Julien Christmann

Suqing Shi

Ahmad Ibrahim

Christian Ley

Céline Croutxé-Barghorn

Marjolaine Bessières

Xavier Allonas

Mechanistic Investigation of a Dual Bicyclic Photoinitiating System for Synthesis of Organic -Inorganic Hybrid Materials

Keywords: Photocyclic initiating system, dual-cure polymerization, cationic polymerization, free radical polymerization, photoacid generator

One-pot synthesis of organic -inorganic hybrid materials under light requires specific photoinitiating systems which are able to release several different initiating species after light absorption. In this paper, the reaction mechanism of a photocyclic three-component initiating system based on isopropylthioxanthone as photoinitiator, an iodonium salt and a thiol as coinitiators was studied. It is shown that this system enables simultaneous release of both radicals and protons which are able to initiate respectively a free radical photopolymerization and the hydrolysis-condensation of a sol-gel network. Time-resolved investigations by Laser Flash Photolysis show that the initiating species are produced within two concomitant cyclic reaction mechanisms depending on the relative quantities of the co-initiators. Protons resulting from the secondary dark reaction of a photocyclic system are detected for the first time at the microsecond scale by means of a proton-sensitive molecular probe and corresponding quantum yields are measured. Finally, synthesis of organic, inorganic and hybrid materials under LED light at 395 nm is evaluated with respect to the mechanistic considerations demonstrating the dual initiating character of the system.

Introduction

Following the pioneering work of G. Ciamician [START_REF] Ciamician | The Photochemistry of the Future[END_REF] , light is nowadays a significant sustainable energy source for the triggering of chemical processes, especially material synthesis. Light offers several major advantages over classic thermal sources, such as spatial and temporal control, ease of use, cost reduction, room temperature process… Polymeric materials can be easily synthesized under light using various processes such as free radical photopolymerization (FRP) [START_REF] Fouassier | Photoinitiation, Photopolymerization, and Photocuring: Fundamentals and Applications[END_REF][START_REF] Allonas | Lasers in the Photopolymer Area in Lasers in Chemistry, Influencing Matter[END_REF][START_REF] Schnabel | Polymers and Light: Fundamentals and Technical Applications[END_REF][START_REF] Odian | Principles of Polymerization -Fourth Edition[END_REF][START_REF] Decker | UV-Radiation Curing Chemistry[END_REF][START_REF] Yagci | Photoinitiated Polymerization: Advances, Challenges, and Opportunities[END_REF][START_REF] Ibrahim | High-Perfomance Photoinitiating Systems for Free Radical Photopolymerization. Application to Holographic Recording[END_REF] , cationic ring opening photopolymerization [START_REF] Fouassier | Photoinitiation, Photopolymerization, and Photocuring: Fundamentals and Applications[END_REF][START_REF] Allonas | Lasers in the Photopolymer Area in Lasers in Chemistry, Influencing Matter[END_REF][START_REF] Schnabel | Polymers and Light: Fundamentals and Technical Applications[END_REF][START_REF] Odian | Principles of Polymerization -Fourth Edition[END_REF][START_REF] Decker | UV-Radiation Curing Chemistry[END_REF][START_REF] Yagci | Photoinitiated Polymerization: Advances, Challenges, and Opportunities[END_REF][START_REF] Crivello | Cationic Polymerization -Iodonium and Sulfonium Salt Photoinitiators in the series Advances in Polymer Science[END_REF][START_REF] Yagci | Externally Stimulated Initiator Systems for Cationic Polymerization[END_REF][START_REF] Crivello | The Discovery and Development of Onium Salt Cationic Photoinitiators[END_REF][START_REF] Sangermano | Cationic UV-Curing: Technology and Applications[END_REF] , anionic photopolymerization [START_REF] Schnabel | Polymers and Light: Fundamentals and Technical Applications[END_REF][START_REF] Odian | Principles of Polymerization -Fourth Edition[END_REF][START_REF] Yagci | Photoinitiated Polymerization: Advances, Challenges, and Opportunities[END_REF][START_REF] Salmi | Quaternary Ammonium Salts of Phenylglyoxylic Acid as Photobase Generators for Thiol-Promoted Epoxide Photopolymerization[END_REF] , thiol-ene step growth reaction [START_REF] Hoyle | Chemistry of the Past with Promise for the Future[END_REF][START_REF] Hoyle | Thiol-Ene Click Chemistry[END_REF] , photoclick polymerization [START_REF] Tasdelen | Light-Induced Click Reactions[END_REF][START_REF] Tasdelen | Externally Stimulated Click Reactions for Macromolecular Syntheses[END_REF] or hybrid organic-inorganic curing [START_REF] Belon | Combination of Radical and Cationic Photoprocesses for the Single-Step Synthesis of Organic-Inorganic Hybrid Films[END_REF][START_REF] Nishibayashi | Photo-Induced Sol-Gel Processing for Low-Temperature Fabrication of High-Performance Silsesquioxane Membranes for Use in Molecular Separation[END_REF][START_REF] Crivello | Preparation and Cationic Photopolymerization of Organic-Inorganic Hybrid Matrixes[END_REF][START_REF] Chemtob | Concomitant Organic-Inorganic UV-Curing Catalyzed by Photoacids[END_REF][START_REF] Ni | Direct-to-Metal UV-Cured Hybrid Coating for the Corrosion Protection of Aircraft Aluminium Alloy[END_REF] . Several types of photoinitiating systems (PISs) have been developed in order to optimize the conversion of light into chemical energy and to improve the initial quantum yield in reactive species. Among the materials formed under light, organic/inorganic hybrid materials present interesting properties due to the elegant combination of the properties of organic polymers and inorganic materials through the in-situ formation of interpenetrated network or generation of chemical bonds between organic and inorganic components. [START_REF] Belon | Combination of Radical and Cationic Photoprocesses for the Single-Step Synthesis of Organic-Inorganic Hybrid Films[END_REF][START_REF] Nishibayashi | Photo-Induced Sol-Gel Processing for Low-Temperature Fabrication of High-Performance Silsesquioxane Membranes for Use in Molecular Separation[END_REF][START_REF] Crivello | Preparation and Cationic Photopolymerization of Organic-Inorganic Hybrid Matrixes[END_REF][START_REF] Chemtob | Concomitant Organic-Inorganic UV-Curing Catalyzed by Photoacids[END_REF][START_REF] Ni | Direct-to-Metal UV-Cured Hybrid Coating for the Corrosion Protection of Aircraft Aluminium Alloy[END_REF] They are conventionally synthesized through a two-step process: a first sol-gel thermal reaction produces a liquid organic based polysilicate network which is further photopolymerized, leading to a solid cross-linked hybrid system. Beside this approach, an innovative process has been proposed in which a concomitant organic-inorganic photopolymerization occurs in a one-step water-and solvent free approach. UV light triggers the generation of acids or bases capable of catalyzing simultaneously both sol-gel reaction and organic photopolymerization. [START_REF] Soppera | New Insights into Photoinduced Processes in Hybrid Sol-Gel Glasses Containing Modified Titanium Alkoxides[END_REF][START_REF] Chemtob | Bridged Polysilsesquioxane Films Via Photoinduced Sol-Gel Chemistry[END_REF][START_REF] De Paz | Insights into Photoinduced Sol-Gel Polymerization: An in Situ Infrared Spectroscopy Study[END_REF][START_REF] Shi | Activation of the Sol-Gel Process by Visible Light-Emitting Diodes (LEDs) for the Synthesis of Inorganic Films[END_REF] The fabrication of such hybrid materials generally involves both the photopolymerization of multifunctional (meth)acrylates or epoxides, and the photocatalyzed hydrolysis-condensation of metal alkoxide inorganic precursors (i.e. alkoxysilanes). In most cases, the widely available (meth)acrylate monomers (or oligomers) and free radical photoinitiators make poly(meth)acrylates-based hybrid materials much more attractive in the photocuring field. As a matter of course, the photoinitiating system responsible for simultaneous generation of radical and proton initiating species becomes definitely essential in the design and synthesis of poly(meth)acrylates-based hybrid materials.

Due to the lack of light absorption in the visible light region, the photoacid generator which is often an iodonium or sulfonium salt, seems to be the limits for developing poly(meth)acrylatesbased hybrid materials under more environmental and sustainable light (i.e. visible light LEDs).

