Winnie Nzahou Ottou

Haritz Sardon

David Mecerreyes

Joan Vignolle

Daniel Taton
email: taton@enscbp.fr

Update and challenges in organo-mediated polymerization reactions

Keywords: Organic catalyst, Ring-opening polymerization, Step-growth polymerization, Group transfer polymerization, Acid catalysis, Basic catalysis

published or not. The documents may come

Table of contents

Introduction

Organocatalysis, i.e. the use of small organic molecules to catalyze chemical reactions, has gained increased attention over the past 15 years, with an emphasis on asymmetric variants, allowing a variety of biologically active compounds to be accessed. [START_REF] Macmillan | The advent and development of organocatalysis[END_REF] For a long time, highly stereoselective transformations have been mainly achieved using enzymes and transition metal catalysts. However, organocatalysis has become the third branch of catalysis, providing various advantages, including: (i) environmentally more friendly and inherently lower toxicity of organic small molecules, (ii) higher availability of organic reagents, (iii) lower sensitivity toward oxygen and moisture and (iv) large chiral pool. [START_REF] Macmillan | The advent and development of organocatalysis[END_REF] The scope of organocatalytic systems, and their role in various elementary reactions of molecular chemistry have been discussed in detailed reviews. [START_REF] Wende | Evolution of asymmetric organocatalysis: multi-and retrocatalysis[END_REF][START_REF] Bugaut | Organocatalytic umpolung: N-heterocyclic carbenes and beyond[END_REF][START_REF] Volla | Catalytic C-C bond-forming multi-component cascade or domino reactions: pushing the boundaries of complexity in asymmetric organocatalysis[END_REF][START_REF] Dalko | Comprehensive enantioselective organocatalysis: catalysts, reactions, and applications[END_REF] They have also been introduced in macromolecular synthesis, where organic reagents can promote polymerization reactions, either as catalysts or as direct initiators, producing polymeric materials exempted of any metallic residues. Related metal-free polymers are thus expected to be employed in highvalue and sensitive domains, such as biomedical and personal beauty care applications, microelectronic devices or food packaging.

Several classes of organic activators (catalysts or initiators), including Brønsted/Lewis acids or bases, and mono-or bicomponent bifunctional catalytic systems have been utilized, not only for step-growth and chain-growth polymerizations, but also for depolymerization reactions in a context of recycling polymeric materials.

In recent years, organic activators have also been applied in biorefining processes for biomass conversion and upgrading into sustainable chemicals, materials, and biofuels as alternatives to petroleum-based compounds. [START_REF] Liu | Organocatalysis in biorefining for biomass conversion and upgrading[END_REF] Metal-free polymerizations of bio-based monomers offer a new strategy for the development of high-performance bioplastics with enhanced thermal stability and solvent resistance.

A general review on organo-catalyzed polymerizations has been published in 2010 by Hedrick, Waymouth et al., [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF] and related specific topics, such as, ring-opening, [START_REF] Dove | Organic catalysis for ring-opening polymerization[END_REF][START_REF] Mespouille | Implementation of metal-free ring-opening polymerization in the preparation of aliphatic polycarbonate materials[END_REF] grouptransfer, [START_REF] Fuchise | Recent progress in organocatalytic group transfer polymerization[END_REF] anionic, [START_REF] Hirao | Advances in living anionic polymerization: from functional monomers, polymerization systems, to macromolecular architectures[END_REF] zwitterionic [START_REF] Brown | Zwitterionic ring-opening polymerization for the synthesis of high molecular weight cyclic polymers[END_REF] polymerizations, and polymerizations induced by Hbond catalysts [START_REF] Thomas | Hydrogen-bonding organocatalysts for ring-opening polymerization[END_REF] have also been reported. Our group has also reviewed the general field of organo-catalyzed polymerizations at the end of 2011 [START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] and, more specifically, in 2013, that regarding the use of N-heterocyclic carbenes (NHCs) as organic catalysts for metal-free polymer synthesis. [START_REF] Fèvre | N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis[END_REF] While preparing this manuscript, Naumann and Dove reported recent trends of polymerizations utilizing NHCs either as organic catalysts or as direct initiators. [START_REF] Naumann | N-heterocyclic carbenes as organocatalysts for polymerizations: trends and frontiers[END_REF] From the end of 2011 to early 2015, roughly 150 papers describing the use of organic catalysts in polymerization reactions have appeared (Figure 1), demonstrating the ever increasing interest of organocatalysis as a new tool for macromolecular engineering. Data were obtained by a search in SciFinder and Web of science using the keywords "organocatalyzed polymerization", "metal-free polymerization" and derivatives (April 2015). Fig updated

The present review focuses on these recent developments and on applications of related metal-free polymers. The structure of main organic activators (catalysts or initiators) employed in polymerization reactions is first presented, and classified according to the nature of their functional group (i.e. acid or basic compounds, or activators featuring hydrogen-bonds with a donating or an accepting capability). Main monomer substrates and general polymerizations mechanisms are also briefly described. A focus on each category of activators is then given in the context of organo-catalyzed polymerizations although, in some cases, the catalyst is not employed in substoichiometric amounts relative to the initiator. The last section of this review deals with organo-initiated (non-catalyzed) polymerizations utilizing specific organic initiators, i.e. in absence of any other co-activator.

Scope of organic activators, monomer candidates and general polymerization mechanisms 1.1. Organic activator platform

A variety of metal-free compounds have been employed as catalysts or initiators in polymerization reactions (Figure 2), and most of them have been discussed in previous reviews. [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Dove | Organic catalysis for ring-opening polymerization[END_REF][START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] They include Brønsted acids (e.g. sulfonic, phosphoric and carboxylic derivatives), Lewis acid (e.g. trimethylsilyltrifluoromethanesulfonimide: Me3SiNTf2), Brønsted or Lewis bases (e.g. phosphazenes, N-heterocyclic carbenes, amines, phosphines, amidines or guanidines). Combination of antagonist properties, within the same molecule or not, leads This review

to bifunctional ambiphilic systems. Generally, such a dual activation is based on hydrogenbond interactions (e.g. thioureas and phenols derivatives). [START_REF] Thomas | Hydrogen-bonding organocatalysts for ring-opening polymerization[END_REF] 1.2. Scope of monomers Figure 3 illustrates representative monomer families recently polymerized using organic activators. Cyclic esters (lactides and lactones) have been the most studied in the context of organo-catalyzed ring-opening polymerization (OROP). [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Dove | Organic catalysis for ring-opening polymerization[END_REF][START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] This is not only due to the significant importance of corresponding biodegradable, non-toxic and in vivo bioresorbable polyesters, but also to the relative easiness for polymerizing these polar cyclic monomers.

Cyclic carbonates have received a special attention, because related polycarbonates exhibit a lower degradation rate in water and allow for an easier introduction of reactive functionalities compared to other aliphatic polyesters, such as polylactide (PLA) or polylactones. [START_REF] Mespouille | Implementation of metal-free ring-opening polymerization in the preparation of aliphatic polycarbonate materials[END_REF] Other cyclic monomers, such as cyclic ethers, cyclosiloxanes, N-carboxyanhydrides (NCAs), Ocarboxyanhydrides (OCAs) and phosphoesters, have also been reported to undergo metalfree ROP.

Besides ROP, (meth)acrylic monomers have been subjected to the group transfer polymerization (GTP), in the presence of silyl ketene acetal (SKA) initiators; [START_REF] Fuchise | Recent progress in organocatalytic group transfer polymerization[END_REF] they were also evidenced to undergo a direct 1,4-conjugate addition polymerization, using a specific nucleophile (e.g. NHC). [START_REF] Zhang | Conjugate-addition organopolymerization: rapid production of acrylic bioplastics by N-heterocyclic carbenes[END_REF][START_REF] Naumann | Polymerization of methyl methacrylate by latent pre-catalysts based on CO2-protected N-heterocyclic carbenes[END_REF][START_REF] Zhang | Organocatalytic conjugate-addition polymerization of linear and cyclic acrylic monomers by N-heterocyclic carbenes: mechanisms of chain initiation, propagation, and termination[END_REF] A few examples of step-growth polymerizations leading to polyurethanes or polyaldols have also been reported. [START_REF] Alsarraf | Cyclic guanidines as efficient organocatalysts for the synthesis of polyurethanes[END_REF][START_REF] Sardon | Highly tunable polyurethanes: organocatalyzed polyaddition and subsequent post-polymerization modification of pentafluorophenyl ester sidechains[END_REF][START_REF] Liu | Polyaldol synthesis by direct organocatalyzed crossed polymerization of bis(ketones) and bis(aldehydes)[END_REF] 1.

Polymerization mechanisms

The mechanism involved in organo-catalyzed polymerization reactions obviously depends on both the nature of the catalyst/chain starter and the monomer polymerizability.

Polymerization can occur through an activated monomer mechanism (AMM), following either an electrophilic or a nucleophilic pathway. Polymerization operating by activation of a purposely-added initiator (or via activation of the polymer chain-ends) refers to as the activated chain-end mechanism (ACEM). A cooperative dual activation of both the monomer and the initiator can also take place with specific mono-or bicomponent hetero-bifunctional organic catalysts.

Activated monomer mechanisms (AMM)

Electrophilic reagents, such as Brønsted organic acids (E in Scheme 1), alkylating or acylating agents, can activate the monomer, e.g. heterocycles such as lactide (LA) or ethylene oxide (EO), by interaction with the heteroatom, e.g. the oxygen atom of the carbonyl group (C = O) of cyclic esters and carbonates, the phosphoryl group (P = O) of phosphoesters, or the oxygen atom of epoxides. Representative families of organic molecules used as catalysts or initiators for organopolymerization (in gray: compounds already discussed in ref [START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF]; in black: more recently employed organocatalysts). Representative monomers that were subjected to an organopolymerization pathway (in gray: monomers already discussed in ref [START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF]; in black: monomers polymerized more recently by an organocatalytic pathway). Fig updated In absence of the alcohol (see section 7), the nucleophile can remain bound to the polymer chain (e.g. in the case of -butyrolactone β-BL), leading to a non-catalytic process (Scheme 3a). In the same way, (meth)acrylate monomers, such as MMA, can directly be polymerized by some Lewis bases (e.g. NHCs), in absence of any other external initiating species. The reaction proceeds via successive and selective 1,4-conjugate additions (via the Michael addition reaction) of the Lewis base (e.g. NHC) onto the monomer, forming a zwitterionic enolate intermediate 10 that can further propagate (Scheme 3b). In the latter case also, the process is not catalytic, the Lewis base acting as a direct organic initiator.

N N R R N N R 1 R 2 NHC1 R 1 =R 2 =Me NHC2 R 1 =R 2 =iPr NHC3 R 1 =R 2 =tBu NHC4 R 1 =R 2 = C 12 H 25 (Doc) NHC5 R 1 =R 2 =Cy NHC6 R 1 =R 2 =Ad NHC7 R 1 =R 2 =2,
O O O R R = H, TMC R = OCH 2 Ph, BTMC R = O(Me) 2 DMTMC O O O O Et O O O R 2 R 1 R 1 ,R 2 = CH 3 DTC R 1 ,R 2 =CH 2 Br DTC-Br 2 R 1 =CH 3 R 2 =COH 2 CH 2 CH=CH 2 , MAC R 1 =CH 3 R 2 =COH

Scheme 3.

Nucleophilic and non-catalytic AMM mechanism (see also section 7): a) in zwitterionic ROP (ZROP) of cyclic monomers (e.g. β-BL); b) in the zwitterionic polymerization (ZP) of (meth)acrylates (e.g. MMA).

Initiator/chain-end activated mechanism (ACEM)

In the ACEM, interaction of an initiating alcohol or of polymer chain-ends with a catalyst (Brønsted basic or H-bonding catalysts) leads to an activated initiating species 11 (Scheme 4a), which is nucleophilic enough to add and ring-open the heterocyclic monomer. The ensuing hydroxyl-terminated mono-adduct 12 can then propagate the polymerization by subsequent ring-openings of the monomer upon activation with the catalyst. The structure of the initiating and propagating species, 11 and 12, can be described by two limit forms, that are, an activated alcohol-Brønsted base complex and an alkoxide (with a bulky and soft counter cation), depending on the pKa difference between the alcohol and the base (Scheme 4a). Chain growth can then occur by repeated nucleophilic additions of the propagating chain end onto incoming monomers.

In the context of the GTP of (meth)acrylics, organic catalysts such as NHCs or

Bifunctional activation

Cooperative dual activation of both the monomer and the initiator/chain-end can be achieved with specific organic catalysts, possessing Brønsted acidic/basic properties and/or a H-bonding capability (e.g. thiourea). For instance, a combination of a weak Brønsted acid (E), such as diphenyl phosphate (DPP), and a Brønsted base (B), such as dimethylaminopyridine (DMAP), can be employed in a bicomponent catalytic system to activate a cyclic monomer (Scheme 5a). Such a bifunctional activation is generally triggered using molecules capable of developing hydrogen bonding (e.g. DMAP, thiourea-amino derivatives or the guanidine named 1,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD). [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] A synergy between the two components is expected, providing that the pKa difference between E and B entities is not too high. Very recently, this dual bifunctional activation has been extended to Lewis acid/Lewis base systems.

In particular, boranes such as B(C6F5)3 combined with phosphines such as P(tBu

Polymerization catalyzed by Brønsted and Lewis acids

As mentioned above, acid-promoted polymerizations mainly proceed through an electrophilic activation of the monomer, i.e. by a pseudo-cationic mechanism, using protic compounds (typically alcohols) as initiators. It is important to note that some of these acids have been reported to behave as dual catalysts, and this specific activation mode will be detailed in section 6. The ring-opening polymerization (ROP) of cyclic monomers, including esters, carbonates, siloxanes and epoxides, has been the most investigated, using a wide range of sulfonic or sulfonimide Brønsted acids (see Figure 4). Lately, phosphoric acid derivatives have also emerged as efficient promoters of these polymerizations. In contrast to ROP, only a few examples of acid-catalyzed group transfer polymerization (GTP) of (meth)acrylates and stepgrowth polymerizations have been reported. In the following lines, polymerization reactions will be described according to the nature of the functional group of these acid organocatalysts (i.e. sulfonic, sulfonimide, phosphonic, carboxylic-derivatives, etc.), that in fine governs their relative acidity.

RO

H B d+ + E d- n O O O O E* d+ Nu R' O O E* n Nu O O n O O E* Nu a) b) E d + d- (1) O O O O O O O O B d- d+ (12) RO O O O H O RO O O O O n O O H O O B d+ d- d- d+ d+ d- (5)

Sulfonic and sulfonimide acids

Strong and "super strong" sulfonic acids have been widely investigated to promote the ROP of cyclic esters and carbonates, e.g. lactide, -caprolactone (-CL), -valerolactone (-VL), and trimethylene carbonate (TMC). Synthesis of polyesters and polyethers by polycondensation reactions in sulfonic acid-tagged imidazolium ionic liquids (referred to as Brønsted acid ionic liquids, BAILS) has also been described. In this case, the BAIL plays both the role of the solvent and the catalyst. [START_REF] Zhang | Poly(oxyalkylene) synthesis in Bronsted acid ionic liquids[END_REF][START_REF] Zhang | Synthesis of linear and hyperbranched polyesters in Bronsted acid ionic liquids[END_REF] An overview of main acidic catalysts employed and related monomers that have been polymerized in this way is shown in Figure 5. as monomers investigated. Note that the strength of these acids is given as an indication (pKa values are given in H2O and have been determined using the Hammett acidity function (Ho)) [START_REF] Bordwell | Equilibrium acidities in dimethyl sulfoxide solution[END_REF],

O R'O O O O H n R'O O H O n O O k R ' O O H O k n O O O O O H O n R'O R 1 O O R 2 Me 3 SiO O n-1 O O R 1 O O R 1 OSiMe 3 O R 2 R 2 R'OH n O C N -R 1 -N C O N H R 1 N H O R' O O O n n H O -R '-O H R n n R'OH R'OH R'OH n n n N H S CF 3 O O S O O F 3 C Tf 2 NH (pKa ~ -1.7) S OH F 3 C O O S OH
keeping in mind that these values may vary dramatically as a function of the solvent. [START_REF] Raamat | Acidities of strong neutral Brønsted acids in different media[END_REF] Methanesulfonic acid (MSA; pKa ~ -0.6 in H2O) [START_REF] Bordwell | Equilibrium acidities in dimethyl sulfoxide solution[END_REF] was demonstrated to catalyze the ring-opening copolymerization (ROcP) of both ε-CL and [4,4′-bioxepane]-7,7′-dione (BOD) in a one pot-fashion, with benzyl alcohol (BnOH) or propargyl alcohol (PgOH) as initiator, in dichloromethane, at r.t (Scheme 6). [START_REF] Ren | Organic catalyst-mediated ring-opening polymerization for the highly efficient synthesis of polyester-based star polymers[END_REF] In this case, BOD played the role of a crosslinking comonomer. Alternatively, the ROP of BOD was also induced by a pre-synthesized hydroxylated poly(ε-CL) (PCL-OH) as macroinitiator. PCL-based stars consisting of a cross-linked core (nanogel), with a total number average molar masses (Mn) ranging from 9,900 to 36,200 g.mol -1 and relatively low dispersity (Đ ≤ 1.3) were thus successfully obtained, from an initial ratio

[BnOH]0/[-CL]0/[BOD]0/[MSA]0 = 1:100:30:3. Although, the polymerization mechanism was not discussed, it was previously postulated by Bourissou, Martín-Vaca et al. [START_REF] Gazeau-Bureau | Organo-catalyzed ROP of ε-caprolactone: methanesulfonic acid competes with trifluoromethanesulfonic acid[END_REF] that the ROP of ε-CL likely proceeded by an AMM, in presence of ROH and MSA. With the stronger triflic acid (TfOH; pKa ~ -14 in H2O) [START_REF] Bordwell | Equilibrium acidities in dimethyl sulfoxide solution[END_REF], however, deactivation of the initiating/propagating alcohol was shown to compete with the activation of the monomer. In other words, the catalytic activity of these organic acids did not simply correlate to their relative acidity. [START_REF] Gazeau-Bureau | Organo-catalyzed ROP of ε-caprolactone: methanesulfonic acid competes with trifluoromethanesulfonic acid[END_REF] Scheme 6. Synthesis of PCL-based stars via ROP and the " arm-first" approach. [START_REF] Ren | Organic catalyst-mediated ring-opening polymerization for the highly efficient synthesis of polyester-based star polymers[END_REF] In a more recent study, Zinck et al. investigated the catalytic efficiency of MSA, p-toluenesulfonic acid (PTSA, pKa ~ -2.8 in H2O) and (1R)-(2)-10-camphorsulfonic (CSA, pKa ~ 1.2 in H2O) as organocatalysts (1 mol% relative to the monomer) for the ROP of ε-CL in water (Scheme 7a). [START_REF] Stanley | Brønsted acid-catalyzed polymerization of ε-caprolactone in water: a mild and straightforward route to poly(ε-caprolactone)-graft-water-soluble polysaccharides[END_REF] The reaction was quantitative at 100 °C, leading to PCL's of Mn up to 5,000 g.mol -1 (Đ = 1.7-1.9). The same reaction conducted at lower temperatures (25 or 50 °C), using polysaccharides such as dextran or methylcellulose as macroinitiators and PTSA as catalyst, afforded water-soluble polysaccharide-graft-PCL copolymers of Mn = 6,200 -8,700 g.mol -1 (Scheme 7b). [START_REF] Stanley | Brønsted acid-catalyzed polymerization of ε-caprolactone in water: a mild and straightforward route to poly(ε-caprolactone)-graft-water-soluble polysaccharides[END_REF] The authors proposed that the polymerization was

Scheme 7.

Acid-catalyzed polymerization of -CL in water. [START_REF] Stanley | Brønsted acid-catalyzed polymerization of ε-caprolactone in water: a mild and straightforward route to poly(ε-caprolactone)-graft-water-soluble polysaccharides[END_REF] The ROP of β-butyrolactone (β-BL) initiated by iso-propanol (iPrOH) was shown to be catalyzed by TfOH (20-40 mol% relative to iPrOH) in dichloromethane at r.t. [START_REF] Basko | Cationic copolymerization of racemic βbutyrolactone with L,L-lactide: one-pot synthesis of block copolymers[END_REF] The polymerization was controlled for a degree of polymerization (DP) lower than 30, producing poly(β-butyrolactone)s (PBL's) of Mn equal to 2,400 g.mol -1 (Đ = 1.3). The observed deviation from linearity at higher monomer to initiator ratios was attributed to the formation of cyclic oligomers by end-to-end ring closure. The same catalytic system was further used for the direct and one-pot ROcP of β-BL and L-lactide (LLA). Interestingly, this led to gradient copolymers of molar masses Mn = 1,400 -3,500 g.mol -1 (Đ = 1.3-1.7), owing to the different reactivity of the two monomers, PBL being formed during the first stage.

Catalytic activities of MSA and TfOH were compared for the ROP of -BL in presence of n-pentanol (nPenOH) as initiator, in benzene at 30 °C. [START_REF] Couffin | Selective O-acyl ring-opening of betabutyrolactone catalyzed by trifluoromethane sulfonic acid: application to the preparation of well-defined block copolymers[END_REF] With an initial ratio of acid/ROH = 3/1, TfOH proved significantly more active than MSA, in relation with its higher acidity (pKa TfOH ~ -14

vs. pKa MSA ~ -0.6). Indeed, 20 eq. of β-BL could be converted within only 15 min with TfOH, whereas 60 min were required with MSA, leading to PBL of controlled molar masses (Mn up to 8,200 g.mol -1 ) and narrow distributions (Đ < 1.25). A variety of well-defined di-, tri-and pentablock copolymers were also prepared using dihydroxylated initiators (i.e. butan-1,4-diol) or macroinitiators (i.e. dihydroxylated poly(ethylene oxide) HO-PEO-OH or ,-dihydroxylated poly(butadiene) HO-PB-OH), upon successive addition of -BL and -CL.

