 2015, Denning et al. 29 reported the photochemical reduction of an imidazolium-2-carboxylate (NHC-CO 2 ) using an excited state electron donor, N,N,N′,N′-tetramethylbenzidine. The reduced formate complex might then decompose into NHC and formate ion. However, the authors only detected the corresponding imidazolium (the conjugated acid of the NHC), which was not an issue because the principal goal was actually the controlled capture and release of CO 2 . The limited research effort on NHC photogenerator is in contrast with the strong interest for other photobase generators (PBGs), capable of releasing amidine or guanidine organosuperbases. 30 Most PBGs proceed through a photoinduced proton transfer reaction employing a salt containing the protonated superbase and an anion chromophore based on ketoprofene, 31 thioxanthone derivatives, 32 and tetraphenylborate. [33][34][35] Today, there are strong motivations for developing photoNHC-mediated reactions:

temporal and spatial control of reaction, use of a contactless stimulation, and NHC release at ambient condition by precise energetic and spatial dosage of radiation. . 36 First, ITX is an inexpensive, commercially available aryl ketone, known for its ability to mediate single electron-transfer after photochemical excitation in the near UV-Vis region (350 -420 nm); in particular, excited ITX is used as an electron acceptor sensitizer in many photoreactions. 37 On the other hand, the tetraphenylborate anion may act as suitable electron donating partner to make PET thermodynamically feasible. 38,39 Therefore, the expected primary photochemical process is electron-transfer from BPh 4 -to triplet ITX. In Thirdly, using a mechanism-guided approach, NHC yield are optimized by investigating the effect of irradiation time, sensitizer concentration and structure. In a last part, the NHC photogenerator is harnessed to prepare a variety of polymer structures through their two fold role. As organocatalyst, polyurethane and polyester were prepared by step-growth polymerization and ring-opening copolymerization, respectively. As ligand, polynorbornene was synthesized by ring-opening metathesis polymerization (ROMP) using a NHCcoordinated Ru catalyst.

EXPERIMENTALS

Materials

b. Synthesis of polyurethane

A spectroscopic cell capped with a rubber septum was charged with IMesH + BPh 4 -(9.18 mg, 0.015 mmol, 1 equiv.), ITX (3.82 mg, 0.015 mmol, 1 equiv.) and 0.5 mL of THF.

A Schlenk tube containing the diol oligomer PEG 400 (2050 mg, 5.125 mmol, 342 equiv.

c. Electron spin resonance (EPR) of NHC-radical adduct

IMesH + BPh 4 -(9.18 mg, 0.015 mmol, 3 equiv.), ITX (1.27 mg, 0.005 mmol, 1 equiv.), TPO (10.4 mg, 0.033 mmol, 6 equiv.) were added to 1 mL of THF. The mixture was prepared in a small vial capped with a rubber septum. The solution was degassed with N 2 , then 0.2 mL were transferred into a quartz EPR tube with an internal diameter of 2.5 mm, the solution was degassed with N 2 . Subsequently, the EPR tube was irradiated with a 365 nm LED (LC-L1V3, Hamamatsu, 65 mW•cm -2 ). EPR spectra were recorded before irradiation, and after 2 and 5 min irradiation. For the control experiment, an identical procedure was followed using the mixture of IMes (4.56 mg, 0.015 mmol, 3 equiv.) and TPO (1.04 mg, 0.003 mmol, 0.6 equiv.) in THF, cautiously prepared in a gloves box.

c. Steady-state photolysis

A stock solution containing ITX (1.27 mg, 0.005 mmol) was prepared in acetonitrile

G et = E ox (D) -E red (A) -E T + C (2)
where G et is the free energy change of photoinduced electron-transfer (PET); E ox (D), the oxidation potential of the electron donor; E red (A), the reduction potential of the electron acceptor; E T , the triplet state energy level of electron acceptor; and C, the Coulombic stabilization energy (-0.06 V in acetonitrile). 41 For ITX, the measured oxidation/reduction potentials (1.5 eV/ -1.57 eV) and triplet state energy (2.64 eV) were taken from previous report. 42 The other potentials were determined experimentally by cyclic voltammetry.

The free energy change of electron transfer ( ) values were determined below: TX (E red = -1.62 eV, E T = 2.84 eV), 43 EAB (E red = -2.48 eV, E T = 3.0 eV) 44,45 and CQ (E red = -1.25 eV, E T = 2.18 eV) 46 were also calculated:




f. UV-Vis absorption

Absorption spectra were recorded on a Lambda 35 UV/vis spectrometer from PerkinElmer with a bandwidth of 1 nm and a scan speed of 400 nm•min -1 . All the solutions were analyzed in a quartz cuvette of 1 cm length.

g. Size exclusion chromatography (SEC)

An ) and enthalpy of reactions between IMes and TPO were calculated by utilizing the Gaussian 03 package. 47,48 The uB3LYP method with the 6-31G* was applied to optimize the relaxed geometries which were regularly checked. In the absence of light, the mixture is stable supporting that in its ground state, ITX cannot participate in an electron-transfer reaction. In either case, the 1 H NMR spectrum shows the disappearance of the NCHN proton after exposure during 2 min to UV radiation (365 nm, 65 mW•cm -2 ). In addition, reaction of the as-irradiated media with CS 2 gives the expected IMes-CS 2 complex whose structure has been identified by 13 C NMR (Figure 2.

3).

This characterization suggests deprotonation of the azolium salt to form the expected NHCs

IMes. Given the high reactivity of NHCs, it is however important to develop new techniques able to characterize these species just after their formation, i.e. before secondary reactions take place. quantitative reaction between IMes and CS 2 . 36 However, the zwitterionic IMes-CS 2 adduct is only partially soluble in solvents such as THF and acetonitrile, making yield calculation complex because the adduct is partitioned between solid and liquid phases. To obtain quantitatively a soluble and stable NHC adduct, we turned our attention towards the recently reported NHC-carbodiimide (CDI) adduct 51,52 demonstrating that this reaction may also cause the disruption of the IMesH + -IMes hydrogen bonds. [53][54][55] Having proved that the synthesis of NHC-CDI can be quantitative and easily monitored by 1 H NMR, 1,3-di-p-tolylcarbodiimide was added to the as-irradiated solutions of

Optimization of NHC yield a. Effect of ITX concentration

As discussed above, it is known that 3 ITX* is not only involved in electron-transfer with BPh 4 -, but also with photogenerated IMes (or SIMes) leading to the degradation of the desired carbene under irradiation. Therefore, it is crucial to investigate the influence of ITX concentration on the yield of photogenerated NHC. For this purpose, the influence of the ITX/IMesH + BPh 4 -molar ratios (0 -1.33) was assessed while maintaining the irradiation conditions unchanged (5 min, 65 mW•cm -2 ). As can be seen in Figure 2.9a, the increase of the ITX/IMesH + BPh 4 -ratio, until stochiometry is reached, results in a nearly linear increase of IMes yield. A maximum amount of 70 % can be generated upon using a stochiometric ratio. However, further increase of ratio ITX/IMesH + BPh 4 -causes the yield to slightly decrease (56 %). Presumably, a higher concentration in ITX contributes to more side reactions between IMes and 3 ITX*.

b. Effect of irradiation time and irradiance

The effect of irradiation time on IMes yield is illustrated in Changing irradiance has also a substantial impact on NHC yield. Keeping the ratio ITX/ IMesH + BPh 4 -of 1/3, a ten-fold decrease of light intensity, from 65 mW•cm -2 to 6.5 mW•cm -2 , causes conversion of IMes to decrease from 22.1% to 11.3 %. Because less ITX is excited, deprotonation of the IMesH + cation becomes slower. Consequently, less NHC is produced after a given irradiation time of 5 min.

c. Effect of photooxidant

The influence of other aryl ketone photooxidants than ITX on NHC yield was estimated using thioxanthone (TX), 4,4′-bis(diethylamino)benzophenone (EAB) and camphorquinone (CQ) (Table 2.1). CQ 0

[a] Determined by 1 H NMR spectroscopy after CDI addition.

[b] Irradiation with a 450 nm LED (55 mW•cm -2 )

As expected, the nearly similar redox properties of excited ITX (entry a) and TX (entry b) resulted in a comparable amount of IMes released, 22 % and 24 %, respectively.

Conversely, the replacement of ITX by EAB (entry c) and CQ (entry d) prevents the formation of IMes. In accordance with this result, the electron-transfer between BPh 4 -and the excited state of these two electron donors was found to be non-thermodynamically favorable (G et, EAB = 0.44 eV, G et, CQ = 0.07 eV). 44,46

Use of NHC photogenerator in polymerization a. Synthesis of polyurethane

There have been very few studies utilizing NHC organocatalyst in PU polymerizations in the literature. 16,24 Nevertheless, NHC photogenerators were evaluated in the model linear step-growth polymerization of isophorone diisocyanate (IPDI) and poly(ethylene glycol) (M n = 400 g•mol -1 , PEG 400 ) (Scheme 2.6).

Scheme 2.6. Synthesis of polyurethane catalyzed by photogenerated NHC.

Through a possible alcohol activation mechanism (Scheme A.II.1), 16,59 a polymerization was accomplished at 60 o C by mixing a stoichiometric amount of PEG 400 and IPDI followed by addition of the photogenerated NHC (0.3 mol %).

/ITX (entry b).

For comparative purposes, the free NHC IMes (entry a') was also included as polymerization control as well as an uncatalyzed reaction (entry c).

[a] Experimental number average molar mass M n obtained by SEC.

[b] Experimental weight average molar mass Mw obtained by SEC.

[c] Polymer dispersity index Đ= M w / M n .

[d] Glass transition temperature determined by DSC.

[e] Isolated yield.

Without catalyst, full conversion was not reached and a low molecular weight polyurethane was obtained (M n = 1700 g•mol -1 , entry c). In contrast, the two photoNHCs (entry a-b) exhibited greater polymer yields (> 95 %), higher molecular weights (4000 -5000 g•mol -1 ) and lower dispersity values Ð in the range from 1.5 to 1.7 (see SEC traces in Figure

b. Synthesis of polyester

Encouraged by this first result, the NHC photogenerator's catalytic activity was also evaluated in ring-opening copolymerization (ROCOP) of epoxide and anhydride. This reaction offers a controlled route to polyesters, although it has been much less studied than ring-opening polymerization of cyclic esters. 60,61 ROCOP generally requires organometallic initiators to reach efficient curing, but the use of NHC was reported very recently. 62,63 In a model copolymerization, the as-irradiated azolium salt/ITX mixture (1/1 equiv.) was added to a mixture of cyclohexene oxide (CHO, 500 equiv.) and phthalic anhydride (PA, 100 equiv.) (Scheme 2.7). Polymerization was conducted at 110 o C using thus CHO as both the monomer and the solvent. o C for 60 min in THF after the addition of the as-irradiated azolium salt/ITX solution (1/1 equiv.) or free NHC (1 equiv.).

Entry

Catalyst system PA conv. [a] (%) Ester conv. [b] (%) By contrast, it is well-known that conventional catalytic systems such as metalloprophyrin or salen can form a significant proportions of ether linkages resulting from CHO homopolymerization. 60,61 The alternating structure was also evidenced in the MALDI-

M n-SEC [c] (g•mol -1 ) Đ [d]
ToF

c. Synthesis of polynorbornene

In the field of catalyst precursors for olefin metathesis, NHC-coordinated ruthenium complexes with the general formula RuCl 2 (arene)(NHC) stand out by their ease of preparation. They can be produced in situ from commercial and inactive ruthenium dimers such as [RuCl 2 (p-cymene)] 2 (Scheme 2.8) after addition of a suitable free NHC such as IMes or SIMes, which causes the dissociation of this precatalyst. 64,[START_REF] Ohm | A New Synthetic Rubber Norsorex® Polynorbornene[END_REF] The resultant active complexes, RuCl 2 (p-cymene)-IMes or RuCl 2 (p-cymene)-SIMes, have emerged as versatile and efficient catalysts to promote the ROMP of both strained and low ring-strained cyclic olefins. [66][67][68] Building on our previous study, 36 [b] Cis content of the polymers calculated from 1 H NMR spectrum cis (%) = 100.

[c] Theoretical number average molar mass

M n-th = [ ] [ ]
.

[d] Experimental number average molar mass M n obtained by SEC.

[e] Polymer dispersity index Đ= M w / M n obtained by SEC.

[f] The solution was kept in dark for 1 min, n.d. means not determined due to no polymerization or insoluble polymer.

CONCLUSIONS

In presence of ITX, imidazolium or imidazolinium salts bearing tetraphenylborate as anion are able to generate photochemically the corresponding free imidazolylidene and 2.5

INTRODUCTION

Of high importance in photochemistry are arylborates, which are four-coordinate boron compounds bearing a tetrahedral geometry, a formal negative charge, and at least one aryl substituent. Although their photochemical reactivity has been studied since the late 1970s, 1,2 progress in this field has been made at the cost of intense debates, 3 and many unknowns still remain. 4 -Photolysis upon exposure to 254 nm irradiation was focused on tetraarylborate compounds such as NaBPh 4 . The most accepted mechanism involves a di-π-borate rearrangement leading to a three-membered biradical anion, evolving towards a more stable borirane anion isomer. Most studies on direct irradiation attempted to identify photoproducts (biphenyl, 1-phenyl-cyclohexadienne mostly) and elucidate photochemical mechanism. 1,6,7 Its utility in preparative photochemistry was demonstrated only recently. Observing that intermediate anion species could abstract acidic protons from water and alcohol, 7,8 Sun et al. proposed in 2008 a tetraphenyl borate salt containing as cation the conjugated acid of a nitrogen base (BH + ) such as bicyclic guanidine TBD (TBDH + ). 6 The base was photogenerated through photoinduced proton abstraction of BH + during the photolysis at 254 nm. For example, TBDH + BPh 4 -was used as photobase generator (PBG) for the organocatalyzed ringopening polymerization of cyclic esters 6 or thiol-epoxy polymerization. 9 Several authors added an aryl ketone derivative to extend absorption to near UV-Vis range via a supposed triplet-triplet energy transfer. [10][11][12][13] -Due to their relatively low oxidation potential, 14 arylborates such as Ph 4 B -or triarylalkylborate salts (Ph 3 BR -) were also used as electron donor in interand intra-PET reactions with cationic or neutral electron acceptor (EA) sensitizers such as carbocyanine, 15 fullerene, 16 coumarine, 17 fluorone, 18 benzophenone derivatives. 19 A boranyl radical is formed, which in turn undergoes rapid cleavage yielding a triphenylborane (BPh 3 ) and an alkyl/phenyl radical (R • /Ph • ). Only alkyl radicals have been applied for alkylation of the electron acceptor, 20 or initiation of radical polymerizations. 17 Recently, we proposed another application of photoinitiated oxidation of arylborates, the photogeneration of N-heterocyclic carbene (NHC) using azolium arylborate salts (NHCH + BPh 4 -) (Scheme 3.2). 21 When paired with an EA sensitizer such as isopropythioxanthone (ITX), an electron transfer from the borate (BPh 4 -) to the excited triplet state of ITX ( 3 ITX * ) may take place as described above. Unlike the previous case, the azolium cation (NHCH + ) enables a subsequent step of proton abstraction with ITX radical anion ITX -(1) to take place, yielding the expected free NHC (2). 21 Our recent investigations proved that it was possible to form 1,3-bis(mesityl)imidazol-2-ylidene (IMes) and its saturated analogue SIMes, the two most employed NHCs. 21 The utility of this twocomponent NHC photogenerator NHCH + BPh 4 -/ITX was also demonstrated in photopolymerization reactions to form polyurethane, polyester and polynorbornene. 22 Indeed, although NHCs have brought profound changes in catalytic organic synthesis, [23][24][25] they generally require inert atmosphere and harsh conditions for their generation. As a result, an air-stable system able to generate NHC on demand and on simple UV exposure has the potential to significantly simplify implementation of the broad spectrum of NHC-catalyzed reactions. While it is true that thermally latent NHC precursors 26,27 also exist,

EXPERIMENTALS

Materials

Synthesis of IMesH + BPh 4 -

The IMesH + BPh 4 -was synthesized as described previously. 21 1):

Characterization methods

Nanosecond

[ 3 ITX*](t) = [ 3 ITX*](0) (1)
where is the lifetime of the ITX triplet without quencher. Computational procedure. Triplet energy of IMesH + BPh 4 -molecule and ITX were calculated by utilizing the Gaussian 03 package. 30,31 The uB3LYP method with the 6-31G* was applied to optimize the relaxed geometries which were regularly checked.

Determination of photogenerated

where [IMesH + BPh 4 -] is the initial concentration of IMesH + BPh 4 -.

The validity of the method was checked by titrating a free IMes solution (1  10 -4 M in acetonitrile) using a similar acetonitrile PR solution as titrant (2  10 -4 M). An acceptable uncertainty of the order of 10 % was obtained on the value of the initial IMes concentration.

This difference was not only assigned to the measurement error, but also to the fact that IMes is not stable in air.

RESULTS AND DISCUSSION

Assignment of transient species a. Nanosecond laser spectroscopy

To identify the intermediate species formed in the 2-component system ITX/IMesH + BPh 4 -(Scheme 3.2), excitation transfer reactions of ITX with different quenchers were recorded by means of a nanosecond laser spectroscopy. Laser flash exposure was performed at 355 nm in deaerated and anhydrous acetonitrile, where only the sensitizer (ITX) absorbs the light. Given the short lifetime of ITX singlet state as well as its high intersystem crossing quantum yield, 32,33 ITX is assumed to interact mainly via its triplet state.

As can be seen in Figure 3.1, the transient absorption spectra of ITX alone (trace a) shows an absorption maximum at 630 nm, fully consistent with the ITX triplet ( 3 ITX*) in acetonitrile reported in the literature. 29,32,[34][35][36] Slow decrease of the transient absorption was attributed to the deactivation process of the 3 ITX* with a decay of 5.7 µs at an initial ITX concentration of 10 -4 M. To study the effect of the azolium cation, the reactivity of the triplet state was evaluated in presence of IMesH + Cl -since no quenching is expected from Cl -anion. 37 The absorption at 630 nm was weakly quenched when this imidazolium salt was added to a deoxygenated solution of ITX. A low rate constant (round to 4.7  10 6 M -1 •s -1 ) was determined by constructing a Stern-Volmer plot of the triplet lifetime as a function of [IMesH + Cl -], with the triplet being monitored at 600 nm (see experimental section for details).

A typical decay experimental trace for the triplet decay of ITX in presence of IMesH + Cl -is given in Figure 3.2a, showing clean first-order kinetics. The existence of very weak interactions is consistent with the fact that photooxidation of 3 ITX* by IMesH + is thermodynamically unfavorable (G et = + 0.74 eV) 21 and that the triplet energy transfer between ITX (E T = 2.77 eV) and IMesH + (E T = 3.34 eV) exhibits a fairly endothermic character (Table 3.1). It can thus be concluded that the azolium cation plays a minor role in the primary photochemical reaction.

Similarly, triplet quenching experiment was carried out with NaBPh 4 to investigate the specific role of the tetraarylborate anion given that Na + cannot quench or donate proton (Figure 3.1b). As can be seen in Figure 3.2b, the decay rate was much faster, illustrating the more efficient quenching of 3 ITX* by BPh 4 -. Additionally, it can be noticed that the decay trace does not return to zero in this case; deviation to first order kinetics was attributed to the fact that the triplet absorption probably overlaps with another species. To clarify this point, Subsequent spectra show the slow decay of this broad peak. We hypothesized that this transient species could be the radical anion ITX -generated after photoreduction of 3 ITX* by BPh 4 -. To support our assignment, we rely first on the fact that the electron transfer for this reaction is slightly exothermic (G et = -0.13 eV), 21 because of the high oxidation potential of the donor anion BPh 4 -. Second, triplet energy transfer, which is the second possible interactions process between ITX (E T = 2.77 eV) and BPh 4 -(E T = 3.64 eV) (Table 3.1), is not energetically favorable. Third, our transition absorption spectrum strongly resembles that of thioxanthone radical anion observed by Schuster et al., who reported also a similar broad absorption at 650 nm. 38,39 To the best of our knowledge, the spectrum of ITX -has not been previously reported. Because of the absorption overlap between 3 ITX* and ITX -species, the quenching rate (round to 4.1 10 7 M -1 •s -1 ) was obtained by analyzing the first short component of the transient absorption decay. Additionally, the triplet transient of ITX in the presence of BPh 4 -was quickly quenched if an oxygen-saturated solution is used, with a typical rate constant in order of 10 9 M -1 •s -1 (Figure 3.2c). 40,41 This lends additional confidence to the involvement of 3 ITX* and the operation of a charge transfer mechanism.

However, there is no additional band in the transient spectrum that might be attributed to the boranyl radical 4 or other boron species derived from the oxidation of tetraphenyl borate anion. 7 In borate salts, it is well established that back electron is not significant but that boronyl radical can dissociate very rapidly and irreversibly (Scheme 3.2). The failure to observe Ph 4 B  (4) indicates that its very short-lived character 16 as it probably dissociates or rearranges during the laser pulse.

Having investigated the individual role of IMesH + and BPh 4 -, quenching experiment was then carried out using IMesH + BPh 4 -(Figure 3.2d). The similarity of quenching rate obtained by this quencher (round to 4.9 × 10 7 M -1 •s -1 ) and NaBPh 4 (round to 4.1 × 10 7 . M -1 •s -1 ) is indicative that the photoreduction of ITX by BPh 4 -is the primary photochemical process.