Recently, a visible light photoacid generator with fast protonic acid release behavior has been reported by combining isopropylthioxanthone (ITX) with an iodonium salt (IOD + ). [START_REF] Shi | Activation of the Sol-Gel Process by Visible Light-Emitting Diodes (LEDs) for the Synthesis of Inorganic Films[END_REF] The addition of a thiol (RSH) to a blend of ITX and IOD + is able to initiate simultaneously the free radical photopolymerization of dimethacrylates and the cationic polymerization of epoxides under visible light LED with high monomer conversion rates. [START_REF] Shi | Photoinitiating Systems for LED-Cured Interpenetrating Polymer Networks[END_REF] This innovative three-component photoinitiating system is very promising to design and prepare poly(meth)acrylate-based organic-inorganic hybrid materials. Nevertheless, the photochemical mechanism for efficient production of radical and protonic acid initiating species by photolysis of this three-component photoinitiating system is still unclear up to now. The aim of this paper is to understand the photochemical mechanism of this innovative dual-mechanistic photoinitiating system by Laser Flash Photolysis. Primary photoreactions of ITX with either IOD + or RSH are first studied. The resulting ITX photoproducts are found to be involved in secondary dark reactions with these co-initiators, leading to additional formation of reactive species such as radicals that are evidenced by LFP. The photogenerated protons are detected at the microsecond scale for the first time with a proton-sensitive molecular probe and the proton production quantum yield is also calculated. These mechanistic considerations are finally correlated to the synthesis results of poly(meth)acrylates, polysiloxane and the corresponding hybrid materials.

Experimental and Theoretical Methods

Isopropylthioxanthone (ITX) purchased from Fluka was a mixture of 2-and 4isopropylthioxanthone (75% and 25%, respectively, as checked by 1 H-NMR). Quinine sulfate dihydrate (98%) was purchased from Sigma. Pentaerythritol tetrakis(3-mercaptopropionate) (RSH, 95%) was supplied by Aldrich. Cumenyl(tolyl)iodonium tetrakis(pentafluorophenyl) borate (IOD + , 98%) was obtained from TCI. Acetonitrile (99.97%) was got from Biosolve. Ethoxylated (3) bisphenol A dimethacrylate (SR348C) was provided by Sartomer. Poly(dimethoxysiloxane) (PDMOS) is a non-hydrolyzed oligomeric silicate precursor obtained from ABCR. Further information concerning PDMOS structure can be found elsewhere. [START_REF] De Paz | Insights into Photoinduced Sol-Gel Polymerization: An in Situ Infrared Spectroscopy Study[END_REF] All chemicals were used as received. Their structures are presented in Chart 1.

Chart 1. Molecular structures of the compounds used in this study.

Absorption and fluorescence spectra were recorded at room temperature with an Analytik Jena Specord 210 UV-visible spectrometer and a Horiba Jobin Yvon Fluoromax 4 spectrofluorimeter respectively. Compounds were dissolved in acetonitrile and placed into 1 cm  1 cm quartz cuvette.

The fluorescence quantum yield (Φfluo) of ITX was determined with quinine sulfate dihydrate in 1N H2SO4 as reference (Φfluo = 0.546 [START_REF] Melhuish | Quantum Efficiencies of Fluorescence of Organic Substances: Effect of Solvent and Concentration of the Fluorescent Solute[END_REF] ), using a conventional procedure. [START_REF] Rosspeintner | Photophysical Properties of 2-6,dicyano-N,N,N',N'-tetramethyl-p-phenylenediamine[END_REF] Maximal optical density above 300 nm was maintained below 0.1 in order to avoid inner filter effects. Two excitation wavelengths (360 and 375 nm) and two slit sizes (1 and 2 nm) were used and the results were averaged on these four experiments.

Fluorescence lifetime of ITX was obtained by Time-Correlated Single Photon Counting (TCSPC) using a Horiba Jobin Yvon Fluoromax 4 spectrofluorimeter equipped with a Horiba Jobin Yvon Fluorohub module. Light source was ensured by a Horiba Jobin Yvon 374 nm nano-

ITX RSH IOD + 4 -

SR348C PDMOS

LED (< 200 ps) and maximal optical density above 300 nm was also maintained below 0.1.

Samples were previously deaerated by argon bubbling (15 min).

The relaxed triplet state energy of ITX was calculated in the same manner as reported elsewhere [START_REF] Christmann | Elucidation of the Key Role of (Ru(bpy)3) 2+ in Photocatalyzed RAFT Polymerization[END_REF][START_REF] Dietlin | Theoretical and Experimental Study of the Norrish I Photodissociation of Aromatic Ketones[END_REF] . Redox potentials were determined by cyclic voltammetry, as described in Ref

  -1 = 0 -1 + kq[Q] Eq. 1
 and 0, respectively, refer to the triplet state lifetime with and without any quencher Q. [Q] is the concentration of this quencher in the cell.

Photoacid production quantum yield (ΦH+) was estimated by a spectroscopic method using Quinaldine Red (QR) as an acid sensor in a similar way to that already reported. [START_REF] Suzuki | Interaction Mechanism in Pyrromethene Dye/Photoacid Generator Photosensitive System for High Speed Photopolymer[END_REF] Threecomponent systems were dissolved in acetonitrile where a small amount of QR was added. The obtained solution was then irradiated by the Powerlite 9010 pulsed laser at 355 nm (absorption by the other compounds was neglected at their respective concentrations). The absorbed light energy (Eabs) was calculated with the following equation: from the ratio between the incident energy and that of one photon at 355 nm. The number of generated protons Nprotons was monitored through the decrease of the 520 nm band of QR, assuming a quantitative relation between QR disappearance and H + production in the considered range.

Eabs = I 0 ∫ (1 -10
Finally, quantum yield was calculated as the ratio of these two quantities (Eq. 4). Results were averaged on three repetitions and the associated standard deviation was estimated.

ΦH+ =

N protons

N abs

Eq. 4

ITX based photoinitiating systems were used to trigger the free radical photopolymerization and the sol-gel process. The concentration of ITX was 2.5 mol% with respect to the monomer and/or organic precursor. For two-component systems, 1.5 mol% of co-initiator (IOD + or RSH) was contained. For three-component systems, 1.5 mol% of co-initiator (IOD + or RSH) and 0.15 mol% of another component (RSH or IOD + , respectively) were both incorporated. To prepare the hybrid materials, a mixture of PDMOS (80 mol%) and SR348C (20 mol%) was used and all other mol% were calculated on the basis of the total amount of PDMOS and SR348C.

Photopolymerization kinetics and monomer conversions were followed by real-time FTIR spectroscopy using a Vertex 70 from Bruker Optics operating in a rapid scan mode and being equipped with a nitrogen liquid cooled MCT detector. [START_REF] Decker | Real-Time Kinetic Study of Laser-Induced Polymerization[END_REF][START_REF] Ibrahim | Investigation of Termination Reactions in Free Radical Photopolymerization of UV Powder Formulations[END_REF] The sampling interval was 0.1 s and the resolution was 4 cm -1 . The irradiation was provided by a 395 nm LED device (Roithner LaserTechnik) at 20 mW.cm -2 under air. The intensity of the LED was measured using a calibrated fiber optic spectrometer (Ocean Optics, USB4000). The photopolymerization reaction was carried out under air at room temperature, using polypropylene film and BaF2 pellet. Room humidity was kept constant at almost 43 -45% by a hygrometer and consistent with previous work. [START_REF] Shi | Activation of the Sol-Gel Process by Visible Light-Emitting Diodes (LEDs) for the Synthesis of Inorganic Films[END_REF] Each experiment was repeated at least three times to ensure a good reproducibility. Conversion of SR348C was followed by the decrease of the area of its twisting vibration band at 1310 cm -1 and calculated from:

Conversion (%) = A 0 -A t A 0 100 Eq. 5
A0 and At represent respectively the absorption band area before exposure and at exposure time t. Areas were corrected with a reference band (at 1510 cm -1 ) in order to avoid any error from displacement of the 1310 cm -1 band. The absorbance decrease of the CH3 symmetric stretching vibration band centered at 2848 cm -1 (distinctive from CH3 methanol vibration modes) was monitored to follow the methoxysilyl hydrolysis ratio in PDMOS. The maximum rate of conversion Rc max was determined as the maximum of the first derivative of the conversion with time.

