Alexander M Kalsin

Tatyana A Peganova

Iana S Sinopalnikova

Ivan V Fedyanin

Natalia V Belkova

Éric Deydier

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr.

Eric Deydier

Mechanistic diversity in acetophenone transfer hydrogenation catalyzed by ruthenium iminophosphonamide complexes

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

With the discovery of metal-ligand bifunctional catalysts, 1 transfer hydrogenation of ketones has become a highly efficient method for the synthesis of secondary (chiral) alcohols. [START_REF] Wang | [END_REF]3 In this regard, the Noyori-Ikariya catalysts [(Arene)RuCl(Tsdpen)] 4 and structurally similar complexes [5][6][7][8][9][10][11] have drawn much attention and the effect of structural changes on the activity and on the catalytic mechanism have been extensively studied both experimentally and theoretically. [12][13][14][15][16][17][18][19][20] The "accepted" mechanism for bifunctional catalytic hydrogenation by the Noyori-Ikariya catalysts has been postulated to proceed as a concerted outer-sphere proton/hydride transfer from the metal centre to the substrate carbonylic carbon via a highly-ordered pericyclic transition state involving the ligand NH group, 12 the formation of which is responsible for high activities and chemo-/enantioselectivities. However, the degree of synchronism of the hydride and proton transfer has been further discussed to suggest alternatively the existence of a spectrum of possible reaction pathways ranging from concerted to stepwise proton/hydride transfer, with the proton either transiting through a ligand or being directly transferred from H2 to the alkoxide product. [21][22][23][24] In most cases the presence of an NH function is a crucial prerequisite rendering the catalyst highly efficient.

Recently we have demonstrated for the first time that ruthenium iminophosphonamide [(p-Cymene)RuCl(Ph2P(N-p-Tol)2)] (1f) can catalyse transfer hydrogenation of acetophenone in isopropanol under basic conditions. 25 In contrast to isoelectronic Noyori-Ikariya catalysts, the NPN ligand in 1f is not deprotonatable and therefore no ligand assistance in the hydrogen transfer step is possible. We have proposed a catalytic cycle that involves an outer-sphere transfer of the hydridic atom from the in situ generated hydride complex [(p-Cymene)RuH(Ph2P(N-p-Tol)2)] (2f) to the carbonyl atom via the zwitterionic intermediate [(p-Cymene)Ru + (NPN)(H…C (O - )MePh], while the proton is delivered by the alcohol solvent. This mechanistic proposition was supported by model experiments, kinetic studies and DFT calculations. Arene ruthenium iminophosphonamides are electron-rich complexes due to zwitterionic nature of the R2P + (NR' -)2 (NPN) ligand, which makes the 18ē [(Arene)Ru(NPN)X] complexes coordinatively labile and stabilizes their 16ē [(Arene)Ru(NPN)] + X - counterparts. 26 The 18ē complexes with electron-donating Nand P-substituents are prone to dissociation and are susceptible to electrophilic attack of the basic N-atoms. Therefore, such electron-donating N-and P-substituents may significantly change the propensity of the N-atoms to protonation, which can ultimately realize the concerted proton/hydride transfer in catalytic reduction of ketones.

In this paper we report on the acetophenone transfer hydrogenation with isopropanol catalyzed by arene ruthenium iminophosphonamide complexes [(Arene)RuCl(NPN)] (1a-e, Chart 1) bearing various N-and P-substituents on the NPN ligand (A-E), including kinetic studies and DFT calculations in support of the catalytic mechanism. The exploration of a wider This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins range of complexes differing by the nature of the arene, N and P substituents, has revealed the presence of two alternative mechanisms, with the preferred pathway controlled by the NPN ligand basicity.

Results and Discussion

The synthesis and properties of the complexes 1a-c and 1f have been previously reported, 26,27 while the new complexes 1d and 1e bearing the nonsymmetrical N-methyl-N'-p-tolyl-substituted ligand D and the ligand E with electron-withdrawing N-Ar* (Ar* = p-C6H4CO2Et) groups, respectively, are described here for the first time. The new arene ruthenium complexes 1d and 1e were synthesized according to the previously published procedure, 27 i.e. by reacting [(Arene)RuCl2]2 with the known iminophosphonamine D 29 or the new proligand E, respectively, deprotonated with 1 equiv. NaHMDS in benzene (Scheme 1).