Additionally, a broad feature characteristic of ITX -was also observed in the region of 560 -680 nm (Figure 3.1c). However, there is no additional absorption band in the 380 -430 nm region that could be assigned to a thioxanthyl ketyl radical (ITXH • ) derived from subsequent proton transfer step of ITX -with IMesH + . 29 This result can be understood on the basis of a much slower proton transfer step compared to electron transfer. The only indication for protonation of the radical anion is the shorter lifetime of ITX -in presence of IMesH + BPh 4 -(29.5 µs) compared to NaBPh 4 (40.1 µs). A final quenching experiment was also carried out by using IMes, the targeted NHC. A significant acceleration of the decay at 600 nm is observed, leading to an increased quenching rate (round to 7.2 × 10 9 M -1 •s -1 ). Such a result is consistent with the stronger donor properties of IMes (E ox = -0.84 eV) compared to BPh 4 -(E ox = 0.94 eV). 22 It also highlights a more complex mechanism where photogenerated NHC can also interact with triplet ITX and compete with BPh 4 -for charge transfer.

a. EPR

As indicated in the previous section, failure to observe the boranyl radical (4) in the nanosecond experiment may indicate that this short-lived species dissociates very rapidly to give the phenyl radical ( 5) and triphenylborane (6). Spin trapping and EPR detection (with the trapping reagent α-phenyl-N-tert-butylnitrone, PBN) were employed to detect the free radical Ph  formed during in situ irradiation of ITX in presence of tetraphenyl borate anion.

In are in agreement with a PBN spin adduct of the phenyl radical. In addition, these values correspond reasonably well to previously reported values for these spin adduct in literature. 16,19 This results strongly suggests that the transition state of oxidized BPh 4 -could dissociate into Ph  and BPh 3 . 16,42 Moreover, it is noted that the detection of this phenyl radical is generally very challenging, 16 and failures are generally reported in the literature for both electrochemically 1,6,7 and photochemically 43 induced oxidation of BPh 4 -. By contrast, n-butyl radical was more easily detected from a similar oxidation of Ph 3 BBu -species. 43 As discussed later, the formation of biphenyl upon radical coupling being unlikely, the most plausible c). 21 Further evidence for the generation of IMes was given upon adding CS 2 to the as-irradiated medium. A red precipitate forms immediately that was straightforwardly assigned to IMes-CS 2 using 1 H NMR analysis (Figure 3.5d). With these results and the identification of ITX -described above, it is thus possible to formulate a tentative second step where NHC are created through proton abstraction of IMesH + cation by ITX -. A method to determine NHC concentration that were described in previous paper (formation of NHC-carboddiimide adduct 22 ) revealed an IMesH + conversion of 70 % after 5 min irradiation. revealed the progressive release of NHC and led to a maximum yield of 50 % achieved after 5 min irradiation. This result emphasizes that IMes was created and that this species coexists in the reaction medium with its conjugated acid IMesH + and may form H-bonded bis(carbene)-proton adduct. [44][45][46] Formation of this C•••H-C type adduct has been already observed spectroscopically, e.g. UV, NMR. 46 Evidence for the formation of bis(carbene)proton adduct is visible in the spectrum of Either increasing amount of ITX or prolonging irradiation afford the rising of 3 ITX* amount, which postulates the photooxidaztion of IMes. In fact, occuring this undesired reaction is undesrable but the exclusion of this undesired reaction is clearly impossible owning to their chemical nature. Therefrom, it is crucial to adjust ratio between IMesH + BPh 4 -and ITX in order to sufficiently produce IMes and preclude prevalence of side reaction to gain a good yield of photogenerated IMes.

c. GC-MS

To move forward in the identification of other photoproducts, the freshly irradiated photolysis medium (N 2 -saturated acetonitrile solution of ITX/IMesH + BPh 4 -) was analyzed by GC-MS. Three products were detected: biphenyl, isopropylthioxanthene and ITX (Figure

and Table A.III.1).

Biphenyl was obtained in good yield and its formation was also observed in other (photo)oxidation experiments of BPh 4 -anion by iron (II) complex, 42 1,4dicyanonaphthalene 7 or coumarin derivative. 43 Labelling studies 7,43 revealed that two phenyl groups of the biphenyl come from the same borate, suggesting that the biphenyl was formed from carbons that were bound to boron. Having the evidence that phenyl radicals are formed in our system (EPR data), it is therefore reasonable to assume that biphenyl comes from triphenylboron. The most plausible route would be a rapid auto-oxidation of BPh 3 by Ph  followed by generation of biphenyl and Ph 2 B  (BPh 3 + Ph   Ph-Ph + Ph 2 B  ).The fate of the boron radical will be discussed in the 11 B NMR section. Since it does not require the addition of oxygen in the reaction medium, biphenyl is thus a primary photochemical product. However, except ITX, none of these thioxanthone reduction products were detected by GC-MS. This is apparently due to their thermal instability in the GC injection port as established by Schuster et al.. 38 Isopropylthioxanthol is known to disproportionate upon heating to form ITX and isopropylthioxanthene. Additionally, pinacol can be easily oxidized under air to regenerate ITX. 38,48 This set of reactions is consistent with the formation of a ketyl radical and the thioxanthone products detected in GC-MS. Note also that 1-phenyl-1,4cyclohexadiene is not detected while it is a major product of the direct photolysis of BPh 4 -in presence of H donors. 1 Its absence confirms that energy transfer from ITX to BPh 4 -is not the predominant mechanism.

d. 11 B NMR spectroscopy

In arylboron photochemistry, the identification of boron photoproducts represents a considerable difficulty. Boron products can be involved in complex reactions where both the nature and the role of the boron species can be hard to determine; additionally, their proportion generally differ depending on whether irradiation was performed under N 2 or air.

As emphasized in Scheme 3.2, the oxidized borate is assumed to evolve in triphenylborane. 16,21,50,51 Nishida et al. continued this investigation and proposed subsequent degradation reactions. 42 In our specific case, the photogenerated BPh 3 may also form ate complexes with NHC Lewis base. 52,53 Despite these challenges, 11 B NMR spectroscopy remains the best tool to identify the boron species in our systems. In the case of photoinitiated oxidation of arylborates, the NMR identification has rarely been studied 7,54 while it can serve to provide insights into the mechanism. Nevertheless, accurate identification of the groups attached to boron is not trivial due to the lack of detailed structural information that can be obtained from 11 B NMR data. that is absent in the photolysis medium. In addition, the irradiation of BPh 3 with or without ITX (1 equiv.) did not cause any change in the spectrum. In contrast, the addition of IMes (1 equiv.) resulted in the formation of a white precipitate and a downfield chemical shift (-9 ppm, see Figure 3.9) characteristic of NHC-borane species, that was also not found in spectrum of reaction medium. Though the above experiments do not necessarily demonstrate the non-existence of BPh 3 in the photolysis solution, it is interesting to note that both free BPh 3 or BPh 3 -IMes adducts are not present after irradiation, and that degradation reaction with triplet excited ITX cannot be advanced as explanation. As hypothesized in the last section, it is thus more reasonable to conclude that BPh 3 may react with Ph  to form Ph 2 B  . To shed further light into the identity of these boron-containing species, CS 2 was added to the photolysate solution (ITX/IMesH + BPh 4 -) just after irradiation (Figure 3.8c).

This led to immediate conversion of the intermediate at 1.1 ppm to a new broad resonance at 45.2 ppm while the position of the second resonance at 5.9 ppm remained unchanged. The change of chemical shift suggests that a NHC-borane species was initially present; the addition of CS 2 causing its dissociation and the subsequent formation of the more stable IMes-CS 2 zwitterionic adduct. By this means, a free boron species can be released whose value of 11 B chemical shift at 45.2 ppm can be useful for its assignment. Accordingly, observed 55 and calculated 11 B NMR spectra 56 are consistent with the exact chemical shift of diphenylborinic acid Ph 2 BOH. Additionally, the singlet signal at 1.1 ppm could be ascribed to IMes-Ph 2 BOH adduct present in the photolysis solution. However, the second weak signal at 5.9 ppm could not be assigned. Notably, this second boron-containing complex is not able to form ate-complex with IMes due to the absence of change in the chemical resonance when

CONCLUSIONS

The photochemical mechanism underlying the release of NHC upon irradiation of IMesH + BPh 4 -with ITX has been clarified. As summarized in Scheme 3.6, the first step is an electron transfer (k q = 4.9 × 10 7 M•s -1 ) between electronically excited ITX and borate anion BPh 4 -acting as electron donor to yield the ion pair IMesH + /ITX - and the boranyl radical Ph 4 B  . This latter is subjected to a rapid decomposition into Ph  and Ph 3 B. A subsequent reaction of the ion pair is proton abstraction of IMesH + by ITX -to form the IMes (NHC) and the ketyl radical (ITXH  ). Of high importance for NHC reactivity was the fate of organoboron products. Ph 3 B and Ph  were proved to undergo a multi-step oxidation reaction, yielding biphenyl and diphenylborinic acid.

Ph 2 BOH was found to react with the photogenerated IMes to give an IMes-Ph 2 BOH ate-complex. While it is difficult to avoid the formation of NHC-boron adduct, our recent investigation showed that they did not prevent the activity of its NHC photogenerator for ring-opening metathesis polymerization or synthesis of polyurethane, thus demonstrating its practical interest.

CHAPTER IV. COMBINING A LIGAND PHOTOGENERATOR AND A RUTHENIUM PRECATALYST: A PHOTOINDUCED APPROACH TO CROSS-LINKED ROMP POLYMER FILMS

INTRODUCTION

As one of the most valuable tools for the generation of C=C double bonds, olefin metathesis has been exploited in many useful ways, including complex architectures, 1 pharmaceutical products, 2 or biodegradable copolymers. 3 In all these cases, progress has been mainly driven by the development of advanced and well-defined metal alkylidene complexes, like the notorious Grubbs and Schrock catalysts. 4,5 Ligand fine-tuning has resulted in a portfolio of catalysts featuring a broad range of reactivity, functional group tolerance and bench stability. In this respect, a major breakthrough was in 1999 the introduction of Nheterocyclic carbene (NHC) ligands in the place of the conventional phosphine compounds. 6,7 In particular, sterically bulky and strong electron donating NHCs have proven to be highly effective, resulting in NHC-coordinated ruthenium complexes with longer life-time and superior catalytic activity. 6,8 However, the main developments in olefin metathesis catalysis have responded more to the needs of synthetic chemistry (as the above mentioned examples show), but have been less useful for the preparation of polymer materials. Ring-opening metathesis polymerization (ROMP) of cyclic olefins is currently the main type of olefin metathesis polymerizations. 6,9 To date, the use of ROMP polymers has been limited to the manufacturing of specific structural and engineering materials. This despite the broad spectrum of properties achievable by the major ROMP polymers, derived from norbornene (NB), cyclopentene, cyclooctene or dicyclopentadiene (DCPD). For example, polycyclooctene (Vestenamer®) and polynorbornene (pNB) (Norsorex®) elastomers are employed after vulcanization as shock absorbers and anti-grip materials. In addition, hydrogenated pNB (Zeonex®) is a high performance optical thermoplastic, while polydicyclopentadiene (pDCPD) (Metton® or Telene®) is known as a prime thermoset with mechanical properties similar to engineering thermoplastics. 10 Most of these reported ROMP polymers have been utilized only as structural materials because their processing methods are restrictive. For example, pDCPD is prepared essentially by reaction injection molding, which requires separate monomer and catalyst compartments, efficient mixing and titration heads, nitrogen inerting of the mold, etc. [11][12][13] To unlock the potential of ROMP polymer materials, more efficient and -smarter‖ catalysts are crucial, especially to extend the shaping of ROMP polymer as film, particle or fiber, but also in general, to foster more versatile, energy-efficient and facile process conditions. To address those challenges, latent olefin metathesis catalysts complexes have emerged, i.e. catalysts able to release active species in situ and -on demand‖ by external stimulation. Heat, [14][15][16] mechanical force, 17 and radiation [18][19][20] are the most common stimuli. A separate category concerns catalysts activated after the addition of an external chemical agent. [21][22][23] Among these different options, the activation brought about by visible or ultraviolet irradiation is particularly attractive. 24 Photolatent ROMP catalysts can be activated at ambient temperature, and make possible the use of storable and ready-to-use formulations combining together the catalyst and the monomer(s). As standard product of a photoactivated polymerization, thin films based on cross-linked ROMP polymer are also expected with prospect for the development of novel UV-curable coatings or adhesives. Finally, unlike the other triggers, the reaction can be spatially-controlled. 23,25 In the field of photochemically activated olefin metathesis, three main approaches have been proposed, in which the activation proceeds generally by a photochemical reaction involving the ligand of a transition metal complex (one must nevertheless note an original metal-free redox-mediated approach for ROMP using vinyl ethers as initiator and pyrilium salt as photo-oxidant 13 ). The two most reported routes rely on ligand photodissociation 26,27 and ligand cis-to-trans photoisomerization. 18,20 The third route, which is the least investigated, is the focus of the present study. In the latter, the active olefin metathesis catalyst is generated from an inactive and non-absorbing precatalyst after reaction with a ligand liberated by a second photosensitive compound (ligand photogenerator). This methodology was first introduced by Grubbs et al. in 2009 by using a Ru-alkylidene complex ligated by acetylacetonate (acac) as precatalyst and a photoacid generator (PAG) based on triphenylsulfonium chloride. 21 Upon irradiation at 254 nm, HCl was released, and the subsequent displacement of acac by Cl -resulted in an active metathesis catalyst. A similar method was also employed by Pier et al. using a ruthenium carbide and Ph 3 S triflate (OTf). 22 Recently, we replaced the PAG with a NHC photogenerator and used a stable, commercially available dimeric Ru(II) complex as metathesis inactive catalyst. ,1) and a dimeric Ru(II) complex, inactive catalyst (Ru-1), results in a active olefin metathesis photocatalyst (Ru-1-

IMes).

The NHC (IMes) can be photogenerated through two distinct routes: a direct photolysis or a sensitized reaction using an electronically excited electron acceptor.

In this contribution, the main aim is to demonstrate the utility of this tandem approach for the photoinduced preparation of cross-linked ROMP films. To this purpose, we report first our efforts to improve the photocatalytic system by addressing a number of issues:

optimizing the yield of photogenerated IMes, ensuring a long-term photolatency, identifying the active metathesis catalyst and the initiating species (Ru-NHC complex), and finally, collecting some preliminary results on photoROMP in solution of common monomers such as NB. In a second part, a range of copolymer films was prepared by photoinduced ROMP between NB and DCPD using the photocatalytic mixture based on and tetrahydrofuran (THF) received from VWR were distilled and stored over molecular sieve (4 Å) under nitrogen atmosphere before using.

Experimental protocols

Quantification of IMes yield. Quantitative conversion of IMes into IMes-CDI adduct was described elsewhere. 29 The irradiation sources include a medium pressure Xe-Hg

Preparation of the active metathesis catalyst (p-cymene)RuCl 2 (IMes) (Ru-1-IMes).

Noels' complex starting from Ru-1 and IMes was synthesized based on a protocol developed by Nolan et al. 34 while Noels' complex starting from the tricomponent system 1/ITX/Ru-1 was prepared following procedure below.

A mixture of Ru-1 (0.6 mg, 1 equiv.), 1 (3 mg, 5 equiv.) and ITX (0.62 mg, 2.5 equiv.) in 1 mL THF-d 8 was charged into a borosilicate NMR tube. Then, the tube was irradiated at 365 nm (Xe-Hg lamp, 20 mW•cm -2 ). 1 H NMR spectra were recorded at different given time (t = 0, 10, 20 and 30 min). Experiments were performed at ambient temperature with a 4mm double resonance MAS Bruker probe at a speed of 12kHz, a 1 H π/2 pulse duration of 5.6 μs, a contact time of 1 ms and a recycle delay of 2 s. Chemical shifts reported thereafter are relative to tetramethylsilane and the de-convolution of the experimental spectra were carried out with the DMfit software.

The fraction of pendant cyclopentene rings involved in olefin coupling reaction can be determined from the 13 , 36 or a sensitized reaction involving an additional electron acceptor species such as isopropythioxanthone (ITX). 28,29 Photolysis was carried out by exposure of 1 in THF-d 8 (0.03 M) to 254 nm, while the sensitized reaction was conducted in similar conditions with a mixture of 1 and ITX (1/1 equiv.) upon irradiating this time at 365 nm or 405 nm (thanks to the higher wavelength absorption of the sensitizer). The precise reaction mechanisms in both cases have been commented on elsewhere. 28,29,37,38 And Table 4.1 only compares the efficiency of the two methods in the light of the amount of NHC generated after 5 min irradiation. 39 Clearly, the combination of 1 and ITX at 365 nm (entry a) and 405 nm (entry b)

shows improved activity in comparison with 1 irradiated alone at 254 nm (entry c). Only 11 %

IMes was liberated in this latter case versus 29 % (405 nm) and 67 % (365 nm) in the sensitized reaction. This suggests that the photoinduced route using the 2-component system 1/ITX is more efficient. However, since the two photochemical mechanisms are completely different, there is no obvious explanation. To account for the poor NHC yield after the direct photolysis, one possible reason is the fact that IMes has a non-negligible absorption band at 254 nm (ε 254 = 9.1 10 3 M -1 .cm -1 ), while it does not absorb in the UVA range. This could reduce the amount of photons absorbed by 1, and undermine the yield of the photolysis reaction. Therefore, the mixture 1/ITX was employed as NHC photogenerator in the rest of the study. 34 This suggests that (p-cymene)RuCl 2 (IMes) could be actually the active catalytic species when the tandem system NHC photogenerator/Ru precatalyst is irradiated. However, all attempts to characterize by 1

c. Catalyst photolatency

Photochemical latency is a very important characteristic for practical implementation of a photocatalytic system. For this, the catalytic activities of Ru-1 (in presence of ITX) has been tested with exclusion of light by using standard ROMP substrates such as NB. Results are presented in Table 4.2 using CD 2 Cl 2 as a solvent. The photochemical latency of Ru-1 was found to be poor since 18 % conversion (entry a) was obtained after 4 h at room temperature. Although the mechanism of this secondary reaction is still elusive, we propose that ROMP could proceed by a decoordination of the η 6 -arene ligand to afford a highly unsaturated ruthenium center that evolves into a propagating alkylidene after addition of a NB molecule. 40,41 In agreement with this hypothesis, no premature reaction took place with ENB (entry b) due to its higher steric hindrance which limits the reaction with the ruthenium center. In the search of other ruthenium pre-catalysts able to ensure a complete photolatency, did not increase the conversion, illustrating the minor role played by the NHC precursor in the instability issue. Conversely, the Ru-3 complex exhibited nearly no latency since 90 % conversion was reported after 4 h (entry e). These results showed the size of the halogen as a crucial parameter affecting the equilibrium for olefin binding. 42,43 As expected, a larger halogen than chloride such as iodide disfavors olefin binding due to steric hindrance, resulting in a better latency. In all instances, a low content of cis double bond (ζ cis = 23-30 %)

Ru

was determined in the polymer structure, which is consistent with the literature data for ROMP relying on ruthenium dimers as catalyst. 44,45 In conclusion, Ru-2 seems the best precatalyst candidate, however it remains an open question whether it will form an active metathesis catalyst when coupled with IMes.

d. Photoactivated ROMP in solution

The catalytic activity of the three-component system Ru precatalyst/1/ITX (1/5/2.5 equiv.) was evaluated for the ROMP of NB (Table 4.3). The 1/ITX acts as NHC photogenerator in this case, and the reaction was performed in CD 2 Cl 2 inside a NMR tube. As expected, the irradiation (365 nm, 1 min) of a solution devoid of Ru dimer resulted in no reaction (entry a). By contrast, a small conversion was observed when the Ru-1 precatalyst was irradiated alone (1 %, entry b) or in presence of ITX (7 %, entry c). This result can be reconciled with the poor latency of Ru-1 noted earlier (see section c), and suggests that the η 6 -arene ligand displacement could be enhanced by a photochemical activation. The vacant site created on the Ru can form subsequently an active Ru-NHC complex after the addition of a monomer molecule. 31,40 The question of the mechanism and the nature of the metathesis initiating species will be addressed in the next section. A much higher catalytic activity was achieved in presence of 1 (entry d), with 78 % conversion after 1 min, suggesting the successful formation of a highly active Ru-arene complex, which is likely to be Ru-1-IMes. The content of cis double bond (ζ cis = 52 %) was similar to that obtained with previous systems and supports this assumption. 40,46 By comparison, much lower cis contents are generally reported with phosphine-coordinated ruthenium complex. 47 Upon increasing the proportion of ITX proportion from 2.5 equiv. to 5 equiv., a significant increase of conversion was noted (94 %, entry e). In contrast, irradiation at 405 nm instead of 365 nm led to drop the conversion (22 %, entry f). This can be rationalized on the basis of a lower IMes yield at higher excitation wavelength (see section a). For the polymer produced with the tandem approach precatalyst/NHC photogenerator (entries d-f), the molecular weight distributions were monomodal (SEC data), suggesting that only one mechanism is involved in the catalysis.