The ROP of -CL, 1,5-dioxepan-2-one (DXO) and racemic (rac-LA) was also achieved in dichloromethane at r.t., using 3-phenyl-1-propanol (PPA) as initiator and bis(trifluoromethane)sulfonimide (Tf2NH, pKa ~ -1.7 in H2O) [START_REF] Foropoulos | Synthesis, properties, and reactions of bis((trifluoromethyl)sulfonyl) imide, (CF3SO2)2NH[END_REF] as catalyst (0.1-3 eq.) [START_REF] Makiguchi | Synthesis of block and end-functionalized polyesters by triflimide-catalyzed ring-opening polymerization of ε-caprolactone, 1,5-dioxepan-2-one, and rac-lactide[END_REF] Under these conditions, PCL's, poly(1,5-dioxepan-2-one)s (PDXO's), and PLA's of molar masses (Mn) up to 56,500 g.mol -1 (Đ = 1.14-1.19) were synthesized.

In contrast to MSA, TfOH and Tf2NH, o-benzenedisulfonimide (OBS; pKa ~ -4.1 in H2O) [START_REF] King | Sulphonic acids, esters and their derivatives[END_REF] was presented as a non-toxic, non-volatile and non-corrosive strong acid. Its catalytic performance in the ROP of δ-VL and ε-CL was studied, using BnOH as initiator and toluene as solvent. [START_REF] Jin | o-Benzenedisulfonimide as a recyclable cationic organocatalyst for the controlled/living ringopening polymerization of δ-valerolactone and ε-caprolactone[END_REF] The polymerization proceeded at 30 °C in a controlled fashion, affording PVL and PCL homopolymers, as well as PVL-b-PCL diblock copolymers with Mn = 3,000 -6,000 g.mol -1 and Đ values in the range of 1.09-1.2. Overall, the catalytic efficiency of OBS was found equivalent to that of Tf2NH, making OBS a more sustainable alternative than other strong acids.

While the acid-catalyzed ROP of small and medium ring sized lactones has been widely studied, only a few examples describing the ROP of macrolactones have been reported. Three of such macrolactones, namely, ω-pentadecalactone (ω-PDL), globalide (GB) and ambrettolide (AMB), were polymerized at 80 °C in bulk or miniemulsion conditions, using dodecylbenzenesulfonic acid (DBSA, pKa ~ -2,8 in H2O) and TfOH as catalysts (0.5-10 mol% relative to the monomer) (Scheme 8), as reported by Mecerreyes et al. [START_REF] Pascual | Acid catalyzed polymerization of macrolactones in bulk and aqueous miniemulsion: ring-opening vs. condensation[END_REF] In bulk, although both catalysts induced relatively fast polymerizations (> 98% of conversion after 24 h), TfOH was found the most efficient, yielding polyesters of Mn around 10,000 g.mol -1 . However, the high dispersity (Đ = 2-3) observed suggested the occurrence of significant transesterification side reactions. In contrast, only DBSA was active in miniemulsion conditions. Based on 1 H NMR analyses, a mechanism involving the polycondensation of the hydroxyl-carboxylic ring-opened macrolactone was actually proposed, only oligoesters (Mn = 2,000 g.mol -1 ) being produced under these conditions. The inactivity of TfOH in waterborne miniemulsion was attributed to the strong solvation of the proton by water molecules, restricting monomer activation.

Scheme 8. Acid-catalyzed polymerization of macrolactones in bulk and miniemulsion. [START_REF] Pascual | Acid catalyzed polymerization of macrolactones in bulk and aqueous miniemulsion: ring-opening vs. condensation[END_REF] Later on, the same group reported the acid-catalyzed ROP of the renewable macrolactone ethylene brassylate (EB, see Figure 5), both in bulk and in toluene solution, using BnOH (1 eq.) as initiator. [START_REF] Pascual | Organocatalyzed synthesis of aliphatic polyesters from ethylene brassylate: a cheap and renewable macrolactone[END_REF] Poly(ethylene brassylate)s of molar masses Mn = 5,900 g.mol -1 (Đ = 1.9) and Mn = 2,000 g.mol -1 (Đ = 2.7) were obtained at 80 °C, in presence of DBSA and PTSA catalysts (1 eq.), respectively. The use of a solvent allowed the viscosity to be decreased, but at the expense of the polymerization rate, 58% and 73% of monomer conversion being reached in toluene (96 h) and in bulk (44 h), respectively, for a DP = 100.

As observed with other macrolactones, although the analysis of chain-ends by MALDI-ToF mass spectrometry indicated an initiation by BnOH, the presence of species generated by Step-growth polymerization ROP

BnOH inter-and intra-molecular transesterifications were also detected, in agreement with the broad dispersity observed.

Besides cyclic esters, cyclic carbonates have also been subjected to the ROP by an acid catalysis. In 2010, Bourissou et al. first demonstrated the ability of TfOH and MSA to efficiently catalyze the ROP of trimethylene carbonate (TMC), in presence of n-PenOH as initiator. [START_REF] Delcroix | Ring-opening polymerization of trimethylene carbonate catalyzed by methanesulfonic acid: activated monomer versus active chain end mechanisms[END_REF] MSA was found to be a better catalyst than TfOH, avoiding side decarboxylation reactions, presumably due to its lower acidity. However, despite its higher selectivity, two distinct polymer populations (denoted as A and B in Scheme The MSA-catalyzed ROP of TMC was next investigated by Ribeiro, Peruch et al., using a mono-alcohol (biphenyl-4-methanol, BPM) and a diol (1,4-phenylene-dimethanol, PDM) as initiators, in toluene at 30 °C. [START_REF] Campos | A new insight into the mechanism of the ring-opening polymerization of trimethylene carbonate catalyzed by methanesulfonic acid[END_REF] With BPM, PTMC's with molar masses ranging from 3,000 to 15,000 g.mol -1 (Đ = 1.15-1.25) were obtained. The authors also confirmed the occurrence of both the AMM and the ACEM as initially proposed by Bourissou et al. [START_REF] Delcroix | Ring-opening polymerization of trimethylene carbonate catalyzed by methanesulfonic acid: activated monomer versus active chain end mechanisms[END_REF] Additionally, they demonstrated that the use of PDM as diol initiating system (a situation where all chain propagations are bidirectional) enabled the preparation of PTMC's exhibiting a unimodal and narrow dispersity (Đ = 1.07-1.10) with Mn up to 16,800 g.mol -1 .

The MSA-catalyzed sequential copolymerization of -CL and TMC was then successfully achieved, producing PCL-b-PTMC diblock copolymers of Mn = 5,900 -10,600 g.mol -1 (Đ <1. and PCL-b-PTMC-b-PCL (ROP of TMC first using H2O as initiator) triblock copolymers of Mn = 9,100 -29,400 g.mol -1 (Đ <1.2). [START_REF] Campos | Copolymerisation of εcaprolactone and trimethylene carbonate catalysed by methanesulfonic acid[END_REF][START_REF] Couffin | Mild and efficient preparation of block and gradient copolymers by methanesulfonic acid catalyzed ring-opening polymerization of caprolactone and trimethylene carbonate[END_REF] Simultaneous polymerization of the two monomers gave rise to PCL-gradient-PTMC copolymers, with a preferential insertion of ε-CL.

To gain a better mechanistic insight into the acid catalyzed ROP of TMC, computational calculations by density functional theory (DFT), including a continuum dielectric representation of dichloromethane, with MSA, TFA or TfOH as catalysts, and methanol as model initiator, were realized by Coady, Hedrick et al.. [START_REF] Coady | Polymerizing base sensitive cyclic carbonates using acid catalysis[END_REF] Theoretically, in the case of MSA, only the AMM was predicted to be operative instead of the two distinct and competitive mechanisms observed experimentally by Bourissou et al. [START_REF] Delcroix | Ring-opening polymerization of trimethylene carbonate catalyzed by methanesulfonic acid: activated monomer versus active chain end mechanisms[END_REF] and Peruch et al. [START_REF] Campos | A new insight into the mechanism of the ring-opening polymerization of trimethylene carbonate catalyzed by methanesulfonic acid[END_REF]. Indeed, an activation barrier of 17 kcal.mol -1 and 45 kcal.mol -1 were calculated for the ring-opening of the monomer, in the case of the AMM and ACEM, respectively. Thus, the presence of adventitious water was most likely the cause of the two populations observed in those previous studies, although calculations involving a "real initiator" may decrease the difference of energetic barrier between the two mechanisms. Interestingly, DFT calculation also demonstrated that, instead of promoting the AMM, sulfonic acids could eventually display a bifunctional activity, resulting not only in the activation of the carbonyl moiety of the monomer by protonation, but also in the activation of the propagating hydroxyl group through H-bonding with the sulfonate counter anion (Scheme 10).

A similar mechanism was previously reported by Bourissou et DFT calculations suggested the occurrence of a dual hydrogen-bonding mechanism, where both the alcohol initiator and the isocyanate were activated by interaction with the sulfonate anion (XSO3 -) and the proton of the catalyst, respectively. Note that protonation of the isocyanate might either occur at the nitrogen or oxygen atoms (see insert in Scheme 12). In both cases, the reaction followed a preference for N-activation over O-activation pathways.

Scheme 12. Organic acid-catalyzed synthesis of PUs from diisocyanate and diol. [START_REF] Sardon | Organic acid-catalyzed polyurethane formation via a dualactivated mechanism: unexpected preference of N-activation over O-activation of isocyanates[END_REF] As depicted in Scheme 13, the same group reported the post-polymerization modification of PUs derived from such an acid-catalyzed polymerization. [START_REF] Sardon | Highly tunable polyurethanes: organocatalyzed polyaddition and subsequent post-polymerization modification of pentafluorophenyl ester sidechains[END_REF] Using 5 mol% of TfOH, the polyaddition between a pentafluorophenyl ester-containing diol derived from of Mn of 25,000 -37,000 g.mol -1 (Đ = 1.3-1.6). As previously reported, [START_REF] Engler | Accessing new materials through polymerization and modification of a polycarbonate with a pendant activated ester[END_REF] these poly(bis-MPA-C6F5-HDI) urethanes could also be post-functionalized by various amines, yielding highly functional PU derivatives.

Scheme 13. Synthesis of polyurethanes from bis-MPA-C6F5 and hexamethylene diisocyanate using TfOH as catalyst. [START_REF] Sardon | Highly tunable polyurethanes: organocatalyzed polyaddition and subsequent post-polymerization modification of pentafluorophenyl ester sidechains[END_REF] Besides the ROP of cyclic esters and carbonates, methyl acrylate (MA) was reported to undergo an acid-catalyzed GTP, using various trialkylsilyl ketene acetal (SKA) as initiators, such as 1-trimethylsiloxy-, 1-triethylsiloxy-, and 1-triiso-propylsiloxy-1-methoxy-2-methyl-1-propene (MTS Me , MTS Et , and MTS iPr , respectively). GTP was performed using 2 mol% (relative to the initiator) of pentafluorophenylbis(triflyl)-methane (C6F5CHTf2, pKa ~ 1.5 in AcOH). [START_REF] Ishihara | Polystyrene-bound tetrafluorophenylbis(triflyl)methane as an organic-solvent-swellable and strong Brønsted acid catalyst[END_REF][START_REF] Takada | Controlled polymerization of methyl acrylate for high-molecular-weight polymers by pentafluorophenylbis(triflyl)methane-promoted group transfer polymerization using triisopropylsilyl ketene acetal[END_REF] The for the GTP of (meth)acrylates using nucleophilic catalysts (Scheme 4b). [START_REF] Fuchise | Recent progress in organocatalytic group transfer polymerization[END_REF] The more common N-(trimethylsilyl)bis-(trifluoroethanesulfonyl)imide (Me3SiNTf2) Lewis acid could also catalyze the GTP of nBuA in toluene at r.t., using MTS iPr derivatives as functional initiators. [START_REF] Takada | Synthesis of α-, ω-, and α,ω-end-functionalized poly(n-butyl acrylate)s by organocatalytic group transfer polymerization using functional initiator and terminator[END_REF] Under these conditions, α-functionalized (hydroxyl, ethynyl, vinyl, and norbornenyl) poly(nbutyl acrylate)s (PnBuA's) of Mn = 4,000 -142,700 g.mol -1 (Đ =1.03-1.10) could be achieved. Moreover, addition of functional 2-phenyl acrylate terminating agents allowed synthesizing, not only -functionalized (ethynyl, vinyl, hydroxyl, and bromo), but also as ,-functionalized polyacrylates of Mn = 4,000 -4,800 g.mol -1 (Đ =1.06-1.08). Scheme 15. Synthesis of ,-functionalized poly(nBuA)'s by Me3SiNTf2-catalyzed GTP using functional initiators and terminators. [START_REF] Takada | Synthesis of α-, ω-, and α,ω-end-functionalized poly(n-butyl acrylate)s by organocatalytic group transfer polymerization using functional initiator and terminator[END_REF] Triarylsulfonium hexafluorophosphate salts were employed as photoactive acidic catalysts (0.33 mol% with respect to monomer) for the ROP of -CL, -VL and TMC in propylene carbonate at r.t, using BnOH as initiator (Scheme 16a). [START_REF] Barker | Triarylsulfonium hexafluorophosphate salts as photoactivated acidic catalysts for ring-opening polymerisation[END_REF] Under UV irradiation, the sulfonium indeed decomposed into sulfide species with concomitant release of the strong HPF6 acid (Scheme 16b), allowing homopolymers of Mn = 1,600 -27,700 g.mol -1 (Đ = 1.09-1.26), as well as PVL-b-PLLA-b-PVL block copolymers to be prepared in a "one-pot" reaction. However, the bimodal molar mass distribution observed by SEC suggested the occurrence of side reactions, likely resulting from competitive mechanisms (cationic ACEM and BnOH-initiated AMM). It should be emphasized here that these sulfonium hexafluorophosphate salts are not true organic Very recently, TfOH was evidenced to be an effective catalyst for the chain-growth cationic polymerization of 2-halogenated-3-substituted-thiophenes featuring alkyl, alkylthio, alkoxy, and dialkoxy substituent groups (see Figure 5 for related structures). [START_REF] Balasubramanian | Chaingrowth cationic polymerization of 2-halogenated thiophenes promoted by Bronsted acids[END_REF] Polythiophenes of Mw = 2,500 -6,100 g.mol -1 (Đ = 1.31-1.91) were obtained in tetrachloroethane at 100 °C, with low amounts of TfOH (0.05 eq.), while MSA did not give reproducible results. The chain growth process was proposed to involve nucleophilic aromatic substitution-type reactions, where the nucleophile is a non-activated thiophene and the electrophile is a protonated monomer. Upon loss of HX (X = Br, Cl), the aromaticity is recovered, leading to the conjugation of the polymer units. As expected, the polymerization was found to be dependant on both the strength of the acid and the electron density of the thiophene monomers. For instance, the polymerization of chlorinated monomers proved much slower than that of bromo-analogues. In addition, a copolymer of Mw = 7,000 g.mol -1 (Đ = 1.63) was obtained upon sequential polymerization of 2chloro-3-butylthiothiophene and 2-chloro-3-dodecylthiothiophene (Scheme 17).

OSiR 3 O SKA MTS Me (R=CH 3 ) MTS Et (R=C 2 H 5 ) MTS iPr (R=CH(CH 3 ) 2 ) F C F F F F H O O MA + S S O O CF 3 O O F 3 C C 6 F 5 CHTf 2 toluene, r.t n-1 O O H O O H OSiR 3 O PMA OSiR 3 O H C 6 F 5 CHTf 2 + C 6 F 5 C(SiR 3 )Tf 2 O O O O H MA O O H Si R R R C 6 F 5 CTf 2

Carboxylic acid compounds

In comparison with sulfonic and sulfonimide acids (pKa ≤ 1.5 in H2O), carboxylic acids and amino-acids are weak acids (pKa~ 3-5 in H2O) that have been sparingly investigated as catalysts in polymerization reactions. Related studies have mainly focused on cyclic esters and carbonates as monomer substrates (Figure 6). For instance, the catalytic activity of salicylic acid (SAA; pKa ~ 2.9 in H2O) was recently investigated for the ROP of -CL and -VL, with BnOH as initiator (Scheme 18). [START_REF] Xu | Controlled/living ring-opening polymerization of εcaprolactone with salicylic acid as the organocatalyst[END_REF] The polymerization successfully proceeded in bulk at 80 °C, leading to narrowly distributed polyesters with molar masses, Mn, in the range of 1,300 -41,000 g.mol -1 (Đ = 1.03-1.16) and Mn = 3,200 -12,100 g.mol -1 (Đ = 1.05-1.12) for PCL and PVL, respectively. Well-defined PCLb-PVL diblock copolymers were also synthesized by sequential addition of -CL and -VL. In addition, end-functionalized PCL's with vinyl, propargyl and azido groups, as well as PEO-b-PCL diblock copolymers could be prepared, using functionalized initiators, such as propargyl alcohol (PgOH), 6-azido-1 hexanol (AHA), 2-hydroxyethyl methacrylate (HEMA), and a methoxy poly(ethylene oxide) (MeOPEO) macroinitiator. Interaction developing between SAA and -CL, as observed by NMR, suggested that the polymerization likely proceeded by an AMM (see Scheme 1a), although a bifunctional activation cannot be completely ruled out. Recently, TFA (0.5 eq.) was yet found suitable to catalyze the cationic polymerization of 2-halogenated-3-substituted-thiophene, giving similar results in various halogenated solvents at r.t. In contrast, AcOH was too weak to initiate the polymerization. [START_REF] Balasubramanian | Chaingrowth cationic polymerization of 2-halogenated thiophenes promoted by Bronsted acids[END_REF] Polythiophenes with molar masses, Mw, in the range of 1,300 -3,900 g.mol -1 (Đ = 1.18-4.77) were obtained from TFA, the dispersity increasing for prolonged reaction times. Polymerization of electron-poor 2bromothiophene was only effective with the most acidic TfOH catalyst, while some electron-rich monomers, such as 2-bromo-3-alkoxy thiophenes and bromo-ethylenedioxythiophene (EDOT), could be polymerized with TFA. This metal-free methodology affords highly solutionprocessable and conductive poly(alkylthiophene) with a conductivity greater than 180 S.cm -1 .

[51]

Synthesis of PUs by polyaddition between , bis-hydroxy poly(ethylene oxide) and HDI was conducted in dichloromethane, at 20 °C, using TFA and AcOH as catalysts (0.05 eq.). [START_REF] Sardon | Organic acid-catalyzed polyurethane formation via a dualactivated mechanism: unexpected preference of N-activation over O-activation of isocyanates[END_REF] In comparison with sulfonic acids (see Scheme 12 above), carboxylic acids were ineffective, however, monomer conversions less than 3% being reached after 6 h of reaction, as a consequence of the low acidity of these catalysts.

Phosphoric acid and their derivatives

Phosphoric acids (pKa ~ 2 -4 in DMSO) [START_REF] Christ | pKa values of chiral Brønsted acid catalysts: phosphoric acids/amides, sulfonyl/sulfuryl imides, and perfluorinated TADDOLs (TEFDDOLs)[END_REF] have attracted a renewed interest as relatively mild Brønsted acids in organocatalysis in general, [START_REF] Parmar | Complete field guide to asymmetric BINOLphosphate derived bronsted acid and metal catalysis: history and classification by mode of activation; Bronsted acidity, hydrogen bonding, ion pairing, and metal phosphates[END_REF] and in polymerization reactions in particular. [START_REF] Dove | Organic catalysis for ring-opening polymerization[END_REF][START_REF] Akiyama | Stronger Brönsted acids[END_REF] As highlighted in Figure 7, they can provide a dual activation due to both hydrogen-bonding donating and accepting properties of the acidic O-H and basic P=O moieties. [START_REF] Akiyama | Stronger Brönsted acids[END_REF] Although these catalysts could have been discussed in the section devoted to bifunctional catalytic systems, they have been included in the present section to facilitate comparison with the other acidic catalysts. The DPP/ROH catalytic system was also applied by Mecerreyes et al. for the ROP of macrolactones, -PDL, AMB, GB and EB (see Figure 7 for related structures), [START_REF] Pascual | Acid catalyzed polymerization of macrolactones in bulk and aqueous miniemulsion: ring-opening vs. condensation[END_REF][START_REF] Pascual | Organocatalyzed synthesis of aliphatic polyesters from ethylene brassylate: a cheap and renewable macrolactone[END_REF] both in bulk and in aqueous miniemulsion at 80 °C, with BnOH as initiator. For a DPP concentration of 0.5-10 mol% relative to BnOH, aliphatic polyesters of Mn = 5,600 -21,000 g.mol -1 , but with a rather high dispersity, Đ = 1.65-2.53, were obtained. The polymerization was slower than in the case of stronger acids, such as TfOH, DBSA, or PTSA, in agreement with the lower acidity of DPP. An AMM was proposed in the case of the ROP of these macrolactones under bulk conditions.

O O O R 1 R 2 R 1 , R 2 = CH 3 R 1 , R 2 = CH 2 Br R 1 =H, R 2 = OCH 2 C 6 H 5 R 1 = CH 3 , R 2 = CO 2 CH 2 CH=CH 2 R 1 = CH 3 , R 2 =
DPP was also shown to catalyze the ROP of TMC in bulk or in dichloromethane at r.t., using PyBuOH, or PPA as initiators. [START_REF] Coady | Polymerizing base sensitive cyclic carbonates using acid catalysis[END_REF][START_REF] Makiguchi | Diphenyl phosphate as an efficient acidic organocatalyst for controlled/living ring-opening polymerization of trimethylene carbonates leading to block, end-functionalized, and macrocyclic polycarbonates[END_REF] Polymerizations reached full conversion after 22 h, producing PTMC's of controlled molar masses (Mn up to 11,000 g.mol -1 ; Đ = 1.06), in accordance with the initial monomer-to-initiator ratio.