The isolated products were fully characterized by multinuclear NMR spectroscopy and elemental analysis, and their molecular structures were confirmed by single crystal Xray diffraction studies (Figures 1 and2, Table S1 in ESI).

Complexes 1d and 1e exhibit a three-legged piano stool geometry with a pseudo octahedral configuration of the ligands around the ruthenium atom. Their structural parameters are similar to those of C6Me6 and p-cymene analogues 1a-c and 1f, respectively, for which a detailed structural analysis has previously been reported. 26,27 Expectedly, the Ru-N and Ru-Cl distances are slightly shorter for complex 1e (2.146(8) Å and 2.411(2) Å, respectively) than for 1d (2.1555(14) Å and 2.4276(5) Å, respectively), because of the effect of the pcymene ligand and the electron-withdrawing N-aryl groups of the iminophosphonamide moiety. As a result of the delocalization of the unpaired electron density of the nitrogen atoms on the N-aryl substituents, the pyramidalisation of the Please do not adjust margins Please do not adjust margins nitrogen atoms for 1d,e is small, Σ(N) = 357.7 -359.5°, similarly to complexes 1a,b,f. The Ru(1)N(1)P(1)N(2) metallacycle is slightly bent by 0.5° (1d) and 8.2° (1e) from planarity. The pcymene ring and one of the OEt fragments in 1e are disordered among two positions with almost equal occupancies for each component (refined as 0.49:0.51).

Analogously to 1b and 1c, 26 the Ph groups at the phosphorus atom in the complex 1d experience degenerative exchange due to facile chloride dissociation with an exchange rate constant kex = 3.4 s -1 and a Gibbs activation energy ΔGex ≠ = 16.5 kcal/mol, which is 2.0 kcal/mol higher than that for 1c having two donating N-Me substituents (Figure S1, Table S2). The same exchange for 1e is expected to be even slower than for 1f, for which the ΔGex ≠ in benzene was estimated to be greater than 18.5 kcal/mol. 26 Indeed, no exchange for 1e was detected in C6D6, while even in the relatively polar CDCl3 the ortho-H of the two different phenyl groups are observed separately (Figure S2), indicating a very slow exchange.

(b) Generation and stability of [(Arene)Ru(NPN)H] species.

We have recently described the generation of a hydride complex [(p-Cymene)RuH(NPN)] (2f, NPN = Ph2P(N-p-Tol)2) from the corresponding chloride 1f and sodium isopropoxide via β-H elimination in the isopropoxide intermediate [(p-Cymene)Ru(O i Pr)(NPN)] (3f). 25 Here we show that this procedure is operational for the generation of a wider array of ruthenium hydrides [(Arene)RuH(NPN)] (2a-e) from the corresponding chloride complexes 1a-e, although the stability of the products is very different.

The chloride complexes 1a-e were treated with 1-1.5 equiv. of NaHMDS in C6D6/ i PrOH = 500/10-40 μL at room temperature and the generation of the corresponding hydride complexes 2ae (Scheme 2) was monitored by 31 P and 1 H NMR spectroscopy. The formation of [(С6Me6)RuH(Ph2P(N-p-Tol)2)] (2a) proceeds via the observable isopropoxide intermediate [(С6Me6)Ru(O i Pr)-(Ph2P(N-p-Tol)2)] (3a), which predominates in the mixture during the first 15 min of the reaction, and is complete within 2 hrs (Figure 3). Complex 3a was identified by the characteristic septet of the methyne group of the isopropoxy ligand in the initial 1 H NMR spectrum (Figure S3).

Unlike the p-cymene analogue 2f, the hydride 2a is stable for days. It does not decompose or undergo rearrangement into an isomeric cyclohexadienyl complex, even upon prolonged heating at 60°C (though traces of the iminophosphonamine A due to NPN ligand protonolysis were visible after 1 day), apparently due to steric bulk of the C6Me6 ligand. Complex 2a was isolated from the reaction mixture and characterized by 1 H, 31 P and 13 C NMR spectroscopy (Figures S4). In the 1 H NMR spectrum, the RuH signal is observed at δ -3.25 in pure benzene-d6. Despite many attempts, we were not able to obtain suitable crystals for an X-ray analysis.