Results and Discussion

Isopropylthioxanthone (ITX) has been considered as one of the most important photoinitiator in the field of photopolymerization. [START_REF] Dadashi-Silab | Shining a Light on an Adaptable Photoinitiator: Advances in Photopolymerization Initiated by Thioxanthones[END_REF] It has been reported to act as both efficient photosensitizer for iodonium salts to trigger the cationic polymerization or sol-gel process, and effective type II photoinitiator for free radical polymerization, owing to its attracting photochemical and redox properties. Table 1 shows the photochemical and redox properties of ITX which were found very close to those reported in the literature. [START_REF] Amirzadeh | On the Photoinitiation of Free Radical Polymerization -Laser Flash Photolysis Investigations on Thioxanthone Derivatives[END_REF][START_REF] Steidl | Non-Ionic Photo-Acid Generators for Applications in Two-Photon Lithography[END_REF][START_REF] Segurola | Photocuring Activity of Several Commercial, Near UV Activated Photoinitiated in Clear and Pigmented Systems[END_REF][START_REF] Lalevée | New Thioxanthone and Xanthone photoinitiators Based on Sylil Radical Chemistry[END_REF][START_REF] Manivannan | Primary Processes in the Photosensitized Polymerization of Cationic Monomers[END_REF][START_REF] Andrzejewska | Heteroatomic Thiols as Co-initiators for Type II Photoinitiating Systems Based on Camphorquinone and Isopropylthioxanthone[END_REF][START_REF] Zhu | On the Photolysis of bis[2-(o-Chlorophenyl)-4,5-diphenylimidazole] sensitized by 2-Isopropylthioxanthone or Michler's Ketone[END_REF][START_REF] Pohlers | Ionic vs Free Radical Pathways in the Direct and Sensitized Photochemistry of 2-(4'-Methoxynaphthyl)-4,6-bis(trichloromethyl)-1,3,5triazine: Relevance for Photoacid Generation[END_REF][START_REF] Fouassier | Radical Polymerization Activity and Mechanistic Approach in a New Three-Component Photoinitiating System[END_REF][START_REF] Charlot | Photopolymerization of Methyl Methacrylate: Effects of Photochemical and Photonic Parameters on the Chain Length[END_REF][START_REF] Dossot | Role of phenolic derivatives in photopolymerization of an acrylate coating[END_REF] Due to the known short lifetime of the first excited singlet state (τS) and the high intersystem quantum yield (ΦISC), ITX is expected to react from its lower triplet state which can be detected using laser flash photolysis. The transient absorption spectrum of ITX in argon saturated acetonitrile (Figure S1) exhibits a photobleaching at 385 nm and two peaks at 310 and 640 nm corresponding to the triplet -triplet absorption. ES (eV) b 3.13 3.11 [START_REF] Lalevée | New Thioxanthone and Xanthone photoinitiators Based on Sylil Radical Chemistry[END_REF] ; 3.57 [START_REF] Zhu | On the Photolysis of bis[2-(o-Chlorophenyl)-4,5-diphenylimidazole] sensitized by 2-Isopropylthioxanthone or Michler's Ketone[END_REF] kq 3ITX/O2 (L.mol -1 .s -1 ) f / 4.810 9 52 τS (ps) b 280 200 [START_REF] Zhu | On the Photolysis of bis[2-(o-Chlorophenyl)-4,5-diphenylimidazole] sensitized by 2-Isopropylthioxanthone or Michler's Ketone[END_REF] Eox/red / SCE (V) 1.51 / -1.73 1.54 / -1.63 [START_REF] Dossot | Role of phenolic derivatives in photopolymerization of an acrylate coating[END_REF] -/ -1.596 52g a. in benzene. b. "S" refers to the first excited singlet state. c. in methanol. d. "T" refers to the lowest triplet state. e. calculated by quantum mechanics. f. quenching rate constant of the reaction 3 ITX + O2. g. vs. AgCl.

ITX / IOD + as primary photoreaction

The reaction of ITX triplet state ( 3 ITX) with strong electron acceptors such as iodonium salts should result in ITX radical cation (ITX •+ ). Therefore, the photoinduced electron transfer reaction between [START_REF] Allonas | Lasers in the Photopolymer Area in Lasers in Chemistry, Influencing Matter[END_REF] ITX, cumenyl(tolyl)iodonium tetrakis(pentafluorophenyl)borate (IOD + , Chart 1) was firstly evaluated to clearly address the roles of each component in generation of proton and radical species. The corresponding Gibbs free energy can be calculated from the Rehm-Weller equation (Eq. 6) [START_REF] Rehm | Kinetik und Mechanismus der elektronbertragung bei der fluoreszenzloshung in acetonitril[END_REF][START_REF] Rehm | Kinetics of Fluorescence Quanching by Electron and H-Atom Transfer[END_REF] :

ΔGET = Eox(D) -Ered(A) -ET + C Eq. 6
where Eox(D) refers to the oxidation potential of the electron donor D and Ered (A) to the reduction potential of the electron acceptor A. C stands for a coulombic term which is often neglected in polar solvents such as acetonitrile. Ered(IOD + ) was measured as -0.70 V / SCE, in agreement with reported values for iodonium salts [START_REF] Bachofner | The Electroreduction of Diphenyliodonium Salts[END_REF][START_REF] Timpe | Excited States of Ketones as Electron Donors -Ketone-Iodonium Salt Systems as Photoinitiators for Radical Polymerization[END_REF][START_REF] Kunze | Triplet Quenching by Onium Salts in Polar and Nonpolar Solvents[END_REF] . The reaction was found to be exergonic with ΔGET = -0.50

eV.

The kinetic of the reaction between 3 ITX and IOD + was studied by Laser Flash Photolysis (LFP) at 640 nm. By increasing the concentration of IOD + , [START_REF] Allonas | Lasers in the Photopolymer Area in Lasers in Chemistry, Influencing Matter[END_REF] ITX was found to decay faster. The quenching rate constant kq 3ITX/IOD+ was determined from the Stern-Volmer plot as 6.210 9 L.mol - 1 .s -1 , a value which is in agreement with the high exergonicity of the electron transfer reaction and is similar to the reported values of TX derivative/iodonium salt in acetonitrile. [START_REF] Fouassier | Photochemistry and Photopolymerization Activity of Diaryliodonium Salts[END_REF][START_REF] Manivannan | Primary Processes in the Photosensitized Polymerization of Cationic Monomers[END_REF][START_REF] Timpe | Excited States of Ketones as Electron Donors -Ketone-Iodonium Salt Systems as Photoinitiators for Radical Polymerization[END_REF][START_REF] Kunze | Triplet Quenching by Onium Salts in Polar and Nonpolar Solvents[END_REF][START_REF] Fouassier | Time-Resolved Laser Spectroscopy of the Sensitized Photolysis of Iodonium Salts[END_REF][START_REF] Toba | Onium Butyltriphenylborates as Donor-Acceptor Initiators for Sensitized Photopolymerizations of Vinyl Monomer[END_REF] The reduced form of IOD + (IOD • ) is known to decompose itself into an aryl radical and an iodobenzene derivative. [START_REF] Crivello | Cationic Polymerization -Iodonium and Sulfonium Salt Photoinitiators in the series Advances in Polymer Science[END_REF] Interestingly, when the triplet state signal at 310 nm and 640 nm was progressively quenched by increasing amounts of IOD + , a long-lived transient could be detected at 340 nm and around 800 -850 nm (Figure 1). The electron transfer reaction being exergonic, this new transient was confidently attributed to the radical cation of ITX (ITX •+ ). A similar band has also been reported above 800 nm in the literature. [START_REF] Pohlers | Ionic vs Free Radical Pathways in the Direct and Sensitized Photochemistry of 2-(4'-Methoxynaphthyl)-4,6-bis(trichloromethyl)-1,3,5triazine: Relevance for Photoacid Generation[END_REF] It should be noted that weak transient signal between 400 and 450 nm is traditionally attributed to cation radicals originating from TX derivatives. [START_REF] Kunze | Triplet Quenching by Onium Salts in Polar and Nonpolar Solvents[END_REF][START_REF] Fouassier | Time-Resolved Laser Spectroscopy of the Sensitized Photolysis of Iodonium Salts[END_REF][START_REF] Toba | Onium Butyltriphenylborates as Donor-Acceptor Initiators for Sensitized Photopolymerizations of Vinyl Monomer[END_REF][START_REF] Manivannan | Chlorothioxanthone-Onium Salts: Efficient Photoinitiators of Cationic Polymerization[END_REF][START_REF] Rodrigues | Mechanistic Study of Tetrahydrofuran Polymerization Photoinitiated by a Sulfonium/Thioxanthone System[END_REF] Such a band is also visible on Figure 1, albeit at much lower intensity than those at 340 and 800 -850 nm.

Therefore, it appears reasonable to study the behavior of ITX •+ at 340 nm and 800 -850 nm. These results clearly reveal the photocyclic behavior of the ITX / IOD + / RSH photoinitiating system. Radicals are produced by the reaction between 3 ITX and IOD + followed by a secondary dark reaction between ITX •+ and RSH which produces both radicals and protons (Scheme 1). This combination is expected to initiate both free radical polymerization and sol-gel process, enabling the simultaneous one-pot synthesis of the organic-inorganic hybrid material.

Scheme 1. Mechanism of the ITX / IOD + / RSH photocyclic system.