In the same way, Kakuchi et al. reported the ROP of cyclic carbonates possessing functional groups, such as, 5,5-dimethyl-1,3-dioxan-2-one, 5,5-dibromomethyl-1,3-dioxan-2one, 5-benzyloxy-1,3-dioxan-2-one, 5-methyl-5-allyloxycarbonyl-1,3-dioxan-2-one, and 5methyl-5-propargyloxycarbonyl-1,3-dioxan-2-one (Figure 7), with PPA as initiator and DPP as catalyst. [START_REF] Makiguchi | Diphenyl phosphate as an efficient acidic organocatalyst for controlled/living ring-opening polymerization of trimethylene carbonates leading to block, end-functionalized, and macrocyclic polycarbonates[END_REF] Diblock copolymers consisting of a PTMC segment and a second PCL or PVL block were obtained in bulk at r.t., with molar masses increasing from 4,900 (Đ = 1.11) to 10,500 g.mol -1 (Đ = 1.12) for the TMC/-CL system, and from 5,000 (Đ = 1.09) to 9,800 g.mol -1 (Đ = 1.14) for the TMC/-VL one (Scheme 19b).

Surprisingly, in contrast to TfOH (vide supra), DPP did not allow the ROP of MTC-OC6F5 using alcohols as initiators. Given the efficiency of DPP to polymerize TMC, this inactivity towards MTC-OC6F5 was ascribed to the occurrence of side reactions between the activated ester moiety and DPP. [START_REF] Engler | Accessing new materials through polymerization and modification of a polycarbonate with a pendant activated ester[END_REF] Binaphthol (BINOL)-derived monophosphoric acids (17, Figure 7) represent a special class of chiral Brønsted acid catalysts widely used for enantioselective organic transformations that have been recently applied to polymer synthesis. [START_REF] Werner | Phosphonium salt organocatalysis[END_REF][START_REF] Terada | Phosphoric acid catalysis of reactions not involving imines[END_REF] In particular, the BINOL-derived phosphoric acid, namely, binaphthyl-diyl hydrogen phosphate (17 in Figure 7 with X = H), was shown to efficiently catalyze the ROP of ε-CL and -VL in bulk at 40-60 °C, with BnOH as initiator, [START_REF] Miao | Ring-opening polymerization of lactones using binaphthyl-diyl hydrogen phosphate as organocatalyst and resulting monosaccharide functionalization of polylactones[END_REF] yielding polymers with Mn = 4,500 -12,000 g.mol -1 (Đ = 1.08-1.17). Interestingly, the use of selectively protected monosaccharides (mono-and polyols D-glucose derivatives) as initiators led to functionalized-carbohydrate polyesters. [START_REF] Miao | Polyester functionalized carbohydrates via organocatalyzed ring-opening polymerization[END_REF] The axial chirality displayed by those catalysts has recently enabled the enantiomerselective polymerization of rac-LA using 3-phenyl-1-propanol (PPA) as initiator in toluene at 75 °C. [START_REF] Makiguchi | Binaphthol-derived phosphoric acids as efficient chiral organocatalysts for the enantiomer-selective polymerization of raclactide[END_REF] Depending on the nature of the 3,3'-flanking substituents on the catalysts, a maximum selectivity factor (kD/kL) of 28.3 could be obtained at 49% of LA conversion (Mn = 3,700 g.mol -1 ;Đ = 1.13). From a mechanism point of view, a dual activation of the monomer and chain-end by the bifunctional catalyst was proposed to contribute to the high kD/kL observed. [START_REF] Makiguchi | Binaphthol-derived phosphoric acids as efficient chiral organocatalysts for the enantiomer-selective polymerization of raclactide[END_REF] Besides phosphoric acid catalysts, imidodiphosphoric acid (IDPA, 18 in Figure 7) have been employed for the ROP of δ-VL and ε-CL in toluene at r.t., with BnOH as initiator. [START_REF] Kan | Imidodiphosphoric acid as a bifunctional catalyst for the controlled ring-opening polymerization of δ-valerolactone and εcaprolactone[END_REF] Welldefined PVL's and PCL's of Mn values in the range of 4,300 -20,200 g.mol -1 (Đ = 1.14-1.24)

were thus obtained. IDPA was also studied for the BnOH-initiated ROP of TMC in toluene at r.t and 50 °C. [START_REF] He | Bifunctional imidodiphosphoric acid-catalyzed controlled/living ring-opening polymerization of trimethylene carbonate resulting block, α,ω-dihydroxy telechelic, and star-shaped polycarbonates[END_REF] No side decarboxylation was noted at 50 °C, giving rise to well-defined PTMC's, with Mn = 1,800 -18,000 g.mol -1 and Đ = 1.12-1.19. Additionally, telechelic and star-shaped PTMC's were synthesized using various initiators. Well-defined PTMC-b-PVL and PTMC-b-PCL diblock copolymers could also be obtained by sequential polymerization of TMC with δ-VL and ε-CL, using IDPA as catalytic system.

In summary, organic acids, from weak to strong acids, have (re)emerged as a novel class of organic catalysts for polymerization reactions. In some cases, the catalyst efficiency does not simply correlate to its acidity, suggesting a different mechanism such as a dual activation of both the monomer and the initiator. Such an acidic organocatalysis proves efficient not only for the ROP of cyclic esters and carbonates, but also for the GTP of (meth)acrylics and for polyurethane synthesis.

Polymerization catalyzed by phosphorous-containing

Brønsted and Lewis bases: phosphazenes and phosphines

Phosphazenes and phosphines are basic compounds (Figure 8), which have been investigated as catalysts mainly for the ROP of cyclic monomers, but also for the GTP polymerizations of (meth)acrylic substrates (Figure 9).

Phosphazenes

Initially developed by Schwesinger, [START_REF] Schwesinger | Peralkylated polyaminophosphazenes-extremely strong, neutral nitrogen bases[END_REF] phosphazenes behave as strong neutral Brønsted bases (pKa = 26-47 in MeCN) [START_REF] Ishikawa | Superbases for organic synthesis: guanidines, amidines and phosphazenes and related organocatalysts[END_REF] and are weak nucleophiles. In the context of polymer synthesis, they have been used to catalyze the ROP of cyclopropanes and heterocyclic monomers (e.g. esters, ethers or carbonates), and to promote the anionic polymerization of vinyl monomers (e.g. alkyl (meth)acrylates) as well. [START_REF] Boileau | Activation in anionic polymerization: why phosphazene bases are very exciting promoters[END_REF] In the presence of protic initiators, phosphazenes operate via an ACEM (see Scheme 4a), in agreement with their high Brønsted basicity and low nucleophilicity. The deprotonation of a protic alcohol initiator results in the formation of an alkoxide associated with a soft and bulky phosphazenium cation, which allows the metal-free polymerization to be controlled. [START_REF] Boileau | Activation in anionic polymerization: why phosphazene bases are very exciting promoters[END_REF] Other initiating systems based on thiol-, amide-, or even some CH-containing precursors can also be used. Most of phosphazenes used in this context and monomers investigated are shown in Figures 8 and9.

The ROP of cyclic ethers with phosphazenes as catalysts have been (re)investigated by Hadjichristidis et al. For instance, the 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2bis[tris(dimethylamino) phosphoranylidenamino]-2 5 ,4 5 -catenadi(phosphazene), denoted as t-BuP4 (see Figure 8) was employed (1.15 eq.) to trigger the ROP of several epoxides, including ethylene oxide (EO), propylene oxide (PO), 1,2-butylene oxide (BO) and tert-butyl glycidyl ether (tBGE), using poly(N,N-dimethylacrylamide-co-acrylamide) (CoAm) as a macroinitiator (Scheme 20). [START_REF] Zhao | A facile metal-free "grafting-from" route from acrylamide-based substrate toward complex macromolecular combs[END_REF] In contrast to EO and PO, BO and tBGE could not undergo direct anionic ringopening graft polymerization (AROGP) from the CoAm precursor, most probably to due a steric hindrance. In toluene, at 45 °C, these polymerizations yielded macromolecular combs with various side chains, including single-or double grafted homopolymers, block and statistical

O O O O O O O R R'O O O O H n O O k R O O H O k n O O O H O n R'O R 1 O O R 2 OSiMe 3 O n -1 O O R 1 O O R 1 OSiMe 3 O R 2 R 2 R'OH R n n R'OH R ' O H n n P NR'"' 2 R' 2 N NR" 2 N R O R n R'OH O H R n R'OH or R'COOH P R R R O O R O O R R'O H O O O O R n R R 'O H o r R ' S H H H O O O R 2 O R' n R 'O H
copolymers (Mn in the range of 600 -1,700 kg.mol -1 ; Đ = 1.17-1.31). Initiation was proposed to occur through deprotonation of pendant primary amides by t-BuP4, followed by ring-opening of the monomer. initiated by carboxylic acids. [START_REF] Zhao | Phosphazene-promoted metal-free ringopening polymerization of ethylene oxide initiated by carboxylic acid[END_REF] Sequential metal-free ROP of epoxides (EO and/or BO) and cyclic esters (δ-VL or ε-CL) or carbonates (TMC) was then reported by Hadjichristidis et al. To this end, the authors switched the polymerization catalyst, from the basic t-BuP4 (for epoxides), to the acidic DPP (for cyclic esters/ carbonate). [START_REF] Zhao | A "catalyst switch" strategy for the sequential metal-free polymerization of epoxides and cyclic esters/carbonate[END_REF][START_REF] Zhao | One-pot synthesis of linear-and threearm star-tetrablock quarterpolymers via sequential metal-free ring-opening polymerization using a "catalyst switch" strategy[END_REF] Thus, EO and BO were first polymerized in THF at 40 °C, using PPA as initiator and t-BuP4 as catalyst (0.2 eq.), giving polyethers of Mn = 1,300 -3,800 g.mol -1 and Đ = 1.04-1.05. An excess of DPP (1.2 eq.) was then introduced, followed by addition of ε-CL, δ-VL or TMC. In this way, polyether-polyester/polycarbonate block copolymers, with molar masses Mn = 6,200 -3,800 g.mol -1 and Đ = 1.04-1.72, could be readily synthesized. Scheme 23. Sequential ROP using a "catalyst switch"strategy, from t-BuP4 to DPP. [START_REF] Zhao | A "catalyst switch" strategy for the sequential metal-free polymerization of epoxides and cyclic esters/carbonate[END_REF][START_REF] Zhao | One-pot synthesis of linear-and threearm star-tetrablock quarterpolymers via sequential metal-free ring-opening polymerization using a "catalyst switch" strategy[END_REF] Later on, the same group studied the catalytic activity of the less-hindered phosphazene base, 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2 5 ,4 5 -catenadi (phosphazene), denoted as t-BuP2 (see Figure 8), for the ROP of EO, ε-CL and LLA , using PPA or even water as initiators. [START_REF] Hadjichristidis | Sequential polymerization of ethylene oxide, ε-caprolactone and L-lactide: a one-pot metal-free route to tri-and pentablock terpolymers[END_REF] Complete EO conversion was achieved in THF after 3 days at 50 °C, leading to PEO's with predictable molar mass (Mn = 4,600 g mol In the same way, tert-octylimino-tris(dimethylamino)phosphorane (t-OctP1), t-BuP2, and t-BuP4-catalyzed ROP of TMC, in the presence of naturally occurring native starch granules as macroinitiators, proceeded in bulk at 150 °C (Scheme 24). [START_REF] Samuel | Highly efficient metal-free organic catalysts to design new environmental-friendly starch-based blends[END_REF] This strategy allowed synthesizing polycarbonate/starch composite materials with enhanced physical and biodegradability properties over the native starch. The phosphazene catalysts showed an excellent catalytic activity (98% of monomer conversion after only 2 min of reaction), even at very low concentration (e.g. with 0,01 mol% of t-BuP4). PTMC's with moderate molar masses Phosphazenes also enabled directly polymerizing methacrylic monomers.

R OH O t-BuP 4 R O O O R O O O R OH O R O O OH R O O O R O O OH H transfer (fast) R O O O R O + O OH O n slow R O O O n O fast R O O O n OH fast + R O O O n O R O O O n OH H R O O O n OH + R O O O n O R O HO O n

Phosphines

Even though phosphines have been widely employed as catalysts in a variety of organic transformations, [START_REF] Methot | Nucleophilic phosphine organocatalysis[END_REF][START_REF] Marinetti | Enantioselective phosphine organocatalysis[END_REF]

H OH + H H O O n t-BuP 1 (cat.) O O O N C O O H N n 1)
2) orthophthaldehyde (PA) 9-anthracene methanol dimethylphenylphosphine (PhMe2P), methyldiphenylphosphine (Ph2MeP) and triphenylphosphine (Ph3P), were shown to be effective catalysts for the ROP of LA, generating narrowly dispersed polylactides with predictable molar masses (Mn = 2,800 -13,000; Đ = 1.11-1.40) through a nucleophilic AMM (Scheme 2). [START_REF] Myers | Phosphines: nucleophilic organic catalysts for the controlled ring-opening polymerization of lactides[END_REF] The substitution pattern of the phosphine was a dominant feature in controlling their reactivity. Thus, alkyl-substituted phosphines, being the most basic and most nucleophilic phosphines, proved more active than their aryl-containing counterparts. [START_REF] Myers | Phosphines: nucleophilic organic catalysts for the controlled ring-opening polymerization of lactides[END_REF] In 2012, Taton et al. reported that tris(2,4,6-trimethoxyphenyl)phosphine (TTMPP; see were also synthesized. TTMPP-derived PMMA's exhibited a tacticity of mm/mr/rr = 0.06/0.41/0.53, i.e. very similar to that of PMMA grown by an anionic process, suggesting the occurrence of a dissociative mechanism (Scheme 4b), though, attempts to identify enolate-type species by NMR analysis were unsuccessful.

Scheme 28. Phosphine-catalyzed GTP of MMA and tBuA. [START_REF] Fèvre | Tris(2,4,6-trimethoxyphenyl)phosphine (TTMPP) as potent organocatalyst for group transfer polymerization of alkyl (meth)acrylates[END_REF] In summary, commercially available phosphazene Brønsted "superbases" are powerful organocatalysts which have been mainly investigated for ROP of epoxides, cyclic esters, cyclic In the context of macromolecular chemistry, NHCs have been applied for both chaingrowth and step-growth polymerization reactions (Figure 11) in the past decade. [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Fèvre | N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis[END_REF][START_REF] Naumann | N-heterocyclic carbenes as organocatalysts for polymerizations: trends and frontiers[END_REF][START_REF] Kamber | Organocatalytic ring-opening polymerization[END_REF] The field of NHC-mediated polymerization reactions has been reviewed in 2013 by our group [START_REF] Fèvre | N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis[END_REF] and by Naumann and Dove in early 2015. [START_REF] Naumann | N-heterocyclic carbenes as organocatalysts for polymerizations: trends and frontiers[END_REF] Main researches have been focused on the use of "free" NHCs derived from imidazol(in)ium salts, but efforts have also been (re)directed towards the design of more air-tolerant "protected" NHCs (called "masked" NHCs). These NHC precursors are indeed generally easier to handle than bare NHCs, avoiding the use of controlled inert atmospheres. [START_REF] Naumann | Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo-and polymerization catalysis[END_REF] Figure 12 illustrates main categories of NHCs used in polymerization.

Free NHCs

Free NHCs are generally prepared by deprotonation of their corresponding conjugated acid imidazol(in)ium salts with a strong base, e.g. NaH / t-BuOK, KHMDS (route a, Scheme 29). [START_REF] Arduengo | Electronic stabilization of nucleophilic carbenes[END_REF] Other routes for NHC preparation include: b) electrochemical reduction of imidazol(in)ium salt, [START_REF] Orsini | Polarity reversal induced by electrochemically generated thiazol-2-ylidenes: the Stetter reaction[END_REF][START_REF] Orsini | Umpolung of α,βunsaturated aldehydes by electrogenerated NHCs in ionic liquids: synthesis of γbutyrolactones[END_REF] c) chemical reduction of imidazol-2(3H)-thiones by a metal (typically potassium) [START_REF] Kuhn | Synthesis of imidazol-2-ylidenes by reduction of imidazole-2(3H)thiones[END_REF] or d) thermal activation of "masked" NHCs.

[111] R RO' O R RO' O n N O O O R n N N N O O O R R R n-2 O O O O n O O O O O O O O O O O O n-2 R'OH R'O O O H O O n R'OH R'OH O O O n RO' O O H O n R'O Si O Si H n n O C N -R -N C O N H R N H O R' O O O n n H O -R '-O H Y X N R 3 R 4 X = N, S Y = C, N R 1 R 2 O O k R O O H O k n Si O Si n R'OH O R'O O H n R'OH R'O O H O R n Si O Si Si O Si Si O Si n-3 O O O O n H N O NH O k k n O O R O O R 'O H R R N N R N N R Tetrahydropyrimidin -type
Tetrahydrodiazepintype

N N R R N N R R N N N R R N N R R

N-Heterocyclic carbenes

Imidazol-type Imidazolin-type Triazol-type

Benzimidazoltype

Scheme 29. Main synthetic routes for the generation of imidazole(in)-2-ylidenes. [START_REF] Naumann | Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo-and polymerization catalysis[END_REF][START_REF] Arduengo | Electronic stabilization of nucleophilic carbenes[END_REF][START_REF] Orsini | Polarity reversal induced by electrochemically generated thiazol-2-ylidenes: the Stetter reaction[END_REF][START_REF] Orsini | Umpolung of α,βunsaturated aldehydes by electrogenerated NHCs in ionic liquids: synthesis of γbutyrolactones[END_REF][START_REF] Kuhn | Synthesis of imidazol-2-ylidenes by reduction of imidazole-2(3H)thiones[END_REF] Hedrick, Waymouth et al. previously demonstrated that, in the presence of alcohol initiators, NHCs could efficiently catalyze the ROP of cyclic esters (lactide and lactones) with a turn over frequency (TOF) of 18 s -1 , while exhibiting many features of a controlled polymerization. [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF] In agreement with experimental observations, DFT calculations suggested that the polymerization operated by a combination of both a nucleophilic AMM (Scheme 3a) and a H-bonding ACEM (Scheme 4a). Competition between both mechanisms might occur, particularly at low alcohol concentration (high monomer/initiator ratios), for which experimental molar masses deviated from those predicted. [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF] The ROP of cyclic esters have been extensively described, with a variety of NHCs. For instance, a series of 1,3-non symmetrically substituted imidazol-2-ylidenes (NHC12-15; Scheme 30) have been employed as catalysts for the ROP of L-LA [START_REF] Wang | Characteristics and mechanism of L-lactide polymerization using N-heterocyclic carbene organocatalyst[END_REF], ε-CL and 2,2dimethyltrimethylene carbonate (DTC) [START_REF] Zhang | Ring-opening copolymerization of ε-caprolactone with 2,2-dimethyltrimethylene carbonate using N-heterocyclic carbene organocatalysts[END_REF], in the presence of BnOH as initiator (Scheme 30).

Homopolymers (Mn up 26,800 g.mol N N Scheme 32. NHC-catalyzed step-growth polymerization of diisocyanates with diols. [START_REF] Coutelier | N-Heterocyclic carbene-catalysed synthesis of polyurethanes[END_REF] Scheme 33. Potential side reactions during polyurethane formation. [START_REF] Duong | N-heterocyclic carbenes as highly efficient catalysts for the cyclotrimerization of isocyanates[END_REF] Despite the efficiency of free NHCs as catalysts for polymerization reactions, their air-and moisture-sensitivity have limited their widespread adoption. Thus, recent studies have focused on the development of protected carbenes, as more air-tolerant precursors.

Protected-NHCs

In 2014, Buchmeiser and Naumann have published a mini-review describing pros and cons of several classes of protected-NHCs used in metal-free polymerization reactions. [START_REF] Naumann | Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo-and polymerization catalysis[END_REF] Protected-or masked-NHCs are adducts or complexes that can release the free carbene upon application of an external stimulus, such as temperature or solvent effect. [START_REF] Naumann | Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo-and polymerization catalysis[END_REF] Examples of these protected NHCs include imidazol(in)ium carboxylates (NHC-CO2), NHC-alcohol-adducts, some NHC-metal complexes and azolium hydrogen carbonates ([NHC(H).HCO3]) (Figure 13).

In this context, various protected-NHCs adducts have been designed for a use as latent catalysts in polymer synthesis.

N N tBu tBu

NHC tBu + isophorone diisocyanate (IPDI) THF, 30°C N C O N C O HO OH 1,4-butanediol H N O O N H O O n N N tBu tBu R' OH N N tBu tBu R O H R NCO R N O O R' H N N tBu tBu R N H O O R' polyurethanes (PUs) R N C O N N O O R R N N N O O O R R R + isocyanurate (trimer) uretdione (dimer) R' OH R N H N O R' O R O allophanate Figure 13
. Thermal generation of free NHC from NHC-adducts. [START_REF] Naumann | Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo-and polymerization catalysis[END_REF] For instance, NHC-carboxylate pre-catalysts (NHC-CO2 adducts) have been studied for the ROP of cyclic esters, such as lactide and lactones. Plasseraud et al. thus reported the solvent-free ROP of rac-LA, using 1-n-butyl-3-methylimidazolium-2-carboxylate (BMIM-CO2;

NHC11; 1 mol% relative to the monomer) and alcohol initiators, such as BnOH, ethylene glycol (EG), glycerol and pentaerythritol. Reactions were carried out in vacuo at 75 °C. [START_REF] Hoppe | n-Butyl-3methylimidazolium-2-carboxylate: a versatile precatalyst for the ring-opening polymerization of ε-caprolactone and rac-lactide under solvent-free conditions[END_REF] Conversions in the range of 50-70% were achieved, leading to PLA-based oligomers (Mn = 500 -1,000 g.mol -1 ; Đ = 1.16-1.62). The incomplete conversion observed was attributed to the increase of the viscosity during PLA formation.