Replacing the phenyl P-substituents with more electrondonating ethyl groups in 1b does not affect the hydride 2b generation time from the corresponding isopropoxide 3b, while it causes side reactions to occur (Figure S5). The initially observed resonance at δ 64.5 in 31 Please do not adjust margins Please do not adjust margins nucleophilic attack of the free isopropoxide-anion on the electrophilic phosphorus atom followed by the P-N bond cleavage and isopropyl group migration onto the nitrogen atom (Scheme 3). Once the excess of O i Pr -is consumed, 2b is rather stable in solution. This side reaction reduces the yield of the hydride complex 2b, and affects the catalytic performance of 1b (see below). Unfortunately, even decreasing the amount of base to 1.0 equiv. did not suppress this side reaction completely, and besides 2b, the product contained 20-60% of 4b in different experiments.

The reaction of 3c with isopropanol in the presence of a strong base (Figure S8) proceeds very quickly leading mostly to decomposition product, namely the aminophosphine oxide Ph2P(O)(NHMe) (4c, δ 25.3). The isopropoxide complex 3c was not detected even after only 5 min after base addition. The corresponding hydride complex [(С6Me6)RuН(Ph2P(NMe)2)] (2с), characterized by a 31 P NMR signal at δ 49.8 and RuH resonance at δ -4.12 in 1 H NMR (Figure S8, insert), was observed in a relative amount lower than 10% while three additional minor impurities with 31 P NMR resonances at δ 32.8, 47.6, 52.7 could not be identified. Complex 2c can alternatively be obtained by reacting 1c with 1 equiv. of NaBHEt3 in benzene-d6 (Figure S9a), 25 presenting the same characteristic resonances at δ 49.8 in 31 P NMR and at δ -4.09 in 1 H NMR (Figure S9b). Thus the hydride complex 2c is indeed generated in the reaction of 1c with isopropoxide, however due to the highly basic Natoms 27 it has very low stability in the presence of isopropanol, perhaps because of its facile protonation and further P-N bond alcoholysis to form the aminophosphine oxide. Indeed, 1c slowly decomposes to aminophosphine oxide in isopropanol even in the absence of a base at room temperature (Figure S10).

Complex 1d, having only one basic N-Me group, reacts with basic isopropanol in a much cleaner way (Figure S11) to give predominantly the hydride complex 2d via the isopropoxide intermediate 3d, which is observed in the first minutes at δ 46.6 in the 31 P NMR spectrum. Complex 2d decomposes in the presence of isopropanol more slowly than 2c, and therefore was characterized in situ by 31 P NMR (δ 41.3) and 1 H NMR (δRuH -3.72) (Figure S12). After 3 days, 2d was almost completely decomposed to aminophosphine oxide Ph2P(O)(NHMe) (4d, δ 25.3) and iminophosphonamine D (δ 4.0), plus unknown compounds with 31 P NMR signals at δ 35.5 and 8.7, apparently due to protonation of the NMe group, followed by NPN ligand decoordination and alcoholysis. The formation of the hydride complex [(p-Cymene)RuН(Ph2P(NAr*)2)] (2е) is slower than that of the known p-cymene hydride 2f, 25 due to the presence of acceptor substituents in the N-Aryl groups (Figure S13). The isopropoxide intermediate [(p-Cymene)Ru(O i Pr)(Ph2P(NAr*)2)] (3е), characterized by a 31 P resonance at δ 43.2, was still present after 2 h at room temperature and could be characterized by 1 H NMR (Figure S14). Complex 2e is relatively stable and was characterized in situ by 31 P and 1 H NMR (Figure S15). However, it slowly isomerizes to the mixture of cyclohexadienyl complexes 5e, 5e' (δP 53.3, 53.7, Figure S13), similarly to 2f (Scheme 4).

It is noteworthy that excess of base (> 1 equiv. NaHMDS) results in a concomitant transesterification of the carboethoxy substituents to form mono-(2e') and di-(2e'') carboisopropoxy derivatives, ultimately giving the mixture of the hydride complexes [(p-Cymene)RuH(Ph2P(N-p-C6H4COOR)2)] (R = Et, i Pr) (Figure S16).