However, high molecular weights (M n -SEC ) and rather broad polydispersities (Đ) were obtained, which reflect a poor control of the initiation step. The source of the problem is the ill-defined initiation mechanism and above all, the poor initiation efficiency resulting in only a small number of propagating centers produced in solution. This issue has been widely reported with such class of NHC-coordinated Ru complex. 31,32,48 The effect of the precatalyst structure was also examined. Upon replacing Ru-1 by Ru-2, the rate of polymerization slightly decreased (69 %, entry g). By the same reasoning used above to explain latency, it was expected that catalyst activity would decrease from Cl to I.

This result is also consistent with previous investigations about the lower catalytic activity of RuI 2 (arene)(phosphine) complexes. 49 In this case, there are consequently less backbiting reactions, resulting in polymer with higher molecular weight and a fairly narrow dispersity index. Ru-3 precatalyst (entry h) led to similar conversions, but the polymer was not soluble in THF, indicative of an even lower initiation efficiency.

Our photocatalytic system was also active in the ROMP of ENB. The results are summarized in Table 4.4. The reaction process was performed under similar conditions to those described for NB, except for irradiation time that was extended to 10 min. Indeed, we expect that a functional NB derivative such as ENB would be less reactive due to a greater steric crowding. Accordingly, only 64 % conversion was achieved after 10 min irradiation at

e. Mechanism of catalyst activation

Our approach relies on the in situ formation of an active Ru-arene complex from reaction between [RuCl 2 (p-cymene)] 2 Ru-1 and the photogenerated IMes ligand. However, the presumed NHC-coordinated catalyst Ru-1-IMes identified in this study (section b) as the active metathesis catalyst lacks a suitable metal-carbene fragment to initiate ROMP. The literature reports an ill-defined mechanism involving p-cymene decoordination and a subsequent reaction between the resultant highly unsaturated ruthenium and a cyclic olefin as being accountable to the formation of active species. 31,50 This last section questions the shows a typical conversion-time curve while alternating between periods of radiation exposure (white area) and dark (grey area). Surprisingly from the mechanistic hypothesis, a temporal control of polymer chain growth was demonstrated through intermittent exposure of the reaction to UV light since an almost stop of polymerization was observed when light was switched off. This result is in apparent contradiction with the hypothesis of a formation of an active Ru catalyst whose activity could be preserved without irradiation. While almost no conversion occurred in the dark (off periods), polymerization could be re-initiated by reexposure to light. This has been attributed to the fact that radiation might promotes the multiple insertion of new NB units into the propagating center. 33,40,51 Further experiments, in particular to study the effect of irradiance and wavelength, are required to understand the complete role by irradiation. Nevertheless, one can propose a putative reaction mechanism (Scheme 4.2) for the ROMP photoinitiation. First, the active complex RuCl 2 (arene)(NHC) Ru-1-IMes is subjected to the rapid loss of p-cymene ring upon photochemical activation to form an unsaturated Ru species A. 33,40,51 As a subsequent step, the insertion of NB into the metal center occurs to form the Ru complex RuCl 2 (IMes)(NB) B.

Then a sigmatropic [1,2-H] rearrangement leads to the formation of a ruthenium-alkylidene complex C (initiating species). 47 The next steps follow the typical chain-growth polymerization mechanism involving the metallacyclization and cycloreversion steps, with the particularity that the insertion of new cyclic olefin molecules could be facilitated by light.

According to this proposed mechanism, unsuccess in the ROMP of cyclooctene probably relates to the restrictive [1,2-H] rearrangement of such monomer. 47 On the other hand, a local excess of IMes photogenerated is supposed to afford a bis(IMes)-Ru. 51 Unfortunately, it is well-known that the bis-NHC ligated Ru analogue is inactive at ambient temperature owing to its difficulty in dissociating of one NHC ligand to form the active species C. 23,52 Thus, total number of active ROMP C decreases and thus producing the polymer with M n that is significantly higher than its theoretical value.

Preparation of cross-linked ROMP polymer films a. Scouting experiments

For the preparation of UV-cured films by photoactivated ROMP, emphasis has been made on DCPD because this monomer is able to form a cross-linked structure with high stiffness and impact resistance. 53 To assess to which extent DCPD could react with our photocatalytic system (Ru-1/1/ITX), preliminary tests were carried in CH 2 Cl 2 by monitoring the photopolymerization kinetics using real-time FTIR.

c. FTIR spectroscopy

Turning to chemical structure, the same series of five films was analyzed by ATR-FTIR spectroscopy. In Figure 4.6A, all spectra show the absence of signal at 1566 cm -1 ascribed to the alkene C=C stretching vibration of norbornenyl groups (present indistinctly in NB or DCPD). 28,61 This suggests that norbornenyl rings were either completely reacted or removed upon drying. Because cyclopentene has a lower ring strain than NB, the frequency of the C=C stretching vibration is decreased and generally arises at ~ 1610 -1650 cm -1 . The υ(C=C) of the internal double bond of cyclopentene is not visible in our system regardless of DCPD content. However, it is difficult to draw conclusions because this vibrational mode as it generally gives rise to a weakly intense band. To estimate the presence of cyclopentene rings, it is more reliable to look at the specific out-of-plane deformation vibration at ~ 710 cm -1 .

Clearly visible at 708 cm -1 , its presence supports an incomplete cross-linking process. 58 The signals at 970 cm -1 and 754 cm -1 are attributed to the deformation vibrations of =C-H bonds on trans and cis acyclic double bonds, respectively. 58,62,62 They indicate that the chain backbone is made up of a mixture of both trans-and cis-disubstituted alkene -(CH=CH)-in accordance with a ROMP mechanism. The =C-H stretching region at 3000 -3100 cm -1 is also informative because it is not overlapped by alkane C-H stretching absorption occurring below 3000 cm -1 . As shown in Figure 4.6B, we found two broad signals at 3047 cm -1 and 3002 cm -1 in the spectra of polymer films containing DCPD, readily ascribed to the =C-H stretching of -CH=CHvinylene groups. The higher frequency can be associated to cyclic olefins (residual cyclopentene) while the second signal at 3002 cm -1 reflects acyclic olefin (cyclopentene reacted by ROMP). 35,61,63 In conclusion, the FTIR data of DCPD-based films show a partial cross-linking, which occurs through a metathesis mechanism. The possibility of cross-linking via olefin coupling was studied in the next section using solid state 13 C NMR.

d. CPMAS 13 C NMR

In carbons (from cyclopentene) into 2 sp 3 carbons (Scheme 4.3). Consequently, the ratio of integrated area of sp 2 -C and sp 3 -C gives access to the fraction x of cyclopentene involved in olefin coupling (see experimental section for calculation details). 35,64 Our calculations reveal a significant contribution from olefin coupling which ranges from 11 to 34 %. Olefin coupling of cyclopentene ring tends to increase with a higher concentration of NB: 11 % (DCPD-100) < 22 % (NB-25) < 26 % (NB-50) < 34 % (NB-75). This trend can be rationalized on the basis that ROMP of NB is known to be highly exothermic and that olefin coupling is activated by heat. 57

e. Thermal analysis

The same series of NB/DCPD-based specimens was analyzed by DSC and TGA under N 2 . As shown in Figure 4.8A, the DSC thermograms indicate a strong impact of DCPD content on the glass transition temperature (T g ). As expected, the lowest T g value was found in Table 4.5 for pure polynorbornene (pNB) (NB-100); the value of 27 °C is fully consistent with literature data. [START_REF] Ohm | A New Synthetic Rubber Norsorex® Polynorbornene[END_REF]66 Upon increasing the content of DCPD, higher T g values were gradually obtained, with a maximum of 114 °C found for DCPD-100. This tendency is likely to be caused by the increase of cross-linking density as more cyclopentene rings react.

In the literature, the T g of polydicyclopentadiene (pDCPD) generally ranges from 53-165 o C. 53,54 Hence, our intermediate T g value agrees with a significant fraction of non-reacted cyclopentene rings as evidenced by FTIR and solid-state NMR. The TGA traces in Figure 4.8B are much less dependent on the DCPD concentration and show systematically two distinct decomposition regions. The gradual weight loss during the first decomposition stage arising at 200 °C is suggestive of low-molecular weight chains decomposition. Because of its sluggish polymerization rates, pure pDCPD is likely to contain a higher fraction of short linear chains. This drives a weight loss at low temperature that is significantly more marked than for NB-samples. During second decomposition stage taking place at 400 -500 o C, all curves exhibited a serious decline in the weight loss, illustrating prompt degradation of main polymer chains. 53,67 DCPD-100 showed the greatest thermal stability. A slight and progressive decrease was noted upon increasing NB loading. This is consistent with the drop of cross-linking density. 67 Also noteworthy is that a single main decomposition peak is observed regardless of NB content, suggesting a good homogeneity of the copolymer network. All samples left practically char coal residues with the highest amount of 23 % from DCPD-100 at high temperature.

f. AFM and microscopy study

The Young's modulus (E) which represents elasticity does not vary largely upon changing the incorporation ratio of NB in copolymers (Table 4.5). Generally, the configuration of polymeric structures can have a highly influence on the E value. Herein, the polymer films, consisting of a binary blend of the trans and cis configuration on the polymeric backbone, are more malleable than the ones initiated by Ru complex without NHC ligand. 55 Besides, a highly brittle structure of pure pNB, showing a higher E value, is likely responsible for a moderate increase of E with NB feeding. The microstructure of the ruptured solid films has been observed by scanning electron microscopy (SEM), as presented in Figure 4.9. Consistent with literature, 67 the higher DCPD loading is, the relative smoother the surface morphology is. This characteristic is in accordance with the increase in πinteraction of double bonds which exhibits better packing of the polymeric chains. 67,68

CONCLUSIONS

We have demonstrated that cross-linked polymer films could be produced under UV irradiation through the copolymerization of NB with DCPD. The approach for photoactivated olefin metathesis used a NHC photogenerator (IMesH + BPh 4 -and ITX) combined with an inactive Ru precatalyst (Ru-1, Ru-2 and Ru-3). Though the detailed mechanism of photoactivation remains uncertain, an active NHC-coordinated Ru-arene complex is presumed to form by reaction between the ruthenium precatalyst and the photogenerated NHC IMes. Irradiation at 365 nm was also proved to have a critical role in the formation of the active metathesis species by promoting the decoordination of the p-cymene ligand and the multiple insertions of new NB units into the propagating center. The films are homogeneous, and the possibility of cross-linking was attested by the measurement of gel contents (10 -70 %) and the rising T g value when increasing DCPD loading. FTIR data show a complete conversion of norbornenyl group in both monomers, but an incomplete ring-opening of the cyclopentene pendant group from DCPD. This lower ring-strained cyclic olefin is subjected to metathesis but also olefin addition reactions as evidenced by 13 C solid-state NMR. We anticipate that photoROMP of NB and DCPD derivatives will enable the development of more UV-curable coatings with chemical, thermal and mechanical properties currently not attainable with a conventional radical photopolymerization.

GENERAL CONCLUSIONS

In ). According to the laser flash photolysis study, upon increasing the amount of BPh 4 -anion, 3 ITX* was found to decay faster. The electron transfer rate k q was evaluated from Stern-Volmer plot as 4.9 × 10 7 M -1 ·s -1 , a value which is similar to the determined value of NaBPh 4 /ITX (4.1 × 10 7 M -1 ·s -1 ). This loin la méthode la plus couramment utilisée pour accéder à un NHC libre. [19][20][21] Cependant, cette méthode nécessite une base forte, qui limite le champ d'application de substrats et cette base peut aussi intervenir dans le procédé de polymérisation en tant qu'amorceur. 21 Dans la recherche de routes plus intelligentes pour libérer le NHC, des NHCs « masqués » ou « protégés » ont été étudiés récemment. 21,22 D'un grand intérêt sont les précurseurs de NHC thermiquement labiles sur la base de produits d'addition NHC-CO 2 stables à l'air, 23 des NHCisothiocyanate 24 ou des complexes NHC-métal. 25 En dépit des résultats prometteurs, cette L'effort de recherche limité sur les photogénérateurs de NHC contraste avec le vif intérêt pour d'autres Générateurs de PhotoBase (PBG), capables de libérer des organosuperbases de type amidine ou guanidine. 29 La plupart des PBGs passent par une réaction de transfert de proton photoinduite en utilisant un sel contenant la superbase sous forme protonée et un chromophore anionique de type kétoprofène, 30 thioxanthone, 31 ou tétraphénylborate. [33][34][35] Aujourd'hui, il existe de fortes motivations pour le développement de réactions induites par des photoNHCs: un contrôle temporel et spatial de la réaction, l'utilisation d'une 37,38 Encouragé par les résultats préliminaires prouvant la libération de NHC lors de l'irradiation, 35 NHCs les plus employés. 16 L'utilité de ce photogénérateur de NHC deux composants NHCH + BPh 4 -/ITX a également été démontrée dans des réactions de photopolymérisation pour former le polyuréthane, le polyester et le polynorbornène. 17 En effet, bien que les NHCs aient apporté des changements profonds dans la synthèse organique catalytique, [23][24][25] ils nécessitent généralement une atmosphère inerte et des conditions difficiles pour leur génération. En conséquence, un photogénérateur stable capables de créer des NHC sous exposition aux UV est très recherché. Il est vrai que les précurseurs NHC thermiquement latents existent également. 21,22 Bien que l'hypothèse mécanistique de photoréduction du thioxantone par l'arylborate soit acceptable, des preuves concluantes sont nécessaires. À l'exception de Outil précieux pour la génération de doubles liaisons C=C, la métathèse des oléfines a été exploitée dans de nombreuses applications, telles que la synthèse d'architectures complexes, 1 de produits pharmaceutiques, 2 ou de copolymères biodégradables. 3 Dans tous ces cas, les progrès ont été principalement obtenus grâce au développement de complexes alkylidène métalliques avancés et bien définis, comme les catalyseurs de Grubbs et Schrock. 4,5 A cet égard, un progrès important a été en 1999 l'introduction de ligand à base de carbène N-hétérocyclique (NHC) à la place des composés de type phosphine. 6 Cependant, les principaux développements dans la catalyse de métathèse des oléfines ont répondu davantage aux besoins de la chimie de synthèse (comme le montrent les exemples mentionnés cidessus), mais ont été moins utiles pour la préparation de matériaux polymères. La polymérisation par métathèse par ouverture de cycle (ROMP) des oléfines cycliques est actuellement le principal type de polymérisations par métathèse. Pour déverrouiller le potentiel de matériaux polymères de ROMP, des catalyseurs « intelligents » sont indispensables. Pour faire face à ce défi, des catalyseurs de métathèse latents complexes sont apparus, en mesure de libérer des espèces actives in situ et « à la demande »par une stimulation externe. Chaleur, 8-10 force mécanique, 11 et rayonnement [12][13][14] sont les stimuli les plus courants. Une catégorie distincte concerne des catalyseurs activés après l'addition d'un agent chimique externe. [15][16][17] Parmi ces différentes options, l'activation provoquée par l'irradiation visible ou ultraviolette est particulièrement attrayante. 18 Les catalyseurs de ROMP photolatente peuvent être activés à la température ambiante, et rendent possible l'utilisation de formulations stables au stockage et prêtes à l'utilisation. Comme produit standard d'une polymérisation photoactivé, des films minces sont également attendus avec la possibilité de développer de nouveaux revêtements photoréticulables. Enfin, à la différence des autres déclencheurs, la réaction peut être contrôlée spatialement. 17,19 Dans le domaine de la métathèse des oléfines activées par photochimie, trois approches ont été proposées, dans lequel l'activation se fait généralement par une réaction photochimique impliquant le ligand d'un complexe de métal de transition. Les deux voies les plus courantes reposent sur une photodissociation de ligand 20,21 et une photoisomérisation de ligand. 12,14 La troisième voie, qui est le moins étudiée, est l'objet de la présente étude. Dans cyclooctène. [25][26][27]

Additional figures in Chapter III

Figure 0 . 1 .

 01 Figure 0.1. Chemical structures of common ROMP polymers and their industrial applications.

Figure 0 . 2 .Scheme 1 . 19 . 29 Scheme 1 . 20 . 30 Scheme 1 . 21 . 30 Scheme 2 . 1 . 4 - 51 Scheme 2 . 2 . 61 Scheme 2 . 3 . 63 Scheme 2 . 4 .Scheme 2 . 5 .Scheme 2 . 6 .Scheme 2 . 7 .Scheme 2 . 8 .Scheme 3 . 1 .Scheme 3 . 2 .Scheme 3 . 3 .Scheme 3 . 4 .Scheme 3 . 5 .Scheme 3 . 6 .Scheme 4 . 1 . 4 -, 1 )Scheme 4 . 2 .Scheme 4 . 3 . 1 . 2 . 1 . 2 .Figure 0 . 1 .Figure 0 . 2 .Figure 1 . 1 . 2 Figure 1 . 2 . 3 Figure 1 . 3 . 4 Figure 1 . 4 . 5 Figure 1 . 5 . 8 Figure 1 . 6 . 10 Figure 1 . 7 . 11 Figure 1 . 8 . 14 Figure 1 . 9 . 24 Figure 1 . 10 . 25 Figure 1 . 11 . 26 Figure 1 . 12 . 28 Figure 1 . 13 . 28 Figure 1 . 14 . 31 Figure 1 . 15 . 34 Figure 1 . 36 Figure 1 . 17 . 37 Figure 1 . 18 . 37 Figure 2 . 1 . 49 Figure 2 . 2 . 59 Figure 2 . 3 . 60 Figure 2 . 4 . 4 -/ 62 Figure 2 . 5 . 1 H 4 -/ 4 -/ 64 Figure 2 . 6 . 4 -] = [SIMesH + BPh 4 -] = 3  64 Figure 2 . 7 . 65 Figure 2 . 8 .Figure 2 . 9 . 4 -]Figure 2 . 10 .Figure 2 . 11 .Figure 2 . 12 . 1 HFigure 3 . 1 .Figure 3 . 2 .Figure 3 . 3 .Figure 3 . 4 .Figure 3 . 5 . 1 HFigure 3 . 6 . 98 Figure 3 . 7 .IMesH + BPh 4 -Figure 3 . 8 .Figure 3 . 9 .Figure 3 . 10 .Figure 4 . 1 . 1 H 2 (Figure 4 . 2 .Figure 4 . 3 .Figure 4 . 4 .Figure 4 . 5 .Figure 4 . 6 .Figure 4 . 7 .Figure 4 . 8 .Figure 4 . 9 . 133 Figure A.II. 1 .Figure A.II. 2 .Figure A.II. 3 . 4 -/ 142 Figure A.II. 5 . 4 -/ 142 Figure A.II. 6 . 4 -/ 142 Figure A.II. 7 . 143 Figure A.II. 8 . 143 Figure A.II. 9 . 143 Figure A.II. 10 .Figure A.II. 12 .IMesH + BPh 4 -/IMesH + BPh 4 -/Figure A.II. 15 . 2 . 3 ) 4 -/ 4 -/Figure A.II. 16 .IMesH + BPh 4 -/ 4 -/Figure A.III. 1 . 4 -/Figure A.III. 2 . 4 -/Figure A.III. 3 . 3 M, [IMesH + BPh 4 -Figure A.III. 4 . 1 H 4 -( 1 /Figure A.III. 5 .Figure A.III. 6 .Figure A.IV. 1 .Figure A.IV. 4 .Figure A.IV. 5 .Figure A.IV. 6 . 4 -/

 021192912030121302145122612363242526272831323334353641414243121201021121231341451581610171118141924110251112611228113281143111534136117371183721492259236024462251446426443642765282942102112121313233343513698374383931041124243444546474849133123414254142641427143814391431012441523441644142433441415614564 Figure 0.2. Summary of the scope of studies presented in this thesis.

  CONCLUSIONS ..................................................................................................................139 APPENDIX ...........................................................................................................................141 SUMMARY IN FRENCH OF CHAPTER II, III AND IV ..............................................156 Chapitre II. Mélange d‗azolium tétraphénylborate avec l‗isopropylthioxanthone: une nouvelle classe de photogénérateur de NHC pour la synthèse de polyuréthane, polyester et polymère de ROMP ......................................................................................157 Chapitre III. Photoréduction d'un dérivé du thioxanthone par un sel d'azolium tétraphénylborate: Un moyen de photogénérer des N-carbènes hétérocycliques ............163 Chapitre IV. La combinaison d'un photogénérateur de ligand et d'un précatalyseur de ruthénium: une approche photoinduite pour la synthèse de film polymère réticulé par ROMP ..............................................................................................................................168 List of Publications ..............................................................................................................173

4

 4

Figure 1 . 1 .

 11 Figure 1.1. General structure of: (a) conventional carbenes and (b) Arduengo carbenes.

Figure 1 . 2 . 1 . 1 . 2 . 12 Figure 1 . 3 .

 121121213 Figure 1.2. General ground-state electronic features of singlet and triplet carbenes. Arrows: electrons.

Figure 1 . 4 .

 14 Figure 1.4. Chemical structures and nomenclatures of main classes of NHCs.27

  C 2 carbon lead to an increase of steric pressure on C carbene -CO 2 bond compared to less bulky substituents. That results in reduce conjugation and subsequently in a rise of temperature dissociation of carbon dioxide. For example, 2f.CO 2 begins to lose its CO 2 moiety at 71 o C, 2e.CO 2 at around 140 o C, whereas 2d.CO 2 is stable up to 162 o C.52

Figure 1 . 5 .

 15 Figure 1.5. Chemical structures of common betaine adducts.

Figure 1 . 6 .