In contrast, the solvent-free ROP of D,L-lactide (D,L-LA) at r.t and at 80 °C, in presence of BnOH (1 eq.) using imidazolium-2-carboxylates compounds, namely,

)imidazolium carboxylate (NHC iPr -CO2; NHC2), 1,3-bis(dodecyl)imidazolium carboxylate (NHC Doc -CO2; NHC4) and 1,3-bis(mesityl)imidazolium carboxylate (NHC Mes -CO2; NHC7), was successfully achieved by Taton et al. [START_REF] Fèvre | Azolium hydrogen carbonates and azolium carboxylates as organic pre-catalysts for N-heterocyclic carbene-catalysed group transfer and ring-opening polymerisations[END_REF] With 0.03-1.5 eq. of NHC-CO2 adduct, PLA's with molar masses Mn of 5,000 g.mol -1 (Đ = 1.13) were obtained. No induction period was noted with such NHC-CO2 adducts, suggesting that the NHC was generated at r.t. Hence, under such experimental conditions, these precursors did not exhibit a latent behavior.

Scheme 34. ROP of LA using NHC-CO2 pre-catalysts and ROH as initiators. [START_REF] Hoppe | n-Butyl-3methylimidazolium-2-carboxylate: a versatile precatalyst for the ring-opening polymerization of ε-caprolactone and rac-lactide under solvent-free conditions[END_REF][START_REF] Fèvre | Azolium hydrogen carbonates and azolium carboxylates as organic pre-catalysts for N-heterocyclic carbene-catalysed group transfer and ring-opening polymerisations[END_REF] A poor efficiency of NHC iPr -CO2, NHC Doc -CO2, NHC Mes -CO2 for the ROP of ε-CL was noted at r.t., presumably due to the limited solubility of these pre-catalysts in the monomer. [START_REF] Fèvre | Azolium hydrogen carbonates and azolium carboxylates as organic pre-catalysts for N-heterocyclic carbene-catalysed group transfer and ring-opening polymerisations[END_REF] In contrast, full conversion of ε-CL was observed at 75 °C using BMIM-CO2, leading to linear and In contrast to oligomers obtained previously with BMIM-CO2 by Plasseraud et al., [START_REF] Hoppe | n-Butyl-3methylimidazolium-2-carboxylate: a versatile precatalyst for the ring-opening polymerization of ε-caprolactone and rac-lactide under solvent-free conditions[END_REF] PCL's of Mn = 5,000 -17,000 g.mol-1 (Đ = 1.20-1.70) were achieved with this series of NHC-CO2 adducts. [START_REF] Naumann | Protected N-heterocyclic carbenes as latent pre-catalysts for the polymerization of ε-caprolactone[END_REF] Less hindered imidazolium-(NHCs 2-3) and imidazolinium (NHC25) carboxylates were the most active for the ROP of ε-CL upon decarboxylation. Importantly, these protected NHCs were completely inactive at r.t., while a fast and quantitative consumption of the monomer was noted at 70 °C, evidencing the thermally latent behavior of these precatalysts. A nucleophilic AMM (Scheme 2) was postulated, but the occurrence of an ACEM (Scheme 4a) could not totally be ruled out. The latter mechanism was likely even more favored with NHCs derived from tetrahydropyrimidinium carboxylates, due to the higher basicity of the related free NHCs compared to that of imidazol-2-ylidenes (pKa DMSO = 27.8 -28.2 for tetrahydropyrimidin-2-ylidenes vs. pKa DMSO = 20 -25 for imidazol-2-ylidenes) [START_REF] Magill | Basicity of nucleophilic carbenes in aqueous and nonaqueous solvents-theoretical predictions[END_REF]. It was noted that 7-membered ring tetrahydrodiazepinium-carboxylates were ineffective pre-catalysts in these conditions.

N N R R N N R R N N R R N N R R N N R R H OR
The ROP of PO catalyzed by imidazolium (NHC Me -CO2, NHC iPr -CO2, NHC tBu -CO2 NHC Mes -CO2), imidazolinium (NHC25) or benzimidazolium (NHC27) carboxylates was also reported. [START_REF] Lindner | Ring-opening polymerization and copolymerization of propylene oxide catalyzed by Nheterocyclic carbenes[END_REF] Diethylene glycol (DEG) was employed as initiator, in bulk at 120 °C. In most cases, diol-functionalized PPO oligomers were obtained after 4 h (40-74% of conversion), with Mn = 300 -1,200 g.mol -1 and Đ = 1.08-1.23. Catalytic activity of NHC iPr -CO2 was close to that observed in a previous work by Taton et al., with the corresponding free NHC. [START_REF] Raynaud | Metal-free and solvent-free access to α,ω-heterodifunctionalized poly(propylene oxide)s by N-heterocyclic carbene-induced ring opening polymerization[END_REF] Imidazolium pre-catalysts were by far the most active (the least-bulky substituents affording the best activity, NHC Me -CO2 > NHC iPr -CO2 > NHC tBu -CO2 > NHC Mes -CO2), compared to the benzimidazol-type (NHC27), which proved ineffective. The same system was also efficient for the sequential ROP of PO and ε-CL and L-LA, leading to poly(ether)-b-poly(ester) copolymers of low Mn (750 -3,700 g.mol -1 and Đ = 1.38-1.77).

Based on DFT calculations and in situ IR spectroscopy, it was proposed that decarboxylation was a slow process, which allowed a continuous delivery of the free carbene catalyst, thus providing a better control over the ROP. ROP of PO would occur via the AMM by the NHC, forming a zwitterionic imidazolium alkoxide 21 (Scheme 35). The latter species would be subsequently displaced by the ROH, generating anα-alkoxide monoadduct 23, whereas an imidazolium would be released, the polymerization proceeding through an anionic mechanism (route A1). Such a mechanism contrasted with that proposed by Taton et al. where NHC 2 could serve either as catalyst (route A2) or as direct initiator (route B). [START_REF] Raynaud | Metal-free and solvent-free access to α,ω-heterodifunctionalized poly(propylene oxide)s by N-heterocyclic carbene-induced ring opening polymerization[END_REF] Further investigations would be definitely needed, however, to clarify the precise mechanism, which could eventually depend on the structure of the NHC catalyst.

Scheme 35.

Possible mechanisms for the NHC-catalyzed ROP of PO in presence of an alcohol initiator. [START_REF] Naumann | Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo-and polymerization catalysis[END_REF][START_REF] Raynaud | Metal-free and solvent-free access to α,ω-heterodifunctionalized poly(propylene oxide)s by N-heterocyclic carbene-induced ring opening polymerization[END_REF] NHC-CO2 adducts were also used to trigger the ROP of cyclic carbonates. For instance, NHC iPr -CO2 (0.1 mol% relative to the initiator) was found particularly active for the ROP of TMC in THF at r.t., using BnOH as initiator. [START_REF] Fèvre | Azolium hydrogen carbonates and azolium carboxylates as organic pre-catalysts for N-heterocyclic carbene-catalysed group transfer and ring-opening polymerisations[END_REF] The polymerization reached 91% of TMC conversion after 1 h, leading to a PTMC of Mn = 3,500 g.mol -1 (Đ = 1.07), confirming that the free NHC was generated under such conditions. TMC was also polymerized in bulk and in THF or toluene solution at 60, 80 and 100 °C, in the presence of NHC Mes -CO2 as pre-catalyst and BnOH as initiator. [START_REF] Reitz | Ring-opening polymerization of organic carbonates using CO2-carbene adducts as effective organo catalyst[END_REF] While at 60 °C and 80 °C, solvent-free polymerizations were quite similar, giving a PTMC of Mn = 11,000 g.mol -1 (Đ = 1.4), slightly higher molar masses (Mn = 13,400 g.mol -1 ) and a broader dispersity (Đ = 1.7) were obtained at 100 °C. Polymerizations in solution at 60 °C were less controlled, yielding PTMC's of lower molar masses (Mn = 2,000-3,000 g.mol -1 ) with higher dispersities (Đ = 1.3-3.0). These observations evidenced the temperature-dependence of this NHC-catalyzed ROP of TMC, with decarboxylation of the monomer occurring at high temperatures.

Taton et al. reported the use of NHC iPr -CO2 as an efficient pre-catalyst also for the GTP of MMA, using MTS as initiator at r.t., both in bulk and THF. [START_REF] Fèvre | Azolium hydrogen carbonates and azolium carboxylates as organic pre-catalysts for N-heterocyclic carbene-catalysed group transfer and ring-opening polymerisations[END_REF] With 3 mol% of NHC iPr -CO2

(relative to MTS), PMMA's of molar masses Mn of 6,300 g.mol -1 and Đ = 1.12-1.37, with a good control over the polymerization, were obtained.

The same group described the ROP of D,L-LA using imidazolium hydrogen carbonates, denoted as [NHC(H)][HCO3], as pre-catalysts (10 mol% relative to the initiator) and BnOH as initiator. [START_REF] Fèvre | Imidazol(in)ium hydrogen carbonates as a genuine source of N-heterocyclic carbenes (NHCs): applications to the facile preparation of NHC metal complexes and to NHCorganocatalyzed molecular and macromolecular syntheses[END_REF] The saturated [NHC Mes (H)][HCO3] (NHC25 in Figure 2) precursor was found to be Anionic polymerization [START_REF] Zhang | Synthesis of linear and hyperbranched polyesters in Bronsted acid ionic liquids[END_REF] the most efficient precursor, high monomer conversions of lactide being achieved in THF at r.t., resulting in PLA's of molar masses (Mn = 4,000 -7,800 g.mol In conclusion, NHCs have been successfully used as organic catalysts for chain-growth polymerization, including ROP of cyclic esters, carboxyanhydrides, as well as for the synthesis of polyurethanes via step-growth polymerizations. Protected NHCs provide a genuine source of carbenes for metal-free polymerizations. However, their catalytic efficiencies often remain lower than the corresponding free NHCs, probably due to a slow or incomplete generation of active species under polymerization conditions. Further developments towards air-stable protected NHCs, which could be efficiently activated by an external stimulus, could further expand the scope of NHC-catalyzed polymerization reactions. Carbohydrate-functionalized polylactides of number-average DP ranging from 2 to 210 (Đ = 1.07-1.48), considering the growth of one polymeric chain per initiating hydroxyl group, were obtained with a high functionalization efficiency.

In the same manner, DMAP served as catalyst for the copolymerization of L-LA and TMC in toluene at 130 °C, using BnOH as initiator. [START_REF] Guerin | Macromolecular engineering via ring-opening polymerization (1): L-lactide/trimethylene carbonate block copolymers as thermoplastic elastomers[END_REF] The ROP of L-LA, using a pre-synthesized BnO-PTMC-OH prepolymer or through direct sequential copolymerization of the two monomers, afforded PTMC-b-PLLA diblock copolymers of molar masses, Mn, of 26,200 g.mol -1 (Đ = 1.32).

DMAP was thought to operate via an ACEM (Scheme 4a). Under the same conditions, the phosphazene BEMP was more effective than DMAP, presumably due to its higher basicity. Aliphatic tertiary amines, including triethylamine (TEA; pKa ~ 11.09 in H2O), N,N,N,Ntetramethylethylenediamine (TMEDA; pKa ~ 8.97 in H2O) and N,N,-N,N,N-pentamethyl diethylenetriamine (PMDETA; pKa ~ 9.10 in H2O), were used to catalyze the BnOH initiated-ROP of TMC in THF at 55 °C (Scheme 37). [START_REF] Mingfa | Controllable ring-opening polymerization of trimethylene carbonate catalyzed by aliphatic tertiary amines in the presence of benzyl alcohol or F127[END_REF] Under those conditions, PMDETA and TMEDA were the most efficient catalysts, yielding PTMC's of Mn = 1,600 -10,400 g.mol -1 and Đ = 1.17-1.55, at full TMC conversion. The catalytic activity correlated not only to the basicity of tertiary amines and to their ability to activate the initiating alcohol by H-bonding, but also with the number of nitrogen atoms in the catalyst (see insert in Scheme 37). [START_REF] Coady | Catalyst chelation effects in organocatalyzed ring-opening polymerization of lactide[END_REF] A particular spatial orientation of nitrogen lone pairs was proposed to explain the catalytic activity of the amines. According to NMR characterizations, this amine-catalyzed ROP of TMC presumably proceeded through the ACEM (Scheme 4a). Using a PEO99-PPO65-PEO99-OH macroinitiator, poly(ether-carbonate) copolymers with a well-defined structure could be easily prepared by this amine-catalytic pathway.

N N R R NR 3 O O O O O O O R R'O O O O H n O O O H O n R'O n R 1 C O -R -C O R 1 n n C H O -R '-C H O R n R'OH

Amidines and guanidines

Compared to trialkyl or aryl amines, amidines and guanidines are stronger organic bases (pKa ~ 23-26 in MeCN) [START_REF] Kaljurand | Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales[END_REF], due to the stability of their corresponding conjugated acid (the positive charge is delocalized over two or three nitrogen atoms). [START_REF] Ishikawa | Superbases for organic synthesis: guanidines, amidines and phosphazenes and related organocatalysts[END_REF] Amidines and guanidines are however slightly less basic (by approximately 2-3 pKa units) than phosphazenes such as 2tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP in Figure 8; pKa ~ 27.6 in MeCN). As phosphazenes, amidines and guanidines have been mainly used as deprotonating agents of acidic O-H, S-H or C-H-containing initiators (see section 3.1), but a particular guanidine, namely, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) (pKa MeCN = 26.0) [START_REF] Kaljurand | Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales[END_REF] could also serve as dual activator in some molecular or macromolecular reactions. [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Dove | Organic catalysis for ring-opening polymerization[END_REF][START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] Representative examples of guanidines and amidines employed in metal-free polymerizations are shown in Figure 15.

Amidines

The amidine 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (pKa = 24.3 in MeCN) [START_REF] Kaljurand | Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales[END_REF] was found to be an excellent Brønsted basic catalyst for the ROP of lactide in presence of alcohol initiators. [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Dove | Organic catalysis for ring-opening polymerization[END_REF][START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] In contrast, the polymerization of lactones required the use of a thiourea cocatalyst to minimize transesterifications. [START_REF] Lohmeijer | Guanidine and Amidine Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF] Based on previous results obtained for DBU-catalyzed ROP of LA, [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF] Coulembier et al.

explored the bulk polymerization of the eutectic L-LA/TMC (50/50) melt, using BnOH as initiator, at 23 °C (Scheme 38). [START_REF] Coulembier | Synthesis of poly(Llactide) and gradient copolymers from a L-lactide/trimethylene carbonate eutectic melt[END_REF] Pure homoPLLA's of molar masses Mn = 15,500 g.mol -1 (Đ = 1.33)

were obtained from this particular mixture of monomers. This was attributed to the limiting diffusion of TMC during the very fast ring-opening process of LA (conversion of 95% in 30 s), resulting from PLLA crystallization out of the eutectic melt. Nevertheless, ROP of TMC was observed when the medium was quickly solubilized in dichloromethane after PLLA formation, affording poly(LLA-g-TMC) gradient copolymers of Mn = 22,000 g.mol -1 and Đ = 1.64 after 16 h (63% of TMC conversion) BnOH. [START_REF] Coulembier | Synthesis of poly(Llactide) and gradient copolymers from a L-lactide/trimethylene carbonate eutectic melt[END_REF] ROP of a particular cyclic carbonate, namely, spiro[fluorene-9,5'- [START_REF] Macmillan | The advent and development of organocatalysis[END_REF][START_REF] Bugaut | Organocatalytic umpolung: N-heterocyclic carbenes and beyond[END_REF]-dioxan]-2'-one (F-TMC) was also reported by Hedrick et al., using BnOH as initiator and DBU as catalyst (1eq) (Scheme 39). [START_REF] Venkataraman | Fluorene-functionalized aliphatic polycarbonates: design, synthesis and aqueous self-assembly of amphiphilic block copolymers[END_REF] Polymerizations proceeded in dichloromethane at r.t, leading to poly (F-TMC)'s of predictable molar masses (Mn of to 4,500 g.mol -1 ; Đ = 1.3). When employing MeO-PEO-OH of different molar masses (Mn = 1,600 -10,000 g.mol -1 ; Đ < 1.2) as macroinitiators, the polymerization was almost completed in 15 min (monomer conversion > 95%), yielding welldefined amphiphilic diblock copolymers of Mn = 5,000 g.mol -1 for the poly(F-TMC) block, as determined by NMR. Depending on the molar mass of the PEO blocks, spheres or elongated rod-like micelles or lamellar phases such as tapes could be generated by self-assembly of these copolymers in aqueous solution.

R 2 N N N N N R 1 R 4 R 3 R 1 R 2 R 3 R 5 R 4 O O O O O O O R R'O O O O H n O H O n O O k R O O H O k n O O R R'OH R n R 'O H R'OH
Scheme 39. DBU-catalyzed ROP of fluorene-functionalized aliphatic cyclic carbonate monomer. [START_REF] Venkataraman | Fluorene-functionalized aliphatic polycarbonates: design, synthesis and aqueous self-assembly of amphiphilic block copolymers[END_REF] Besides polyesters and polycarbonates, polyphosphoesters are attractive materials for bioapplications because they also exhibit biocompatible and biodegradable properties. [START_REF] Zhao | Polyphosphoesters in drug and gene delivery[END_REF] In this context, Iwasaki et al. reported, in 2010, the first solvent-free ROP of cyclic phospholanes such as 2-isopropoxy-2-oxo-1,3,2-dioxaphospholane (iPP) and 2-methacryloyloxyethylphosphoryl choline (PiPP) using DBU as catalyst, in presence of alcohols as initiators. [START_REF] Iwasaki | Synthesis of well-defined thermoresponsive polyphosphoester macroinitiators using organocatalysts[END_REF] Polyphosesters of Mn = 2,400 -7,700 g.mol -1 (Đ = 1.03-1.09) were produced and the polymerization was thought to occur by a basic ACEM, on the basis of NMR analysis.

In the same way, Wooley et al. described the DBU-catalyzed ROP of butynyl phospholane (BYP) in dichloromethane at r.t., using BnOH as initiator (Scheme 40a). [START_REF] Zhang | Facile synthesis of clickable, water-soluble, and degradable polyphosphoesters[END_REF] Using 1.5 eq. of DBU relative to BnOH, poly(BYP)'s of molar masses Mn of 20,000 g.mol -1 (Đ = 1.14-1.19) containing a pendent alkyne function were obtained after only 6 min (conversion of 95%). Postfunctionalization of the obtained polyphosphoesters was accomplished via azide-alkyne Huisgen cycloaddition and thiol-yne reactions.

The ROP of 2-isobutoxy-2-oxo-1,3,2 dioxaphospholane (iBP) was initiated in toluene at 0 °C using 1eq. of BnOH as initiator in presence of 5 eq. of DBU (Scheme 40a). [START_REF] Clément | Metal-free strategies for the synthesis of functional and well-defined polyphosphoesters[END_REF] Although Mn of the obtained poly(iBP)'s (Mn = 6,900-15,800 g.mol -1 ; Đ = 1.19-1.32) evolved with the monomer conversion, this polymerization was very slow (24 h to achieve 80% for DP = 50) and a multimodal molar mass distribution was observed, especially for high monomer conversion diblock copolymer of Mn = 21,900 g.mol -1 and Đ = 1.17. [START_REF] Lim | Construction of a reactive diblock copolymer, polyphosphoester-block-poly(L-lactide), as a versatile framework for functional materials that are capable of full degradation and nanoscopic assembly formation[END_REF] After suitable modifications, the amphiphilic copolymer was manipulated to self-assemble into spherical nanoparticles, with distinct negative and positive surface charges, and uniform size distributions.

Scheme 40. DBU (or TBD)-catalyzed ROP of phospholanes and copolymerization with

L-LA using alcohol initiators. [START_REF] Zhang | Facile synthesis of clickable, water-soluble, and degradable polyphosphoesters[END_REF][START_REF] Clément | Metal-free strategies for the synthesis of functional and well-defined polyphosphoesters[END_REF][START_REF] Lim | Construction of a reactive diblock copolymer, polyphosphoester-block-poly(L-lactide), as a versatile framework for functional materials that are capable of full degradation and nanoscopic assembly formation[END_REF] scheme modified

Guanidines

The most studied bicyclic guanidine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), pKa = 26.0 in MeCN, [START_REF] Kaljurand | Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales[END_REF] has been established as a powerful catalyst for the ROP of cyclic esters and cyclic carbonates.

In presence of alcohol initiators, TBD was reported to specifically operate as a bifunctional activator (Scheme 41). Both the alcohol and the carbonyl groups of the monomer are activated by H-bonding in a cooperative fashion, creating concomitantly an incipient guanidinium ion (see also section 6 for other examples of organic catalyst providing a dual activation). [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] TBD is discussed here for the purpose of comparison with other guanidines or amidines.