To summarize, the above experiments demonstrate that the reaction of 1a-e with sodium isopropoxide leads to the corresponding hydride complexes 2a-e, apparently, via isopropoxide intermediates, similarly to what was observed earlier for 2f. However, the stability of 2a-e under the reaction conditions differs strongly from 2a (stable for days) to hardly observable 2c. Among the hexamethylbenzene complexes 2a-d, those with more basic N-atoms, 2c and 2d, are less stable due to facile protonation and alcoholysis by isopropanol. Although 2b appeared to be of comparably high stability as 2a, the sterically more accessible phosphorus atom in this NPN ligand is prone to O i Pr -nucleophilic attack, resulting in partial degradation of the intermediate 3b to the aminophosphine oxide 4b, thus reducing the yield of the hydride complex 2b. Scheme 4. Generation of the hydride 2e and its rearrangement to 5e/5e'. Owing to the bulkier arene ligand, the hexamethylbenzene complex 3a undergoes β-hydride elimination at a considerably slower rate than its p-cymene counterpart 3f, 25 while this also prevents the corresponding hydride species 2a from transforming to the isomeric 16ē η 5 -cyclohexadienyl complexes, unlike the p-cymene complexes 2e and 2f. The electron-withdrawing N-substituents in 3 retard the β-hydride elimination to 2 (3e vs 3f and 3a vs 3c and 3d) and is in line with observations about the chloride exchange rates in complexes 1. 26 As we have proposed recently for the p-cymene hydride complex 2f, 25 the generation of 2 requires isopropoxide ligand dissociation to the corresponding cationic species 6 and [7] + , the intermediate and the resting state in this process, respectively (Scheme 5).

Ru

(c) Transfer hydrogenation of acetophenone catalysed by 1a-e.

All the complexes 1a-e have been tested in transfer hydrogenation of acetophenone in isopropanol with addition of 1.5 equiv. NaHMDS at 40°C and the results were compared to those reported for 1f 25 (Table 1). The conditions (temperature, incubation time after the addition of base) used for the generation of the catalytically active hydride complexes (2a-f) were in accordance with the model experiments described above. It is important to underline that the incubation period is needed to quantitatively transform the precatalyst 1 to the active species 2. Notably, we have no evidence of darkening of the solution with formation of metallic nanoparticles (in fact, the solution colour becomes fainter as the hydrides 2 are less intensely coloured than the corresponding precursors 1).

The nature of the N-substituents and η 6 -arene affects the initial acetophenone hydrogenation rate, which is in qualitatively accordance with the rate of β-H elimination from 3 to 2: more donating N-groups and less bulky arene significantly improve the catalyst performance. The lowered activity of 1b compared to 1a is due to partial decomposition of the precatalyst during generation of the hydride species 2b, as mentioned in section (b). Similarly, the rapid activity decay observed when using 1c is caused by stability issues for 2c.

For each complex, the kinetic data at different temperatures (Figure S17) were obtained to calculate the activation parameters of the reaction from Eyring plots (Figure 4). The low stability of 1c in basic isopropanol, especially at elevated temperatures, did not allow us to run an Eyring analysis for this system as it rapidly degraded above 40°C (Figure S17c). For 2b the catalytic results were less repeatable, therefore for building the Eyring plot we used the average of the two most consistent rate constants obtained at each temperature.

For catalyst 1f, kinetic experiments with different [Ru] had demonstrated that the rate law is first order in catalyst, as well as first order in ketone. 25 For complexes 1a-e, we have therefore assumed the same rate law and no additional kinetic experiments with variation of [Ru] were carried out. The activation enthalpy ΔН ≠ and entropy ΔS ≠ data for the ratedetermining step thus obtained are summarized in Table 2.

The activation parameters obtained in the presence of the N-Aryl substituted precatalysts (1a, 1b, 1e, 1f) are within narrow ranges (9.7-11.7 kcal/mol for ΔН ≠ and 31-35 cal/(mol•K) for ΔS ≠ ). This suggests the occurrence of a similar catalytic mechanism for these complexes, which apparently involves the hydride species 2 and the zwitterionic intermediate 6 as previously investigated by a combination of experimental and computational methods for the 1f system. 25 The activation enthalpies for 1a and 1e are meaningfully higher than that for 1f, in line with the retardation effect of the bulkier C6Me6 arene ligand and the electron-withdrawing N-substituents.

The activation parameters found for complex 1d, on the other hand, differ significantly. The higher ΔН ≠ (19.1±0.8 kcal•mol -1 ) and the smaller negative ΔS ≠ (-7±3 cal•mol -1 •K -1 )) are indicative of a different rate-determining transition state, less highly ordered than for the bis-N-Aryl complexes. Therefore, the N-Me substituted complexes 1c and 1d are supposedly 1.