ITX / RSH as primary photoreaction

Due to positive electron transfer Gibbs free energy (0.54 eV), neat electron transfer between [START_REF] Allonas | Lasers in the Photopolymer Area in Lasers in Chemistry, Influencing Matter[END_REF] ITX and RSH can be excluded. One should then consider the well-known photoreduction of aromatic ketones by hydrogen donors (such as thiols), which could occur through coupled electron/proton transfer. [START_REF] Andrzejewska | Heteroatomic Thiols as Co-initiators for Type II Photoinitiating Systems Based on Camphorquinone and Isopropylthioxanthone[END_REF][START_REF] Yates | Photoreduction of Triplet Thioxanthone by Amines: Charge Transfer Generates Radicals That Initiate Polymerization of Olefins[END_REF] In the present case, this reaction led to the formation of ketyl ITXH • and thiyl RS • radicals. Indeed, [START_REF] Allonas | Lasers in the Photopolymer Area in Lasers in Chemistry, Influencing Matter[END_REF] ITX signal at 640 nm was efficiently quenched when increasing amounts of RSH and the quenching rate constant kq 3ITX/RSH was determined as 1.110 9 L.mol -1 .s -1 . Interestingly, this value is much higher than for aliphatic thiols [START_REF] Amirzadeh | On the Photoinitiation of Free Radical Polymerization -Laser Flash Photolysis Investigations on Thioxanthone Derivatives[END_REF][START_REF] Lalevée | Thiyl Radical Generation in Thiol or Disulfide Containing Photosensitive Systems[END_REF] , and in the same order of magnitude for aromatic thiols. [START_REF] Andrzejewska | Heteroatomic Thiols as Co-initiators for Type II Photoinitiating Systems Based on Camphorquinone and Isopropylthioxanthone[END_REF][START_REF] Lalevée | Thiyl Radical Generation in Thiol or Disulfide Containing Photosensitive Systems[END_REF] Simultaneously, ITXH • appeared as a long-lived transient at 420 nm (blue curve on Figure 3), in full agreement with the reported spectrum of ITXH • in the literature. 47,52,55-

ITX •+

3 ITX 0 ITX [START_REF] Charlot | Photopolymerization of Methyl Methacrylate: Effects of Photochemical and Photonic Parameters on the Chain Length[END_REF][START_REF] Rodrigues | Mechanistic Study of Tetrahydrofuran Polymerization Photoinitiated by a Sulfonium/Thioxanthone System[END_REF][START_REF] Yates | Photoreduction of Triplet Thioxanthone by Amines: Charge Transfer Generates Radicals That Initiate Polymerization of Olefins[END_REF] Ketyl radicals are able to further react with strong electron acceptors. [START_REF] Fouassier | Photochemistry and Photopolymerization Activity of Diaryliodonium Salts[END_REF][START_REF] Charlot | Photopolymerization of Methyl Methacrylate: Effects of Photochemical and Photonic Parameters on the Chain Length[END_REF][START_REF] Erddalane | Efficiency and Excited State Processes in a Three-Component System, Based on Thioxanthene Derived Dye/Amine/Additive, Usable in Photopolymer Plates[END_REF] Therefore, IOD + was added to an ITX solution containing 1.710 -1 M of RSH. Kinetics at 420 nm show that the ITXH • signal was decreased when IOD + concentration was increased (Figure 3), resulting in the decrease of the photobleaching signal of ITX at 385 nm (Figure S3). This feature indicates that ITX is efficiently recovered during the reaction. A rate constant of about 10 6 L.mol -1 .s -1 was measured by a Stern-Volmer treatment of the ketyl radical signal at 420 nm in presence of increasing amounts of IOD + , in agreement with previous study of a ketyl radical associated to a diphenyliodonium salt in methanol. 69

A dual bicyclic initiating system

The previous results showed that the three-component system based on ITX, IOD + and RSH can react through two photocyclic processes. Free radicals are produced during the primary and secondary reactions while protons are generated only via the secondary reactions involving the ITX photoproduct (Scheme 3). Therefore, two limiting cases have to be considered, depending on the relative concentrations of IOD + and RSH: i) when kq 3ITX/IOD+ [IOD + ] >> kq 3ITX/RSH [RSH], the primary photoreaction occurs between [START_REF] Allonas | Lasers in the Photopolymer Area in Lasers in Chemistry, Influencing Matter[END_REF] ITX and IOD + leading to ITX 3 ITX and RSH is favored yielding ITXH • which is further oxidized by IOD + (photocycle (b)). It should be noted that when kq 3ITX/IOD+ [IOD + ] ≈ kq 3ITX/RSH [RSH], two competitive photocycles take place which are both able to initiate the polymerization, as recently reported for different systems. [START_REF] Ley | Tailoring of Organic Dyes with Oxidoreductive Compounds to Obtain Photocyclic Radical Generator Systems Exhibiting Photocatalytic Behavior[END_REF] It is then possible to favor one of the two pathways by finely adjusting the relative concentrations of IOD + and RSH, knowing the rate constants in the medium. Scheme 3. Proposed mechanism of reactions for the photocyclic initiating system involving ITX, IOD + and RSH.

Photogeneration of proton

The two different mechanisms described above show that whatever the photoreaction which primarily occurred ( 3 ITX / IOD + or 3 ITX / RSH), radicals were undoubtedly formed. Protons should also be generated in both cases, which is a hypothesis that needs to be verified. Detection of photogenerated protons at the microsecond scale by LFP had been already reported in the literature. [START_REF] Pohlers | A Novel Photometric Method for the Determination of Photoacid Generation Efficiencies Using Benzothiazole and Xanthene Dyes as Acid Sensors[END_REF] Therefore, in order to prove the photogeneration of acid, we decided to apply a similar method. It should be noted that the detection of protons generated within the second reaction of a three-component photocyclic system has never been reported up till now.

Quinaldine Red (QR) was used as the molecular probe to detect the release of protons. QR exhibits an absorption maximum at 520 nm, far above from the absorption range of ITX and without overlap with the spectra of the transient species (Figure S4). Preliminary study of a solution of QR in acetonitrile under the same experimental conditions showed that no transient absorption bands appeared in this range upon laser excitation. Moreover, the absence of photochemical reaction between QR and any other components was also assessed.

The basic form of QR is pink in acetonitrile while its protonated form (QRH + ) is colorless (λabs max = 365 nm). A small amount of QR (< 10 -5 M) was added to solutions of ITX, IOD + and RSH, these two latter being introduced at various concentrations in order to modify the nature of the photocyclic reaction. Because QRH + absorbs in the spectral region of the ITX photobleaching, only the evolution of QR at 520 nm was recorded. When introducing increasing concentrations of thiol to a solution of ITX containing 10 -3 M of IOD + (photocycle (a)), a clear photobleaching appeared at 520 nm (Figure 4a) which is attributed to the disappearance of QR to form QRH + .

UV-vis spectra were recorded before and after laser irradiation (Figure 4b), confirming the decrease of the 520 nm band of QR after laser excitation, and the growing shoulder at 365 nm was attributed to QRH + . The same behavior is also observed at 520 nm for photocycle (b) (Figure 5a). UV-visible spectra show a shoulder similar to that previously observed (Figure 5b). It can be conclude that the photogeneration of acid actually occurs through the two photocyclic pathways. UV-vis spectra shown in Figures 4 and5 were obtained with the same number of laser pulses.

However, it clearly appears that the photolysis signal of QR at 520 nm was lower for photocycle (a) as compared to (b) (Figures 4 and5). It suggests that the photoacid generation efficiency is higher when 3 ITX first reacted with IOD + . In order to quantify this difference in reactivity, proton production quantum yield ΦH+ was estimated by steady-state spectroscopic measurements. Values of ΦH+ = 0.12 (standard deviation σ = 0.012) and 0.017 (σ = 0.003) for photocycles (a) and (b), respectively, confirmed the differences observed by LFP. Pathway (a) is then almost 7-times more efficient than pathway (b). This may be explained by a better reactivity of ITX •+ with RSH compared to ITXH • with IOD + in acetonitrile, as attested by the rate constants of these reactions (1.210 9 and 10 6 M -1 .s -1 respectively).

Application of dual bicyclic initiating system for synthesis of organic-inorganic material

The ability of the bicyclic system to generate initiating radicals and protons simultaneously can be employed to perform simultaneously a free radical photopolymerization (FRP) and a photoacid catalyzed sol-gel reaction (PSG) under LED at 395 nm. A difunctional methacrylate monomer (SR348C) was selected for attesting radical initiation of organic polymerization and a poly(dimethoxysiloxane) sol-gel precursor (PDMOS) was used for proton-catalyzed photoinduced sol-gel process (Chart 1).