 16 Figure 1.6. Chemical structures of thermal latent NHCs protected by organic compounds halogenated and alcohols.45,62

Scheme 1 . 5 .Scheme 1 . 6 .

 1516 Scheme 1.5. Ultrasound activation of Ag(I)-NHC-polymer complex and subsequent transesterification catalyzed by NHC formed in situ.

Scheme 1 . 7 .

 17 Scheme 1.7. Schematic generation of free carbene based on NHC-carboxylic acids.

Figure 1 . 8 .Scheme 1 . 8 .

 1818 Figure 1.8. A range of novel photogenerator NHCs based on decarboxylation.

2a.CO 2 Scheme 1 . 10 .

 2110 Scheme 1.10. Schematic representation of chain-end activated ROP of lactone mediated by NHC.CO 2 precursors.

Scheme 1 . 11 .Scheme 1 . 12 .

 111112 Scheme 1.11. Formation of polyester based on Lewis acid/base activation of thermally labile NHCmetal adducts.b. ROP of epoxidesPreliminary results obtained by Taton et al.39,82 indicate that NHCs are very promising catalysts for the ROP of epoxides (ethylene oxide and propylene oxide (PO)). As usual, the application of NHCs in their free forms being problematic due to the need of strictly waterfree conditions, Limbach et al.83 used NHC.CO 2 as thermo-latent precatalysts for the ROP of PO at 120 o C, in the presence of diethylene glycol as initiator. The precatalyst 2e.CO 2 bearing the least bulky N-substituents was sought to be the most active species. With a catalyst loading of 5 % mol (relative to PO), this species provided the highest conversion of 73.7 %. However, only PO oligomers resulted. With regards to DFT calculations, ROP of epoxides is supposed to prefer a free anionic mechanism, causing an inherently slow process and side reactions (transfer to monomer) (Scheme 1.12).

Scheme 1 . 13 .

 113 Scheme 1.13. Schematic initiation step of a latent one-component epoxide/ anhydride mixture in the presence of protected NHCs. 41,84 1.3.2. Step-growth polymerization a. Polyurethanes In 2012, Taton et al. reported that NHCs can trigger the formation of polyurethane by

Scheme 1 . 16 .

 116 Scheme 1.16. General mechanism of metal-mediated ROMP. Among the well-defined NHC-ruthenium complexes, Noels' catalysts with principle formula RuCl 2 (NHC)(p-cymene) have high utility due to their great performance towards ROMP. 89,90 Such complexes can be generated in situ via a combination of [RuCl 2 (pcymene)] 2 , azolium salts and strong bases such as t-BuOK, NaH or Cs 2 CO 3 . It is a much simpler procedure in comparison to the multi-step exchange of ligands required to obtain Grubbs' second-generation catalysts and its derivatives. 91 Accordingly, NHC.CO 2 betaines that show strong lability upon heat or dissolution can act as efficient NHC precursors for in situ preparation of ROMP catalysts without the need for additional bases. 91,92 Delaude et al. have explored the synthesis of Noels' analogues using two-fold excess of NHC.CO 2 adducts (Scheme 1.17). The metathesis activity of these complexes was assessed in visible light induced ROMP of cyclooctene and compared to those prepared by in situ deprotonation of azolium salt counterparts such as NHCH + Cl -and NHCH + BF 4 -

Figure 1 . 9 .

 19 Figure 1.9. The first three routes (1-3) rely on a photolatent transition metal complex converted into an active catalytic species through different photochemical reactions. Route 1involves a photodissociation of one or more ligand(s) from the metal center. Route 2 uses a photosensitive exogenous species (PL) to liberate ligand(s), subsequently reacting with the precatalyst to generate an active catalyst. Route 3 relies on a cis-to-trans photoisomerization of ligand in chelated ruthenium alkylidene complexes. Unlike previous approaches, route 4 is metal-free and does not follow the conventional metathesis mechanism. It consists in a

Figure 1 . 9 .Scheme 1 . 18 .

 19118 Figure 1.9. 4 strategies to develop photochemically activated ROMP.

W1Scheme 1 . 18 .

 118 Scheme 1.18. Instead, elimination of tetramethylsilane occured upon α-H abstraction reaction from two trimethylsilylmethyl groups in a cis position, thus generating the active tungstenalkylidene complexes.

Figure 1 . 10 .

 110 Figure 1.10. Structure of well-defined tungsten catalysts developed by Van der Schaaf et al.. Ruthenium based precatalysts. In the mid 1990's, the development of rutheniumbased catalysts, displaying tolerance towards moisture and functional groups, motivated Mühlebach et al. to investigate the behavior of η 6 -arene sandwich, half-sandwich and nitrile complexes Ru1-3 (Figure 1.11) for the photopolymerization of strained cyclic olefins (NB

Figure 1 . 11 . 1 . 13 .

 111113 Figure 1.11. Ruthenium and osmium based photocatalysts prepared by Mühlebach et al..

Figure 1 . 12 .

 112 Figure 1.12. Ruthenium photocatalysts with NHC ligands.

Figure 1 . 13 .

 113 Figure 1.13. Monomers employed for photoROMP by Bushmeiser et al..

(route 2 ,

 2 photoacid generator (PAG) triphenylsulfonium chloride and the precatalyst Ru12 (Scheme 1.19).[START_REF] Keitz | A Tandem Approach to Photoactivated Olefin Metathesis: Combining a Photoacid Generator with an Acid Activated Catalyst[END_REF] Upon irradiation at sub-300 nm, HCl was released which allowed protonation and

Scheme 1 . 19 .

 119 Scheme 1.19. Photogeneration of an active catalyst by irradiation of solution containing a photoacid generator (PAG) and a ruthenium complex.

Figure 1 .

 1 Figure 1.14. S chelated latent ruthenium benzylidene catalysts used in photoROMP (Ru14 and Ru16). Ru15 is the first isomerizable chelated ruthenium catalyst, the Ru15-cis form being much more latent than Ru15-trans.

a

  Reaction conditions : CH 2 Cl 2 , ratio monomer/ Ru-14-cis 300:1.

Ru18 Ru19- 1 Ru17 Ru19- 2 Figure 1 . 15 .

 12115 Figure 1.15. N-Chelated latent ruthenium benzylidene catalysts used in photoROMP. Only Ru17 and Ru19 proceed via a presumed photoisomerization mechanism.

  photoROMP via photoinduced electron transfer Genesis and general principle. Before the breakthrough of metal-free ROMP by Boydston and coworkers in 2015, 130 it was deeply rooted that the development of ROMP was closely linked to that of metal transition catalysts. Metal alkylidene reacts with cyclic olefin in a [2+2] cycloaddition to form a metallo cyclobutane, which is recognized as a key intermediate in the polymerization mechanism. In this context, it is not surprising that most efforts to improve photoROMP have focused on photosensitive metal complexes (routes 1 and 3). For Boydston et al., the point of departure was the observation that most olefin [2+2] cycloaddition reactions in organic chemistry proceed without the aid of metal catalyst to form a cyclobutane, which is reminiscent of the metallo cyclobutane. 131 The most inspiring example was the electron transfer-induced intermolecular olefin cross-coupling established by Chiba et al. involving an electron rich olefin and an unactivated olefin. 132,133 Mechanistically, the group of Chiba used a vinyl ether (electron rich olefin) which was oxidized at a carbon fiber anode in lithium perchlorate/nitromethane electrolyte solution (Figure 1.16A). The resulting radical cation adds intermolecularly to an unactivated olefin to form radical intermediate, which cyclizes to cyclobutane prior to back electron transfer (reduction).

Figure 1 .

 1 Figure 1.16. A. [2+2] cyclo-addition of an electron rich olefin mediated by single electron transfer to an unactivated olefin. B. Putative mechanism of photoinduced electron transfer ROMP of NB as envisaged by Boydston et al..

  photocatalyst (λ excitation = 450 -480 nm) (Figure1.17) and ethyl 1-propenyl ether as initiator (NB:initiator:P1 = 100:1:0.03 equiv.). P1 turned out to be able to oxidize selectively the vinyl ether initiator into the reactive radical cation, to be regenerated subsequently by back electron transfer. In 1998, Steckhan and coworkers were the first to use P1 to catalyze the reaction between 2-vinylbenzofuran and styrene.136 Use of photocatalytic process instead of

Figure 1 . 17 .

 117 Figure 1.17. Pyrilium salt (P1) and thiopyrilium salt (P2) used as photo-oxidant in metal-free PET ROMP.

Figure 1 . 18 .

 118 Figure 1.18. A range of vinyl ether initiators used by Boydston et al..130

  J. Pinaud, T. K. H. Trinh, D. Sauvanier, E. Placet, S. Songsee, P. Lacroix-Desmazes, J.-M. Becht, B. Tarablsi, J. Lalevée, L. Pichavant, V. Héroguez and A. Chemtob, In Situ Generated Ruthenium-Arene Catalyst for Photoactivated Ring-Opening Metathesis Polymerization through Photolatent N-Heterocyclic Carbene Ligand, Chem.: Eur. J., 2018, 24, 337-341. T. K. H. Trinh, J. P. Malval, F. Morlet-Savary, J. Pinaud, P. Lacroix-Desmazes, C. Reibel, V. Héroguez and A. Chemtob, Mixture of azolium tetraphenylborate with isopropylthioxanthone: a new class of N-heterocyclic carbene (NHC) photogenerator for polyurethane, polyester and ROMP polymers synthesis, Chem.: Eur. J., 2019, DOI: doi.org/10.1002/chem.201901000. ©Wiley-VCH 2016 69451 Weinheim, Germany

Figure 2 . 1 .

 21 Figure 2.1. IMes and SIMes are among the most frequently employed NHCs.

Scheme 2 . 1 .

 21 Scheme 2.1. Photolysis mechanism for the photogeneration of IMes (or SIMes) from the bicomponent photogenerating system ITX/IMesH + BPh 4 -(or ITX/SIMesH + BPh 4 -).Encouraged by preliminary results proving the release of NHC upon irradiation,36 this current work describes a comprehensive study of this new class of bicomponent NHC photogenerator ─ ITX/IMesH + BPh 4 -and ITX/SIMesH + BPh 4 -─ with four distinct parts. Firstly, we propose two novel analytical methods to characterize the photochemically released NHCs through their conversion in suitable adducts: carbene-radical adduct enables an identification method by using electron paramagnetic resonance (EPR) spectroscopy, while stable NHC-carbodiimide adduct facilitates the quantification of photoNHC yield using 1 H NMR. Secondly, to support the 2-step photochemical pathway based on electron/proton transfer reactions, experiments of real-time photobleaching of ITX are performed as well as calculation of the free energy change of electron-transfer with Rehm-Weller equation.

2 . 2 . 3 IMesH + BPh 4 - 4 -/

 22344 photoreactor was constructed by winding a 365 nm LED strip (SMD3528, 60 LED/Meter, Lightingwill, length: 1000 mm) around a quartz cylinder (internal diameter: 60 mm, length: 200 mm) (see Figure A.II.2). The NMR tube was introduced in the axis of the quartz cylinder where it received an irradiance of 6.5 mW•cm -2 . All samples were irradiated for 60s and gels obtained (see Figure A.II.3). Monomer conversion and cis/trans ratio were monitored by 1 H NMR analysis. Reactions were quenched by adding an excess amount of vinyl ethyl ether, the resultant polymer was precipitated in acetone. The collected polynorbornene was washed several times with acetone and air dried for 48 h before SEC analysis.

( 2  4 -/ 4 -( 1 . 4 -. 4 -) and BPh 4 -(IMesH + BPh 4 -). 4 -(IMesH + BPh 4 -

 2441444444 photoinduced electron-transfer was estimated through the Rehm-Weller equation (2):40

  ), 4,4′-bis(diethylamino)benzophenone (EAB) or camphorquinone (CQ), which have well-known reduction potentials (E red ) and triplet energies (E T ) in the literature,

8 (δ 1 .

 81 All NMR spectra were recorded in appropriate deuterated solvents with tetramethylsilane (TMS) as the internal reference on a Varian 300 -MR. All chemical shifts are reported in parts per million (ppm) relative to the residual CD 2 Cl 2 (δ 5.32 ppm), THF-d 72 ppm and 3.58 ppm), CDCl 3 (δ 7.26 ppm), DMSO-d 6 S (δ 2.50 ppm) or ACN-d 3 (δ 1.94 ppm). Peaks multiplicities in 1 H NMR spectra are abbreviated as s (single), d (double), t (triplet), m (multiplet), br (broad).

  Agilent 1260 Infinity series equipped with Polymer Laboratories ResiPore columns, a G1314B wavelength detector, a MDS refractive index detector and a MDS viscosimeter detector was employed to determine the molar mass values and distributions of polyurethane and polyester samples. Precise concentration solutions of polymer in THF were prepared and injected with flow rate of 1 mL•min -1 . A set of EasiVial polystyrene PS-M standards was used to establish the universal calibration. High molecular weights of polynorbornene samples were determined using a SEC instrument from a Varian apparatus equipped with TOSOHAAS TSK gel columns and a refractive index detector. The mobile phase was THF at a flow rate of 1 mL•min -1 ; the injected volume was 100 µL. The MALS instrument was normalized using a THF solution of polystyrene standard.h. Electron paramagnetic resonanceEPR measurements were performed in a Bruker Elexsys E500 spectrometer with X band frequency in continuous wave (around 9.8 GHz) at room temperature. Spectra were recorded with a modulation amplitude of 1 G, a modulation frequency of 100 kHz and a microwave power of ∼ 2 mW. Both Bruker WIN-EPR and SimFonia software were used for spectra treatment and simulation. i. Computational procedure Triplet energy of IMesH + BPh 4 -(E T,BPh4-= 83.97 kcal•mol -1 and E T,IMesH+ = 77.04 kcal•mol -1

1

 1 j. MALDI-TOF analysis of poly(PA-alt-CHO)MALDI-TOF MS spectra were recorded by a Bruker RapiFlex mass spectrometer equipped with a nitrogen laser for MALDI (λ = 337 nm). Mass spectra were accumulated in positive mode at 20 kV acceleration voltage potentials. All the polymers were dissolved in THF at a concentration of 1 mg•mL -1 . Sodium trifluoroacetate at a concentration of 5 mg•mL -1 was utilized as the cationization agent. Trans-2-[3-(4-tert-butylphenyl)-2-methyl-2propenylidene]malononitrile (DCTB) is the matrix dissolved in THF at a concentration of 40 mg•mL -1 . The mixed solution of matrix/salts/polymer (1:4:4 in vol. ratio) was hand spotted on a MALDI target until drying. Data were analyzed by using the FlexAnalysis version 3.0 (Bruker) software. Synthesis and characterization of photogenerated NHC To photogenerate the well-established IMes, we propose the azolium tetraphenylborate salt IMesH + BPh 4 -as a potential NHC photogenerator. For its synthesis, simple treatment in ethanol of NaBPh 4 with the commercial azolium salt IMesH + Cl -resulted in a quantitative anion exchange to provide the desired photolatent NHC cleanly and in excellent yield (93.2 %). Compound IMesH + BPh 4 -appears as white crystals with a melting point at 212 °C and a decomposition threshold at 292 °C (Figure A.II.8). Its electronic absorption (Figure 2.2) spans only the UV-C region (100-280 nm) with an intense peak at 198 nm ( 198 = 1.38  10 5 •M -1 •cm -1 ) and weak peak at 254 nm ( 254 = 7.30  10 3 M -1 •cm -1 ). A comparison with NaBPh 4 and IMesH + Cl -salts clearly established that both the anion (BPh 4 -) and cation (IMesH + ) of IMes are chromophores.

Figure 2 . 2 .

 22 Figure 2.2. UV-vis absorption spectra of NaBPh 4 (black), IMesH + Cl -(blue) and IMesH + BPh 4 -(red) in acetonitrile (4  10 -4 M).

2. 3 . 2

 32 Identification and quantification of photogenerated NHC a. EPR spectroscopy of NHC-radical adduct: Identification of NHC IMesH + BPh 4 -(3 equiv.) was mixed with ITX (1 equiv.) under ambient conditions in THF-d 8 .

Figure 2 . 3 . 4 - 4 -/

 2344 Figure 2.3. 13 C NMR spectra in DMSO-d 6 of the IMes-CS 2 adduct formed after addition of CS 2 to the as-irradiated mixture IMesH + BPh 4 -/ITX.

Scheme 2 . 2 .Figure 2 . 4 ,Figure 2 . 4 ,

 222424 Scheme 2.2. Reaction pathway to generate NHC-radical adduct after irradiation of ITX/IMesH + BPh 4 -

Figure 2 . 4 . 4 -/

 244 Figure 2.4. EPR spectrum of: (a) a solution of ITX/IMesH + BPh 4 -/TPO (1/3/6 equiv., [ITX] = 5 10 - 3 M) in THF after different irradiation times and (b) EPR spectrum obtained after 30 s irradiation of a solution of IMes/TPO in THF (5/1 equiv., [TPO] = 3 10 -3 M). The experimental and simulated spectra are both consistent with the formation of the mesitoyl-NHC adduct radical IMes • -M.

Scheme 2 . 3 .

 23 Scheme 2.3. Formation of zwitterrionic IMes-CDI adduct by reaction between CDI and NHC IMes.

IMesH + BPh 4 -/ITX and SIMesH + BPh 4 -/Figure 2 . 5 ) 2 . 3 . 3 Figure 2 . 6 SIMesH + BPh 4 -

 4425233264 Figure 2.6 shows in the same plot the UV-vis spectra of ITX, IMesH + BPh 4 -and SIMesH + BPh 4 -in acetonitrile. ITX has an absorption spectrum with two characteristic maxima at 257 nm (π-π* transition) and 383 nm (n-π* transition). The band centered at 383 nm allows excitation (λ exc = 365 nm) to form triplet ITX ( 3 ITX*), displaying a much higher reduction ability than its ground state analogue. The two azolium salts absorb only in the UVC range (100-280 nm), what is desired to avoid electronic excitation and ensure high reactivity of ITX.

Figure 2 . 5 . 1 H 4 -/IMesH + BPh 4 -/

 25144 Figure 2.5. 1 H NMR spectra in THF-d 8 of: (a) IMes (0.21 M), (b) CDI (0.21 M), (c) IMes/CDI, (1/1 equiv., [IMes] = 0.1 M), and (d) IMesH + BPh 4 -/ITX/CDI (3/1/3 equiv., [ITX] = 0.01 M). In sample d,

Figure 2 . 6 . 4 -] = [SIMesH + BPh 4 -] = 3 Figure 2 . 7 , 4 -/

 26443274 Figure 2.6. UV spectra of ITX, SIMesH BPh 4 -and IMesH + BPh 4 -in acetonitrile ([ITX] = 10 -4 M, [IMesH + BPh 4 -] = [SIMesH + BPh 4 -] = 3  10 -4 M).

Figure 2 . 7 .

 27 Figure 2.7. Photobleaching kinetics of ITX (2  10 -4 M in acetonitrile) monitored at 365 nm in the presence of different quenchers: azolium salts (a) and tetraphenylborate salts (b) (Quencher concentration is 6  10 -4 M).

Scheme 2 . 4 .Figure 2 . 7 , 4 -,Scheme 2 . 5 .Figure 2 . 7 , 4 -

 2427425274 Scheme 2.4. Photochemical mechanism for the generation of NHC IMes and SIMes combining electron-transfer and proton transfer.

Figure 2 . 8 .

 28 Figure 2.8. Temporal evolution of absorbance at 365 nm of ITX solution in acetonitrile (2 10 -4 M) in the absence of quencher and presence of IMes and SIMes (6 10 -4 M).

Figure 2 .

 2 photodegradation can be minimized, but at the same time, only moderate yields can be achieved. The kinetic profile is significantly changed upon increasing the concentration of ITX to reach stoichiometry (ITX/IMesH + BPh 4 -= 1/1 equiv.). Under these conditions, more NHC can be released because of the higher 3 ITX* concentration, and a maximum of 70 % is formed after 5 min. However, after longer radiation exposure, there is now a significant decrease of IMes yield (56 % after 10 min) due to the photochemical degradation of IMes by 3 ITX*.

Figure 2 . 9 . 4 -

 294 Figure 2.9. (a) Effect of ITX concentration on NHC IMes yield produced by irradiation of ITX/IMesH + BPh 4 -([IMesH + BPh 4 -] = 0.03 M) and (b) Effect of irradiation time on NHC IMes yield following the photolysis of ITX/IMesH + BPh 4 -mixture ([IMesH + BPh 4 -] = 0.03 M). Irradiation

IMesH + BPh 4 -/

 4 Photooxidant (3/1 equiv.) was irradiated in C 4 D 8 O ([Photooxidant] = 0.03 M).Irradiation conditions: 365 nm, 65 mW•cm -2 , 5 min irradiation).

2. 10 ).

 10 The two catalysts proceeded at relatively similar rates with a significant acceleration compared to the uncatalyzed experiment. The slightly slower kinetics for photogenerated IMes was assigned to lower amount of NHC IMes present in the reaction media (as compared to the preformed-NHC IMes) since a maximum yield of 70 % in released NHC can be reached as demonstrated before.

Figure 2 . 10 .