≠ , cal•(mol -1 •K -1 ) -32±4 -35±5 - -7±3 -32±4 -31±4
* The data for 1f is given for comparison from 25 . This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins promote an alternative hydrogenation mechanism, the elucidation of which required additional kinetic experiments and DFT calculations.

(d) Acetophenone transfer hydrogenation mechanism in the absence of base.

Whereas systems 1a, 1b, 1e and 1f 25 require the presence of a strong base for activity, the precatalysts 1c and 1d demonstrate high activity even without a base. The acetophenone hydrogenation kinetic curves catalysed by 1c and 1d in neat isopropanol at different temperatures are shown in Figure 5. In fact, these complexes are much more stable under the reaction conditions when a base is not present and can catalyse the hydrogenation process for longer times and at higher temperatures. However, this stability increase is still insufficient to yield perfectly repeatable results for system 1c, as reflected in the lower precision of the Eyring analysis. The activation parameters obtained from the Eyring analysis are ΔН ≠ = 24±5 (1c), 22.5±0.5 (1d) kcal/mol and ΔS ≠ =12±10 (1c), 4±3 (1d) kcal/(mol•K). Note that the activation parameters obtained for 1d with and without base are close to each other. An interesting observation is that the dissolution of both 1c and 1d in isopropanol immediately leads to dissociation to the corresponding 16ē complexes [(η 6 -C6Me6)Ru(NPN)] + Cl -([7c] + and [7d] + , respectively), which is reflected by the colour change from red to purple and the downfield shift of the 31 P NMR resonance from δ 59.8 to 77.8 for 1c 26 and from δ 52.0 to 68.4 for 1d (Figure S18). This behaviour differs from that of the chloro derivatives 1a, 1e and 1f, which maintain their neutral state under the same conditions and only produce the corresponding 16ē complexes when a reagent is added to scavenge the chloride ion. These cationic complexes [7c] + and [7d] + are apparently the catalyst resting state, since they are the major identified species by 31 P NMR during the transfer hydrogenation catalysis. Although 1b also dissociates in i PrOH to cationic complex [7b] + , its catalytic performance is very poor yielding 4.5% conversion of acetophenone after 24 h at 40°C. It should also be mentioned that, as shown in our previous contribution, 25 compound 1f is stoichiometrically transformed to the catalytically active hydride complex 2f in situ by the strong base in iPrOH, hence the same process can be proposed for the other chloride complexes. Since the hydride complexes

Please do not adjust margins

Please do not adjust margins cannot be isolated, a slight excess of base (up to 1.5 equiv) is generally used. In one case (catalyst 1f) the 1f/NaHMDS ratio was varied (1/1 and 1/2) but the observed rate constant was essentially identical (0.095 and 0.106 h -1 , respectively, at 25°C) Thus, whereas a first equivalent of base is needed to activate 1a, 1b, 1e and 1f, it appears that the excess amount has no effect on the catalytic cycle.

(e) Mechanistic discussion.

The possibility of a Meerwin-Ponndorf-Verley (MPV) mechanism can be easily discarded for two reasons: (i) the catalyst works under base-free conditions, whereas the MPV pathways requires alkoxide binding to the catalyst; (ii) these catalysts cannot offer a second open coordination site for coordination of the ketone substrate because of the strong chelating power of the NPN ligand. Taking into account the MeN high basicity in the 18ē ruthenium complexes with ligands C and D, we propose that the acetophenone hydrogenation in neat isopropanol follows the classical Noyori mechanism 14 , in which a simultaneous transfer of the isopropanol methyne H atom to the Ru atom and of the OH proton to the basic N atom occurs through a pericyclic transition state in the rate-determining step (cycle A in Scheme 6). We suppose that the protonated hydride complex [2-H + ] might be the true catalytically active species, having a relatively high energy and therefore being present in the catalytic system in a low stationary state concentration. For the less basic aryl-substituted NPN systems, on the other hand, an external base is necessary and the reaction occurs via cycle B, implicating the neutral hydride complex 2, as validated by a DFT study in our previous contribution. 25 When the transfer hydrogenation is catalysed by complex 1c or 1d in the presence of a base, the resting state is the hydride complex (2c or 2d, respectively) rather than the cationic complex ([7c] + or [7d] + , respectively). NMR monitoring of the hydride complex 2d in the presence of iPrOH (10 equiv) in C6D6 indicates no change (see Figure S19), hence no spontaneous proton transfer with formation of [2-H] + with an iPrO - counterion. Addition of acetophenone to this solution at room temperature only showed the slow formation of the 1phenylethanol product (see Figure S20), without any change for the resonances of 2. This means that 2 is an off-loop resting state under these conditions.