The diffusion rate constant kdiff in these monomers can be estimated according to the Stokes-Einstein equation. [START_REF] Ibrahim | High Performance Photoinitiating Systems for Holography Recording: Need for a Full Control of Primary Processes[END_REF] Values of 510 6 and 10 9 M -1 .s -1 were found, respectively, for SR348C and PDMOS. Because kq ITX/IOD+ and kq ITX/RSH measured in acetonitrile are both greater than these values, they can be considered as equal in SR348C and PDMOS. Therefore, the two photochemical pathways were differentiated by introducing ten times more (in mol%, based on the monomer) of one of the co-initiators with respect to the second one. In SR348C, the dark reaction rate constants for photocycles (a) and (b) are greater than (or almost equal to) kdiff and these reactions are also diffusion-limited. The same holds true for pathway (a) in PDMOS. However, the dark reaction in pathway (b) is clearly not limited by the diffusion as k ITXH•/IOD+ is lower than kdiff.

For sake of comparison, the two-component systems ITX/IOD + or ITX/RSH were first analyzed.

The conversion profiles of the acrylate monomer during light exposure under air are displayed in Figure 6. Final conversions and maximum polymerization rates are collected in Table 2. Final conversions were found to be low for these two-component systems, most probably because of a relatively low production of radicals. In addition, aryl radicals produced from the interaction ITX/IOD + are very sensitive to molecular oxygen, leading to inactive peroxy radicals (kO2 ≈ 10 10 M -1 .s -1 73 , also limited to kdiff). On the contrary, thiyl radicals are known to overcome oxygen inhibition in free radical polymerization. [START_REF] Hoyle | Chemistry of the Past with Promise for the Future[END_REF][START_REF] Hoyle | Thiol-Ene Click Chemistry[END_REF][START_REF] Karash | Reactions of Atoms and Free Radicals in Solution. XXV. The Reactions of Olefins with Mercaptans in the Presence of Oxygen[END_REF][START_REF] O'brien | Oxygen Inhibition in Thiol-Acrylate Photopolymerizations[END_REF] This could explain the difference between the two-component systems. The efficiency of the photosol-gel reaction was studied by monitoring the conversion of the SiOCH3 groups of PDMOS. In the absence of IOD + , only 29% of the alkoxy functions were hydrolyzed after 200 s without IOD + (Figure 7). This 29% hydrolysis ratio observed in presence of only RSH may be directly due to residual water present in the reaction medium. By contrast, the presence of IOD + led to fast and efficient hydrolysis. Comparing the two photocycles, it appears that pathway (a) led to a faster hydrolysis of the PDMOS. This can be related to the value of acid quantum yield of 0.12 determined previously. As the PDMOS is a low-viscous liquid (kdiff ≈ 10 9 M -1 .s -1 ), the secondary dark reaction between ITX •+ and RSH is quite efficient and occurs with a high rate constant (k ITX•+/RSH = kdiff).

Finally, the ability to induce concomitant reactions was studied for a 20/80 mol% SR348C/PDMOS mixture under LED irradiation (Figure 8). Photocycle (a) led to a fast hydrolysis of PDMOS while photocycle (b) is in favor of a better methacrylate polymerization. Interestingly, the final conversion was found higher in the case of the hybrid than for the pure SR348C for pathway (a) (40% vs. 17%). The presence of low-viscous PDMOS increases the rate of polymerization of SR348C. However, the simultaneous conversion of PDMOS rapidly increases the viscosity, thereby limiting the final conversion of SR348C. On the opposite, the low acid quantum yield of photocycle (b) allows good conversion of methacrylate, although at slow rate.

These results nicely illustrate the conclusion obtained from the previous mechanistic study, and the successful synthesis of both organic and inorganic networks demonstrates the simultaneous generation of free radicals and protons through the bicyclic photoinitiating system. Moreover, the ability of this system to mediate the one-pot light-induced synthesis of organic-inorganic hybrids has also been clearly established.

Conclusions

The photochemical mechanism of a new bicyclic photoinitiating system for concomitant synthesis of organic-inorganic hybrid materials under LED irradiation was elucidated. Triplet state of ITX can primary react with an iodonium salt IOD + or a tetrafunctional thiol RSH, and the ITX photoproduct (ITX •+ or ITXH • ) further reacts (respectively RSH and IOD + ) to increase the formation of acid and radicals. The proton generation was assessed at the microsecond scale with a proton-sensitive probe, and the acid quantum yield was also calculated. Free radical photopolymerization of dimethacrylate monomer and sol-gel reaction were performed using this photocyclic initiating system. Polymerization results were in good agreement with the conclusions drawn from mechanistic study. Finally, a hybrid material was successfully produced, attesting the dual initiating character of the bicyclic system.

42 .

 42 Nanosecond transient absorption experiments were realized with an Edinburgh transient absorption spectrometer. It combines a 50 W xenon lamp (Xe 900), a TM300 monochromator and a photomultiplier tube monitored by a digital oscilloscope. Light excitation was ensured by the third harmonic (355 nm) of a Continuum Powerlite 9010 Nd:YAG pulsed laser operating at 10 Hz. 43 Optical density of the solution was set at 0.10 for quenching and 0.55 for other experiments at the excitation wavelength. Incident intensity was set around 4-5 mJ/pulse for transient absorption spectra recording and ITX triplet state quenching analysis, and 10 mJ/pulse for the other experiments. Solutions were previously saturated with argon (15 min) in order to eliminate molecular oxygen. Quenching rate constants (kq) were obtained by the usual Stern-Volmer treatment:

Figure 1 .

 1 Figure 1. Transient absorption spectrum of ITX in acetonitrile at different times after laser flash

Figure 2 .

 2 Figure 2. Transient kinetics of ITX •+ observed at 340 nm for different IOD + and RSH

Figure 3 .

 3 Figure 3. Transient kinetics of ITXH • recorded at 420 nm for various RSH and IOD +

Scheme 2 .

 2 Scheme 2. Mechanism of the ITX / RSH / IOD + photocyclic system.

  Photocycle (a)Photocycle (b)

Figure 4 .

 4 Figure 4. a) Transient kinetics measured at 800 nm (ITX •+ ) and 520 nm (QR bleaching) in the

Figure 5 .

 5 Figure 5. Transient kinetics recorded at 420 nm (ITXH • ) and 520 nm (QR bleaching) in the

Figure 6 .

 6 Figure 6. Acrylate double bonds conversion profiles of SR348C (measured at 1310 cm -1 ) using

Figure 7 .

 7 Figure 7. Hydrolysis ratio profiles of PDMOS (recorded at 2848 cm -1 ) using various combinations

Figure 8 .

 8 Figure 8. Acrylate double bonds conversion profiles of SR348C (▲) and hydrolysis ratio of

Table 1 .

 1 Photochemical and redox properties of ITX in acetonitrile.

	Properties	Present work	Literature	Properties	Present work	Literature
	λabs max ( 0 ITX) (nm)	383	386 47a ; 382 48-50 ; 383 51 ; 384 52	λabs max ( 3 ITX) (nm)	310, 630 -640	605 51c ; 630 47a ; 635 54 ; 640 52,55-56 ; 650 50
			5200 50 ;
	ε(λabs max ) (L.mol -1 .cm -1 )	5,600	5300 48 ; 6900 47a ;	ΦT d	/	0.85 ± 0.1 47a
			6920 49
						2.64 55 ;
	λem max (nm)	415	415 48 ; 420 53	ET (eV) d	2.71 e	2.66 51 ;
						2.71 54
	Φfluo	0.011	0.013 53	τT (µs) d	4 -5	3 -4 56

Table 2 .

 2 Final conversion, maximum conversion rate Rc max and final hydrolysis ratio for the different photoinitiating systems.

		SR348C	PDMOS	SR348C/PDMOS (20/80 mol%)
	System	Conversion (%) / Rc max (s -1 )	Hydrolysis ratio (%)	Conversion (%) / Rc max (s -1 )	Hydrolysis ratio (%)
	ITX/IOD + (1.5 mol%)	14 / 0.7	100	/	/
	ITX/RSH (1.5 mol%)	27 / 0.9	29	/	/
	ITX/IOD + (1.5 mol%)/
	RSH (0.15 mol%)	17 / 0.8	100	34 / 16	100
	= photocycle (a)
	ITX/RSH (1.5 mol%)/
	IOD + (0.15 mol%)	50 / 10.5	100	38 / 6	71
	= photocycle (b)

Table of Contents Graphic and Synopsis

 of

	IOD +	RSH
	3 ITX
	IOD •		RS •
	ITX •+		ITXH •
			IOD +
	0 ITX
		IOD
		+	+
			+

• + H + RSH RS • + H +

Acknowledgement

The authors would like to thank Mäder and ANR for funding the DeepCure project (#ANR-13-CHIN-0004-01)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol)


Pr Julien X Christmann

Xavier Allonas
email: xavier.allonas@uha.fr

Christian Ley

Ahmad Ibrahim

Céline Croutxé-Barghorn

Pr C Allonas

Dr A Ley

Pr C Ibrahim

Croutxé-Barghorn

Triazine-Based Type-II Photoinitiating System for Free Radical Photopolymerization: Mechanism, Efficiency and Modeling

Isopropylthioxanthone, a versatile photoinitiator for free radical photopolymerization, is combined with a triazine derivative in a Type-II photoinitiating system (PIS). Initiation ability of this system for acrylate photopolymerization is assessed using a diacrylate monomer.