 210 Figure 2.10. NCO conversion during polyaddition reaction between PEG 400 and IDPI. Conversion values were determined based on the NCO stretching band (υ-NCO ) at 2255 cm -1 using CH 2 symmetric stretch at 2870 cm -1 as reference (see FTIR spectra in Figure 2.11). NCO conv. (%) = 100 where A o and A t are the integrated absorbance at 2255 cm -1 corresponding to reaction time at 0 min and t min.

Figure 2 . 11 . 4 -/

 2114 Figure 2.11. FT-IR spectra of aliquots taken at different times during the bulk polyaddition between PEG 400 and IPDI (PEG 400 /IPDI (342/342 equiv.). Polymerization was conducted at 60 °C after the addition of the as-irradiated IMesH + BPh 4 -/ITX solution (1/1 equiv.). For each measurement, the

Scheme 2 . 7 .

 27 Scheme 2.7. ROCOP of PA and CHO using photogenerated NHC IMes and SIMes as catalyst.

Table 2 . 3 4 -/ITX (entry a) or SIMesH + BPh 4 -/

 2344 shows PA conversion, molecular weight and polydispersity (Ð) after 1 h reaction using IMesH + BPh ITX (entry b) along with two control experiments (entries c-d). As expected, no reaction took place in the uncatalyzed control reaction (entry c). 62 Conversely, almost full conversions were achieved with IMesH + BPh 4 -(98 %) and with SIMesH + BPh 4 -(84 %). In addition, the polyesters produced exhibited a highly alternating structure with a high fraction of ester linkages (> 95 %, see 1 H NMR spectra in Figure A.II.13).

  MS spectrum of polymer obtained from entry a (Figure A.II.14) which displayed an m/z increment of 246.26 g•mol -1 corresponding to the exact molar mass of the [PA + CHO]repeating unit. Interestingly, no IMes end-capped chains were identified due to quenching of the reaction mixture in methanol. As a result, the five main populations of peaks were all assigned to α-methyl-ω-hydroxyl OH/CH 3 -terminated polymers.

Figure 2 . 12 . 1 H 4 -/

 21214 Figure 2.12. 1 H NMR spectra of in CDCl 3 of aliquots taken at different times during the ROCOP of CHO/PA (500/100 equiv.). Polymerization was conducted at 110 o C after the addition of the asirradiated IMesH + BPh 4 -/ITX solution (1/1 equiv.). PA conv.(%) =  100 where I 7.85 -8.02 is the intensity of methine protons (δ = 7.85 -8.02 ppm) of residual PA and I 7.31 -7.62 is the intensity of methine protons (δ = 7.31 -7.62 ppm) of poly(PA-alt-CHO).

  [a] Determined by 1 H NMR NB conv.(%) = 100 where I 5.21 and I 5.36 are the intensity of vinylic protons (δ= 5.21 ppm and δ= 5.36 ppm) of polynorbornene and I 5.99 is the intensity of vinylic proton (δ= 5.99 ppm) of residual NB.

  imidazolidinylidene species upon irradiation at 365 nm. The mechanism has been investigated by means of steady-state photolysis. It involves a first electron-transfer reaction from the easily oxidized electron donor BPh 4 -to excited ITX acting as an electron acceptor. The thioxanthone radical anion formed can then abstract a proton from the azolium cation to yield the free NHCs. The formation of the NHCs, IMes and SIMes, was proved by EPR spectroscopy in THF through the formation of a persistent NHC-mesitoyl radical adduct. A maximum amount of NHC of 70 % was produced in THF, based on the azolium cation precursor, as determined by 1 H NMR of a stable and soluble NHC-CDI adduct. Irradiation time should be carefully controlled to avoid overexposure resulting in secondary electrontransfer reaction between triplet ITX and the NHC formed. The easy synthesis of NHC progenitors IMesH + BPh 4 -and SIMesH + BPh 4 -opened the way to novel photomediated polymerization reactions where NHCs advantageously acted as organocatalysts in place of metals. Linear polyurethanes were synthesized by the step-growth polymerization of diols and diisocyanate. Additionally, the ring-opening copolymerization of anhydride with epoxy yielded polyesters with narrow polydispersity. Finally, in combination with an inactive Ru precatalyst, photolatent NHCs could also drive the formation of highly active ruthenium-NHC complexes for ring-opening polymerizations of norbornene with > 76 % conversion reached after 1 min of irradiation. It is expected that a wide variety of other NHC-mediated organic chemical reactions and polymerization reactions could benefit from this innovative NHC photogenerating system; in particular, those including thermally sensitive substrates or requiring a spatial control.

5 Scheme 3 . 1 .

 531 Scheme 3.1. Photochemical reactivity of arylborates. Route 1 is based on direct photolysis while route 2 is a photoinduced electron transfer (PET) involving an electron acceptor (EA) sensitizer. The asterisk designates an excited state.

Scheme 3 . 2 .

 32 photochemically produced NHCs have net advantages including low temperature reaction, thermal stability and the possibility to finely tune NHC concentration upon adjusting the energetic dosage.

  laser spectroscopy. All nanosecond time-resolved transient absorption spectra were performed in a 1 cm optical path length and monitored by Edinburgh Instruments LP920 laser flash photolysis spectrometer. The solutions were excited at 90° from the probe beam by a Q-switched nanosecond Nd/YAG laser (λ exc = 355 nm, 8 ns pulse duration; energy reduced down to 5 mJ per pulse from Continuum (Surelite II-10). A filter was used to remove the residual excitation light at 355nm (Schott GG385). As preliminary step, all acetonitrile solutions were purged with nitrogen for 5 min prior to measurement. The decay of the triplet-triplet state absorption of ITX (1 10 -4 M) at 600 nm was measured and follows a first-order kinetics. The temporal change of triplet concentration [ 3 ITX*] can be represented in equation (

( 2 ) 29 [ ] ( 3 ) 4 -( 9 . 11 B

 22934911 Exponential fitting of of the experimental decay Abs.(t, 600nm) provides (cf. Equation2). In presence of a triplet quencher (IMesH + BPh 4 -, NaBPh 4 , IMesH + Cl -), a pseudo-first-order kinetic equation can be applied to extract a triplet lifetime that depends on quencher concentration [ ]. Note that all the triplet lifetimes were determined by using a double exponential equation in the time range 0 -40 µs the longer time-constant being due to the formation of the ITX radical ion at monitored wavelength. By measuring at different [ ], the quenching rate constants, k q , can be obtained by the linear plot based on the Stern-Volmer equation(3).Electron paramagnetic resonance (EPR). EPR measurements were performed with aBruker Elexsys E500 spectrometer with X band frequency in continuous wave (around 9.8 GHz) at room temperature. Spectra were recorded with a modulation amplitude of 1 G, a modulation frequency of 100 kHz and a microwave power of ∼ 2 mW. Both Bruker WIN-EPR and SimFonia software were used to note the spectra and carry out the simulation afterwards. In a typical experiment, an acetonitrile solution (1 mL) containing IMesH + BPh 18 mg, 5 equiv.), ITX (1.27 mg, 3 equiv.) and PBN (0.54 mg, 1 equiv.) was degassed prior to transfer into an aqueous EPR cell. The cell was irradiated for different times (0 s, 30 s and 60 s) with a 365 nm LED light-guide (LC-L1V3, Hamamatsu, 65 mW•cm -2 ) then an EPR spectrum was acquired. 1 H NMR and 13 C NMR spectroscopy. All 1 H NMR and 13 C NMR spectra were recorded in appropriate deuterated solvents with tetramethylsilane (TMS) as the internal reference on a Varian 300 -MR. In a typical measurement, IMesH + BPh 4 -(9.18 mg, 0.015 mmol, 3 equiv.) and ITX (1.27 mg, 0.005 mmol, 1 equiv.) were charged into a borosilicate NMR tube. The mixture was dissolved in 0.3 mL of acetonitrile-d 3 (0.03 M, relative to IMesH + BPh 4 -). Before exposure to a 365 nm LED light spot during 5 min (see details in previous section), the NMR tube was capped with a rubber septum and degassed with N 2 . The as-irradiated tube was then analyzed by 1 H NMR. Excess amount of CS 2 (0.02 mL) was added subsequently into the tube. It caused a sudden color change from yellow to red followed by a gradual precipitation (Figure A.III.1). 1 H NMR measurement was performed to confirm the formation of the IMes-CS 2 zwitterion adduct. NMR spectroscopy. All 11 B NMR were recorded in THF-d 8 with BF 3 .Et 2 O as the internal reference on a Varian Bruker 500 MHz. All solutions were prepared under argon using a glove box technique. Prior to utilization, quartz NMR tubes were dried under vacuum overnight at 80 o C. The THF-d 8 solvent was dried over molecular sieves.Gas chromatography-mass spectrometry (GC-MS). The GC instrument is aShimadzu GC-2010 model completed with the QP-2010 mass spectrometer system. Helium is the gas vector, used at the constant linear velocity of 25 cm•s -1 . The sample was injected into a Supelco® BP-X5 column (0.15 mm of diameter, 25 m of length and 0.25 µm of film thickness) by using split mode with a ratio of 75:25. The temperature of the injector is maintained at 330 °C. The column temperature was heated from 70 o C to 340 o C in two subsequent steps: the first heating state to 250 o C with heating rate of 15 o C•min -1 and the second to 340 o C at the rate of 4 o C•min -1 . Then, the sample was ionized using an Electronic ionisation source heated at 200 o C. The mass spectrum was recorded from 4 min to end of the program with scan mode from 50 m/z to 600 m/z. Mass Spectra of each peak were compared with spectra of NIST05 and NIST05s databases to identify compounds. In a typical experiment, 2 mL of acetonitrile solution including IMesH + BPh 4 -(1.71 mg, 0.003 mmol, 3 equiv.) and ITX (0.24 mg, 0.001 mmol, 1 equiv.) were placed into a UV quartz cuvette (1 cm optical path) and closed by a rubber septum. The cuvette was degassed with N 2 during 5 min then immediately exposed to a 365 nm LED spot light (65 mW•cm -2 ) for 5 min while keeping the stirring. Finally, the irradiated solution was transfer into Agilent 2 mL glass vial with a screw top (PTFE septum) under inert condition for GC-MS analysis.

4 -

 4 IMes yield by phenol red spectrophotometric titration. 10 mL of acetonitrile solutions containing IMesH + BPh 4 -(1.85 mg, 3 10 -4 mmol, 3 equiv.) and ITX (0.25 mg, 10 -4 mmol, 1 equiv.) were prepared (3 10 -4 M relative to IMesH + BPh 4 -and 10 -4 M relative to ITX). 2 mL of solution were transferred into a spectroscopic quartz cell capped with a rubber septum. The colorless mixture was purged with nitrogen before exposing to a 365 nm LED spot light for a given irradiation time (0, 1, 2, 5 and 10 min). Subsequently, a phenol red (PR) titrating solution (2  10 -4 M in acetonitrile) was added gradually (0.1 ml) from a 1 ml syringe into the cuvette. After each injection, an UV/vis spectrum was recorded. At the start of the spectrophotometric titration, the indicator solution in acetonitrile was completely colorless, because the bis-protonated form, referred to as H 2 PR, is the prevalent form. Before the equivalence point, the gradual addition of H 2 PR solution to the irradiated solution of IMesH + BPh 4 -/ITX caused the appearance and intensification of a pink color assigned to the bivalent anion PR 2-(λ max = 580 nm) formed after reaction with photogenerated IMes (Figure A.III.2). After the equivalence point was reached, i.e. when all NHC species reacted, the deep pink color disappeared gradually and the solution became pale yellow marking the conversion of PR 2-into the univalent anion HPR - (λ max = 390 nm). The two acid-base reactions before and after equivalence points were described in the scheme below. Plotting the absorbance at 580 nm (A 580 ) as a function of the titrant volume gave two intersecting straight lines, thereby enabling the detection of the titration end-point. Performing the titration for different irradiation times revealed the progressive release of NHC and led to a maximum yield of 50 % achieved after 5 min irradiation. At the equivalence point in the acid-base titration: concentration of photogenerated IMes released in the UV cuvette, is initial volume of IMesH + BPh 4 -/ ITX solution, [PR] is concentration of PR and is total volume of PR added into the UV cuvette at the titration end-point. Therefore, the yield of IMes released upon irradiation of IMesH + BPh

Figure 3 . 1

 31 Figure 3.1 shows the temporal evolution of the transient absorption spectrum of ITX/NaBPh 4 (trace b). After 100 ns, it seems that the narrow 630 nm peak due to triplet absorption has disappeared and has been replaced by a new broad absorption in the region of 560 -680 nm.

Figure 3 . 1 .

 31 Figure 3.1. Transition absorption spectra of (a) [ITX] = 10 -4 M, (b) ITX/NaBPh 4 = 6 10 -3 M and (c) ITX/IMesH + BPh 4 -= 6 10 -3 M for 100 ns -90 µs delay time in N 2 -saturated acetonitrile with excitation wavelength at 355 nm. (Delta OD is a change in absorbance).

Figure 3 . 2 .

 32 Figure 3.2. Decays of 3 ITX* at 600 nm in the presence and absence of: (a) IMesH + Cl -, (b) NaBPh 4 , (c) NaBPh 4 (under O 2 ) and (d) IMesH + BPh 4 -([ITX] = 10 -4 M, [IMesH + Cl -] = [NaBPh 4 ] = [IMesH + BPh 4 -]= 6 10 -3 M, respectively, recorded in N 2 -saturated acetonitrile).

Figure 3 . 3 ,

 33 the experimental EPR spectra from ITX/NaBPh 4 /PBN (a) and ITX/IMesH + BPh 4 -/PBN (b) are shown. Similar in both instances, the EPR spectra show a strong triple-doublet signal suggestive of a single paramagnetic species and gradually increasing with irradiation time (see Figure 3.4). Hyperfine coupling (hfc) constants of the spin adduct that were retrieved from simulation (a N = 14.7 G and a H = 2.7 G, Figure A.III.3)

  explanation is that Ph• reacts rapidly with Ph 3 B to produce Ph-Ph and Ph 2 B • , making the detection challenging.42

Figure 3 . 3 .

 33 Figure 3.3. EPR spectra of radical adduct from a solution: (a) ITX -NaBPh 4 -PBN and (b) ITX -IMesH + BPh 4 --PBN after 60 s of irradiation. [ITX] = 5 10 -3 M, [IMesH + BPh 4 -] = 1.5 10 -2 M, [NaBPh 4 ] = 1.5 10 -2 M and [PBN] = 3 10 -3 M in acetonitrile.

Figure 3 . 4 .

 34 Figure 3.4. EPR spectra of Ph-PBN radical from a solution ITX -IMesH + BPh 4 --PBN in acetonitrile after exposure under LED 365 nm at given time (concentration: [ITX] = 5 10 -3 M, [IMesH + BPh 4 -] = 1.5 10 -2 M and [PBN] = 3 10 -3 M, respectively).

Using 1 H

 1 NMR spectroscopy, we find that irradiation of an oxygen-free acetonitrile-d 3 solution containing ITX only (Figure 3.5a) is stable; whereas, as-irradiated solution of IMesH + BPh 4 -(1 equiv.)/ ITX (3 equiv.) at 365 nm results in a complete deprotonation at C2 position (H a , δ = 8.70 ppm) of the imidazolium cation (Figure 3.5, trace b and

Figure 3 . 5 . 1 H 21 Scheme 3 . 3 .

 3512133 Figure 3.5. 1 H NMR spectra in ACN-d 3 of: (a) ITX after 5 min irradiation, (b) ITX/ IMesH + BPh 4 -mixture prior to exposure, (c) after 5 min irradiation of (b) and (d) after addition of CS 2 into (c). A substantial change in 1 H NMR is the emergence of protons H e" (δ = 2.36 ppm) and protons H d" (δ = 2.31 ppm), which are attributed to methylene protons of mesityl moieties from IMes-CS 2 adduct, and the shifting towards to the original resonance of protons H e and H d of unreacted IMesH + BPh 4 -(concentration: ITX (0.07 M) and IMesH + BPh 4 -(0.21 M)).

Figure 3 .

 3 5b with the slight shift to lower resonance of methylene protons H e' (δ = 2.07 ppm) and H d' (δ = 2.37 ppm) observed after irradiation. Similar upfield shift was evidenced when adding 1 equiv. of free IMes to 9 equiv. of IMesH + BPh 4 -(Figure A.III.4). In this spectrum, the bridging proton H a (δ = 8.70 ppm) of the C•••H-C complex disappeared entirely due to H-bonding. This means that protonation degree cannot be used to quantitatively assess the NHC yield.

Figure 3 . 6 .

 36 Figure 3.6. (a) Change of UV-Vis spectra of an acetonitrile solution of IMesH + BPh 4 -(3.0  10 -4 M) and ITX (1  10 -4 M) irradiated during 2 min (LED, 365 nm, 65 mW cm -2 ) upon gradual addition of PR (2  10 -4 M) and (b) Titration plot showing the absorbance at 580 nm for the same

Figure 3 . 7 . 49 Scheme 3 . 4 .

 374934 Figure 3.7. GC chromatogram of photoproducts obtained from the photolysis media of ITX -IMesH + BPh 4 -after 5 min of irradiation ([ITX] = 5 10 -4 M and [IMesH + BPh 4 -] = 1.5 10 -3 M,

Figure 3 . 8 Figure 3 . 4 -, the 11 B

 383411 Figure 3.8 shows the 11 B-NMR spectrum of an argon saturated solution of ITX/IMesH + BPh 4 -(1/1 equiv.) prior (a) and after 10 min irradiation (b). As expected, Figure 3.8a exhibits only a narrow singlet attributed to BPh 4 -anion (δ = -6.3 ppm). This chemical shift is characteristic of tetraorganoborate anions experiencing a strong deshielding. After 10 min irradiation (Figure 3.8b), in addition to the resonance of BPh 4 -, the 11 B spectrum of the photolysis solution reveals two new features: an intense resonance at 1.1 ppm, and a broad and much weaker signal at 5.9 ppm. Acting on the assumption that these species could derive from triphenylborane, a set of experiments using BPh 3 was implemented. The 11 B NMR spectrum of BPh 3 , shown in Figure A.III.6, reveals a broad singlet at 50.3 ppm

Figure 3 . 8 .

 38 Figure 3.8. 11 B NMR spectra of a mixture of ITX (0.03 M) and IMesH + BPh 4 -(0.03 M) in THF-d 8 : (a) prior irradiation, (b) after 10 min UV exposure, and (c) after adding CS 2 into medium. Irradiation conditions: LED 365 nm, 65 mW•cm -2 .

CS 2

 2 is introduced. To provide further evidence for this assignment, Ph 2 BOH was generated in situ by moderate hydrolysis of the BPh 3 in non-dried THF-d 8 upon exposing the solution in the NMR tube to atmosphere57,58 This solution revealed two broad signals centered at 45 ppm (Ph 2 BOH) and 51 ppm (residual BPh 3 ) in the 11 B NMR spectrum (Figure 3.10). Addition of IMes into the medium caused the two signals to shift at -9 ppm (IMes-BPh 3 adduct) and 1.1 ppm (IMes-Ph 2 BOH adduct), lending further confidence in our assignment.

Figure 3 . 9 . 4 -

 394 Figure 3.9. 11 B-NMR spectra in THF-d 8 : (a) IMesH + BPh 4 --IMes, (b) IMesH + BPh 4 --IMes -BPh 3 (concentrations: [IMes] = 0.01 M, [IMesH + BPh 4 -]= 0.03 M, [BPh 3 ] = 0.01 M). A white precipitation which is IMes -BPh 3 adduct, formed immediately as BPh 3 introduced into the mixture IMesH + BPh 4 -

Figure 3 .Scheme 3 . 5 .

 335 Figure 3.10. 11 B NMR spectra in non-dried THF-d 8 : (a) Hydrolyzed BPh 3 , (b) Addition of IMes into hydrolyzed BPh 3 .

Scheme 3 . 6 .

 36 Scheme 3.6. The mechanistic pathway involved in the formation of IMes during the photolysis of a mixture ITX/ IMesH + BPh 4 -.

  lamp@254 nm (75 mW•cm -2 , Hammamatsu, LC-9588/01A) (Figure A.IV.1) a LED@365 nm (65 mW•cm -2 , Hammamatsu LC-L1V3) and LED@405 nm (100 mW•cm -2 , Thorlabs M405L2) (Figure A.IV.2). The intensities of lamps were determined by using an optical power meter PM100D from Thorlabs.