Scheme 41. Activation mode of TBD catalyst during the ROP of cyclic esters. [START_REF] Fèvre | Organocatalyzed ring-opening polymerizations[END_REF] Hillmyer et al. reported the solvent-free ROP of the renewable -decalactone (-DL), using 1,4-phenylenedimethanol (PDM) diol as initiator and TBD as catalyst (0.3 eq). [START_REF] Martello | Bulk ring-opening transesterification polymerization of the renewable δ-decalactone using an organocatalyst[END_REF] At 100 °C, full monomer conversion was reached, leading to high molar masses PDL's (Mn of 83,000 g.mol - Similarly, starting from either 1-pyrenemethanol or PEO-OH as macroinitiator, the TBDcatalyzed ROP of LA and -CL was investigated in bulk at 90 °C. [START_REF] Guillerm | A one-pot two-step efficient metal-free process for the generation of PEO-b-PCL-b-PLA amphiphilic triblock copolymers[END_REF] This allowed synthesizing PCL's and PLA's, as well as PEO-b-PCL(or PLA) diblock and amphiphilic PEO-b-P(CL-co-LA) triblock copolymers of molar masses Mn in the range of 3,500 -23,700 g.mol -1 (Đ = 1.4-2.0). These amphiphilic copolymers were shown to self-organize into micelles of hydrodynamic radius Rh = 27-35 nm.

The ROP of -VL catalyzed by TBD (0.17 eq), using BnOH as initiator (1 eq.) was reported in toluene at r.t. [START_REF] Appel | Toward biodegradable nanogel star polymers via organocatalytic ROP[END_REF] The corresponding PVL of Mn = 3,000 g.mol -1 (Đ = 1.06) was obtained within 15 min of reaction for an initial DP = 30. Addition of a crosslinking agent, such as and [4,4′-bioxepane]-7,7′-dione (BOD) to this PVL macroinitiator, led to a polyester nanogel star of Mn = 333,000 g.mol -1 (Đ = 1.21). Alternatively, and more practically, such branched polyesters could also be obtained by mixing all reagents at the early stage of the reaction. Scheme 42. Synthesis of a polyester-based nanogel star polymer by TBD-catalyzed ROP of -VL and BOD initiated by BnOH: a) one-pot/two-step strategy and b) one-pot/one step strategy. [START_REF] Appel | Toward biodegradable nanogel star polymers via organocatalytic ROP[END_REF] The TBD-catalyzed ROP of functionalized lactones, namely, 3-benzyl mercaptovalerolactone (BMVL), 2-(2-methoxyethoxy) ethanethiolvalerolactone (EG2MVL) and 1mercapto-3,6,9,12-tetraoxotridecanevalerolactone (EG4MVL) (Scheme 43a), was also described. [START_REF] Kim | Facile Synthesis of Functionalized Lactones and Organocatalytic Ring-Opening Polymerization[END_REF] 1,4-Pyrenebutanol (PyBuOH) was used as initiator (1-5 mol% of TBD relative to the alcohol) and reactions were carried out in bulk or in toluene at r.t. [START_REF] Kim | Facile Synthesis of Functionalized Lactones and Organocatalytic Ring-Opening Polymerization[END_REF] Well-defined functionalized polyesters (Mn = 500 -8,900 g.mol -1 ; Đ = 1.32-1.45), featuring benzyl mercaptans or oligoethylene glycol pendant groups, could thus be obtained. In addition, aliphatic functionalized polyesters (Mn = 5,800 -6,900 g.mol -1 ; Đ = 1.21-1.47) were successfully prepared by the copolymerization of 3-mercaptovalerolactones with -CL (Scheme 43b). In the same manner, various functionalized lactones, including -allyl--valerolactone (AVL), -propargyl--valerolactone (PgVL), TMS-protected PgVL (TMS-PgVL), and propargyl--caprolactone (PgCL), could be polymerized at r.t in toluene, using BnOH and TBD (2 mol% of relative to the monomer) as initiator and catalyst, respectively. [START_REF] Silvers | Functional aliphatic polyesters and nanoparticles prepared by organocatalysis and orthogonal grafting chemistry[END_REF] Functionalized aliphatic homopolymers, as well as copolymers of molar masses Mn = 8,800 -22,000 g.mol -1 and Đ = 1.01-1.23, incorporating pendant alkene and alkyne groups were prepared. Postpolymerization modification of these polyesters by "click-type" chemistries, allowed hydrophilic solubilizing groups and fluorescent moieties to be incorporated for delivery and controlled release applications.

As an example of base-mediated polymerization of macrolactones, ROP of pentadecalactone (-PDL) and its copolymerization with -CL was investigated by Duchateau el al. [START_REF] Bouyahyi | ω-Pentandecalactone polymerization and ω-pentadecalactone/ ε-caprolactone copolymerization reactions using organic catalysts[END_REF] The combination of TBD/BnOH (1/1) proved catalytically active, in bulk or toluene at 100 °C, producing PCL, PDL homopolymers (Mn = 2,500 -27,000 g.mol -1 ; Đ = 1.3-2.1), as well as random poly(PDL-co-CL) copolymers (Mn = 9,500 and 11,500 g.mol -1 ; Đ = 1.5). At high monomer conversion, a broadening of the molar mass distribution was observed, suggesting the occurrence of transesterification reactions. Accordingly, a general ACEM was ruled out due to the inactivity of the methylated guanidine homologue, N-methyl-1,5,7-triazabicyclododecene (MTBD) under such conditions. Although experimental evidences were lacking, a dual activation of monomer and growing polymer chain by TBD was the most likely mechanism for such macrolactone polymerization.

The TBD-catalyzed ROP of ethylene brassylate (EB) was also investigated by Mecerreyes et al., leading to polyester of Mn = 7,000 -13,900 g.mol -1 (Đ = 1.6-1.9). [START_REF] Pascual | Organocatalyzed synthesis of aliphatic polyesters from ethylene brassylate: a cheap and renewable macrolactone[END_REF] Compared to the acid catalysis by sulfonic acids, the TBD/BnOH combination proved the fastest system, and side reactions were limited. DFT calculations revealed that both TBD and PTSA acted as a bicomponent dual activating system via H-bonding, but the polymerization was kinetically more favored when TBD was used. R'OH/TBD macromonomer featuring a lactide polymerizable group (PEO-LLA) has been reported, using BnOH/TBD (1/0.5), in dichloromethane at r.t (Scheme 44). [START_REF] Castillo | Well-defined poly(lactic acid)s containing poly(ethylene glycol) side chains[END_REF] PLA-g-PEO copolymers were thus obtained with molar masses Mn = 11,000 -14,000 g.mol -1 and Đ = 1.4-2.1. Scheme 44. Synthesis of PEO-grafted PLLA from L-lactide. [START_REF] Castillo | Well-defined poly(lactic acid)s containing poly(ethylene glycol) side chains[END_REF] Similarly to DBU, TBD could also catalyze the ROP of some phospholane monomers. In particular, ROP of 2-isobutoxy-2-oxo-1,3,2 dioxaphospholane (iBP) was carried out in toluene at 0 °C, using BnOH as initiator (Scheme 40a). [START_REF] Clément | Metal-free strategies for the synthesis of functional and well-defined polyphosphoesters[END_REF] In comparison with DBU, TBD provided faster reaction kinetics, complete conversion being obtained within only 2 min (DP = 100, 78% conversion after 11 h with DBU). However, TBD-derived polyphosphoesters poly(iBP)'s exhibited a bimodal molar masses distribution, presumably originating from chain coupling by intermolecular transesterification.

The sequential copolymerization of 2-ethylbutyl phospholane (EBP) and butynyl phospholane (BYP) (EBP being polymerized first) was performed in dichloromethane at 0 °C by Wooley et al. (Scheme 40a). [START_REF] Zhang | Rapid and versatile construction of diverse and functional nanostructures derived from a polyphosphoester-based biomimetic block copolymer system[END_REF] In this way, PEBP50-b-PBYP50 diblock copolymers (Mn = 7,100 -17,200 g.mol -1 ; Đ < 1.17) could be prepared successfully. In contrast, DBU was reported to be only active towards the polymerization of BYP, potentially because of the bulkier ethylbutyl group in EBP. Post-functionalization of the above copolymers by thiol-yne reactions, with various thiols, resulted in nonionic, anionic, cationic and zwitterionic amphiphilic block copolymers that were shown to self-assemble into micelle-like nanostructures.

The TBD-catalyzed ROP of cyclic carbonates proved a practical synthetic strategy to polycarbonates with enhanced thermo-mechanical properties. For instance, Guillaume, Carpentier et al. reported the ROP of the L-LA/TMC binary system in toluene at 110 °C, using TBD in combination with BnOH (1/5). [START_REF] Guerin | Macromolecular engineering via ring-opening polymerization (2): L-lactide/trimethylene carbonate copolymerization-kinetic and microstructural control via catalytic tuning[END_REF] Random P(LLA-TMC) copolymers of molar masses in the range Mn = 6,500 -11,000 g.mol -1 (Đ = 1.4-2.1) were thus obtained. Although this copolymerization proceeded without decarboxylation, extensive transesterification sidereactions were observed upon prolonged reaction time.

The same group achieved the ROP of racemic trans-cyclohexene carbonate (rac-CHC) in toluene at 60 °C, using TBD in combination with BnOH as co-initiator (Scheme 45a). [START_REF] Guerin | Enantiopure isotactic PCHC synthesized by ring-opening polymerization of cyclohexene carbonate[END_REF] The reaction proved efficient (75-85% conversion in 1-4 h), affording well-defined isotactic PCHC's with Mn up to 18,100 g.mol -1 (Đ = 1.16 -1.54).

Glucose-derived polycarbonates (Mn = 4,100 -13,000 g.mol -1 ; Đ = 1.11-1.15) were also successfully produced in dichloromethane at r.t, by the TBD-catalyzed ROP of a glucose-based cyclic carbonate initiated by 4-methylbenzyl alcohol (Scheme 45b). [START_REF] Mikami | Polycarbonates derived from glucose via an organocatalytic approach[END_REF] DSC analysis of these polycarbonates revealed glass transition temperatures significantly higher than those of PTMC (106 °C for glucose-polycarbonate vs. -18 °C for PTMC).

Scheme 45. TBD-catalyzed ROP of a glucose-based monomer initiated by 4-methylbenzyl alcohol. [START_REF] Guerin | Enantiopure isotactic PCHC synthesized by ring-opening polymerization of cyclohexene carbonate[END_REF][START_REF] Mikami | Polycarbonates derived from glucose via an organocatalytic approach[END_REF] Similarly, starch-g-PTMC copolymers were synthesized by TBD-catalyzed ROP of TMC in bulk at 150 °C, in the presence of native starch granules as macroinitiators (Scheme 24). [START_REF] Samuel | Highly efficient metal-free organic catalysts to design new environmental-friendly starch-based blends[END_REF] However, PTMC-based homopolymers (Mn = 4,700 g.mol -1 ; Đ = 1.24) were also detected by NMR spectroscopy, suggesting initiation by propan-1,3-diol generated from hydrolysis of TMC.

Recently, Coulembier et al. reported a CO2-based switchable "on/off" ROP of -CL, using a mixture of TBD/DBU (10/1) as catalytic system and BnOH as initiator (Scheme 46). [START_REF] Coulembier | External and reversible CO2 regulation of ring-opening polymerizations based on a primary alcohol propagating species[END_REF] The ROP of -CL was performed in toluene at r.t., yielding PCL's of molar masses Mn = 1,400 -37,000 g.mol -1 (Đ = 1.1-1.8). While TBD alone was active toward the ROP of -CL, DBU was inactive, but the association of both TBD and DBU resulted in the formation of a hydrogen-bond based-complex (see insert in Scheme 46) that allowed polymerizing this monomer by tempering the reactivity of the TBD catalyst. The switchable "on/off" behavior between active and dormant propagating species was demonstrated by repetitive cycles of CO2/N2 additions and eliminations, where the addition of CO2 resulted in a dormant carbonate "off" state. In the same manner, the TBD-catalyzed ROP of TMC could also be activated and deactivated on demand by the reversible fixation of CO2. Scheme 46. CO2-based switchable ROP of CL using TBD/DBU (10/1) catalytic mixture with BnOH as initiator. [START_REF] Coulembier | External and reversible CO2 regulation of ring-opening polymerizations based on a primary alcohol propagating species[END_REF] There are also some examples of step-growth polymerization reactions utilizing guanidines as organocatalysts. [START_REF] Alsarraf | Cyclic guanidines as efficient organocatalysts for the synthesis of polyurethanes[END_REF][START_REF] Xu | Polyurethane-coated silica particles with broad-spectrum antibacterial properties[END_REF] A series of cyclic guanidines was thus shown to trigger the polyaddition of oligomeric diols (PEO-600 and PTMO-650) and IPDI, in bulk at 60 °C (Scheme 47), affording PUs with Mn = 21,700 -39,800 g.mol -1 (Đ = 1.33-1.89). [START_REF] Alsarraf | Cyclic guanidines as efficient organocatalysts for the synthesis of polyurethanes[END_REF] In contrast to NHC catalysis, [START_REF] Coutelier | N-Heterocyclic carbene-catalysed synthesis of polyurethanes[END_REF] where traces of uretdione (dimer) and isocyanurate (trimer) were detected (see Scheme 33), the guanidine catalysis proceeded without any detectable side events, demonstrating the high selectivity of these catalysts for this reaction. The mechanism of this polyaddition was not clearly established, however, and the authors suggested two possible pathways (Scheme 47). In the nucleophilic mechanism, the addition of the guanidine onto the carbonyl of the isocyanate (AMM) is followed by the reaction of the activated zwitterionic acyl intermediate with the alcohol partner, providing the urethane linkage. However, an ACEM where the alcohol is activated by the base is also plausible. [START_REF] Alsarraf | Cyclic guanidines as efficient organocatalysts for the synthesis of polyurethanes[END_REF] Scheme 47. Guanidine catalyzed-synthesis of PUs from IPDI and PEO. [START_REF] Alsarraf | Cyclic guanidines as efficient organocatalysts for the synthesis of polyurethanes[END_REF] The effectiveness of DBU for synthesizing PUs was further demonstrated with the work of Hedrick, Yang et al., where PUs based on the polyaddition of methyldiethanolamine and IPDI, in presence or not of PEO, were prepared in dichloromethane at r.t. [START_REF] Xu | Polyurethane-coated silica particles with broad-spectrum antibacterial properties[END_REF] Those polymers were next used to decorate silica nanoparticles, following a « grafting onto » approach. Upon quaternization of the amino groups in the main chain polymer, an antibacterial poly(ammonium)type coating was generated.

In the context of a sustainable synthesis of PUs starting from non-isocyanate-based monomers, [START_REF] Maisonneuve | Structure-properties relationship of fatty acid-based thermoplastics as synthetic polymer mimics[END_REF] the organo-catalyzed aminolysis of cyclic carbonates, to form (poly)hydroxyurethanes (PHUs) is an interesting alternative to the isocyanate-based chemistry. PHU's of molar masses Mn = 12,400 -53,400 g.mol -1 and Đ = 1.18-1.38 were thus obtained by step-growth polymerization of the difunctional cyclic carbonate (4-(phenoxymethyl)-1,3dioxolan-2-one derivative (B5CC) and diamines, in DMSO at r.t., using TBD as catalyst (Scheme 48). [START_REF] Lambeth | Organocatalytic synthesis of (poly)hydroxyurethanes from cyclic carbonates and amines[END_REF] Although PHUs have distinct properties than PUs, this strategy proved interesting to avoid toxic isocyanate precursors, while using the rather poor reactivity of 5-membered ring cyclocarbonates. DFT calculations performed with methylbenzoate (MB) as model substrate predicted that the nucleophilic addition of the activated alcohol (or amine) onto the carbonyl group of the ester was the rate-determining step. [START_REF] Horn | Mechanisms of organocatalytic amidation and trans-esterification of aromatic esters as a model for the depolymerization of poly(ethylene) terephthalate[END_REF] Even though both TBD and DBU activated alcohols better than amines, theoretical studies suggested that transesterification and amidation reactions were kinetically comparable. From a thermodynamic standpoint, however, the transesterification was thermo-neutral (reversible) while the amidification was exergonic, hence, the former process was likely to revert to the starting materials.

N N N R N N N R R 1 N=C=O R 1 N O R'OH R 1 HN O OR' R'OH N N N R H R'O d+ d-

General base catalysis Nucleophilic catalysis

Scheme 49. Depolymerization of PET using TBD or DBU catalysis. [START_REF] Horn | Mechanisms of organocatalytic amidation and trans-esterification of aromatic esters as a model for the depolymerization of poly(ethylene) terephthalate[END_REF][START_REF] Fukushima | Advanced chemical recycling of poly(ethylene terephthalate) through organocatalytic aminolysis[END_REF] In summary, among nitrogen-containing bases, amidines and guanidines can efficiently catalyze the ROP of cyclic esters, cyclic carbonates and phospholanes, as well as the depolymerization of PET. While the DBU amidine proves to only activate the initiating alcohol, TBD can behave as a bifunctional activator for both the monomer and the alcohol. In contrast, only a few examples of amine-catalyzed ROPs (i.e. lactide and cyclic carbonates) have been reported, presumably due to their lower basicity compared to those of amidines and guanidines.

The variety and commercial availability of these nitrogen-containing catalysts, combined with their ability to promote control over the polymerization reactions under mild conditions, are key advantages that make them attractive catalytic options for polymer synthesis.

Mono-or bicomponent dual catalytic systems

One of the simplest strategies to provide simultaneous activation of both the monomer and the initiator/chain-end is to combine a weak Brønsted acid (A) with a weak Brønsted base (B) in a bicomponent catalytic system. However, ambiphilic properties can be also found in a single -monocomponent-molecule, providing a more practical reaction set-up. Generally, such a dual activation is based on the formation of two different H-bound complexes (Figure 16): a Brønsted acid-monomer (complex 1), and a Brønsted base-initiator (or growing chain; complex 2). For this purpose, some of the organocatalysts previously described (e.g. amidines,

amines and phosphoric acids) can be employed. [START_REF] Barker | Triarylsulfonium hexafluorophosphate salts as photoactivated acidic catalysts for ring-opening polymerisation[END_REF]. [START_REF] Coady | Catalyst chelation effects in organocatalyzed ring-opening polymerization of lactide[END_REF] Among the various amines investigated, Me6TREN was found the most efficient, yielding PLLA's of molar masses Mn = 23,000 g.mol -1 and Đ = 1.06.

The specificity of Me6TREN was proposed to arise from the presence of multiple basic sites and the locked spatial geometry of the catalyst, allowing for a better alcohol activation through chelative effect (see insert in Scheme 50). Scheme 50. ROP of L-LA using TU/amine catalytic system in the presence of pyrenebutanol. [START_REF] Coady | Catalyst chelation effects in organocatalyzed ring-opening polymerization of lactide[END_REF] The ability of (-)sparteine/TU or DBU/TU catalytic systems for the ROP of functionalized cyclic carbonates, using alcohol initiators, has been investigated in detail by Dove et al.