Involvement of a photoinduced electron transfer mechanism is demonstrated by time-resolved spectroscopic measurements. Further insights in this mechanism are obtained through the use of a photopolymerization kinetic model taking into account the main reaction steps from the absorption of photons to the formation of the polymer. Prediction ability of the model is also tested with different initial concentrations of photoinitiator and co-initiator, as well as a different triazine derivative. This last experiment reveals the noticeable role of back electron transfer in the free radical photopolymerization mechanism of Type-II PIS. -1 .

Introduction

Use of light for the synthesis of materials is an old topic: Egyptians used photocured bitumen of Judea to protect their mummies during the Antiquity. [1] Impressive progresses have been made since and photoinduced synthesis of materials is nowadays a large-scale industrial area.

Among the large number of fields covered by this technology, radical photopolymerization has emerged as an attractive process for a wide range of applications such as coatings, varnishes, dental materials, graphic arts, laser direct imaging, holographic recording, etc. [1][START_REF] Fouassier | Photoinitiation, Photopolymerization, and Photocuring -Fundamentals and Applications[END_REF][START_REF] Decker | [END_REF][START_REF] Schnabel | Polymers and Light -Fundamentals and Technical Applications[END_REF][START_REF] Allonas | Lasers in Chemistry[END_REF][START_REF] Yagci | [END_REF] Indeed, light irradiation provides numerous advantages over classic thermal operations: spatio-temporal control of the reaction, room temperature and solvent-free process, cost reduction… Light initiation of such a process is enabled by the use of a photoinitiating system (PIS). It converts photons into chemical energy through the production of radicals capable of reacting with acrylic monomers to initiate macromolecular chains. Three main families of PIS have been developed, each with their own advantages and limitations. [START_REF] Fouassier | Photoinitiation, Photopolymerization, and Photocuring -Fundamentals and Applications[END_REF][START_REF] Schnabel | Polymers and Light -Fundamentals and Technical Applications[END_REF][START_REF] Allonas | Lasers in Chemistry[END_REF][START_REF] Yagci | [END_REF][START_REF] Fouassier | Photochemistry and Photophysics of Polymer Materials[END_REF][START_REF] Fouassier | Photoinitiators for Polymer Synthesis[END_REF][START_REF] Ibrahim | [END_REF] Type-I PIS rely on the photoinduced dissociation of the initiator to produce primary radicals. Despite a high quantum yield of radical production, typical bond dissociation energies require the use of UV lamps that are known to be harmful and to release ozone in the atmosphere. Therefore, Type-II photoinitiating systems (PIS) have been developed, which combine a photoinitiator (PI) able to absorb light and a coinitiator capable of reacting with the excited photoinitiator through hydrogen abstraction or electron transfer. Nevertheless, radical production is limited by the diffusion of the species into the viscous monomer medium and competition of the bimolecular reaction with PI deactivation pathways. In the case of hydrogen abstraction, hydrogenated PI radicals (PIH • , such as ketyl radicals) could also act as terminating agents and reduce final conversion. [10][11] Photocyclic initiating systems (PCIS) have then been developed in order to combine advantages of previous PIS, i.e. high radical production quantum yield under visible light. A redox additive is added to a Type-II photoinitiator/co-initiator system with the aim to react with the photoproducts created during the photochemical reaction. It has three main advantages: i) the fundamental state of the PI is regenerated and can participate to a new cycle, ii) supplementary initiating radicals are produced, iii) potential terminating agents are consumed. Among the numerous PCIS redox additives reported into the literature, triazine derivatives (Tz) have been extensively used as electron-acceptors in combination with amines [12][13][14][15][16][17] , thiols [18] or borate salts [18][19][20][21] . They have been found to enhance final conversion and rate of polymerization by reacting with the semireduced form of the PI. Indeed, fundamental state of the PI is regenerated and can participate to a new cycle while the semi-reduced form of the triazine (Tz •-) is expected to produce a triazinyl initiating radical (Tz-Cl • ) by dissociation of a C-Cl bond. [22] According to their electron acceptor nature, triazines would be able to react directly with an excited photoinitiator to generate triazinyl radicals. It has been reported that triazine A (TA) is not able to initiate acrylate photopolymerization when combined to several different dyes. [12][13] However, recent studies have proved that triazine derivatives are actually efficient to act as co-initiators, via a photoinduced electron transfer from the singlet state of pyrromethene derivatives. [15][16][17] Interestingly, isopropylthioxanthone (ITX) which is one of the most versatile photoinitiator for free radical photopolymerization (FRP) [23] has never been combined with triazine derivatives in a two-component photoinitiating system for FRP, although that such a combination has been studied for photoacid generation. [24] In the present paper, we studied the ability of a Type-II photoinitiating system combining isopropylthioxanthone (ITX) as photoinitiator and 2,4,6-tris(chloromethyl)-1,3,5-triazine (triazine S, TS) as electron-accepting co-initiator to initiate acrylate photopolymerization. First, initiating ability of the system was studied with the LED-induced radical polymerization of a diacrylate monomer. Then, the nature of the photochemical interaction between ITX and TS was assessed by time-resolved spectroscopic measurements. In order to get more insights into the key-steps of the actual initiating mechanism with triazine derivatives, we adapted a FRP kinetic model in order to take into account specificities of such a Type-II PIS.

Experimental Section

Experimental Methods

Formulations for kinetic analysis were based on a SR349 with eventually 10 wt% DMSO mixture. For Type-I PIS, 0.5 or 1 wt% (based on the resin total weight) of TPO was introduced.

For Type-II PIS, different amounts of ITX and triazine (TS or TA) were introduced.

Formulations were kept in the dark and stirred overnight.

Photopolymerization kinetics and monomer conversions were followed by real-time FTIR spectroscopy using a Vertex 70 from Bruker Optics operating in rapid scan mode and being equipped with a nitrogen liquid cooled MCT detector. [25][26] The sampling interval was 0.12 s and the resolution 4 cm -1 . In order to prevent the diffusion of atmospheric oxygen into the sample during exposure, laminate experiments were carried out by placing the formulation between two polypropylene films and two BaF2 pellets. Thickness of the sample was adjusted by using a 25 µm Teflon spacer. The irradiation was provided by a 395 nm LED device (Roithner LaserTechnik). The intensity of the LED was measured using a calibrated fiber optic spectrometer (Ocean Optics, USB4000) on which a PP film and a BaF2 crystal were placed in order to measure the intensity actually reaching the sample. Kinetics of photopolymerization were measured by following the disappearance of the C=C bond stretching vibration band at 1637 cm -1 and degree of conversion was calculated as:

Conv(%) = (𝐴 1637 ) 0 -(𝐴 1637 ) t (𝐴 1637 ) 0 × 100 (1)
where (A1637)0 and (A1637)t are, respectively, the areas of the 1637 cm -1 vibration band of the sample before exposure and at a given irradiation time t. Rate of polymerization Rp (in M s -1 )

was obtained from the Conv(%) vs time curve and the initial acrylate double bonds concentration in the formulation [DB]0:

Rp = dConv(%) d𝑡 × [DB] 0 100 (2)
Conversion data were smoothed (adjacent-averaging method -5 points) in order to avoid spikes in the Rp curve due to local fluctuations of the conversion values, especially at early times. Each experiment was repeated at least three times in order to ensure a good reproducibility and presented results are the average of these data.

Dark polymerization experiments were performed with formulations containing 0.

 -1 = 0 -1 + kq[Q] (3)
 and 0, respectively, refer to the triplet state lifetime with and without any quencher Q and

[Q] is the concentration of this quencher in the cuvette.

UV-visible absorption spectra were recorded on a Cary 4000 spectrophotometer at room temperature in a quartz cuvette. ITX fluorescence lifetime was determined by time-correlated single photon counting, as detailed in [27]. Polymer volumetric mass P was determined by picnometry and monomer glass transition temperature by differential scanning calorimetry (DSC), as explained in SI.