1 H

 1 ), the same NB/ITX/1/Ru-1 mixture was introduced into an IR liquid cell (Specac® Omni cell) (FigureA.IV.4) containing two sandwich CaF 2 windows. A 1 mm Teflon spacer in the middle of pellets was used to fix the specimen thickness. Samples were then exposed synchronously to an IR beam and an UV irradiation resource equipped with a light guide. Two types of LED were used: a LED@365 nm and LED@405 nm (see above for details).Preparation of cross-linked ROMP polymer films.A mixture of NB and DPCD was prepared at different NB concentration ranging from 25 to 75 mol %. Ru-1 (9 mg, 1 equiv.), ITX (9.3 mg, 2.5 equiv.) and 1 (45 mg, 5 equiv.) were first dissolved in CH 2 Cl 2 (2 mL), then the monomer mixture (510 equiv.) was added and left 10 min under stirring to obtain a homogeneous solution. The catalyzed mixture was then poured into a Teflon mold covered with a borosilicate glass plate. Clamps were used to prevent air entry as well as evaporation of solvent and monomers. The weight of mold containing the reactive mixture was determined before irradiation to be used subsequently for gravimetry measurements. The mold was placed under a LED panel (365 nm, 5.5 mW•cm -2 , top UV Curing light) (Figure A.IV.5) for 1 h. The wet gel was removed from the mold and left overnight. Residual monomers and solvent were removed by drying under vacuum at 50 o C until a constant weight. NMR spectra were recorded in appropriate deuterated solvents with tetramethylsilane (TMS) as the internal reference on a Varian 300 -MR. All chemical resonances were documented in parts per million (ppm) relative to the residual THF-d 8 (δ 1.72 ppm and 3.58 ppm), and CD 2 Cl 2 (δ 5.32 ppm). Peaks multiplicities in 1 H -NMR spectra were abbreviated as s (single), d (double), t (triplet), m (multiplet), br (broad). -Molecular weights of pNB and poly(5-ethylidene-2-norbornene) (pENB) were determined using a size exclusion chromatography (SEC) instrument from a Varian apparatus equipped with TOSOHAAS TSK gel columns and a refractive index detector. The mobile phase was THF at a flow rate of 1 mL•min -1 ; the injected volume was 100 µL. The MALS instrument was normalized using a THF solution of polystyrene standard. -Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments were performed using, respectively, a TGA/DSC3+ (Metter Toledo) and a DSC1 (Metter Toledo) under nitrogen atmosphere. For TGA analysis, the samples (8 -15 mg) were heated from 30 o C to 600 o C at a heating rate of 10 o C•min -1 . For DSC analysis, the samples (3 mg) were scanned from -50 o C to 250 o C at a heating rate of 10 o C•min -1 . -Atomic force microscopy (AFM) characterization was performed on a Bruker-MultiMode 8 controlled by NanoScope V. Information for the tip is NCLR-50, k= 48 N/m. The tip calibration was carried out prior to the analysis (f o = 161.71 kHz, ϕ = 475 and k=22.2 N/m). -Film morphology was observed by scanning electron microscopy (SEM), (SEM Fei Quanta 400, tungsten gim). -To determine gel content, a polymer specimen (~50 mg) was placed into a 5 ml vial containing 4 mL of CH 2 Cl 2 . After magnetic stirring for 24 h at room temperature, the solvent was removed and insoluble component was dried under vacuum at 40 o C until weight constant. The gel content was finally measured. -1 H-13 C ramped CPMAS (Cross-Polarization Magic Angle Spinning) NMR spectra were recorded by using an Advance II Bruker Spectrometer (Ascend™ magnet) (B 0 = 9.4 T) operating at 400.18 MHz for the 1 H and 100.64 MHz for the 13 C and controlled by TOPSPIN 3.2 software. The solid films were grounded into fine powders prior to measurement.

1 ( 4 . 3 . RESULTS AND DISCUSSION 4 . 3 . 1 .

 143431 C spectrum. DCPD contains initially 4 sp 2 and 6 sp 3 carbons. The ratio of sp 2 -C to sp 3 -C does not change when cyclopentene reacts by ROMP. Conversely, olefin coupling leads to the replacement of 2 sp 2 -C by 2 sp 3 -C. This change can be used to estimate the fraction of cyclopentene reacting via olefin coupling (x) in pDCPD film (eq. 1) 35 : (1) where I(sp 2 -C) and I(sp 3 -C) are the integrated areas of the signals associated to olefinic carbons (~ 125 -130 ppm) and aliphatic carbons (30 -60 ppm), respectively. This equation was changed to adapt to our specific copolymer structure resulting from copolymerization of NB and DCPD (eq. 2): (2) where 0.43 is the value of the experimental ratio I(sp 2 -C)/ I(sp 3 -C) in pNB (calculated from sample NB-100), and x NB is the molar fraction of NB in the NB/DCPD monomer mixture.-FTIR spectra were recorded with a Nicolet 8700 Spectrophotometer (Thermo Scientific) using a mercury cadmium telluride detector. Two modes were used, transmission for the real-time kinetics of photoROMP in solution and ATR for characterization of polymer films. All the spectra were recorded in range of 4000 cm -1 -600 cm -1 . The real-time measurements used a scanning velocity of 11.45 cm sec -1 corresponding to a resolution of 0.964 cm -1 in the infrared spectra. Monomer conversion was determined from the temporal evolution of different stretching modes at 1566 cm -1 (NB, C=C stretch) and 3047 cm -DCPD, sp2 =CH-) (Figure A.IV.6). To ensure a good reproducibility of these data, each experiment was performed at least three times. Study and optimization of the tandem photocatalytic system a. NHC photogeneration There are two possible photochemical pathways to deprotonate the IMesH + imidazolium cation in 1, and thereby generate the targeted NHC IMes (Scheme 1). Both approaches rely on the photochemistry of tetraphenylborate counter-anion: a direct photolysis of BPh 4 -

  H NMR the photogenerated active catalyst have failed, preventing to confirm our hypothesis. The spectra of the catalytic reaction mixture 1/Ru-1/ITX show after irradiation too many signals, preventing the unequivocal identification of the chelated NHC-Ru complex (see Figure A.IV.7).

Figure 4 . 1 . 1 H

 411 Figure 4.1. 1 H NMR spectrum of the reaction medium after reaction of [(p-cymene)RuCl 2 ] 2 (Ru-1, 1 equiv.) with IMes (2.02 equiv.) in THF-d 8 ([Ru-1] = 0.05 M).

- 1 [

 1 RuCl 2 (p-cymene)] 2 was substituted with [RuI 2 (p-cymene)] 2 (Ru-2) and [RuCl 2 (COD)] 2 (Ru-3, with COD = 1,5-cyclooctadiene) to probe separately the effect of the halogen and the ligand in the Ru dimer. Remarkably, Ru-2 afforded less than 1 % conversion after 4 h (entry c) and only 1.4 % monomer consumption after 168 h (entry d). Addition of 1

  365 nm and lower M n values were obtained (entry a). The influence of sensitizer concentration (entry b), excitation wavelength (entry c) and precatalyst nature (entries d-e) are consistent with previous observations.

  role(s) played by irradiation (365 nm) in this mechanism. As established earlier, light has a clear role in the generation of NHC ligand (section a). Bench stability experiments (section b) also demonstrate a second role in the disengagement of the η 6 -arene ligand. To shed further light on the other possible effects of light on polymerization mechanism, NB conversion kinetics was measured in situ by real-time FTIR spectroscopy. For this, the reactive monomer mixture was introduced in a liquid IR cell. IR spectra were acquired continuously during UV exposure (LED spotlight 365 nm) in order to construct a conversion-time curve.

Figure 4 . 2

 42

Figure 4 . 2 .Scheme 4 . 2 .

 4242 Figure 4.2. Conversion-time curve for the ROMP of NB in solution in CH 2 Cl 2 : light off (grey area), light on (white area). NB/Ru-1/1/ITX = 510/1/5/0.5 equiv. Irradiation conditions: LED@365 nm, 65 mW•cm -2 .

Figure 4 . 3

 43 shows a typical conversion kinetics of DCPD, together with that of NB. As expected, DCPD exhibited a much slower polymerization rate compared to NB.[54][55][56] To increase reactivity and achieve a cross-linked structure unattainable with NB only, we proposed a copolymerization of DCPD with NB.13,55 As shown on the same plot, a mixture of DCPD/NB (50/50 mol%) resulted in intermediate polymerization rates and almost full conversion after 30 min irradiation.

Figure 4 . 3 .

 43 Figure 4.3. Photopolymerization kinetic of NB, DCPD and NB/DCPD (50/50 mol%) using photocatalyst system Ru-1/1/ITX in CH 2 Cl 2 (Monomer/Ru-1/1/ITX = 510/1/5/2.5 equiv.). Conversions of NB, NB/DCPD (50/ 50 mol%) and DCPD were 100 %, 94 % and 70, respectively.

Figure 4 . 4 ,

 44 Figure 4.4, a mixture of monomer and photocatalyst dissolved in CH 2 Cl 2 was placed in aTeflon mold, covered with a glass slide to avoid evaporation, and irradiated for 1 h through a LED panel (365 nm, 5.5 mW•cm -2 ) (see experimental section for details). The gels were then dried, and characterized using a range of spectroscopic techniques (FTIR-ATR,13 C CPMAS), gravimetry, gel content, TGA, DSC, AFM and SEM. All these techniques are intended to

Figure 4 . 4 .Scheme 4 . 3 .

 4443 Figure 4.4. Photographs of: (a) General set up for the preparation of cross-linked films and (b) Images of polymeric films (i) NB-100, (ii) NB-75, (iii) NB-50, (iv) NB25 and (v) DCPD-100.

Figure 4 . 5 .

 45 Figure 4.5. Characterizations of different polymeric specimens: (a) Polymer yield determined by gravimetry method and (b) Gel content determined by extracting with CH 2 Cl 2 .

Figure 4 . 6 .

 46 Figure 4.6. FTIR-ATR spectra of polymer films obtained by photoROMP at 365 nm: (a) NB-100, (b) NB-75, (c) NB-50, (d) NB-25 and (e) DCPD-100: (A) Expanded spectra in a range of 675 -1700 cm -1 and (B) Expanded spectra in a range of 2960 -3100 cm -1 .

Figure 4 . 7 ,

 47 the13 C CPMAS spectra of the different NB/DCPD samples display strong similarities. The region at ~ 130 ppm represents olefinic carbons (sp 2 -C) while the signals at 30 -60 ppm is reflective of aliphatic carbons (sp 3 -C). As mentioned above, the C=C double bonds of cyclopentene ring can undergo either metathesis or olefin coupling reactions. Of interest is that CPMAS13 C-NMR can be used to estimate the contribution of olefin coupling. Occurrence of olefin coupling is manifested by the conversion of two 2 sp2

Figure 4 . 7 .

 47 Figure 4.7. 13 C NMR CPMAS spectra of polymeric films: (a) NB-100, (b) NB-75, (c) NB-50, (d) NB-25 and (e) DCPD-100.

Figure 4 . 8 .

 48 Figure 4.8. (A) Glass transition temperature as measured by DSC of different polymer films in nitrogen with (a) NB-100, (b) NB-75, (c) NB-50, (d) NB-25 and (e) DCPD-100 and (B) TGA thermograms of different polymer films obtained as heating under nitrogen environment.

Figure 4 . 9 .

 49 Figure 4.9. SEM images of the fracture surface of homo and copolymer specimens with 5000× magnification.

  highlights the role of BPh 4 -as electron donating species, and ITX as an electronically electron acceptor sensitizer, while excluding the role of azolium cation NHCH + in PET. The ITX •-species is supposed to subsequently deprotonate the NHCH + cation to yield a ketyl radical (ITXH • ) and the targeted NHC. Meanwhile, the BPh 4 • radical undergoes further reactions: C-B bond cleavage and auto-oxidation give a biphenyl and diphenylborinic acid (Ph 2 BOH), as proved by a combination of different characterization techniques including EPR, GC-MS and 11 B NMR spectroscopy. The liberation of the desired NHC IMes is successfully identified via the transformation into IMes-CS 2 and IMes-radical adducts visible in 13 C NMR and EPR spectra, respectively. Unexpectedly, the carbene released can react with other species such as Ph 2 BOH and 3 ITX*, thus affecting the overall yield of NHC. Evidence for the formation of IMes-Ph 2 BOH complex was provided by the appearance of a new chemical resonance (δ = 1.1 ppm) in 11 B NMR, while the interaction between NHC and 3 ITX* was inferred from the observation of ITX bleaching during steady-state photolysis and high triplet quenching rate of ITX in the presence of the free NHC. Amount of the NHC photoliberated in the mixture of NHCH + BPh 4 -/ITX was evaluated based on an acid/base titration, using phenol red as titrant or a quantitative reaction between NHC and 1,3-di-p-tolylcarbodiimide (CDI). Basically, the acid/base titration grounded on the deprotonation of phenol red by the carbene photognerated owing to its high pK a and gave the basic form of phenol red. Thereby, based on absorbances of the phenol red

Figure A.II. 4 .

 4 Figure A.II.4. A LED at 365 nm (LC-L1V3, Hamamatsu, 65 mW•cm -2 ): a) an image of the set-up and (b) its emission spectra.

Figure A.II. 5 . 4 -/

 54 Figure A.II.5. Color changes of an as-irradiated IMesH + BPh 4 -/ ITX in THF-d 8 at different given

Figure A.II. 7 .

 7 Figure A.II.7. A set-up for photo-bleaching experiment.

Figure A.II. 8 . 4 -:

 84 Figure A.II.8. Thermal analysis of tetraphenylborate imidazolium IMesH + BPh 4 -: (a) DSC

Figure A.II. 9 .

 9 Figure A.II.9. Enthalpy calculation of reactions of IMes with phosphonyl (P • ) and mesitoyl (M • ) radicals by using uB3LYP/6-31G* level of theory.

Figure A.II. 10 .

 10 Figure A.II.10. EPR spectrum obtained at different given irradiation times of a solution of IMes/TPO in THF (5/1 equiv., [TPO] = 3 10 -3 M).

Figure A.II. 11 . 1 H 4 -( 1 /

 11141 Figure A.II.11. 1 H NMR spectra in THF-d 8 of: (a) IMesH + BPh 4 -(0.03 M), (b) IMes/IMesH + BPh 4 -

Figure A.II. 13 . 1 H

 131 Figure A.II.13. 1 H NMR spectrum of refined (precipitated) polyester in CDCl 3 obtained by the ROCOP of CHO/PA (500/100 equiv.) at 110 o C for 60 min using an as-irradiated IMesH + BPh 4 -/ITX

Figure A.II. 14 .

 14 Figure A.II.14. Positive MALDI-ToF MS spectrum of isolated poly(PA-alt-CHO) obtained by the ROCOP of CHO/PA (500/100 equiv.) at 110 o C for 60 min using an as-irradiated IMesH + BPh 4 -/ITX solution (1/1 equiv.) (photogenerated IMes) as an initiator (entry a in Table 2.3). Note: A 15 is structure A with repeating unit n = 15.

Figure A.II. 16 .IMesH + BPh 4 -/ 4 -/

 1644 Figure A.II.16. SEC traces of precipitated polynorbornene prepared by the ROMP of NB (510 equiv.) irradiated for 60 s with LED 365 nm (6.5 mW cm -1 ) using different catalysts: (a) IMesH + BPh 4 -/ITX/[RuCl 2 (p-cymene)] 2 solution (5/2.5/1 equiv.) and (b) SIMesH + BPh 4 -/ITX/[RuCl 2 (p-cymene)] 2 solution (5/2.5/1 equiv.). Three detectors were used: 1/. LS: light scattering detector, 2/. UV, and 3/. dRI: refractive index detector.

Figure A.III. 1 . 4 -/

 14 Figure A.III.1. Color changes of an as-irradiated IMesH + BPh 4 -/ ITX (3/ 3 equiv.) in ACN-d 3 at different given time and formation of IMes-CS 2 adduct after CS 2 added.

Figure A.III. 2 .

 2 Figure A.III.2. Color changes of an as-irradiated IMesH + BPh 4 -/ ITX (3/ 1 equiv.) in ACN before and

Figure A.III. 3 .

 3 Figure A.III.3. EPR spectra of Ph-PBN radical from a solution ITX -IMesH + BPh 4 --PBN in acetonitrile after expososing under LED 365 nm for 60 s (concentration: [ITX] = 5 10 -3 M, [IMesH + BPh 4 -] = 1.5 10 -2 M and [PBN] = 3 10 -3 M, respectively).

Figure A.III. 4 . 1 H 4 -

 414 Figure A.III.4. 1 H NMR spectra in THF-d 8 of: (a) IMesH + BPh 4 -, (b) IMes , (c) the mixture of IMesH + BPh 4 -and IMes (9/1 equiv.) and (d) irradiated ITX -IMesH + BPh 4 -(1/1 equiv.).

Figure A.III. 5 .

 5 Figure A.III.5. Amount of IMes existing in acetonitrile against exposure time determined by an acid/base titration with phenol red.

Table A.III. 1 .Figure A.III. 6 .

 16 Figure A.III.6. 11 B-NMR spectra of photolysis of [ITX] = 0.03 M -[BPh 3 ] = 0.03 M in THF-d 8 :(a) prior to exposure, (b) after exposure for 10 min (LED 365 nm, 65 mW•cm -2 ).

Figure A.IV. 3 .

 3 Figure A.IV.3. (a) A set-up for photopolymerization with LED at 405 nm (SMD2835-60LED/Meter, Banggood, 4.7 mW•cm -2 ) and (b) Emission spectra of the Banggood LED.

Figure A.IV. 4 .

 4 Figure A.IV.4. (a) An image of Specac® sealed flow cell for IR spectroscopy and (b) Insight component parts of the Specac® sealed flow cell (the cell windows are CaF 2 pellets, Teflon space has a thickness of 0.1 mm).

Figure A.IV. 5 .

 5 Figure A.IV.5. A set-up of top UV curing LED at 365 nm (Tao Yuan , 5.5 mW•cm -2 ).

Figure A.IV. 6 . 4 -/

 64 Figure A.IV.6. Changes in FT-IR profiles during photopolymerization in solution CH 2 Cl 2 using the catalytic system Ru-1/ IMesH + BPh 4 -/ ITX of various monomers: (a) NB and (b) DCPD.

Figure A.IV. 7 . 1 H

 71 Figure A.IV.7. 1 H NMR of 1/Ru-1/ITX in THF-d 8 : (a) Prior to exposure to light and (b) After 30 min of irradiation (Conditions: 1/Ru-1/ITX = 5/1/2.5 equiv., [Ru-1]=0.002M, Xe-Hg lamp@ 254 nm with intensity of 20 mW•cm -2 ).

Figure R.II. 1 . 1

 11 Figure R.II.1. 1 et 2 sont parmi les NHCs les plus fréquemment employés

21 Schéma R.II. 1 .

 211 approche a des limites importantes liées au clivage réversible, la faible solubilité de nombreux adduits NHC et la latence incomplète. Mécanisme de photolyse pour la photogénération de 1 (ou 2) à partir du système bicomposant ITX / 1H + BPh 4 -. Malgré les nombreux développements dans le domaine de photoamorceurs, 26,27 très peu d'attention a été accordée aux précurseurs de NHC photolatents. En 2015, Denning et al. 28 a étudié la réduction photochimique d'un imidazolium-2-carboxylate d'éthyle (NHC-CO 2 ) en utilisant un donneur d'électrons à l'état excité, la N,N,N',N'-tétraméthylbenzidine.

5 Schéma R.III. 1 . 9 -

 519 photogénérateur de NHC bicomposant avec quatre parties distinctes. Tout d'abord, nous proposons deux nouvelles méthodes d'analyse pour caractériser les NHCs libérés photochimiquement grâce à leur conversion en produits d'addition. D'autre part, pour soutenir la voie photochimique en deux étapes sur la base de réactions de transfert d'électrons/protons, des expériences de « photobleaching » en temps réel sont effectuées ainsi que le calcul de la variation d'énergie libre de transfert d'électrons avec l'équation de Rehm-Weller. En troisième lieu, le rendement en NHC est optimisé en étudiant notamment l'effet du temps d'irradiation, de la concentration du sensibilisateur. Dans une dernière partie, le photogénérateur de NHC est mis à profit pour préparer une variété de structures polymères grâce à son double rôle. En tant qu'organocatalyseur, des polyuréthane et polyester ont été préparés par polymérisation par étapes et copolymérisation par ouverture de cycle, respectivement. En tant que ligand, du

Chapitre IV.

  La combinaison d'un photogénérateur de ligand et d'un précatalyseur de ruthénium: une approche photoinduite pour la synthèse de film polymère réticulé par ROMP

6 , 7 À

 67 ce jour, l'utilisation de polymères ROMP a été limitée à la fabrication de matériaux massifs. Cela malgré le large spectre de propriétés des principaux polymères de ROMP, dérivés du norbornène (NB), cyclopentène, cyclooctène ou dicyclopentadiène (DCPD). Par exemple, les élastomères polycyclooctène (Vestenamer®) et polyNB (Norsorex®) sont utilisés après vulcanisation comme amortisseurs et matériaux anti-adhérence. Le polyNB hydrogéné (Zeonex®) est un thermoplastique à haute performance optique, tandis que polyDCPD (Metton® ou Telene®) est connu comme un thermodurcissable ayant des propriétés mécaniques semblables aux thermoplastiques d'ingénierie. La plupart de ces polymères de ROMP ont été utilisés en tant que matériaux de structure, parce que leurs méthodes de fabrication sont restrictives.

  photosensible (un photogénérateur de ligand). Cette méthodologie a été introduite par Grubbs et al. en 2009 en utilisant un complexe de Ru-alkylidène ligaturé par l'acétylacétonate (acac) comme précatalyseur et un générateur de photoacide (PAG) à base de chlorure de triphénylsulfonium.15 Lors d'une irradiation à 254 nm, HCl a été libéré, et le déplacement ultérieur de acac par Cl -a donné lieu à un catalyseur actif en métathèse. Une méthode similaire a également été employée par Pier et al. en utilisant un carbure de ruthénium et Ph 3 S triflate (OTf).16 Récemment, nous avons remplacé le PAG avec un photogénérateur NHC utilisé et un complexe stable, dimérique Ru (II) disponible dans le commerce en tant que catalyseur de métathèse inactif. Notre système en tandem composé du précurseur de NHC 1,3-bis (mésityle) imidazolium tétraphénylborate IMesH + BPh 4 -(1) et du précatalyseur [RuCl 2 (p-cymène)] 2 (Ru-1) s'est révélé être très efficace pour la ROMP du NB. 22,23 En comparaison avec les exemples décrits ci-dessus, un rayonnement UVA (320 -400 nm) a été utilisé, et il n'y pas de déplacement de ligand. Comme le montre le Schéma R.IV.1, le clivage des ponts Ru-chlorido de Ru-1 est induit enthalpiquement par l'addition de 2 équivalents du NHC IMes, photogénéré à partir du composé 1. En conséquence, le catalyseur actif présumé (Ru-1-IMes) dispose d'un cycle p-cymène, de deux ligands chlorido et d'un ligand IMes. D'abord présenté par Herman et al. 24 cette classe de catalyseur RuCl 2 (pcymène)NHC a été ensuite largement étudié par Noels et al. pour la ROMP du NB et du

Table 1 . 2 .