[147] [START_REF] Stevens | Nanosponge formation from organocatalytically synthesized poly(carbonate) copolymers[END_REF][START_REF] Todd | Benzyl bispidine as an efficient replacement for (-)-sparteine in ring-opening polymerisation[END_REF][START_REF] Tempelaar | Organocatalytic synthesis and post-polymerization functionalization of propargyl-functional poly(carbonate)s[END_REF][START_REF] Brannigan | Block copolymer materials from the organocatalytic ring-opening polymerization of a pentaerythritol-derived cyclic carbonate[END_REF] and by Hedrick et al. [START_REF] Venkataraman | 2-Amino-1,3-propane diols: a versatile platform for the synthesis of aliphatic cyclic carbonate monomers[END_REF][START_REF] Edward | Organocatalytic synthesis of quinine-functionalized poly(carbonate)s[END_REF] For instance, the ROP of 5-methyl-5allyloxycarbonyl-1,3-dioxan-2-one (MAC) was successfully achieved in chloroform at r.t., in presence of BnOH as initiator and (-)sparteine/TU3 as organocatalyst, leading to PMAC's of

Monomer activation

F 3 C F 3 C
n molar masses Mn = 4,600 -10,500 g.mol -1 and Đ = 1.12-1.49 (Scheme 51a). [START_REF] Tempelaar | Organocatalytic synthesis and postpolymerization functionalization of allyl-functional poly(carbonate)s[END_REF] Block copolymers based on PMAC and PLA (Mn = 4,300 -7,100 g.mol -1 ; Đ = 1.17-1.28) were also synthesized, either by ROP of LA from a PMAC prepolymer in a one-pot process, or by ROP of MAC using a PLA macroinitiator. Copolymerization of MAC with 5-methyl-5-ethyloxycarbonyl-1,3-dioxane-2-one (MTC-Et) was also reported under the same conditions, producing allylfunctionalized PMAC-b-PMTC-Et copolymers of Mn = 5,000 g.mol -1 (Đ = 1.25-1.56). [START_REF] Stevens | Nanosponge formation from organocatalytically synthesized poly(carbonate) copolymers[END_REF] Also under the same conditions, dibenzyl-functional bispidine, in combination with TU3 co-catalyst, achieved rates almost identical to those observed using (-)sparteine for the ROP of TMC and MAC. [START_REF] Todd | Benzyl bispidine as an efficient replacement for (-)-sparteine in ring-opening polymerisation[END_REF] The polymerization was well-controlled, leading to polycarbonates of molar masses Mn = 6,100 -9,100 g.mol-1 and Đ = 1.03-1.31, establishing that benzyl bispidine is an excellent substitute of (-)sparteine that becomes difficult to obtain commercially. The (-)sparteine/TU3-catalyzed ROP of acrylamide-functionalized cyclic carbonates in dichloromethane at r.t. gave well-defined tert-butylacrylamide-functionalized homo-and copolycarbonates of Mn = 4,600 -5,100 g.mol -1 and Đ = 1.12-1.32 (Scheme 51b). [START_REF] Venkataraman | 2-Amino-1,3-propane diols: a versatile platform for the synthesis of aliphatic cyclic carbonate monomers[END_REF] With the same catalytic system, the ROP of 5-methyl-5-propargyloxycarbonyl-1,3-dioxan-2-one (MPC) afforded PMPC's of molar masses Mn = 6,800 g.mol -1 (Đ = 1.18) in chloroform at r.t. [START_REF] Tempelaar | Organocatalytic synthesis and post-polymerization functionalization of propargyl-functional poly(carbonate)s[END_REF] However, the polymerization of this monomer could be improved by switching to a DBU/TU3 catalysis. In the latter case, a PMPC of Mn = 16,500 g.mol -1 (Đ = 1.11) was obtained. The synthesis of telechelic PMPC's, poly(ether)-b-poly(carbonate)s and poly(ester)-bpoly(carbonate)s copolymers was also reported by using various hydroxyl-functional initiators, including disulfides, diols and MeOPEO-OH, as well as by copolymerization with L-LA. [START_REF] Tempelaar | Organocatalytic synthesis and post-polymerization functionalization of propargyl-functional poly(carbonate)s[END_REF] Post-polymerization modification of these polymers by 'thiol-yne click-chemistry" allowed accessing a range of functional polycarbonates. Scheme 51. Synthesis of functional polycarbonates using (-)sparteine/TU3 as catalyst and BnOH as initiator. [START_REF] Stevens | Nanosponge formation from organocatalytically synthesized poly(carbonate) copolymers[END_REF][START_REF] Venkataraman | 2-Amino-1,3-propane diols: a versatile platform for the synthesis of aliphatic cyclic carbonate monomers[END_REF][START_REF] Tempelaar | Organocatalytic synthesis and postpolymerization functionalization of allyl-functional poly(carbonate)s[END_REF] In the same manner, the DBU/TU3 system served to catalyze the BnOH-initiated ROP of carbonates, including benzyl-MTC (MTC-Bn), quinine-MTC (MTC-Q) and Boc-MTC (MTC-bocG), in dichloromethane at r.t. Polycarbonates of molar masses, Mn = 2,400 -7,300 g.mol -1 (Đ < 1.3), in agreement with values predicted from the initial monomer-to-initiator ratio, were

O O O O O O O O O O MAC MTC-Et + BnOH (1 eq) (-) sparteine (1eq)/TU3 (2eq) CHCl 3 or CH 2 Cl 2, 25 °C O O O O O H O O O O O O x y P([MAC] x -co-[MTC-Et] y ) O O O N R 1 H R 2 O acrylamide-functionalized cyclic carbonate CH 2 Cl 2, 25 °C O O O H O NH R 1 x R 2 O a) b) BnOH (1 eq) (-) sparteine (1 eq)/TU3(1 eq) R 2 = O-alkyl = O(CH 2 ) 2 Cl = OCH 2 CHCH 2 = O(CH 2 ) 2 NHCO 2 tBu = NHPh R 1 = H or CH 3
x y 57 thus synthesized. [START_REF] Edward | Organocatalytic synthesis of quinine-functionalized poly(carbonate)s[END_REF] Copolymerization of MTC-Q with MTC-bocG afforded, after deprotection, water-soluble cationic poly[(MTC-Q)-b-(MTC-G)] copolymers (Mn = 3,900 g.mol -1 ; Đ = 1.5), functionalized with both quinine and pendant guanidinium groups (Scheme 52).

Scheme 52. DBU/TU-catalyzed synthesis of (co) polymers bearing both quinine and guanidinium groups. [START_REF] Edward | Organocatalytic synthesis of quinine-functionalized poly(carbonate)s[END_REF] The benzyl-protected dihydroxylated bicyclic carbonate, namely, 9-phenyl-2,4,8,10 tetraoxaspiro [START_REF] Dalko | Comprehensive enantioselective organocatalysis: catalysts, reactions, and applications[END_REF][START_REF] Dalko | Comprehensive enantioselective organocatalysis: catalysts, reactions, and applications[END_REF]undecan-3-one (PTO), was readily polymerized in dichloromethane at r.t., using 1,4-butanediol, MeO-PEO-OH or HO-PCL-OH as (macro)initiator and DBU/TU3 as catalytic system (Scheme 53). [START_REF] Brannigan | Block copolymer materials from the organocatalytic ring-opening polymerization of a pentaerythritol-derived cyclic carbonate[END_REF][START_REF] Aguirre-Chagala | Organocatalytic copolymerization of a cyclic carbonate bearing protected 2,2-bis(hydroxymethyl) groups and D,L-lactide. Effect of hydrophobic block chemistry on nanoparticle properties[END_REF] A PEO-b-poly(PTO) copolymer of controlled molar mass, Mn = 6,300 g.mol -1 and Đ = 1.06, was obtained using MeOPEO-OH as macroinitiator.

Sequential copolymerization with D,L-lactide provided an amphiphilic functional PEO-bpolyester-polycarbonate (Mn = 12,000 g.mol -1 ; Đ = 1.07). The physicochemical properties of these copolymers could be tuned by optimized post-modification, enabling their potential use as biodegradable materials. ring-opening, whereas those of the MODec and MBoc were typical of planar amides, favoring the ring-opening. [START_REF] Blake | Organocatalytic ring-opening polymerization of morpholinones: new strategies to functionalized polyesters[END_REF] Scheme 54. Polymerization of morpholin-2-ones with ROH initiator and catalyzed by DBU/TU3. [START_REF] Blake | Organocatalytic ring-opening polymerization of morpholinones: new strategies to functionalized polyesters[END_REF] Phenol derivatives represent another class of small molecules with H-bond donor capabilities and that can be involved in polymerization reactions, by analogy with the activation of monomer by thiourea derivatives.

Combining DBU and phenol thus proved suitable to catalyze the ROP of -VL and -CL, with 4-biphenylmethanol (BPM) as initiator (Scheme 55). [START_REF] Thomas | Ring-opening polymerization of lactones using supramolecular organocatalysts under simple conditions[END_REF] Reactions were performed in dichloromethane at r.t., under bench conditions (no pre-drying of reactants, 4 Å molecular sieves). Quantitative conversion of -VL was obtained after 24 h, leading to PVL's of controlled molar masses Mn = 1,200 -6,900 g.mol -1 , and narrow dispersities Đ = 1.08-1.12. In contrast the DBU/phenol-catalyzed ROP of -CL required longer reaction times (48 h and 120 h), and reached only 57-79% of conversion. This lower reactivity of -CL compared to -VL, in presence of H-bonding catalysts, was attributed to the occurrence of transesterification in the case of -CL. [START_REF] Lohmeijer | Guanidine and Amidine Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF] Using this strategy, block copolyesters PVL-b-PLA and PCL-b-PLA were successfully prepared. A supramolecular mechanism involving formation of H-bond phenol:monomer and DBU:initiator (or chain-end) complexes was proposed (see insert in Scheme 55). Compared to the DBU/TU catalytic system, DBU/phenol has the practical advantage that phenol derivatives are commercially available, operating at r.t under mild experimental conditions.

Dual activation mechanism

Scheme 57. ROP of LA using acid-base salts as catalytic systems in presence of DMAP/BnOH as initiating system. [START_REF] Miao | New acid/base salts as co-catalysts for the organocatalyzed ring-opening polymerization of lactide[END_REF] Later on, Peruch et al. associated DMAP to TfOH or MSA (0.5 eq.) as a catalytic system for the ROP of L-LA, -CL and -VL initiated by alcohols, in bulk at 100 °C. [START_REF] Kadota | Controlled bulk polymerization of L-lactide and lactones by dual activation with organo-catalytic systems[END_REF] DMAP/DMAP-TfOH appeared as the most active catalytic system, leading to linear and/or starlike PLLA's of molar masses Mn = 3,200 -140,000 g.mol -1 (Đ = 1.10-1.25) and polylactones of Mn = 1,800 -10,800 g.mol -1 (Đ = 1.12-1.38). PCL-b-PLLA and PVL-b-PLLA block copolymers of Mn = 12,800 (Đ = 1.36) and 14,300 g.mol -1 (Đ = 1.16), respectively, were successfully synthesized through sequential polymerization, with lactones being first polymerized.

An ionic catalyst with the cation and the anion activating both the monomer and the ROH initiator has also been described by Coulembier et al. [START_REF] Coulembier | An imidazole-based organocatalyst designed for bulk polymerization of lactide isomers: inspiration from Nature[END_REF][START_REF] Guillerm | Ammonium betaines: efficient ionic nucleophilic catalysts for the ring-opening polymerization of L-lactide and cyclic carbonates[END_REF] More specifically, an imidazolium trifluoroacetate salt was employed for the controlled ROP of L-LA, using BnOH as initiator (Scheme 58). [START_REF] Coulembier | An imidazole-based organocatalyst designed for bulk polymerization of lactide isomers: inspiration from Nature[END_REF] A PLLA of molar mass Mn = 8,700 g.mol -1 (Đ = 1.28) was rapidly obtained after 3.7 h of reaction in bulk at 140 °C (84% of monomer conversion). Polymerization proceeded by a bifunctional mechanism resembling that operating with lipase B (produced by the yeast Candida antartica), where the imidazolium activates the carbonyl moiety through Hbonding, while the trifluorocarboxylate interacts with the initiating/propagating alcohol (see insert in Scheme 58).

In the same manner meta-(trimethylammonio)phenolate betaine proved efficient catalyst for the ROP of L-LA, -CL and TMC in chloroform at 21 °C using 1-pyrenemethanol as initiator, leading to polymers of Mn = 12,300 -48,000 g.mol -1 (Đ = 1.04-1.47). [START_REF] Guillerm | Ammonium betaines: efficient ionic nucleophilic catalysts for the ring-opening polymerization of L-lactide and cyclic carbonates[END_REF] In this case, the ammonium betaine was proposed to activate the initiating and propagating alcohol site, allowing for a rapid and controlled polymerization. The ROP of TMC was also investigated using an acid-base binary organocatalytic system composed of the guanidine MTBD and the acid TfOH, with BnOH as the initiator. [START_REF] Wang | A baseconjugate-acid pair for living/controlled ring-opening polymerization of trimethylene carbonate through hydrogen-bonding bifunctional synergistic catalysis[END_REF] Using an optimal ratio MTBD/TfOH of 2/1, well-defined PTMC's of molar masses ranging from 2,800 -14,400 g.mol -1 (Đ = 1.1-1.4) were obtained in dichloromethane at r.t.. Moreover, this MTBD/TFA catalytic system allowed synthesizing a PTMC-b-PLLA diblock copolymer of Mn = 13,000 g.mol -1 (Đ = 1.2) upon sequential polymerization of TMC and L-LA.

Recently, Cramail, Taton et al. reported the first example of polyaldol synthesis via a stepgrowth cross-polymerization of bis-aldehydes and bis-ketones using a combination of pyrrolidine and acetic acid as catalytic system (Scheme 59). [START_REF] Liu | Polyaldol synthesis by direct organocatalyzed crossed polymerization of bis(ketones) and bis(aldehydes)[END_REF] After 72 h of reaction in THF at r.t., polyaldols of Mn = 6,700 g.mol -1 and Mw = 12,900 g.mol -1 (Đ = 1.9) could be prepared. The presence of acetic acid (AcOH, 150 mol% relative to the monomers) allowed increasing the molar masses, while broadening the molar mass distribution (Mn = 11,100 g.mol -1 and Mw = 55,000 g.mol -1 ; Đ = 2.6-4.9). Analysis of as-obtained polymers by NMR spectroscopy confirmed the formation of -keto alcohol units (polyaldols), but also evidenced the presence of 20-30% of conjugated ketones, as a result of the dehydration of -keto alcohol units (see insert in Scheme 59). Polymerization between bis(ketone)s and bis(aldehydes)s forming poly(aldol)s. [START_REF] Liu | Polyaldol synthesis by direct organocatalyzed crossed polymerization of bis(ketones) and bis(aldehydes)[END_REF] The direct bulk aldol step-growth polymerization of acetaldehyde at r.t., catalyzed by proline-derived catalysts, in presence of acetic acid was also described (Scheme 60). [START_REF] Kusumoto | Direct aldol polymerization of acetaldehyde with organocatalyst/Bronsted acid systems[END_REF] Only polyene-type (I; Mn = 200 g.mol -1) and polyol-type (II; Mn = 200 -300 g.mol -1 and Mw = 800 -1,100 g.mol -1 ) oligomers were obtained in bulk at r.t.. Poly(vinyl alcohol-coethylene)-type oligomers were subsequently prepared, under hydrogenation and deacetalization conditions. Scheme 60. Direct aldol polymerization of acetaldehyde using proline derivatives and acetic acid as catalysts. [START_REF] Kusumoto | Direct aldol polymerization of acetaldehyde with organocatalyst/Bronsted acid systems[END_REF] Cramail, Taton et al. also reported the synthesis of unprecedented hyperbranched polyacetals (HBPA's) with a degree of branching (DB) equal to unity, by an acid-catalyzed stepgrowth polymerization of a simple AB2 type monomer. As mentioned above, instead of using a bi-component system promoting a dual activation, bifunctionality can be provided from one single molecule. For instance, the asymmetric step-growth polymerization of prochiral bis-anhydrides with diols was investigated using by a cinchona-type organocatalyst in THF at 25 °C (Scheme 62). [START_REF] Martin | Organocatalyzed stepgrowth polymerization through desymmetrization of cyclic anhydrides: synthesis of chiral polyesters[END_REF] The reaction achieved 90% of conversion after seven days, leading to chiral polyester oligomers with a DP up to 14 (Mn = 400 -8,900 g.mol -1 ; Đ = 1.25-2.15), and with high levels of stereocontrol (89% of enantiomeric excess). Such a polymerization was thought to proceed through transfer of the chirality from the cinchona alkanoïd catalyst to the growing polymer chain, with the catalyst acting as dual activator of both the alcohol and the anhydride. PLA's with molar masses Mn ≤ 30,000 g.mol -1 and Đ = 1.14-1.31. [START_REF] Kiesewetter | Organocatalysis: opportunities and challenges for polymer synthesis[END_REF][START_REF] Brown | Zwitterionic ring-opening polymerization for the synthesis of high molecular weight cyclic polymers[END_REF] A zwitterionic ringopening polymerization (= ZROP) mechanism was put forward with the carbene playing the role of a nucleophile (Scheme 64a). The same reaction was reported later on, using methyl-3menthoxymethylimidazol-2-ylidene (NHC16 in Figure 2), also leading to cyclic PLA's but of higher molar masses (Mn = 36,500 g.mol -1 and Đ = 1.89). [START_REF] Prasad | Zwitterionic ring opening polymerization of lactide by metal free catalysts: production of cyclic polymers[END_REF] The ZROP of lactones by different NHCs also produced high molar mass cyclic polylactones. For instance, the ROP of -CL and -VL in THF at r.t., from 1-isopropyl-3benzylimidazol-2-ylidene (NHC14 in Figure 2) or methyl-3-menthoxymethyl imidazol-2-ylidene (NHC16) led to cyclic homopolyesters and copolyesters, with molar mass Mn = 31,900 g.mol -1 but exhibiting broad dispersities (Đ = 2.27). [START_REF] Prasad | Syntheses of cyclic poly(lactones) by zwitterionic ringopening polymerization catalyzed by N-heterocyclic carbene[END_REF][START_REF] Zhang | Unsymmetrical N-heterocyclic carbene: 1isopropyl-3-benzylimidazol-2-ylidene as catalyst for ring-opening polymerization of εcaprolactone[END_REF] PCL's with Mn values up to 114,000 g.mol - 1 were also synthesized in toluene at r.t., when using 1,3,4,5-tetramethylimidazol-2-ylidene (NHC18) or 1,3-bis(isopropyl-4,5-dimethyl)imidazol-2-ylidene (NHC19). [START_REF] Brown | Zwitterionic ring-opening polymerization: models for kinetics of cyclic poly(caprolactone) synthesis[END_REF] Molar masses were found to increase with increasing monomer conversion, but remained significantly higher than values predicted from the initial monomer-to-initiator ratio, presumably due to the occurrence of several competitive polymerization mechanisms.

Modeling experiments revealed that initiation of lactones was a slow and reversible process, while propagation was rapid (route A in Scheme 65) but accompanied by several side reactions. [START_REF] Brown | Zwitterionic ring-opening polymerization: models for kinetics of cyclic poly(caprolactone) synthesis[END_REF][START_REF] Acharya | Experimental and computational studies on the mechanism of zwitterionic ring-opening polymerization of δ-valerolactone with N-heterocyclic carbenes[END_REF] These include intramolecular backbiting of alkoxide chain-ends on internal

Nu R 1 R 2 O O R 1 R 2 O O Nu R 1 R 2 O O Nu 1,4 addition Nu O R 2 O R 1 n R 1 R 2 O O n-1 R 1 O R 2 O R 1 = H; acrylates = CH 3 ; methacrylates linear polymers Nu O R 2 O R 1 n R 1 O R 2 O n R 1 CO 2 R 2 CO 2 R 2 R 1 R 1 R 2 O 2 C R 2 O 2 C R 1 cyclic polymers Nu = Y X N R 3 R 4 X = N, S Y = C, N R 1 R 2 NHCs Amidines P NR'"' 2 R' 2 N NR" 2 N R Phosphazenes R 2 N N R 1 R 3 N N S m

Isothioureas

ester moieties of growing zwitterions 29 (route B), and intermolecular attack of active zwitterions on internal esters of other zwitterions (route C), as well as cyclized chains (route D). [START_REF] Brown | Zwitterionic ring-opening polymerization: models for kinetics of cyclic poly(caprolactone) synthesis[END_REF] This caused the formation of both higher molar masses linear (30 in Scheme 65) and cyclic chains (31 and 33). Attack of active zwitterionic imidazolium alkoxides on cyclized chains (route D) was proposed to be a key step, leading to high molar masses cyclic PCL's [START_REF] Foropoulos | Synthesis, properties, and reactions of bis((trifluoromethyl)sulfonyl) imide, (CF3SO2)2NH[END_REF].

Scheme 65. Proposed mechanism for ZROP of -CL using NHCs and related side reactions. [START_REF] Brown | Zwitterionic ring-opening polymerization: models for kinetics of cyclic poly(caprolactone) synthesis[END_REF][START_REF] Acharya | Experimental and computational studies on the mechanism of zwitterionic ring-opening polymerization of δ-valerolactone with N-heterocyclic carbenes[END_REF] The direct ROP of other monomers, such as cyclic siloxanes, cyclic phosphates and Ncarboxyanhydrides initiated by NHCs was also reported. For instance, Waymouth et al.