Computational Methods

Photopolymerization kinetics were modeled with Wolfram Mathematica 9 software. Rate constants were defined as functions of the fractional free volume f and differential equations were numerically solved with the NDSolve function. Time evolution of all the species were generated in form of interpolating functions evaluated each 0.05 s. Conv(%) was directly calculated into the Mathematica notebook with Eq. 4 (where [DB] refers to the acrylate double bond concentration) and Rp values were calculated after export with Eq. 2.

Conv(%) =

[DB] 0 -[DB] t

[DB] 0 × 100 (4)

Results and Discussion

Radical Photopolymerization Initiated by ITX / TS System

Triazine derivatives have often been considered in the development of photoinitiating systems for free radical photopolymerization of acrylates in Type-II PIS. [12][13]19] Therefore, it was found quite interesting to evaluate the initiating efficiency of triazine S (TS) using isopropylthioxanthone (ITX) as versatile photoinitiator. [23] Figure 1a displays the conversion of acrylate double bonds as a function of the irradiation time by a LED emitting at 395 nm with an incident irradiance of 10 mW cm -2 . The inhibition time is low and the maximum Rp value is 0.82 M s -1 at 1.9 s (Figure 1b). As can be seen, rapid and efficient conversion of acrylate double bonds was possible with a final conversion of 57 %, in agreement with the good reactivity already reported. [15,17]

Investigations of the Photochemical Initiating Mechanism

We have recently reviewed photochemical and electrochemical properties of ITX. [27] Due to the low lifetime of its singlet state (200 ps in MeCN), one can assume that photoinduced bimolecular interactions in the resin only occur from its triplet state ( ≈ 6 µs in MeCN). The interaction between triazine derivatives and ITX triplet state has been reported to occur through an energy transfer with a resulting triazine triplet state undergoing a homolytic dissociation of the C-Cl bond followed by a consecutive electron transfer between the radical moieties. [24] The triplet state energy ET of TS is 3.2 eV, [12] a value much higher than that of ITX (ET = 2.71 eV [24] ). Therefore, the triplet-triplet energy transfer from ITX to TS appears thermodynamically disfavored, the Gibbs free energy variation of energy transfer being ΔGPEneT = 0.49 eV. As TS presents a reduction potential similar to that of typical iodonium salts (-1.0 V / SCE measured in this work vs. -0.7 to -0.85 V / SCE for iodonium salts [17,27] ), a similar electron transfer mechanism is expected to occur. The Gibbs free energy variation of such a reaction is given by the Rehm-Weller equation: [28][29] ΔGPEleT = Eox(ITX) -Ered(TS) -ET + C (

where Eox(ITX) is the oxidation potential of ITX, Ered(TS) the reduction potential of TS. The coulombic term C is generally neglected in polar solvents such as acetonitrile. Taking into account the properties of ITX (Eox(ITX) = 1.51 V / SCE [27] ), this leads to a ΔGPEleT value of -0.20 eV. Thus, photoinduced electron transfer appears to be much more exergonic than energy transfer.

Quenching of [START_REF] Decker | [END_REF] ITX by TS was found to be quite effective, with a quenching rate constant kq of 3.1 × 10 9 M -1 s -1 in good agreement with the negative ΔGPEleT previously calculated. In order to ensure this mechanism, transient absorption spectrum of ITX was monitored in presence of TS in acetonitrile (Figure 2). According to the transient spectrum of ITX previously reported in MeCN under similar conditions, absorption signals at 310 and 640 nm are attributed to the triplet-triplet absorption of ITX while the negative one at 380 nm corresponds to the photobleaching of the corresponding fundamental state. [27] The first clue indicating that the energy transfer is not the predominant process is given by the fact that the photobleaching at transient observed could be explained by the uncomplete recovery of 0 ITX due to the formation of the ITX radical cation (ITX •+ ) through electron transfer. This contention is confirmed by the apparition of a new band at 340 nm consecutively to the ITX triplet decay which has been previously assigned to ITX •+ . [27] An additional transient appears within the same time scale at 290 nm which is similar to that reported for a triazinyl radical (Tz-Cl • ) [22] and checked by ourselves by direct irradiation of TA at 355 nm (Figure S1, see SI). This new band can then be confidently attributed to the triazinyl radical of TS (TS-Cl • ). Analysis of [START_REF] Decker | [END_REF] ITX decay and signal growth at 290 and 340 nm is also interesting and reveals similar kinetics (Figure S2), indicating that TS-Cl • and ITX •+ are produced directly from the quenching reaction of 3 ITX. As photoinduced electron transfer from [START_REF] Decker | [END_REF] ITX to TS would produce ITX •+ and TS •-, this analysis shows that the latter loses a chloride anion to form the triazinyl. Two explanations can be proposed: i) the dechlorination of TS •-is faster than reported rate constant values (k-Cl = 8 -20 × 10 4 s -1 [22] ) and we are not able to detect the short-living TS •-with our setup; ii) the band at 290 nm is a mix of the signals of TS •-and TS-Cl • (probably presenting similar spectral properties as their structures are alike).

In order to characterize the initiation mechanism of an acrylate photopolymerization reaction by triazinyl radicals, the reaction rate constant between triazinyl radicals and an acrylate (i.e.

the initiation rate constant ki) was measured by LFP. The triazinyl radical was obtained by direct UV excitation of TA in the presence of increasing amounts of butyl acrylate BA (Figure 3). A value of almost 3 × 10 4 M -1 s -1 was found, showing that triazinyl radicals TA-Cl • are efficient to initiate FRP of acrylates. As TA-Cl • looks more stabilized than TS-Cl • thanks to an extended conjugated system, this value is considered as a lower limit of ki for TS-Cl • .

This photochemical study demonstrated that the interaction between 3 ITX and TS is governed by a photoinduced electron transfer mechanism and that the triazinyl radicals generated from the fast dechlorination of TS •-are efficient to initiate the photopolymerization of an acrylate monomer (Scheme 2).

Kinetic Modeling of FRP Initiated by a Triazine-Based Type-II PIS

As seen before, triazine derivatives behave as efficient co-initiators for Type-II photoinitiating systems. It is quite interesting to model the free radical photopolymerization by taking into account not only the photochemical mechanism of reaction but also the effect of this mechanism on the photopolymerization kinetics. Indeed, different processes are known to affect the conversion-time evolution during FRP reaction such as auto-acceleration and -deceleration [30- 32] as well as termination by reaction diffusion. [30,[START_REF] Schulz | [END_REF][34][35][36][37][38][39][40][41][42][43][44][45] Taking into account these effects into a kinetic model requires to consider the change in the rate constants during the photopolymerization within a free radical photopolymerization model as described in the literature [46][47][48][START_REF] Goodner | Solvent Free Polymerizations and Processes[END_REF][START_REF] Goodner | [END_REF][51] and recently used to predict kinetic rate constants during dark polymerizations experiments under oxygen diffusion [52] and model layer-by-layer UV-curing. [53] Complete description of the model can be found in SI. One of its main advantages is that it does not stick on the pseudo-steady-state approximation which was found to be invalid from the onset of the autoacceleration region. [46] However, a relatively high number of parameters is required to run properly such simulation. Among them, some are easily accessible by experiment (glass transition temperature Tg, density , resin viscosity , etc.) and some others can be found in the literature (initiation kinetic constant without diffusional control ki0, thermal expansion coefficient , etc.). Kinetic parameters governing the evolution of the rate constants (especially reaction diffusion termination parameter Rrd, exponential coefficients A and critical free volumes f,c) are more specific to this work. Experimental procedures have then been proposed

to determine some of them using a Type-I PIS, while the others can be reasonably approximated. [48][START_REF] Goodner | Solvent Free Polymerizations and Processes[END_REF][START_REF] Goodner | [END_REF]54] It was decided to determine these parameters with the Type-I photoinitiator diphenyl 2,4,6-trimethylbenzoyl phosphine oxide (TPO). Complete explanations

kdea3 =  / T (8)
T refers to the triplet state formation quantum yield of ITX and s and T its singlet and triplet state lifetime respectively. The TS •-dissociation rate constant has been reported equal to 1.0 × 10 5 s -1 . [22] As it was proved that ITX •+ can be quenched by hydrogen-donor species, [27] we also assume a similar reaction with impurities and stabilizers inherently present in the formulation.