 12 Photoinduced ROMP employing N-chelating complexes Ru18 a and Ru19 b ........35

). ..............................................................155 List of Tables Title...................................................................................................................................... Page Table 1.1. Photoinduced ROMP employing photoisomerizable complexes Ru-14-cis a ........32

Table 1 . 3 .

 13 Photoinduced ROMP of NB using a photocatalyst P1 a .........................................38

Table 2 . 1 .

 21 Effect of the photooxidant's nature on the release of photogenerated NHC IMes. .......................................................................................................................................69

Table 2 . 2 .

 22 Results of step-growth polymerization of PEG 400 /IPDI (342/342 equiv.). ...........70

Table 2 . 3 .

 23 Results of ROCOP of CHO/PA (500/100 equiv.). ................................................73

Table 2 . 4

 24

				List of Abbreviations
	Chemical compounds
	ACN		: acetonitrile
	BPh 3			: triphenylborane
	BPh 4	-		: tetraphenylborate anion
	CD 2 Cl 2		: deuterium dichloromethane
	CDCl 3		: deuterium chloroform
	CDI			: 1,3-di-p-tolylcarbodiimide
	CH 2 Cl 2		: dichloromethane
	CHO		: cyclohexene oxide
	COD		: cyclooctadiene
	COE			: cyclooctene
	CQ			: camphorquinone
	CS 2			: carbon disulphide
	DCPD		: dicyclopentadiene
	DMSO		: dimethyl sulfoxide
	DTDL		: dibutyltin dilaurate
	EAB			: 4,4'-bis(diethylamino)benzophenone
	ENB			: 5-ethylidene-2-norbornene
	IDPI			: isophorone diisocyanate
	IMes		: 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene
	IMesH + BPh 4	-	: 1,3-bis(2,4,6-trimethylphenyl)imidazolium tetraphenylborate
	IMesH + Cl -		: 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride
	ITX			: isopropylthioxanthone
	NaBPh 4		: sodium tetraphenylborate
	NB			: norbornene

. Results for the photoROMP of NB conducted at ambient conditions. ..................76 Table 3.1. Triplet energy simulation of ITX and IMesH + BPh 4 -estimated. ...........................94 Table 4.1. Effect of excitation wavelength on the release of free NHC IMes in THF-d 8 ([1] = 0.03 M, [ITX] = 0.03 M) after 5 min of irradiation. ..............................................................118 Table 4.2. Photolatency results of Ru dimer/ITX/monomer mixture in CD 2 Cl 2 ([Monomer] = 1 M). All the samples were stored in the dark (Ru dimer/ITX/monomer = 1/2.5/510 equiv.). ................................................................................................................................................120 Table 4.3. Photopolymerization results of NB using Ru dimer/1/ITX photoinitiating system in CD 2 Cl 2 ([NB] = 1 M, 510 equiv.). Irradiation conditions: 365 nm, 6.5 mW cm -2 , 1 min.. ................................................................................................................................................121 Table 4.4. Photopolymerization results of ENB using Ru dimer/IMesH + BPh 4 -/ITX photoinitiating system in CD 2 Cl 2 ([ENB] = 1 M, 510 equiv.). Irradiation conditions: 365 nm, 6.5 mW•cm -2 , 10 min .............................................................................................................122 Table 4.5. Thermo-mechanical results of curing NB/ DCPD films using Ru-1/1/ITX photoinitiating system in CH 2 Cl 2 . Irradiation conditions: 365 nm, 5.5 mW cm -2 . ................132 Table A.III.1. Identification of photoproducts by GC-MS after the photolysis course of ITX/IMesH + BPh 4 -................................................................................................................151 : dichloro(p-cymene)ruthenium(II) dimer Ru-2 : diiodo(p-cymene)ruthenium(II) dimer Ru-3 : dichlorodi-μ-chlorobis[(1,2,3,6,7,8-η-2,7-dimethyl-2,6-octadiene-1,8diyl]diruthenium(IV)

TABLE OF CONTENTS

 OF List of Schemes ........................................................................................................................ vi List of Figures .......................................................................................................................... ix List of Tables ...........................................................................................................................

CONTENT ......................................................................................................................... Page GENERAL INTRODUCTION ............................................................................................... i ACKNOWLEDGEMENTS ...................................................................................................v xv List of Abbreviations ............................................................................................................. xvi

TABLE OF

 OF Catalyst activation through conformational change of chelated ligand ...........30 1.8 Metal-free photoROMP via photoinduced electron transfer ...........................35 1.9 Conclusions ......................................................................................................38 INTRODUCTION .......................................................................................................49 2.2 EXPERIMENTALS .....................................................................................................52

CONTENTS .................................................................................................... xix CHAPTER I. STATE OF THE ART .....................................................................................1 A LIBERATION OF N-HETEROCYCLIC CARBENES FROM LATENT PROGENITORS AND THEIR USE IN POLYMER SYNTHESIS .............................2 1.1 Essential features of N-Heterocyclic carbenes ..................................................2 1.1.1. Electronic structures of N-Heterocyclic carbenes ..................................3 1.1.

2. Stabilization of N-Heterocyclic carbenes ..............................................3 1.1.3. Synthesis of N-Heterocyclic carbenes ...................................................4 1.1.4. Principal applications of N-Heterocyclic carbenes ................................6 1.2 Liberation of NHCs from their latent progenitors .............................................6 1.2.1. By thermal-trigger ..................................................................................7 1.2.2. By mechanochemistry ..........................................................................12 1.2.3. By photochemistry ...............................................................................13 1.3 Latent NHC progenitors for polymerization reactions ....................................15 1.3.1. Ring-opening polymerization ..............................................................15 1.3.2. Step-growth polymerization.................................................................19 1.3.3. Ring-opening metathesis polymerization ............................................21 1.4 Conclusions ......................................................................................................22 B RECENT DEVELOPMENTS IN PHOTOCHEMICALLY ACTIVATED RING-OPENING METATHESIS POLYMERIZATION ......................................................23 1.5 Catalyst activation through ligand photodissociation ......................................24 1.6 Catalyst activation through in situ photogeneration of ligands .......................29 1.7 C REFERENCES ............................................................................................................39 CHAPTER II. MIXTURE OF AZOLIUM TETRAPHENYLBORATE WITH ISOPROPYLTHIOXANTHONE: A NEW CLASS OF N-HETEROCYCLIC CARBENE (NHC) PHOTOGENERATOR FOR POLYURETHANE, POLYESTER AND ROMP POLYMERS SYNTHESIS.............................................................................48 2.1 2.2.1 Materials ..........................................................................................................52 2.2.2 Synthesis ..........................................................................................................52 2.2.3 Characterization methods.................................................................................54 2.3 RESULTS AND DISCUSSION ..................................................................................58 2.3.1 Synthesis and characterization of photogenerated NHC .................................58 2.3.2 Identification and quantification of photogenerated NHC ...............................59 2.3.3 Insight into photochemical mechanism: coupled electron/proton transfer ......63

2.3.4 Optimization of NHC yield ..............................................................................67 2.3.5 Use of NHC photogenerator in polymerization ...............................................69 2.4 CONCLUSIONS..........................................................................................................76 2.5 REFERENCES ............................................................................................................77 CHAPTER III.

PHOTOREDUCTION OF TRIPLET THIOXANTHONE DERIVATIVES BY AZOLIUM TETRAPHENYLBORATE: A WAY TO PHOTOGENERATE N-HETEROCYCLIC CARBENES

  INTRODUCTION .....................................................................................................111 4.2 EXPERIMENTALS ...................................................................................................113 4.2.1 Materials ........................................................................................................113 4.2.2 Experimental protocols ..................................................................................114 4.2.3 Characterization methods...............................................................................116 4.3 RESULTS AND DISCUSSION ...............................................................................117 4.3.1 Study and optimization of the tandem photocatalytic system .......................117 4.3.2 Preparation of cross-linked ROMP polymer films ........................................125

................................................83 3.1 INTRODUCTION .......................................................................................................84 3.2 EXPERIMENTALS .....................................................................................................87 3.2.1 Materials ..........................................................................................................87 3.2.2 Synthesis of IMesH + BPh 4 -...............................................................................87 3.2.3 Characterization methods.................................................................................87 3.3 RESULTS AND DISCUSSION ..................................................................................90 3.3.1 Assignment of transient species .......................................................................90 3.3.2 Assignment of photoproducts ..........................................................................95 3.4 CONCLUSIONS........................................................................................................105 3.5 REFERENCES ..........................................................................................................105 CHAPTER IV. COMBINING A LIGAND PHOTOGENERATOR AND A RUTHENIUM PRECATALYST: A PHOTOINDUCED APPROACH TO CROSS-LINKED ROMP POLYMER FILMS ...............................................................................110 4.1 4.4 CONCLUSIONS........................................................................................................133 4.5 REFERENCES ..........................................................................................................134

  Trinh, A. Chemtob, D. Quémener, J. Pinaud, V. Héroguez, -Emerging trends in ring opening metathesis polymerization‖, Macromolecular Engineering:

	From Precise Synthesis to Macroscopic Materials and Applications, 2019 (Book chapter,
	accepted).
	©Wiley-VCH 2016
	69451 Weinheim, Germany

Scheme 1.2.Principle thermal liberation of NHCs from their corresponding precursors. a. Betaine adducts

	Protecting free NHCs by heteroallene species of general formula X=C=Y to afford
	corresponding betaine adducts is a common strategy to produce thermo-latent NHC
	progenitors shown in Figure 1.5. With a weak electrophilic carbone atom, heteroallene are
	indeed subject to the nucleophilic attack of the carbene of NHCs. 46 It has been proven that the
	stability of such complexes is significantly dependent on the heteroallene nature with, for
	example, the degradation temperature of crystal adducts derived from 2h increasing in the
	order: 2h.CO 2 (110 o C) < 2h.COS (140 o C) < 2h.CS 2 (210 o C). 47 2h.CO 2 and 2h.COS can
	readily dissociate upon heating in the solid state, while the highly stable 2h.CS 2 adduct
	degrades entirely without affording its parent NHC 2h after onset of thermolysis.

1.3). The first one is via hydrolysis

  Two facile routes are described to prepare [NHC(H)][HCO 3 ] salts (Scheme of NHC.CO 2 adducts; whereas the other relates to an anion metathesis process between azolium halides and KHCO 3 , thus avoiding the necessity of NHCs generation prior to be masked by CO 2 .

Scheme 1.3. Schematic preparation of [NHC(H)][HCO 3 ] using NHC.CO 2 adducts and azolium halides and reversible release of corresponding NHCs therefrom.

Figure 1.8) has been recently designed by Placet et al. 73 Subjecting these photogenerator NHCs at 254 nm and 365 nm enables to photorelease the desired NHCs

	71,72 have
	pioneered photolatent NHCs by revealing the formation of 2,3-dihydrothiazol-2-ylidene (6a)
	and 2,3-dihydroimidazol-2-ylidene (2g) upon photolysis of their corresponding carboxylic
	acids under 254 nm UV-light and at a temperature of 10 K (Scheme 1.7). Unexpectedly,
	these primary NHC products (6a and 2g) reacted instantly with carbon dioxide giving air-
	stable crystals, NHC.CO 2 . Furthermore, both 6a and 2g were sought to easily isomerize upon
	warm-up or exposure to light, making their trapping unsuccessful, even when handled under
	vacuum.
	A series of photolatent NHCs containing protonated NHCs and carboxylated-
	fragment chromophores such as ketoprofenate (Keto -), xanthate (Xanth -) and
	phenylglyoxylate (PA -) (via photoinduced electron transfer (PET) followed by decarboxylation and proton transfer
	(Scheme 1.8). Herein, the carboxylated-functional chromophores behave as both electron
	donor and acceptor. Moreover, the irreversible decarboxylation avoids the back electron
	transfer (BET) and therefore increase the yield of generated NHCs. One main drawback of
	such motifs is the need to work under vacuum conditions in order to remove generated CO 2
	and then avoid the formation of insoluble NHC.CO 2 adducts.

IMes. 1.7 Catalyst activation through conformational change of chelated ligand

  which a photoinduced conformational transition drives the change from latent to active form.119 For this purpose, photoswitchable N and S chelated ruthenium complexes have been studied for less than 10 years.

	Chelation by sulfur atom. In 2009, the group of Lemcoff demonstrated that a cis-to-
	trans photoisomerization of ligand in sulfur chelated NHC ruthenium benzylidene complexes
	Scheme 1.20. Photogeneration of an active catalyst by irradiation of solution containing a PAG and a was a way to switch from a latent form (cis-dichloro) stable in air to an active ROMP catalyst
	ruthenium complex. (trans-dichloro). 120 The first structure Ru14-cis (Figure 1.14), cis-(Cl) 2 (SiMes)Ru=CHPhS-
	Ph, is nothing more than a sulfur chelated form of the well-known Hoveyda-Grubbs type
	Using a N-heterocyclic carbene (NHC) photogenerator based on tetraphenylborate complex. Ru14-cis has two primary features that make it a suitable ROMP photocatalyst:
	dimesitylimidazolium salt with respect of generating in situ the Noels' catalyst Ru5 from the
	commercially available ruthenium dimer [RuCl Switching metathesis catalyst's conformation upon exposure to UV irradiation is the
	third strategy to control reactivity. The concept of photoinduced cis-to-trans isomerization
	has been an area of interest for decades, as illustrated by the plethora of azobenzene-
	containing materials. 118 Cis-trans photoisomerization induces both molecular movement and
	geometric change, which account for the activity regulation of many biomolecules. Currently,

2 (p-cymene)] 2 (Scheme 1.21) for the ROMP is reported in this PhD thesis. Motivated by the promising results, the latter use of this catalytic system was broadened to polymerize NB in an aqueous miniemulsion. Finally, resulting PNB particles with diameters in range of 100 nm were obtained.

116,117

Scheme 1.21. In situ formation of Noels' catalyst by sensitized photogeneration of NHC this photochemical reaction has been extensively exploited to devise smart molecular machines able to change form or to move.

118

By contrast, there are much less examples of photoreactive compounds in -Chelation by sulfur resulting in a latent cis isomer. At the time of research for this precatalyst, chelation of Ru metallic by N or O through alkylidene moiety was already a well-established approach to decrease metathesis activity. 121,122 Because the S-Ru bond is generally stronger than O-Ru, Lemcoff et al. postulated that it was possible to completely suppress activity, which is the first step forward to a photolatent ROMP catalyst. Accordingly, Ru14-cis was inert in presence of exo substituted NB, COD or COE at ambient temperature. 120 However, reaction occurred spontaneously with more strained cyclic olefins such as NB or DCPD (Table 1.1).

Table 1 .

 1

1. Photoinduced ROMP employing photoisomerizable complexes Ru-14-cis a

Table 1 . 2 .

 12 Photoinduced ROMP employing N-chelating complexes Ru18 a and Ru19 b

	Entry	Monomer	Catalyst	Latent behavior	λ (nm)	Irradiation time (h)	Conv. (%)	M n (g•mol -1 )/ PDI
	1		Ru18	Excellent 254	2	60	< 2000/ -
	2		Ru18	No	254	2	30	< 2000/ -
	3		Ru19	Excellent 254	24	54	6.7 10 4 / 2.24

Table 1 . 3 .

 13 Photoinduced ROMP of NB using a photocatalyst P1 a

	Entry Initiator	Molar ratio NB : initiator : P1	[NB] o (M) b	Time (min)	Conv.(%)	M n (g•mol -1 )/ PDI
	1	MPB	97 : 1 : 0.03	1.9	30	88	15.1 3 / 1.7
	2	CME	97 : 1 : 0.03	1.9	30	92	14.9 3 / 1.6
	3	EPE	106 : 1 : 0.03	2.0	30	87	15.8 3 / 1.6
	4	EPE	104 : 1 : 0.1	1.9	150	80	13.5 3 / 1.4
	5	EPE	104 : 1 : 0.25	1.9	120	90	19.2 3 / 1.6
	6	EPE	48 : 1 : 0.03	1.8	60	95	8.1 10 3 / 1.4
	7	EPE	57 : 1 : 0.03	1.2	120	93	11.5 3 / 1.4
	8	EPE	491 : 1 : 0.03	5.3	120	51	22.2 3 / 1.5
	9	EPE	494 : 1 : 0.03	1.8	60	72	43.9 3 / 1.5
	10	EPE	1000 : 1 : 0.03	1.9	120	61	60.2 3 / 1.6
	11 c	EPE	103 : 1 : 0.03	1.9	2580	53	7.2 10 3 / 1.3

a Reaction conditions: CH 2 Cl 2 , reaction mixture was irradiated by a 2W Miracle blue LED indoor gardening bulb.

  Our aim is to design a novel NHC photogenerator, both stable in absence of light and capable of releasing in situ free NHC under UV exposure, preferentially UVA (320 -400 nm) to avoid substrate or solvent excitation. For this purpose, imidazolium or imidazolinium salts seem the most appropriate photolatent NHC precursors. Like many ionic liquids, they are air/moisture stable, thermally inert and chemically quite stable. The challenge is thus how to promote a photochemically induced proton transfer reaction that would convert the starting inactive azolium salt into an active NHC. The proposed solution is to produce a suitable radical anion To this end, we begin with a mixture of isopropylthioxanthone (ITX) with an air-stable azolium tetraphenylborate salt, such as 1,3-bis(mesityl)imidazolium

	tetraphenylborate IMesH + BPh 4	-	or 1,3-bis(mesityl)imidazolinium tetraphenylborate
	SIMesH + BPh 4	-

via a photoinduced electron-transfer (PET) reaction, which can subsequently deprotonate the azolium cation (Scheme 2.1).

2.2 Synthesis a. Synthesis of azolium tetraphenylborate salts IMesH + BPh 4 -and SIMesH + BPh 4 -were

  prepared following a typical procedure below.36 Two separate 250 ml RB flasks were charged with IMesH + Cl -(4.00 g, 11.7 mmol) (or SIMesH + Cl -(4.01 g, 11.7 mmol)) and NaBPh 4 (5.38 g, 15.7 mmol). Ethanol (120 ml) was added in each flask. Then, the the IMesH + Cl -(or SIMesH + Cl -) solution was added dropwise to NaBPh 4 solution under continuous stirring during 60 min (Figure A.II.1). A white precipitate formed, which was recovered by filtration, washed with ethanol and then with water, and finally dried at 60 °C during 15 h. The white solid was readily soluble in dichloromethane, tetrahydrofuran or acetonitrile, but less soluble in protic solvents such as ethanol. It can be handled in air and was stable for more than 1 year without any particular storage precautions.1 H NMR (300 MHz, ACN-d 3 ), IMesH + BPh 4 -, isolated yield 93.2 % : δ ppm : 2.14 (s, 12H, 4  o-ArCH 3 ), 2.42 (s, 6H, 2  p-ArCH 3 ), 6.85 -6.89 (t, 4H, J = 6 Hz, 4  ArH), 6.99 -

	1,3-Bis(mesityl)imidazolium chloride (IMesH + Cl -, 98 %), sodium tetraphenylborate (NaBPh 4 , 99.5 %), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, >98%), isophorone diisocyanate (IPDI, 99.0 %), polyethylene glycol 400 g•mol -1 (PEG 400 ), and vinyl ethyl ether (> 98.0 %) were purchased from TCI. 1,3-Bis(mesityl)imidazole-2-ylidene (IMes, 97 %), 1,3-bis(mesityl)imidazolidin-2-ylidene (SIMes, 97 %), 1,3-bis(mesityl)imidazolinium 7.56 (s, 2H, 2  NCH) and 8.63 (s, 1H, NCHN). SIMesH + BPh 4 --, isolated yield 84 % : δ ppm : 2.36 (s, 6H, 4  p-ArCH 3 ), 2.37 (s, 12H, 2  o-ArCH 3 ), 4.34 (s, 4H, 2  NCH 2 ), 6.86 -6.91 (t, 4H, J = 7.5 Hz, 4  ArH), 7.01 -7.06 (t, 8H, J = 7.5 Hz, 8  ArH), 7.12 (s, 4H, 4  ArH), chloride (SIMesH + Cl 7.04 (t, 8H, J = 7.5 Hz, 8  ArH), 7.2 (s, 4H, 4  ArH), 7.30 -7.34 (br, 8H, 2  8  ArH), 7.31 -7.36 (br, 8H, 2  8  ArH), and 8.07 (s, 1H, NCHN).