described the NHC-initiated ZROP of 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane (TMOSC, see Figure 3) ) using a series of NHCs in THF at r.t. [START_REF] Brown | Zwitterionic polymerization to generate high molecular weight cyclic poly(carbosiloxane)s[END_REF] 1,3,4,5-Tetramethylimidazol-2-ylidene (NHC18) and 1,3-bis-isopropyl-4,5-dimethylimidazol-2-ylidene (NHC19) proved the most efficient initiators (> 95% of monomer conversion reached after 5 sec), yielding cyclic PTMOSC's, i.e. poly(carbosiloxane)s, of very high molar masses, Mn = 587,000 g.mol -1 (Đ = 1.74) and 942,000 g.mol -1 (Đ = 3.18), respectively. The more sterically hindered 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene (NHC8) did not exhibit any activity for this polymerization, presumably due to its bulkiness. Under the same conditions, both NHC18 and NHC19 were also found to mediate the ZROP of 2-isopropoxy-2-oxo-1,3,2-dioxaphospholane (iPP, see Figure 3). The polymerization rapidly occurs to generate cyclic poly(alkylene phosphates), namely poly(iPP), of molecular weights ranging from 55,000 to 202,000 g.mol -1 (Đ < 1.3). [START_REF] Stukenbroeker | Synthesis and topological trapping of cyclic poly(alkylene phosphates)[END_REF] new ref Waymouth Macromol 2014. In the same manner, the ZROP of Ncarboxyanhydrides initiated with various imidazol-2-ylidenes (NHCs 2-3; 7-8), allowed synthesizing cyclic polypeptoids in a controlled manner (Mn = 3,000 -30,000 g.mol -1 and Đ = 1.2). [START_REF] Guo | N-heterocyclic carbene-mediated zwitterionic polymerization of N-substituted N-carboxyanhydrides toward poly(α-peptoid)s: kinetic, mechanism, and architectural control[END_REF] The NHC-initiated zwitterionic polymerization of (meth)acrylic monomers, through repeated and selective 1,4-conjugate addition reactions, was also described. In 2012, Chen et al. reported the direct polymerization of both MMA and a biomass-derived cyclic analog, methyl--methylene--butyrolactone (MMBL). Three different NHCs, namely, NHC tBu (= NHC3), NHC Mes (= NHC7) and 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (TPT = NHC26), were tested as chain starters, in DMF at r.t. [START_REF] Zhang | Conjugate-addition organopolymerization: rapid production of acrylic bioplastics by N-heterocyclic carbenes[END_REF][START_REF] Zhang | Organocatalytic conjugate-addition polymerization of linear and cyclic acrylic monomers by N-heterocyclic carbenes: mechanisms of chain initiation, propagation, and termination[END_REF] NHC tBu proved the most active (NHC tBu > NHC Mes > TPT), converting up to 10,000 eq. of MMBL in 5 min only, and leading to PMMBL of Mn = 70,000 -85,000 g.mol -1 (Đ = 1.36-2.11), regardless of the [MMBL]0/[NHC]0 ratio employed. In contrast to MMBL, the polymerization of MMA could only be induced by NHC tBu , leading to PMMA of Mn = 33,300 g.mol -1 (Đ = 1.99). The polymerization was proposed to proceed through a 1,4-conjugate addition reaction of the NHC onto MMA, generating an imidazolium enolate 34 (Scheme 66), that can propagate the polymerization. [START_REF] Zhang | Conjugate-addition organopolymerization: rapid production of acrylic bioplastics by N-heterocyclic carbenes[END_REF][START_REF] Zhang | Organocatalytic conjugate-addition polymerization of linear and cyclic acrylic monomers by N-heterocyclic carbenes: mechanisms of chain initiation, propagation, and termination[END_REF][START_REF] Ottou | Cyclodimerization versus polymerization of methyl methacrylate induced by N-heterocyclic carbenes: a combined experimental and theoretical study[END_REF] In sharp contrast, under the same conditions (in DMF, r.t), Taton et al. found that the reaction of 1,3-bis-(diiso-propyl)imidazol-2-ylidene (NHC iPr = NHC2) with MMA afforded an unprecedented imidazolium enolate cyclodimer (NHC iPr :MMA = 1:2), [START_REF] Ottou | Cyclodimerization versus polymerization of methyl methacrylate induced by N-heterocyclic carbenes: a combined experimental and theoretical study[END_REF] evidencing the remarkable selectivity of NHCs towards MMA. Scheme 66. NHC reactions towards MMA. [START_REF] Ottou | Cyclodimerization versus polymerization of methyl methacrylate induced by N-heterocyclic carbenes: a combined experimental and theoretical study[END_REF] In a recent addition, TPT was found to efficiently catalyze the conversion of dimethacrylates into unsaturated polyesters of Mw up to 28,400 g.mol -1 and Đ = 1.1-1.9, in toluene at 80 °C (Scheme 67). [START_REF] Hong | Proton-transfer polymerization (HTP): converting methacrylates to polyesters by an N-heterocyclic carbene[END_REF] This reaction was thought to involve the initial conjugate addition of TPT onto the monomer, forming the triazolium enolate 35 (Scheme 67). Upon proton transfer, the resulting nucleophilic diaminoalkene 36 could react in a 1,4-fashion with another monomer molecule to yield the new triazolium enolate 37. This new step-growth polymerization, referred to as proton (H) transfer polymerization (HTP), was proposed to proceed by repeated conjugate addition / proton transfer / conjugate addition / proton transfer/ NHC release sequences. [START_REF] Hong | Proton-transfer polymerization (HTP): converting methacrylates to polyesters by an N-heterocyclic carbene[END_REF] Interestingly, the same TPT carbene was employed by Deactivation of the NHC by intra-molecular C-H insertion of the carbene center into a CH3 Mes was proposed to account for the inactivity of the diazepinium-based carbene (Scheme 69). [START_REF] Naumann | Polymerization of methyl methacrylate by latent pre-catalysts based on CO2-protected N-heterocyclic carbenes[END_REF][START_REF] Holdroyd | Intramolecular C-H insertion in ringexpanded N-heterocyclic carbenes[END_REF] Scheme 69. Intra-molecular C-H bond insertion of diazepinium-type NHCs. [START_REF] Naumann | Polymerization of methyl methacrylate by latent pre-catalysts based on CO2-protected N-heterocyclic carbenes[END_REF][START_REF] Holdroyd | Intramolecular C-H insertion in ringexpanded N-heterocyclic carbenes[END_REF] Scheme modified In contrast, NHC tBu -CO2 enabled the direct polymerization of MMA in solution and in bulk at 85 °C. [START_REF] Naumann | Polymerization of methyl methacrylate by latent pre-catalysts based on CO2-protected N-heterocyclic carbenes[END_REF] With this NHC-precursor, PMMA's with molar masses Mn of 25,000 g.mol -1 could be obtained, with a rather broad molar mass distribution (Đ = 2.6-2.7), these values (Mn and Đ) being comparable to those reported by Chen et al. [START_REF] Zhang | Conjugate-addition organopolymerization: rapid production of acrylic bioplastics by N-heterocyclic carbenes[END_REF] for the polymerization of MMA in DMF at r.t. with the bare NHC tBu . Tetrahydropyrimidinium-2-carboxylates were also suitable initiators for the polymerization of MMA in DMSO (Mn = 80,000 g.mol -1 and Đ = 1.82). [START_REF] Naumann | Polymerization of methyl methacrylate by latent pre-catalysts based on CO2-protected N-heterocyclic carbenes[END_REF] In this case, however, the NHC-CO2 adducts likely operated as latent Brønsted bases rather than nucleophiles. Indeed, it was proposed that these highly basic NHCs could deprotonate DMSO, generating the so-called "dimsyl" anion (40) that could further initiate the anionic polymerization of MMA (Scheme 70) with the tetrahydropyrimidinium as bulky counteraction (not a ZP in this case). Alternatively, a GTP like mechanism could also be operative, where the initiator would be a NHC-enolate adduct (instead of an ion pair). In this case, the NHC would be transferred to the growing enolate chain end, based on the higher electrophilicity of these saturated NHCs. The same group also described the direct ROP of -caprolactam (-CLA) initiated by various NHC-CO2 adducts (NHCs 22-25; 34-38 in Figure 2). [START_REF] Naumann | Polymerization of ε-caprolactam by latent precatalysts based on protected N-heterocyclic carbenes[END_REF] Polymerizations were conducted under rather harsh conditions, i.e. in bulk at 180 °C, allowing the preparation of polyamide-6 (PA-6) (Scheme 71). [START_REF] Naumann | Polymerization of ε-caprolactam by latent precatalysts based on protected N-heterocyclic carbenes[END_REF]. Importantly, this polymerization also evidenced that NHC-CO2 adducts and related free NHCs thus delivered could withstand such a high temperature. Tetrahydropyrimidinium-based carbenes were more active than imidazol(in)ium compounds, giving after 45 min PA-6 of molar masses Mη = 140,000 -420,000 g.mol -1 (as determined by viscometric measurements). The broadened dispersity observed and the high molar masses obtained suggested incomplete initiation or other side reactions, such as chain transfer, as expected for the anionic-like ROP of -CLA. [START_REF] Wichterle | The anionic polymerization of caprolactam[END_REF][START_REF] Mikheyev | Chain transfer in anionic polymerization of caprolactam[END_REF] Nevertheless, the inhomogeneity of the system caused by the precipitation of PA-6 from the monomeric melt, could also explain these results.

The activity of tetrahydropyrimidinium carboxylates was related to the basicity of corresponding free carbenes, suggesting a basic mechanism where the monomer was anionically polymerized (see insert in Scheme 71). Thus, polymerization started by deprotonation of the lactam by the NHC, forming the activated monomer amide anion 42 that could initiate the ROP (see insert in Scheme 71).

NHC-CO2 adducts were also effective pre-catalysts for the bulk homopolymerization of laurolactam (LL), leading to PA-12 of molar masses Mn of 14,000 g.mol -1 (Đ = 2.2-3.1). [START_REF] Naumann | Anionic ring-opening homo-and copolymerization of lactams by latent, protected Nheterocyclic carbenes for the preparation of PA 12 and PA 6/12[END_REF] The copolymerization of LL and -CLA gave a gradient copolymer, -CLA being incorporated preferentially at the onset of the polymerization. Scheme 71. Anionic ROP of lactams (-CLA and LL) using NHCcarboxylates. [START_REF] Naumann | Polymerization of ε-caprolactam by latent precatalysts based on protected N-heterocyclic carbenes[END_REF][START_REF] Naumann | Anionic ring-opening homo-and copolymerization of lactams by latent, protected Nheterocyclic carbenes for the preparation of PA 12 and PA 6/12[END_REF] Imidazolium carboxylates, including NHC Mes -CO2, 1,3-bis(2,6-diisopropylphenyl) imidazolium-2-carboxylate (NHC iPrPh -CO2, NHC8 in Figure 2) and 1,3-bis(2,6bis(diphenylmethyl)-4-methylphenyl) imidazolium-2-carboxylate (NHC PrPh *-CO2; NHC9), were applied to the ROP of -BL in THF or/and toluene at 60 °C. [START_REF] Brule | Polymerization of cyclic esters using N-heterocyclic carbene carboxylate catalysts[END_REF] The most bulky NHC iPrPh -CO2 and NHC iPrPh *-CO2 proved less reactive than NHC Mes -CO2, leading to a lesser control over molar masses. The polymerization induced by NHC Mes -CO2 proceeded in 10 min (80%), yielding a PBL of 9,200 g.mol -1 (Đ = 1.46). Molar masses increased with the initial [-BL]0/[NHC]0 ratio, and led to PBL's of Mn = 6,300 -37,800 g.mol -1 (Đ = 1.12-1.47), suggesting a controlled polymerization.

D,L-LA was also polymerized under the same conditions, PLA's with Mn = 6,300 -13,400 g.mol - 1 (Đ = 1.22-1.83) being quantitatively prepared after 30 min. Statistical PBL-co-PLA copolymers could also be prepared by simultaneous copolymerization, while sequential copolymerization (-BL first polymerized) only led to PBL-based homopolymers. [START_REF] Brule | Polymerization of cyclic esters using N-heterocyclic carbene carboxylate catalysts[END_REF] Protected NHCs were also applied to the thermal curing of bisphenol A diglycidyl ether (BADGE), using phthalic acid anhydride (PhA) or hexahydrophthalic anhydride (HHPA) as with Mn ranging from 32,000 to 53,000 g.mol -1 (Đ ≤ 1.6) were obtained in dichloromethane or THF/ dichloromethane, but no clear relationship was found between these molar masses and the initial ratio [LA]0/[DBX]0 (DBX= DBU or DBN). A similar behavior was observed for the ZROP of lactide by NHCs, which was attributed to a slow and reversible initiation step. [START_REF] Brown | Zwitterionic ring-opening polymerization for the synthesis of high molecular weight cyclic polymers[END_REF][START_REF] Culkin | Zwitterionic polymerization of lactide to cyclic poly(lactide) by using N-heterocyclic carbene organocatalysts[END_REF] Theoretical studies supported the occurrence of a zwitterionic mechanism through the formation of amidinium alkoxide zwitterions, generating a mixture of linear and cyclic PLA's, as observed for the ZROP with NHCs (see Scheme 65). [START_REF] Brown | Zwitterionic ring-opening polymerization for the synthesis of high molecular weight cyclic polymers[END_REF][START_REF] Culkin | Zwitterionic polymerization of lactide to cyclic poly(lactide) by using N-heterocyclic carbene organocatalysts[END_REF]. Compared to DBU initiator, isothioureas (see Scheme 64 for related structure) were demonstrated to be more selective for producing cyclic PLA's (Mn = 48,000 -66,000 g.mol -1 (Đ ≤ 1.6) without appreciable linear contaminants. [START_REF] Zhang | Zwitterionic ring-opening polymerization with isothioureas[END_REF] Poly(3-hydroxybutyrate)s (PHB's), with ,-DBU-N-acylcrotonate chain-ends and molar masses Mn = 2,600 -9,400 g.mol -1 (Đ = 1.16-1.52), were achieved by Guillaume, Carpentier et al., when performing the ROP of -BL in bulk at 60 °C with DBU (Scheme 73). [START_REF] Jaffredo | Controlled ROP of β-butyrolactone simply mediated by amidine, guanidine, and phosphazene organocatalysts[END_REF] While similar polymers were obtained when using the guanidine TBD, higher molar masses with a narrow dispersity were obtained with the BEMP phosphazene (Mn = 4,200 -24,600 g.mol -1 ; Đ = 1.07- In summary, small organic molecules were recently employed as direct initiators, i.e. in absence of any other co-activator, to trigger not only the (Z)ROP of cyclic monomers (lactide, lactones and lactams), but also the zwitterionic vinyl polymerization of (meth)acrylic monomers.

For these organo-initiated polymerizations, metal-free activators included Lewis bases such as NHCs, but also amidines or phosphazenes. Only one example of an acid initiator (i.e.

(di)hydrogenphosphate) was reported. However, these direct polymerizations were usually not very well-controlled, giving rather high dispersities. In addition, the reactant structures (initiator and monomer) drastically influenced the topology of the final polymer, with either cyclic or/and linear structures being generated.

Concluding remarks and future challenges

This review has encompassed the contributions over the past 4 years regarding organopolymerization reactions. The diverse reactivity and selectivity of small organic molecules have been efficiently transferred in recent years, from organocatalysis in organic synthesis to precision macromolecular chemistry. Organic catalysts are generally easily available and can improve polymerization kinetics while providing a good control over the polymerization process by favouring selective propagation steps over side reactions. In some cases, they exhibit performances that compare or even outperform those of commonly used organometallic catalysts, making them powerful alternatives for the preparation of polymers free of any metallic residues. Some of metal-free polymeric materials have demonstrated valuable physico-chemical properties as well as in vivo biocompatibility, making them promising materials for applications in tissue engineering, drug delivery and regenerative medicine. For instance, cationic polycarbonates were found to form dynamic micelles that allowed bacterial cell interactions to be increased and proved strong antimicrobial films against drug-resistant bacteria. [START_REF] Qiao | Highly dynamic biodegradable micelles capable of lysing Gram-positive and Gram-negative bacterial membrane[END_REF][START_REF] Ding | Antibacterial and antifouling catheter coatings using surface grafted PEG-b-cationic polycarbonate diblock copolymers[END_REF][START_REF] Chan | Chemically modifiable Nheterocycle-functionalized polycarbonates as a platform for diverse smart biomimetic nanomaterials[END_REF] Copolymers such as PEO-polycarbonates or PEO-polyphosphoesters were demonstrated to self-assemble into micelles with high loading capacity of the hydrophobic anticancer drug paclitaxel (PTX). These PTX-loaded micelles were preferentially accumulated in tumor tissue with limited distribution to healthy organs. [START_REF] Lee | The use of cholesterol-containing biodegradable block copolymers to exploit hydrophobic interactions for the delivery of anticancer drugs[END_REF][START_REF] Zou | Poly(ethylene oxide)-block-polyphosphoester-graft-paclitaxel conjugates with acid-labile linkages as a pHsensitive and functional nanoscopic platform for paclitaxel delivery[END_REF][START_REF] Zhang | Poly(ethylene oxide)-block-polyphosphester-based paclitaxel conjugates as a platform for ultra-high paclitaxel-loaded multifunctional nanoparticles[END_REF] These few examples of metal-free synthesized polymers in bio-applications illustrate the expansion of organopolymerization, from a precise chemistry to specific applications.

The absence of metal traces is not only sought after in biomedicine, but also in applications such as packaging and microelectronics, where metallic residues could interfere with the polymer end use. Moreover, it should be kept in mind that these residues could drastically shorten the polymer lifespan due to an increase in degradation during either polymer processing or weathering. [START_REF] White | Weathering of polymers: mechanisms of degradation and stabilization, testing strategies and modelling[END_REF] Although toxicity issues are often neglected concerning organic activators, health and environmental risks of these compounds in the final material are so far showed cytotoxicity against human hepatocellular carcinoma cells only at a high concentration. [START_REF] Nachtergael | Organocatalysis paradigm revisited: are metal-free catalysts really harmless?[END_REF] As also emphasized in this review, most of organo-catalyzed and organo-initiated polymerization reactions are based on the ring-opening of carbonyl-containing monomers such as, cyclic esters and cyclic carbonates. Nevertheless, few organic compounds, such as Nheterocyclic carbenes (NHCs) have allowed to broaden the scope of organocatalyzed polymerization with the ROP of cyclic ethers and siloxanes, the GTP of alkyl (meth)acrylates, as well as some step-growth polymerizations. So far, and to the best of our knowledge, the field of organopolymerization of less activated non-polar monomers, e.g. conjugated dienes or styrenics is still unexplored and could become one of the next challenges to be addressed. Only very recently, π-conjugated copolymers based on perfluoroaromatics and thiophene or alkynes moieties have been successfully synthesized in absence of any transition metals. [START_REF] Wang | Transition-metal-free synthesis of alternating thiopheneperfluoroarene copolymers[END_REF][START_REF] Sanji | Transition-metal-free controlled polymerization for poly(p-aryleneethynylene)s[END_REF] The polymerization relies on the fluoride-catalyzed activation of Si-C bonds on one hand and on nucleophilic aromatic substitutions on the other hand.

Spectacular achievements, such as the metal-free ring-opening metathesis polymerization (ROMP) of norbornene [START_REF] Ogawa | Metal-free ring-opening metathesis polymerization[END_REF] and the metal-free atom transfer radical polymerization (ATRP) of methacrylates [START_REF] Treat | Metal-free atom transfer radical polymerization[END_REF] and acrylonitrile, [START_REF] Pan | Photoinduced metal-free atom transfer radical polymerization of acrylonitrile[END_REF] have also been reported using organic photo-redox mediators. These two reactions that are traditionally catalyzed by transition metals could be promoted by electron transfer between the photo-sensitizer and the initiator upon light exposure. These recent examples attest to the exciting potential of metal-free techniques for the synthesis of a wide variety of functional polymeric materials.

4 1. 1 . 4 1. 2 . 5 1. 3 . 11 2. 1 . 12 2. 2 . 21 2. 3 .

 414253111122213 Introduction .......................................................................................................................................... 3 Scope of organic activators, monomer candidates and general polymerization mechanisms ........................................................................................................................................ Organic activator platform ............................................................................................... Scope of monomers .............................................................................................................. Polymerization mechanisms ............................................................................................ 5 Polymerization catalyzed by Brønsted and Lewis acids ............................................... Sulfonic and sulfonimide acids ..................................................................................... Carboxylic acid compounds ........................................................................................... Phosphoric acid and their derivatives ....................................................................... 23 Polymerization catalyzed by phosphorous-containing Brønsted and Lewis bases: phosphazenes and phosphines ................................................................................................. 26 3.1. Phosphazenes ..................................................................................................................... 27

Figure 1 .

 1 Figure 1. Evolution of publications on the topic of metal-free polymerizations since 1995.

Figure 2 .

 2 Figure 2. Representative families of organic molecules used as catalysts or initiators for organopolymerization (in gray: compounds

Figure 3 .

 3 Figure 3. Representative monomers that were subjected to an organopolymerization pathway (in gray: monomers already discussed in

2 or 4 Scheme 1 .Scheme 2 .

 2412 Scheme 1. Electrophilic AMM mechanism for: a) the ROP of cyclic esters (e.g. LA); b) the ROP of cyclic ethers (e.g. EO); and c) the GTP of (meth)acrylates (e.g. MMA); E and E* represent a Brønsted and a Lewis acid, respectively.

Scheme 4 .

 4 Scheme 4. Basic actived initiator/chain-end mechanism (ACEM) in: a) the ROP of cyclic monomers and b) the associative or dissociative GTP of (meth)acrylates; B is a Brønsted base and Nu is a nucleophile (Lewis base).

3 B 5 .

 35 Scheme 5. Bifunctional activation mechanism AM/ACEM in: a) the ROP of cyclic monomers and b) the GTP of MMA; E and E* represent a Brønsted and a Lewis acid, respectively; B is a Brønsted base and Nu is a nucleophile (Lewis base).

Figure 4 .

 4 Figure 4. Overview of polymers synthesized by an organic acid catalysis.

Figure 5 .

 5 Figure 5. Representative examples of sulfonic and sulfonimide acids derivatives, as well

  -3-substituted-thiophenes (X = Br or Cl Z = R; O or S)

6 -

 6 hydroxyhexanoic acid formed upon hydrolysis of the lactone, leading to either a ROP of ε-CL or to a polycondensation mechanism (Scheme 7).

  ,poly(PDL) (or poly(GB) or poly(AMB) ) (or poly(GB) or poly(AMB)

9 )Scheme 9 .

 99 Scheme 9. ROP of trimethylene carbonate catalyzed by methanesulfonic acid: a) AMM with mono-alcohol; b) AMM/ACEM combined bidirectional mechanism.[START_REF] Delcroix | Ring-opening polymerization of trimethylene carbonate catalyzed by methanesulfonic acid: activated monomer versus active chain end mechanisms[END_REF][START_REF] Campos | A new insight into the mechanism of the ring-opening polymerization of trimethylene carbonate catalyzed by methanesulfonic acid[END_REF]

Scheme 14 .

 14 Scheme 14. C5F5-CHTf2-promoted GTP of MA using trialkylsilyl ketene acetals initiators.[START_REF] Takada | Controlled polymerization of methyl acrylate for high-molecular-weight polymers by pentafluorophenylbis(triflyl)methane-promoted group transfer polymerization using triisopropylsilyl ketene acetal[END_REF]

  Scheme 16. a) ROP of -VL in presence of BnOH and triarylsulfonium hexafluorophosphate salts; and b) proton generation upon UV irradiation.[START_REF] Barker | Triarylsulfonium hexafluorophosphate salts as photoactivated acidic catalysts for ring-opening polymerisation[END_REF]

Scheme 17 .

 17 Scheme 17. Brønsted acid-catalyzed polymerization of 2-halogenated thiophenes.[START_REF] Balasubramanian | Chaingrowth cationic polymerization of 2-halogenated thiophenes promoted by Bronsted acids[END_REF]

Figure 6 .

 6 Figure 6. Representative examples of carboxylic acids and monomers employed in organo-catalyzed polymerizations investigated; pKa values are given in H2O.

Scheme 18 .

 18 Scheme 18. Salicylic acid (SAA)-promoted ROP of -CL and -VL using alcohols as initiators.[START_REF] Xu | Controlled/living ring-opening polymerization of εcaprolactone with salicylic acid as the organocatalyst[END_REF]

Figure 7 .