In order to validate this proposition, new DFT calculations were carried out for the proposed cycle A. The results, shown in Figure 6, suggest that the proposed mechanism is indeed accessible. The calculations were carried out at the same level of theory as those previously presented for cycle B (see details in the Experimental section), 25 using the full NPN ligand and a benzene ring to model the real arene ligand and included corrections for solvation effects by use of a polarizable continuum in isopropanol and for dispersion effects during optimization. Only the catalyst activation step, involving H2 delivery to [7] + from isopropanol to generate [2-H] + and acetone, was analysed. The energetic profile of the subsequent delivery of H2 to acetophenone, which occurs symmetrically in the reverse direction, should be quite similar. The first step of this process is the chloride ion dissociation from 1. To render the calculated model closer to reality, explicit solvation of the chloride ion by H-bonding interactions with three solvent Please do not adjust margins Please do not adjust margins molecules (modelled by methanol) were considered, yielding a slightly exoergic process. This in agreement with the experimental evidence (see above). The isopropanol molecule first docks onto the NPN ligand by establishing an H-bond as a proton donor with one of the two NMe groups, yielding [7] + •••HOiPr through a second slightly exoergic process. Then, the proton is transferred to the NMe group and the methyne H atom is simultaneously transferred to the Ru centre to become a hydride ligand and yield the H-bonded adduct between [2-H] + (proton donor) and acetone (proton acceptor), via a first-order saddle point (20.3i cm -1 ) located at only 10.7 kcal/mol above the starting complex 1 and 15.6 kcal/mol above system [7] + •••HOiPr. An IRC analysis demonstrates that this concerted proton/hydride transfer is quite asynchronous, the proton transfer occurring on the upslope from [7] + •••HOiPr to the saddle point, whereas the hydride transfer process takes place around the saddle point. Dissociation of the H-bonded adduct yield separate [2-H] + and acetone in a nearly isoergic process. Finally, DFT calculations were carried out to assess the stability of [2-H] + with respect to proton transfer processes to either the isopropoxide ion or isopropanol (modelled by methoxide and methanol, respectively). In order to satisfactorily model the strong base in the alcohol solvent, the H-bonded adduct with three solvent molecules, [MeO(MeOH)3] -was used, leading to a (MeOH)4 cluster and the neutral hydride 2. The calculations yield an exoergic process with ΔGiPrOH = -6.4 kcal/mol, in agreement with the experimental observation that 2 is stable in iPrOH (in other words, iPrOH is not able to protonate 2 and generate [2-H] + ). Therefore, the calculation confirm that 2 is the catalyst resting state when generated from 1 in the presence of a strong base. Conversely, proton transfer from [2-H] + to the solvent, modelled by the (MeOH)3 cluster, to yield 2 and [MeOH2(MeOH)2] + is highly endoergic (ΔGiPrOH = 21.0 kcal/mol). This means that [2-H] + is stable, i.e. is not deprotonated, in the absence of the strong base. Thus, according to Figure 6, the resting state of the catalyst is [7] + . The likely rate-determining step is the conversion of [7]