-, 98 %), acetonitrile-d 3 (ACN-d 3 , 99.8 % D), isopropyl-9H-thioxanthen-9-one, mixture of 2-and 4-isomers (ITX, 97%), 1,3-di-p-tolylcarbodiimide (CDI, 96 %), 4,4′bis(diethylamino)benzophenone (EAB, > 99 %), camphorquinone (CQ, 98 %), phthalic anhydride (PA, > 99 %), cyclohexene oxide (CHO, 98%) and dichloromethane were obtained from Sigma Aldrich. Dichloromethane-d 2 (CD 2 Cl 2 , 99.5 % D), tetrahydrofuran-d 8 (THF-d 8 , 99.5 % D), chloroform-d 1 (CDCl 3 , 99.5 % D) and dimethyl sulfoxide-d 6 (DMSO-d 6 , 99.5 % D) were bought from Eurisotop. Dichloro(p-cymene)ruthenium(II) dimer ([RuCl 2 (pcymene)] 2 , 98 %), was obtained from Strem Chemicals. Other HPLC grade solvents such as ethanol, acetonitrile and tetrahydrofuran (THF) were received from VWR. All the solvents and CHO were distilled and stored over molecular sieve (4 Å) under nitrogen atmosphere before use.

2.

  generated by addition of a CDI derivative to

	IMes (Scheme 2.3). Upon contact with a stoichiometric amount of 1,3-di-p-
	tolylcarbodiimide in THF, IMes yields a soluble carbene adduct within a few seconds, visible
	by the change of color from a slight yellow solution to orange. The product formed in this
	way was characterized by 1 H NMR when the reaction was performed in THF-d 8 (Figure 2.5).
	The reaction caused a complete shift of the methyl protons H e (2.31 ppm) and H d (2.08 ppm,)

of IMes (spectrum a) and the methyl proton H p (2.30 ppm) of CDI (spectrum b). In the spectrum of the mixture IMes/CDI (spectrum c), H e' and H d' form two overlapping peaks at 2.32 ppm, and H p' is subjected to a upfield shift and now arises at 1.92 ppm. Additionally, novel signals assigned to methine protons H m' (6.27 ppm) and H n' (5.77 ppm) appear in the spectrum, which established the quantitative transformation of IMes to the IMes-CDI adduct.

Table 2 . 1 .

 21 Effect of the photooxidant's nature on the release of photogenerated NHC IMes.

Table 2 .

 2

	2 summarizes

-/ITX (entry a) or SIMesH + BPh 4 -

Table 2 . 2

 22 Consistently, the control experiment using the preformed NHC IMes (entry a') resulted in polymers displaying similar M n , Đ and glass transition temperature. Although the molecular weights are moderate, these results demonstrate the role of (photo)NHC in the catalysis of the polymerization. To more precisely compare catalytic activities, kinetic studies were performed for IMes and IMesH + BPh 4

	Entry	Catalyst system	M n [a] (g•mol -1 )	M w [b] (g•mol -1 )	Đ [c]	T g [d] ( o C)	Yield [e] (%)
	a	IMesH + BPh 4 -/ITX	4900	7800	1.59	-17	95.7
	a'	IMes		5100	7900	1.55	-14	98.3
	b	SIMesH + BPh 4	-/ITX	4100	6800	1.67	-18	96.2
	c	No catalyst		1700	4700	2.69	-23	40.1

. Results of step-growth polymerization of PEG 400 /IPDI (342/342 equiv.). Reactions were conducted for 4 h at 60 °C after the addition of the as-irradiated azolium salt/ITX solution (1/1 equiv.) or free NHC (1 equiv.).

A.II.12).

-/ITX during the first hour of reaction (Figure

Table 2 . 3 .

 23 Results of ROCOP of CHO/PA (500/100 equiv.). Polymerization were conducted at 110

+ BPh 4

  we show herein that it is possible to generate in situ the active RuCl 2 (p-cymene)NHC complex bearing ligand IMes or SIMes,66,67,69 by It is well established in the literature that the unsaturation in the imidazole ring of NHC moiety is not a crucial feature altering catalytic efficiency.68 In both cases, ζ cis (≈ 40-50 %) was very similar to that found in the corresponding polymer prepared by the action of a Ru-NHC complex, 68,70 thereby suggesting the formation of the same ROMP catalyst. As control experiments, we also conducted the polymerization of NB utilizing a combination of Ru precatalyst with free NHC

	irradiation of the NHC photogenerator ITX/IMesH + BPh 4	-(or ITX/SIMesH + BPh 4	-) in
	presence of the ruthenium precatalyst [RuCl 2 (p-cymene)] 2 . Using this 3-component system, it
	is thus possible to trigger the ROMP of cyclic olefins such as norbornene (NB) under UV IMes. The mixture of [RuCl 2 (p-cymene)] 2 and IMes was conducted in dark for 1 min showed
	exposure. slightly lower monomer consumption (58.9 %, entry e). This result supports the hypothesis
	that radiation facilitates the decoordination of the p-cymene ligand to form an active
	species. 66
	content after 1 min reaction at ambient temperature. As expected, the irradiation of a solution
	devoid of Ru dimer results in no reaction (entry a). Without photoNHC (entry b), a very
	small conversion was detected (< 2%). By contrast, much higher conversion (78 %) was
	achieved using the 3-component mixture [RuCl 2 (pCy)] 2 /IMesH + BPh 4

Scheme 2.8. Strategy for in situ formation of RuCl 2 (p-cymene)NHC from the inactive precatalyst [RuCl 2 (p-cymene)] 2 dimer and photogenerated NHC IMes and SIMes. The active NHC-coordinated Ru complexes referred to as Noels' catalysts can be used as active for the ROMP of NB. The catalytic activity of the three-component system [RuCl 2 (pCy)] 2 /IMesH -/ITX (1/5/2.5 equiv.) for the ROMP of norbornene in CD 2 Cl 2 was evaluated. In contrast to previous experiments where the as-irradiated photoNHC/ITX was introduced later, NHC photogenerator was initially present in the reaction medium and polymerizations were conducted at room temperature. Table 2.4 displays NB conversion, molecular weight and cis -/ITX (entry c) suggesting the successful formation of the highly active ruthenium-arene complex bearing an NHC ligand. Replacing IMesH + BPh 4 by SIMesH + BPh 4 resulted in similar conversions (76 %, entry d), molecular weight and Đ (Figure A.II.16).

Table 2 . 4 .

 24 Results for the photoROMP of NB conducted at ambient conditions by 1 min irradiation (365 nm, 6.5 mW•cm -2 ) of the mixture NB/Ru dimer/photoNHC/ITX in CD 2 Cl 2 (510/1/5/2.5 equiv., [NB] = 1 M).

	Entry photo-NHC	Ru dimer/ photoNHC/ITX (equiv.)	Conv. [a] (%)	σ cis	[b]	M n -th [c] (kg•mol -1 )	[d] (kg•mol -1 ) M n -SEC	Đ [e]
	a IMesH + BPh 4	-	0/5/2.5	0	n.d	n.d	n.d	n.d
	b -		1/0/0	1.5	9.3	0.36	n.d.	n.d.
	c IMesH + BPh 4	-	1/5/2.5	78	51.9	18.69	146.8	3.02
	d SIMesH + BPh 4 -	1/5/2.5	76	42.5	18.00	138.3	3.14
	e IMes		1/2 [f] /0	59	59.4	14.11	319.2	3.70

Table 3 . 1 .

 31 Triplet energy simulation of ITX and IMesH + BPh 4 -estimated.

	Method uB3LYP/6-31G*	E T (eV)
		2.77
		3.34
		3.64

  IMes) features one p-cymene cycle, two chlorido ligands and one IMes ligand. First introduced by Herman et al.,30 this class of RuCl 2 (p-cymene)NHC catalyst has been extensively studied by Noels et al. for the ROMP of NB and cyclooctene.31-

	Our tandem system made up of the NHC progenitor 1,3-bis(mesityl)imidazolium tetraphenylborate IMesH + BPh 4 -(1) and [RuCl 2 (p-cymene)] 2 precatalyst (Ru-1) proved to be very efficient at ROMP of NB. 28,29 In comparison with the examples discussed above, a safer UVA radiation (320 -400 nm) was employed, and no ligand displacement occurred. As shown in Scheme 4.1, the cleavage of the Ru-chlorido bridges of Ru-1 was rather enthalpically driven through the addition of 2 equivalents of free NHC IMes, that were in situ photogenerated from 1. As a result, the presumed active catalyst (Ru-1-33 Scheme 4.1. Irradiation of a mixture of a NHC photogenerator (IMesH + BPh 4 -

1/ITX/Ru-1. The

	trimethylphenyl)imidazol-2-ylidene (IMes, 97%, Aldrich) and 1,3-di-p-tolylcarbodiimide
	(CDI, 96 %, Aldrich) were used as received without purification. 1,3-bis(2,4,6-
	trimethylphenyl)imidazolium chloride (IMesH + BPh 4	-, 1) was prepared following a
	procedure published alsewhere. 28 Dichloro(p-cymene)ruthenium(II) dimer (Ru-1, 98 %),
	diiodo(p-cymene)ruthenium(II) dimer (Ru-2, 98 %) and dichlorodi-μ-chlorobis[(1,2,3,6,7,8-
	η-2,7-dimethyl-2,6-octadiene-1,8-diyl]diruthenium(IV) (Ru-3, 98 %) were bought from
	Strem Chemicals. Dichloromethane-d 2 (CD 2 Cl 2 , 99.5 % D, Eurisotop), tetrahydrofuran-d 8
	(THF-d 8 , 99.5 % D, Eurisotop) and other HPLC grade solvents such as ethanol, acetonitrile
							cross-
	linked polymer structure was investigated by gel content measurement, FT-IR and solid-state
	13 C NMR spectroscopy. The mechanical behavior, microstructure and the thermal properties
	were also studied through atomic force microscopy (AFM), scanning electron microscopy
	(SEM), TGA and DSC.
	4.2. EXPERIMENTALS
	4.2.1. Materials
	Dicyclopentadiene (DCPD, > 97 %, TCI), 2-norbornene (NB, > 99 %, TCI), 5-
	ethylidene-2-norbornene (ENB, > 98 %, TCI), vinyl ethyl ether (> 98 %, TCI), 2-
	isopropylthioxanthone	(ITX,	analytical	standard,	Aldrich),	1,3-bis(2,4,6-

  NMR tube containing 0.5 mL of CD 2 Cl 2 . All solutions were stored in dark. At given time, a 1 H-NMR spectrum was recorded to determine the extent of monomer conversion and cis/trans ratio of the polymer possibly formed.After 1 min irradiation, 1 H-NMR spectra were recorded; to determine monomer conversion and cis/trans ratio. Excess amount of vinyl ethyl ether (0.1 mL) was added to terminate the polymerization. Polymers were collected through precipitation in acetone and washed 3 times with CH 2 Cl 2 /acetone. SEC measurements were performed after drying under air for 48 h. For measurements of real-time kinetics by RT-FTIR (see characterization section

	placed in borosilicate Photoactivated ROMP in solution. To a borosilicate NMR tube was added 47 mg of
	NB (510 equiv.), 0.63 mg of ITX (2.5 equiv.), 3 mg of 1 (5 equiv.), 0.62 mg of Ru-1 (1
	equiv.), then 0.5 ml of CD 2 Cl 2 . The solutions were irradiated in a circular LED photoreactor.
	For this, a LED strip was wrapped around a quartz cylinder (ϕ int = 60 mm) emitting a
	monochromatic light centered at 365 nm (SMD3528-60LED/Meter, Lightingwill, 6.5
	mW•cm -2 ) or 405 nm (SMD2835-60LED/Meter, Banggood, 4.7 mW•cm -2 ) (Figure A.IV.3).
	1 H NMR (400 MHz, THF-d 8 ): Noels'catatslyst was obtained from Ru-1 and IMes.
	δ ppm : 1.08 (d, J=7Hz, 6H, CH(CH 3 ) 2 ), 1.75 (s, 3H, CH 3 ), 2.22 (s, 12H, Mes-2,6-CH 3 ), 2.31
	(s,6H, Mes-4-CH 3 ), 2.46 (m, 1H, CH(CH 3 ) 2 ), 4.55 (d, J=6Hz, 2H, C 6 H 4 ), 5.00 (d, J=5.8Hz,
	2H, C 6 H 4 ), 6.90 (m, 4H, Mes-3,5-H), 7.05 (s, 2H, NCHCHN).
	Assessment of catalyst photolatency. In a typical experiment, NB (47 mg, 510
	equiv.), ITX (0.63 mg, 2.5 equiv.) and ruthenium pre-catalyst Ru-1 (0.62 mg , 1 equiv.) were

Table 4 . 1 .

 41 Effect of excitation wavelength on the release of free NHC IMes in THF-d 8 ([1] =

	0.03 M, [ITX] = 0.03 M) after 5 min of irradiation.

Entry NHC photogenerator Irradiation wavelength a nm IMes yield b

  IMes is basically acceptable,28,29 experiments were conducted to probe the structure of the active metathesis catalyst. For this, we performed a control experiment in which Ru-1 was reacted with free IMes in THF-d 8 at room temperature. An orange solution formed spontaneously, and subsequent 1 H NMR analysis (performed without prior isolation) established that the expected catalyst structure (p-cymene)RuCl 2 (IMes) was formed in high yield (Figure4.1). The spectrum found was indeed in full agreement with previous literature data.

				%
	a	1/ITX	365	67
	b	1/ITX	405	29
	c	1	254	11
	a Irradiances values: 65 mW·cm -2 (365 nm), 100 mW·cm -2 (405 nm) and 75 mW·cm -2 (254 nm). b NHC yield
	was obtained indirectly after addition of 1,3-di-p-tolylcarbodiimide (CDI) ([CDI] = 0.03 M, 1 equiv). The stable
	IMes-CDI adduct is formed, which can be precisely quantified by 1 H NMR. 29
		b. Nature of the (photo)generated active catalyst
	Having proved the aptitude of 1 to act as NHC precursor under irradiation (in
	combination with a sensitizer), we studied the subsequent reaction of IMes with [RuCl 2 (p-
	cymene)] 2 dimer Ru-1 to form an active catalyst. Though the formation of
	RuCl 2 (arene)(NHC) Ru-1-

Table 4 . 2 .

 42 Photolatency results of Ru dimer/ITX/monomer mixture in CD 2 Cl 2 ([Monomer] = 1 M). All the samples were stored in the dark (Ru dimer/ITX/monomer = 1/2.5/510 equiv.).

	Entry	Precatalyst	Monomer	Reaction time	Conv. a	σ cis	a
				h	%
	a	Ru-1	NB	4	17.6	30
	b	Ru-1	ENB	4	0	n.d
	c	Ru-2	NB	4	<1	n.d
	d	Ru-2	NB	168	1.4	23
	e	Ru-3	NB	4	90	24

a Determined by 1 H-NMR.

Table 4 . 3 .

 43 Photopolymerization results of NB using Ru dimer/1/ITX photoinitiating system in CD 2 Cl 2 ([NB] = 1 M, 510 equiv.). Irradiation conditions: 365 nm, 6.5 mW•cm -2 , 1 min.

	Entry Precatalyst	Pre-catalyst/1/ITX equiv.	Conv. a %	σ cis	b	M n -th c kg•mol -1	d kg•mol -1 M n -SEC	Đ d
	a	Ru-1	0/5/2.5	0	n.d	n.d	n.d	n.d
	b	Ru-1	1/0/0	1	9		0.36	n.d.	n.d.
	c	Ru-1	1/0/2.5	7	23	1.60	n.d.	n.d.
	d	Ru-1	1/5/2.5	78	52	18.69	146.8	3.02
	e	Ru-1	1/5/5	94	52	22.57	108.4	1.96
	f e	Ru-1	1/5/2.5	22	65	5.27	63.4	2.05
	g	Ru-2	1/5/2.5	69	52	16.43	447.2	1.59
	h	Ru-3	1/5/2.5	67	48	16.10	n.d.	n.d.
		[ ] [	]	,

a Determined by 1 H-NMR. b Calculated from 1 H-NMR spectrum, c Theoretical number average molar mass M n-th = d Experimental number average molar mass M n-th obtained by SEC, e Irradiation at 405 nm, 4.7 mW•cm -2 , 1 min. n.d. means not determined due to the absence of reaction or the formation of an insoluble polymer.

Table 4 . 4 .

 44 Photopolymerization results of ENB using Ru dimer/IMesH +

BPh 4 -

						/ITX photoinitiating
	system in CD 2 Cl 2 ([ENB] = 1 M, 510 equiv.). Irradiation conditions: 365 nm, 6.5 mW•cm -2 , 10 min.
	Entry	Precatalyst	Precatalyst/1/ITX equiv.	Conv. a %	M n -th c kg•mol -1	d kg•mol -1 M n -GPC	Đ d
	a	Ru-1	1/5/2.5	64	20	37.9	5.6
	b	Ru-1	1/5/5	84	26	40.7	6.0
	c e	Ru-1	1/5/2.5	11	3	28.1	4.6
	d	Ru-2	1/5/2.5	71	22	55.6	2.7
	e	Ru-3	1/5/2.5	78	24	31.6	4.7

Refer to Table 4.3 for superscript information

Table 4 . 5 .

 45 Thermo-mechanical results of curing NB/ DCPD films using Ru-1/1/ITX photoinitiating system in CH 2 Cl 2 . Irradiation conditions: 365 nm, 5.5 mW•cm -2 .

	Sample	T g	a ( o C)	T 10%	b ( o C)	T max	c ( o C)	E d (GPa)
	NB-100		27	345	454	1.82 ± 0.13
	NB-75		56	397	460	1.51 ± 0.03
	NB-50		67	302	464	1.43 ± 0.07
	NB-25		89	437	472	1.36 ± 0.03
	DCPD-100	114	207	474	1.23 ± 0.08

a Glass transition temperature determined by DSC, b The temperature at which 10 % weight loss occuring determined by TGA, c The temperature at which maximum weight loss occurring determined by TGA and d Young's modulus determined by AFM.

  the search of novel approaches to photolatent ROMP initiators, a NHC

	photogenerating system activable at 365 nm is reported, which is based on a micture of
	azolium tetraphenylborate (NHCH + BPh 4	-) and isopropylthioxanthone (ITX). Unlike
	conventional methods to generate in situ carbene that require strong bases, only mild and air-
	stable precursors are used and NHC ligand can be generated via a step-wise mechanism in
	which electron/proton transfer reactions occur sequentially. The photoinduced electron
	transfer (PET) reaction between BPh 4	-anion and 3 ITX* is exergonic (ΔG et = -0.07 eV) and
	produces the ITX radical anion (ITX •-) and a boranyl radical (BPh 4	•

  stimulation sans contact, et la libération NHC dans des conditions ambiantes grâce un dosage énergétique et spatial précis du rayonnement. Notre objectif est de concevoir un nouveau photogénérateur de NHC, à la fois stable en l'absence de lumière et capable de libérer in situ un NHC libre sous exposition au rayonnement UV, de préférence UVA (320-400 nm) pour éviter l'excitation du substrat ou du solvant. A cet effet, des sels d'imidazolium ou d'imidazolinium semblent les structures les plus appropriées en tant que précurseurs NHC photolatents. Comme beaucoup de liquides ioniques, ils sont stables à l'air et humidité, thermiquement et chimiquement inertes. Le défi est donc de promouvoir une réaction de transfert de protons induite par photochimie qui convertirait le sel d'azolium inactif en un NHC actif. La solution proposée est de produire un anion radical approprié par une réaction de transfert d'électrons photoinduit (TEP), qui sera ensuite capable de déprotoner le cation azolium (Schéma R.II.1). À cette fin, nous proposons un mélange d'isopropylthioxanthone (ITX) avec un azolium stable à l'air à base de tétraphénylborate, tels que le 1,3-

	bis(mésityle)imidazolium tétraphénylborate 1H + BPh 4	-, ou 1,3-bis(mésityle)imidazolinium
	tétraphénylborate 2H + BPh 4

-

.

35

Tout d'abord, l'ITX est bon marché, disponible commerciallement, et connu pour son aptitude à servir de médiateur de transfert d'électrons après excitation photochimique.

36

D'autre part, l'anion tétraphénylborate peut agir en tant que donneur d'électrons approprié pour rendre le transfert d'électron thermodynamiquement possible.

ACKNOWLEDGEMENTS

Today, I submitted my PhD thesis for an external examination. This thesis was conducted at Institution of Materials Science in Mulhouse (IS2M) from 01.2017 to

09.2019. It was financed by French National Research Agency (ANR program: DS0304 2016, contract number: ANR-16-CE07-0016). To accomplish this thesis, it was a long and wonderful journey and would not have been successful without the inspiration and support of many great individuals.

derivatives in UV-Vis spectrocopy, amount of NHC existing in the solution can be estimated.

In the other hand, the generation of NHC-CDI adduct turns out to quantitative and easily monitored by 1 in handling and safety manner.

Preliminary trials using the NHC photogenerator as photolatent organocatalyst were carried out for the synthesis of polyurethane and polyester. In both cases, high yields (> 95 %)

and polymers with narrow polydispersities were obtained within 1 h. These promising results

pave the way for future investigations on photoNHC-driven polymerizations. Additionally, in respect of faster and large-scale polymeric production, the next strategy will focus on broadening the structure of the photolatent NHCs relying on the versatile N-substituents.

APPENDIX 1. Additional figures in Chapter II

Additional figures in Chapter IV