 7 Figure 7. Representative phosphoric acids and monomers polymerized by them, as wellas bifunctional activation of monomer and initiator with Brønsted acid/base pair.[START_REF] Coady | Polymerizing base sensitive cyclic carbonates using acid catalysis[END_REF][START_REF] Makiguchi | Diphenyl phosphate as an efficient acidic organocatalyst for controlled/living ring-opening polymerization of trimethylene carbonates leading to block, end-functionalized, and macrocyclic polycarbonates[END_REF]

Figure 8 .Figure 9 .

 89 Figure 8. Representative examples of phosphazene bases (pKa values in MeCN [66]), and phosphines.

Scheme 20 .Scheme 22 .

 2022 Scheme 20. t-BuP4 promoted anionic graft polymerization of epoxides from acrylamidebased backbone.[START_REF] Zhao | A facile metal-free "grafting-from" route from acrylamide-based substrate toward complex macromolecular combs[END_REF]

  OH + b) Chain growth (with simulataneous proton transfer and transesterification) a) Induction period (transformation of COOH to OH and O -) transesterification (fast) Termination t-BuP 4 -H

  -1 ; Đ = 1.03). Well-defined PEO-b-PCL diblock and PCL-b-PEO-b-PCL triblock copolymers of Mn = 10,900 (Đ = 1.09) and 13,300 g.mol - 1 (Đ = 1.11), respectively, were next generated by addition of ε-CL to the PEO living chains.Pentablock copolymers, PLLA-b-PCL-b-PEO-b-PCL-b-PLLA (Mn = 16,100 g.mol -1 ; Đ = 1.11), were even synthesized by introduction of LLA (nearby complete conversion of LLA within 10 min). It was however essential to follow the EO/ε-CL/L-LA addition in order to avoid any interor intra-macromolecular side reactions, which could occur during the ROP of EO with PCL or PLLA macroinitiators. The same group investigated the influence of solvents over such t-BuP2-catalyzed ROP of ε-CL and L-LA at r.t., using primary alcohols as initiators.[START_REF] Alamri | Phosphazene-catalyzed ring-opening polymerization of ε-caprolactone: influence of solvents and initiators[END_REF] While a higher polymerization rate was noted in more polar solvents (dichloromethane vs. toluene), slower polymerization reactions were noted in cyclic ether solvents (THF, 1,4-dioxane vs. toluene). Due to their slight basicity, these solvents might indeed interact with initiating species, thus slowing down the ROP.Phosphazene catalysts were also applied to the ROP of cyclic carbonates. For instance, Guillaume, Carpentier et al. reported the one-pot, two-step sequential copolymerization of TMC and L-LA in toluene at 100 °C, using the phosphazene 2-tert-butylimino-2-diethylamino-1,3dimethylperhydro-1,3,2-diazaphosphorine (BEMP see Figure8; 0.02 eq.) combined with BnOH as initiator.[START_REF] Guerin | Macromolecular engineering via ring-opening polymerization (1): L-lactide/trimethylene carbonate block copolymers as thermoplastic elastomers[END_REF] TMC was first polymerized prior to the addition of L-LA, yielding PTMC-b-PLLA diblock copolymers of molar masses Mn = 36,200 g.mol -1 (Đ = 1.32). While PLLA is a tough and brittle material, PTMC-b-PLLA block copolymers demonstrated better thermo-mechanical properties, evidencing the advantage of the presence of a rubbery PTMC sequence.

(

  Scheme 24. Sequential ROP of TMC in the presence of native starch granules (anhydroglucose unit) using phosphazenes as catalysts.[START_REF] Samuel | Highly efficient metal-free organic catalysts to design new environmental-friendly starch-based blends[END_REF]

Figure 8 )

 8 Figure8) could efficiently catalyze the GTP of MMA and tert-butyl acrylate (tBuA), using 1methoxy-2-methyl-1-[(trimethylsilyl)oxy]prop-1-ene (MTS) as initiator (Scheme 28).[START_REF] Fèvre | Tris(2,4,6-trimethoxyphenyl)phosphine (TTMPP) as potent organocatalyst for group transfer polymerization of alkyl (meth)acrylates[END_REF] GTP experiments could be conducted in bulk or in THF at r.t. This TTMPP-catalyzed GTP provided poly[alkyl(meth)acrylate]s of Mn = 3,000 -10,600 g.mol -1 (Đ < 1.37 in bulk and Đ < 1.45 in THF), in accordance with the initial [monomer]0/ [MTS]0 ratio. Through sequential polymerization, PMMA-b-PtBuA diblock copolymers of controlled molar masses (Mn = 22,600 g.mol -1 ; Đ = 1.[START_REF] Zhang | Organocatalytic conjugate-addition polymerization of linear and cyclic acrylic monomers by N-heterocyclic carbenes: mechanisms of chain initiation, propagation, and termination[END_REF]

carbonates and cyclopropane- 1 , 1 -Figure 10 .

 1110 Figure 10. Electronic properties of N-heterocyclic carbenes (NHCs).[105] scheme modified: bond added on N atom

Figure 11 .

 11 Figure 11. Scope of NHC-catalyzed polymerization reactions.

Figure 12 .

 12 Figure 12. Representative NHC families used in polymerization reactions.

- 1 ;Scheme 31 .

 131 Scheme 30. ROP of cyclic esters and carbonates catalyzed by dissymmetrical NHCs in presence of BnOH as initiator.[START_REF] Wang | Characteristics and mechanism of L-lactide polymerization using N-heterocyclic carbene organocatalyst[END_REF][START_REF] Zhang | Ring-opening copolymerization of ε-caprolactone with 2,2-dimethyltrimethylene carbonate using N-heterocyclic carbene organocatalysts[END_REF]

  PCL oligomers (Mn = 2,300 -11,700 g.mol -1 ; Đ = 1.2-2.2).[START_REF] Hoppe | n-Butyl-3methylimidazolium-2-carboxylate: a versatile precatalyst for the ring-opening polymerization of ε-caprolactone and rac-lactide under solvent-free conditions[END_REF] Other NHC-NHCs 34-38) and tetrahydrodiazepinium-carboxylates (NHCs 39-40), bearing alkyl-and aryl-substituents (see Figure2for related structures), were also screened byBuchmeiser et al. for the ROP of ε-CL, in presence of BnOH as initiator at 70 °C.[START_REF] Naumann | Protected N-heterocyclic carbenes as latent pre-catalysts for the polymerization of ε-caprolactone[END_REF]

Scheme 36 .

 36 Scheme 36. Catalytic cycle for the photoswichtable NHC-catalyzed ROP of ε-CL and-VL in presence of BnOH initiator.[START_REF] Neilson | Photoswitchable NHC-promoted ring-opening polymerizations[END_REF]

1 .

 1 Alkyl amines and pyridine derivativesIn comparison with NHCs, pyridine derivatives, such as dimethyaminopyridine (DMAP) and alkyl(aryl) amines are relatively moderate Brønsted bases, but they can behave as powerful nucleophiles.[START_REF] Murugan | Applications of dialkylaminopyridine (DMAP) catalysts in organic synthesis[END_REF] These nitrogen-containing organocatalysts have been used for the ROP of various heterocycles, including lactide, lactones, carbonates and O-carboxyanhydrides. An overview of recent polymerization reactions catalyzed by amines as well as related pKa values are illustrated in Figure14.

Figure 14 .

 14 Figure 14. Scope of amine-catalyzed polymerization reactions and representative catalysts (pKa values are given in H2O).

Scheme 37 .

 37 Scheme 37. ROP of TMC catalyzed by tertiary amines (TMEDA or PMDETA) and initiated by BnOH.[START_REF] Mingfa | Controllable ring-opening polymerization of trimethylene carbonate catalyzed by aliphatic tertiary amines in the presence of benzyl alcohol or F127[END_REF]

Figure 15 .

 15 Figure 15. Scope of polymerization reactions catalyzed by amidines and guanidines (pKa values are given in MeCN [134]).

Scheme 38 .

 38 Scheme 38. DBU-catalyzed ROP of eutectic melt of L-LA and TMC monomers initiated by

  time. This was explained by the occurrence of side reactions, such as intra-and/or intermolecular transesterifications. Sequential copolymerization of BYP and L-LA (BYP was first polymerized) gave a poly(butynyl phospholane)-b-poly(lactide) (PBYP-b-PLLA)

1 ;

 1 Đ < 1.25). Sequential addition of L-LA, and -DL produced PLLA-b-PDL-b-PLLA triblock copolymers (Mn = 100,000 g.mol -1 ; Đ = 1.4), that exhibited two distinct glass transition temperatures, at -51 and 54 °C for PDL and PLLA domains, respectively, consistent with a microphase segregation.

Scheme 43 .

 43 Scheme 43. TBD-catalyzed: a) ROP of substituted valerolactones (MVL); b) random copolymerization of MVL and -CL.[START_REF] Kim | Facile Synthesis of Functionalized Lactones and Organocatalytic Ring-Opening Polymerization[END_REF]

  In contrast to the numerous examples of ROP of lactones, only a few reports has described the ROP of lactide catalyzed by guanidines. For instance, the ROP of a PEO-based

Scheme 48 .

 48 Scheme 48. TBD-catalyzed step-growth polymerization between a difunctional cyclic carbonate and diamines.[START_REF] Lambeth | Organocatalytic synthesis of (poly)hydroxyurethanes from cyclic carbonates and amines[END_REF]

Figure 16 .Figure 17 .Figure 18 .

 161718 Figure 16. Unimolecular or bimolecular dual activation based on H-bonding interactions.

Scheme 53 .

 53 Scheme 53. Synthesis of amphiphilic PEO-poly(ester carbonate) using DBU/TU3 catalysis.[START_REF] Aguirre-Chagala | Organocatalytic copolymerization of a cyclic carbonate bearing protected 2,2-bis(hydroxymethyl) groups and D,L-lactide. Effect of hydrophobic block chemistry on nanoparticle properties[END_REF]

Scheme 55 .Scheme 56 .

 5556 Scheme 55. DBU/phenol-catalyzed ROP of -VL and -CL in presence of an alcohol initiator.[START_REF] Thomas | Ring-opening polymerization of lactones using supramolecular organocatalysts under simple conditions[END_REF]

Scheme 58 .

 58 Scheme 58. ROP of L-LA using imidazolium-salt or ammonium betaine as catalyst in the presence of ROH.[START_REF] Coulembier | An imidazole-based organocatalyst designed for bulk polymerization of lactide isomers: inspiration from Nature[END_REF][START_REF] Guillerm | Ammonium betaines: efficient ionic nucleophilic catalysts for the ring-opening polymerization of L-lactide and cyclic carbonates[END_REF] Scheme modified

  Scheme 59. Polymerization between bis(ketone)s and bis(aldehydes)s forming poly(aldol)s.[START_REF] Liu | Polyaldol synthesis by direct organocatalyzed crossed polymerization of bis(ketones) and bis(aldehydes)[END_REF]

Scheme 61 .

 61 Scheme 61. Synthesis of hyperbranched polyacetals (HBPAs) via polytransacetalyzation of monomer 1 (DB≈0.5) and polyacetalyzation of monomer 2 (DB = 1) using acid catalysts.[START_REF] Liu | One-pot synthesis and PEGylation of hyperbranched polyacetals with a degree of branching of 100%[END_REF]

Scheme 62 .Scheme 64 .

 6264 Scheme 62. Synthesis of polyesters by desymmetrizing step-growth polymerization of bisanhydrides with diols.[START_REF] Martin | Organocatalyzed stepgrowth polymerization through desymmetrization of cyclic anhydrides: synthesis of chiral polyesters[END_REF]

Scheme 67 .

 67 Scheme 67. Proposed mechanistic scenario for the HTP of dimethacrylates forming unsaturated polyesters.[START_REF] Hong | Proton-transfer polymerization (HTP): converting methacrylates to polyesters by an N-heterocyclic carbene[END_REF]

Scheme 70 .

 70 Scheme 70. Reaction of DMSO with NHCs and possible anionic polymerization.[START_REF] Naumann | Polymerization of methyl methacrylate by latent pre-catalysts based on CO2-protected N-heterocyclic carbenes[END_REF]

Scheme 72 .

 72 Scheme 72.Proposed mechanism for curing of epoxide/anhydride resins using imidazole-based accelerators (a) and NHCs (b).[START_REF] Naumann | Air stable and latent single-component curing of epoxy/anhydride resins catalyzed by thermally liberated Nheterocyclic carbenes[END_REF][START_REF] Buchmeiser | Latent CO2-protected N-heterocyclic carbene-based singlecomponent system-derived epoxy/glass fiber composites[END_REF] new scheme added

1. 22 )Scheme 74 .Scheme 75 .Scheme 76 .Scheme 77 .

 2274757677 Scheme 73. Direct ROP of β-BL and benzyl β-malolactone (MLABe) using BEMP phosphazene, DBU (amidine) and TBD (guanidine).[START_REF] Jaffredo | Controlled ROP of β-butyrolactone simply mediated by amidine, guanidine, and phosphazene organocatalysts[END_REF][START_REF] Jaffredo | Organocatalyzed controlled ROP of βlactones towards poly(hydroxyalkanoate)s: from β-butyrolactone to benzyl β-malolactone polymers[END_REF][START_REF] Jaffredo | Poly(hydroxyalkanoate) block or random copolymers of β-butyrolactone and benzyl β-malolactone: a matter of catalytic tuning[END_REF]

  almost unknown and need to be clarified. For instance, in a very recent contribution, Mespouille, Coulembier, Dubois et al. demonstrated that functionalized thioureas showed an important cytotoxicity on epithelial intestinal cells (FHs74Int) and on differentiated human hepatocellular carcinoma cells (HepaRG).[222] Interestingly, 4-dimethylaminopyridine (DMAP), 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD), and meta-(trimethylammonio)phenolate betaine (m-BE)

  4,6-Me 3 C 6 H 2 (Mes) NHC8 R 1 =R 2 =2,6-iPr 2 C 6 H 3 NHC9 R 1 =R 2 =2,6-iPr 2 4-Me C 6 H 2 NHC10 R 1 =Me ; R 2 =Et NHC11 R 1 =Me ; R 2 =Bu (BMIM) NHC12 R 1 =CH 2 Ph; R 2 =Me NHC13 R 1 =CH 2 Ph ; R 2 =Et NHC14 R 1 =CH 2 Ph; R 2 =iPr NHC15 R 1 =iPr ; R 2 =PhOCH 3 NHC16 R 1 =Me ; R 2 =CH 2 OC 10

MTC-OC 6 F 5 ) poly(MTC-XR')

	O
	O O O O F F F MTC-OC 6 F 5 F F R OH +	PTSA or TfOH CH 2 Cl 2 , r.t	RO	O F O poly(XR' = NH-R 1 O F F F F H O O n R'-XH N-R 1 R 2 NH-PEO	RO	O	O	O	XR' O	n	H
					X
	OCN hexamethylenene NCO + H poly(ethylene oxide) O OH 34	CH 2 Cl 2 , 20°C H O S O O			O N H O		H N O O	34 n
	diisocyanate (HDI)	(PEO 1500 )	X= CH 3 or CF 3		poly(urethane)s (PU's)
				C O-activation H O O H O S O O X	N	vs	O O	C H	N-activation N H O S O O X
												2,2-
	bis(hydroxymethyl)propanoic acid (bis-MPA), denoted as bis-MPA-C6F5, and HDI produced PUs

  The authors suggested that copolymerization occurred via a hybrid mechanism, involving a combination of vinyl polymerization and ROP through alcohol activation by t-BuP4. The activated alcohol would attack the monomer (ε-CL or MMA, with ε-CL being the most reactive), producing two kinds of active centers, 19 and 20 (of alkoxide-and enolate-type, respectively, Scheme 26).

	These active centers would then be able to react with either MMA or ε-CL, forming new
	propagating centers. However, the reaction mechanism was not fully elucidated, and some side
	reactions such as homopolymerization or transesterification reactions might have occurred.

Zhang et al.

thus reported that t-BuP4 could induce the copolymerization of ε-CL with methyl methacrylate (MMA), tert-butyl methacrylate (tBuMA) or glycidyl methacrylate (GMA) in toluene at r.t., in presence of various initiators, i.e., MeOH, ethylene glycol (EG) or ethyl acetate (EA) (Scheme 26).

[START_REF] Yang | Hybrid copolymerization of ε-caprolactone and methyl methacrylate[END_REF][START_REF] Xu | Synthesis of poly(ε-caprolactone-co-methacrylic acid) copolymer via phosphazene-catalyzed hybrid copolymerization[END_REF][START_REF] Yang | One-step synthesis of hyperbranched biodegradable polymer[END_REF]

According to NMR analyses and thermal characterizations, random copolymers, PCL-co-PMMA or PCL-co-PtBuMA, were obtained with molar masses (Mn) in the range 1,800 -50,000 g.mol -1 and Đ = 1.2-3. Hydrolysis of tert-butyl groups of PCL-co-PtBuMA yielded poly(ε-CL-co-methacrylic acid), poly(CL-co-MAA), i.e. a degradable statistical copolyelectrolyte. Scheme 26. Hybrid copolymerization of cyclic esters ε-CL (ROP) with methacrylic monomers MMA and tBuMA (1,4-conjugate addition polymerization).

[START_REF] Yang | Hybrid copolymerization of ε-caprolactone and methyl methacrylate[END_REF][START_REF] Xu | Synthesis of poly(ε-caprolactone-co-methacrylic acid) copolymer via phosphazene-catalyzed hybrid copolymerization[END_REF][START_REF] Yang | One-step synthesis of hyperbranched biodegradable polymer[END_REF]

Recently, Coulembier et al. investigated the use of t-BuP1 as catalyst in the cyclopolymerization of ortho-phthalaldehyde (PA) initiated by either 9-anthracenemethanol or MeOPEO-OH in dichloromethane at -85 °C (Scheme 26).

[START_REF] Winter | Control over molar mass, dispersity, end-groups and kinetics in cyclopolymerization of ortho-phthalaldehyde: adapted choice of a phosphazene organocatalyst[END_REF]

Poly(ortho-phthalaldehyde)'s (PPA's) are attractive materials due to their spontaneous depolymerization in response to a radiation stimulus.

[START_REF] Köstler | Polyaldehydes: homopolymers, block copolymers and promising applications[END_REF][START_REF] Ito | Chemical amplification in the design of dry developing resist materials[END_REF]

Using the t-BuP1-catalysis, PPA's and PEO-b-PPA diblock copolymers of molar masses Mn = 3,200 -200,000 g mol -1 (Đ = 1.15-1.27) were synthesized with high end-group fidelity. Scheme 27. tBu-P1-catalyzed cyclopolymerization of ortho-phthalaldehyde.

[START_REF] Winter | Control over molar mass, dispersity, end-groups and kinetics in cyclopolymerization of ortho-phthalaldehyde: adapted choice of a phosphazene organocatalyst[END_REF]

  their use in polymerizations reactions has been scarcely

								O	[t-BuP 4 -H]
								RO			O
	O									(19)				O
	O	+	OR'	O	+ R XH	toluene, r.t t-BuP 4	RO		O	e-CL or methacrylate	RO	O	OR' n	O m
	e-CL		R'= CH 3 ; MMA			(20)	OR'	[t-BuP 4 -H]		Random PCL-co-polymethacrylate
			R'= (CH 3 ) 3 ; tBuMA		R-XH							copolymers
											O
						CH 3 OH	HO	OH	O
						methanol	ethylene glycol	ethyl acetate
							(MeOH)	(EG)			(EA)
	investigated.[86]		Alkyl/aryl	phosphines		such	as	tributylphopshines	(Bu3P),

Acknowledgments

The authors are grateful to the Agence Nationale de la Recherche (ANR) -Programme Blanc (CATAPULT Project ANR-11-BS08-0011), and to the European Commission through project SUSPOL-EJD 642671 for financial support.

Triaryl iminophosphoranes featuring a thiourea moiety were reported to efficiently catalyze the ROP of L-LA, -VL and -CL, using PyBuOH as initiator (Scheme 63). [START_REF] Goldys | Organocatalytic ring-opening polymerization of cyclic esters mediated by highly active bifunctional iminophosphorane catalysts[END_REF] Quantitative conversion was obtained within 10 min in dichloromethane at r.t., affording polyesters of molar masses Mn = 4,900 (Đ = 1.23), 32,900 g.mol -1 (Đ = 1.04) and 37,400 g.mol -1 (Đ = 1.08) for PCL, PLLA and PVL homopolymers, respectively. PVL-b-PLA and PCL-b-PLA diblock copolymers were also obtained by sequential addition of monomers. Importantly, mono-functional iminophosphoranes, i.e. with no thiourea moiety, did not induce any polymerization, demonstrating the crucial role of the thiourea co-catalyst for monomer activation (Scheme 63). Scheme 63. ROP of cyclic esters catalyzed by iminophosphoranes in presence of alcohol. [START_REF] Goldys | Organocatalytic ring-opening polymerization of cyclic esters mediated by highly active bifunctional iminophosphorane catalysts[END_REF] In summary, the capability of thioureas (TUs) or phenol derivatives to activate carbonyl substrates combined with the propensity of Brønsted bases to assist the deprotonation of the alcohol initiator have allowed controlling the ROP of cyclic esters and cyclic carbonates, within both mono-and bicomponent catalytic systems. Such an activation, inspired from the multiple H-bonding motifs found in enzymatic active sites, provides high chemoselectivity, very few side reactions, such as transesterification, being observed during the polymerization.

Polymerizations directly induced by organic initiators

Here a short overview of recent polymerization reactions mediated by small organic molecules as direct initiators -and not as catalysts-will be given. Among organic activators described in the previous sections, Brønsted bases, including N-heterocyclic carbenes (NHCs), amidines, guanines and phosphazenes, have indeed served as powerful nucleophiles to directly initiate chain-growth polymerization reactions, either through ring-opening for cyclic monomers (Scheme 64a) or through 1,4 conjugate addition for (meth)acrylic derivatives (Scheme 64b).