This reaction produces radicals reactive towards oxygen and other radical species, and the protonated form of ITX From a general point of view, the model reproduces satisfactorily the shape of the conversiontime curve and the final conversion value (1 % of error). However, the obtained Rp max value is not yet sufficiently accurate to be satisfying (14 % of error). Indeed, the calculated rate of polymerization is higher than the experimental one, indicating that the amount of radicals actually available is lower than calculated. Therefore, one should take into account a possible back electron transfer (bet) which could take place after the photoinduced electron transfer between triplet ITX and TS. bet could explain an experimental quantum yield of radical production (rad) lower than that predicted by our model, even if other side reactions cannot be totally ruled out. Scheme 3 describes such mechanism. In the distinctive case of a dissociative radical anion such as triazine ones, dissociation of the semi-reduced species can take place before separation of the radical ion pair and prevent back-electron transfer (see complete mechanism in SI). k-d and ksep were taken equal to 0.8kd, according to the value proposed by Rehm and Weller in their pioneering work. [29] As measured kq value for ITX + TS reaction was closed to kd in MeCN, we extracted the pure electron transfer rate constant (ket) with Eq. 9. [59] kq =

k et × 𝑘 d k et + k -d (9)
A value of -2.51 eV was found for the bet Gibbs free energy variation Gbet by taking

Ered(ITX •+ ) ≈ Eox(ITX)
and Eox(TS •-) ≈ Ered(TS). As the maximum of the typical log(kPEleT) = f(GPEleT) curve in Marcus' theory is located at GPEleT = - (with  the reorganization barrier, typically comprised between 1 and 2 eV [START_REF] Turro | Principles of Molecular Photochemistry -An Introduction[END_REF] ), it clearly indicates that this reaction relies to the inverted region of Marcus' theory, characterized by a decreasing ket value with decreasing GPEleT. [START_REF] Turro | Principles of Molecular Photochemistry -An Introduction[END_REF][START_REF] Marcus | [END_REF][62][63] The kbet value was adjusted by trial-and-error fit to the experimental data. A kbet value of 4.0 × 10 6 s -1 was found for our system. As this value is lower than kd at early times, we decided to modify the expression of the rate constant of electron transfer between separated ITX •+ and TS •-(kradbet), initially taken equal to kd. In order to take into account the diffusion control at longer times, we defined kradbet as:

kradbet = k bet × 𝑘 d k bet + k -d (10)
According to this change, the Rp max value obtained is slightly increased (of about 0.017 M s -1 ) but one can adjust kbet to 4.5 × 10 6 s -1 in order to recover the agreement for both Conv(%) and

Rp curves (respectively 1 and 5 % of error), as displayed on Figures 5. As can be seen, introduction of the bet into our modeling has a noticeable effect on the Rp curve and its absence may explain the higher Rp max initially calculated than the experimental one.

In order to get more insight into the effect of bet on the FRP process, the results for TS were compared to that obtained with TA. Because TA highly absorbs around 355 nm, it was not possible to analyze its photosensitized mechanism with ITX by LFP. However, positive GPEneT value of 0.19 eV (ET = 2.9 eV [12] ) compared to GPEleT of -0.08 eV (Ered = -1.12 V / SCE [16] )

tends to indicate that a photoinduced electron transfer is also operating for ITX / TA system.

The Type-II PIS model was used with parameters specific to TA (initial concentrations, ket extracted with Eq. 9 from the kq = 3.7 × 10 9 M -1 s -1 reported value [64] ) and a kbet value of 7.5 × 10 5 s -1 was found (Figure 6) for a Gbet value of -2.63 eV. The Gbet of TA being more negative than that of TS (corresponding to a more exergonic reaction), the kbet value is accordingly lower for TA than TS by virtue of the inverted region behavior. [START_REF] Turro | Principles of Molecular Photochemistry -An Introduction[END_REF][START_REF] Marcus | [END_REF][62][63] Good agreement is also found for final conversion (1 % of error) and Rp max (2 % of error) values.

In order to assess the robustness of the kinetic model, it was decided to compare modeling results to experimental data while varying the initial concentration of ITX or TS. Tested systems are listed into Table 1 and results displayed in Figure 7. As can be seen, the modification of

[TS]0 is well reproduced by our model both for conversion (1 to 4 % of error on final conversion) and Rp (4 to 10 % of error on Rp max ) curves. For ITX initial concentration, final conversion and Rp max are slightly over-estimated by the model. This can be explained by the uncertain nature of some photoproducts. Moreover, oxygen inhibition is not perfectly reproduced, especially at low ITX initial concentration. This could originate from side reactions involving peroxy radicals ROO • produced during the reaction of radical species with oxygen (see the mechanism in the SI) which are not taken into account in the model as they are not totally understood yet. [65] However, trends are well reproduced in all cases and this model could be used for qualitative prediction of modifying PIS initial composition.

Conclusions

It was shown that triazine S can be combined with the ITX photoinitiator to initiate FRP of a diacrylate monomer. The photoinduced mechanism of initiation relies on an electron transfer from triplet ITX to TS, followed by TS radical anion dissociation to give initiating triazinyl radicals. Further insights into this complex mechanism were obtained by modeling the whole

5 and 1

 1 photolysis system. It combines a 50 W xenon lamp (Xe 900), a TM300 monochromator and a

FRP process initiated byScheme 1 .Scheme 2 .

 12 photoinitiating systems such as photocyclic ones.

Figures:Figure 1 .Figure 2 .

 12 Figures:

Figure 3 .Figure 4 .

 34 Figure 3. Kinetic traces recorded at 300 nm of an argon-saturated solution of TA at different

Figure 5 . 5 × 10 - 2 M

 55102 Figure 5. Comparison between experimental (black dashed) and simulated (grey plain) acrylate

Figure 7 .

 7 Figure 7. Comparison of experimental (dashed) and simulated (plain) acrylate double bond

  •+ , finally able to go back to 0 ITX after deprotonation (see the associated reactions in SI). At first was considered the electron transfer between[START_REF] Decker | [END_REF] ITX and TS as a diffusion-limited mechanism by setting kq equal to the initial kd value of the formulation (3.2 × 10 7 M -1 s -1 ). The best fits are compared to experimental data in Figures 4.

Table 1 .

 1 Composition of the different ITX + TS systems tested (in SR349 + 10 wt% DMSO) and experimental polymerization results. Errors on modeling data with respect to experimental data are given in parenthesis.

	System	ITX (wt%)	[TS]0 (M)	Final Conv(%)	Rp max (M s -1 )
	1	0.1	8 × 10 -3	74 % (3 %)	1.03 (4 %)
	2	0.1	2 × 10 -2	77 % (1 %)	1.33 (8 %)
	3	0.2	1.5 × 10 -2	80 % (4 %)	1.64 (10 %)

Acknowledgements: ANR and Mäder are fully acknowledged for financial support of the DeepCure project #ANR-13-CHIN-0004-01. Received: Month XX, XXXX; Revised: Month XX, XXXX; Published online: ((For PPP, use "Accepted: Month XX, XXXX" instead of "Published online")); DOI: 10.1002/marc.((insert number)) ((or ppap., mabi., macp., mame., mren., mats.)) Keywords: photopolymerization, radical polymerization, kinetic modeling, initiation mechanism, photochemistry

of the parameter determination as well as the description of the Type-I PIS kinetic model for TPO (mechanism, differential equations, fit to experimental data…) are given in SI. It should be pointed out that 10 wt% of DMSO was introduced into the acrylate resin in order to avoid premature vitrification that would limit the analysis of the photochemical impact on the polymerization. Knowing this set of polymerization parameters, it is then possible to replace photodissociation of a Type-I PIS by reactions specific to Type-II PIS in the kinetic model.

From the photochemical study, it could be assumed that initiating triazinyl radicals and ITX radical anions are produced through a photoinduced electron transfer from ITX to TS. Then, the mechanism, proposed in Scheme 2, was transcribed into a set of differential equations describing the change of the concentration in 0 ITX, 1 ITX, [START_REF] Decker | [END_REF] ITX, TS, TS •-, TS-Cl • and ITX •+ (see SI). All other polymerization parameters (except ki0 taken from our photochemical study) were the same as discussed above. It should be noted that by contrast to TPO, only one initiating radical (TS-Cl • ) is produced during the photochemical reaction. Specific photochemical parameters of ITX are required to solve the whole differential system.

From solvatochromic study using Reichardt's dye, the polarity parameter ET (30) of the mixture of SR349 with 10 wt% of DMSO was found to be 42.9 kcal mol -1 , quite close to that of butyronitrile (42.5 kcal mol -1 [START_REF] Marcus | The Properties of Solvents[END_REF] ). [START_REF] Reichardt | [END_REF] Thus, the medium-dependent parameters such as , singlet and triplet lifetimes, self-quenching rate constant ksq (4.5 × 10 8 M -1 s -1 )… were determined in that butyronitrile. Triplet-triplet annihilation was also taken into account, as its rate constant is quite high (kTTA = 7 ± 3 × 10 9 M -1 s -1 [57] ). Expressions of the rate constants of singlet (kdea1) and triplet (kdea3) deactivation (by either radiative or non-radiative pathways) as well as intersystem crossing rate constant (kISC) have been determined in a previous study (Eqs. 6 -8). [58] kISC = T / S (6)

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.