+ •••HOiPr to [2-H] + •••O=CMe2,
for a calculated free energy span of 15.6 kcal/mol. For system d, the experimentally determined activation parameters (ΔH ‡ = 19.1±0.8 kcal•mol -1 and ΔS ‡ = -7±3 cal•mol -1 •K -1 ) yield a ΔG ‡ of 21.2 kcal/mol at 298 K. The value estimated by DFT is even lower and is thus quite consistent with the feasibility of this concerted pathway. The underestimation of the cycle ΔG ‡ by the DFT calculations may be related at least in part to the small benzene model, which imposes lower steric constraints on the concerted transition state relative to the larger C6Me6 ring in system d. Inaccuracies in the solvation model and in the handling of the entropic contribution in the condensed phase can also contribute to the computational error. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins not have doubts about the correctness of this crystal structure. Detailed crystallographic information is given in Table S3. Crystallographic data have been deposited to the Cambridge Crystallographic Data Centre, CCDC numbers 1956827-1956828. Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033. Computational Details. The calculations were carried out using the Gaussian 09 program package [START_REF] Frisch | Gaussian 09[END_REF] within the DFT approach using the 97D functional, [START_REF] Grimme | [END_REF] which implicitly includes a dispersion correction. The basis set comprised the LANL2DZ basis set, which includes an ECP and augmented with an f polarization function (α = 1.235) 38 for the Ru atom, and the standard 6-311G(d,p) 39 basis sets for other atoms. The solvation effect was included during optimization by use of the SMD polarisable continuum in isopropanol (ε = 19,264). Thermochemical corrections were obtained at 298.15 K on the basis of frequency calculations, using the standard approximations (ideal gas, rigid rotor and harmonic oscillator). A further correction of 1.95 Kcal/mol was applied to bring the G values from the gas phase (1 atm) to the solution (1 mol/L) standard state. 40

Conclusions

With this study we have demonstrated the effect of the N,Psubstituents in the NPN ligand and the arene nature on the performance of the (Arene)Ru(NPN) complexes in acetophenone transfer hydrogenation catalysis. We have discovered that the N-atoms basicity is the feature that defines both the mechanism of the outer-sphere hydrogen transfer to ketone, either involving NPN ligand-assistance or without this precoordination step, and the conditions at which the catalyst can operate.

Under basic conditions, when the precatalyst is converted into the active ruthenium hydride species, the less sterically encumbering p-cymene ligand and the more electron-donating N-substituents favour the catalyst activity, although both decrease the stability of the catalytically active hydride species, due to either side rearrangement to 16ē η 5 -cyclohexadienyl complexes 25 or facile protonation of the N-atoms following by the NPN ligand decoordination, respectively. Replacement of the P-phenyl groups with ethyl substituents has little effect on the catalyst performance, while its stability suffers significantly because of the NPN ligand degradation due to concurrent P-N bond breaking with alkoxide-ion. In general, preliminary generation of the active RuH species strongly limits the overall stability of the catalytic system making it very sensitive to the quality of the solvent and the temperature.

The use of base is not necessary for the complexes with basic N-Me groups, which is a rare occurrence in transfer hydrogenation catalysis; without base these precatalysts are also highly active and significantly more stable. With the help of kinetic analyses and DFT calculations we have suggested an alternative mechanism of transfer hydrogenation under basefree conditions, which involves assistance of the basic N-atom for precoordination of the ketone to form a pericyclic transition state similarly to the proposed one for Noyori-Ikariya catalysts. From the practical point of view, the major advantage of using N-methylated RuNPN catalysts under base-free conditions is the long-term [(Arene)Ru(NPN)] + stability to protonolysis with isopropanol, which seems to be the catalyst main degradation pathway, especially at elevated temperatures.

  (a) Synthesis of [(C6Me6)Ru{Ph2P(N-p-Tol)(NMe)}Cl] (1d) and [(p-Cymene)Ru{Ph2P(N-Ar*)2}Cl] (1e). The unknown iminophosphonamine E was quantitatively obtained from the corresponding diaminophosphonium salt [Ph2P(NHAr*)2]Br 28 by deprotonation with diethylamine. It was fully characterized by NMR spectroscopy and elemental analysis.

Scheme 1 .

 1 Scheme 1. Synthesis of complexes 1d and 1e.

Figure 1 .

 1 Figure 1. ORTEP diagram of 1d. Ellipsoids are shown at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Ru•••Arene(centroid) 1.673, Ru-Cl 2.4276(5), Ru-N1 2.1373(14), Ru-N2 2.1736(14), N1-Ru-N2 68.26(5), Ru-N1-P-N2 179.48(9), Σ(N1) 359.0, Σ(N2) 358.9.

Figure 2 .

 2 Figure 2. ORTEP diagram of 1e. Ellipsoids are shown at 50% probability, hydrogen atoms are omitted for clarity. Only one fragment of the disordered cymene and OEt fragments is shown. Selected bond lengths (Å) and angles (°): Ru•••Arene(centroid) 1.647, 1.673; Ru-Cl 2.411(2), Ru-N1 2.147(8), Ru-N2 2.145(5), N1-Ru-N2 67.9(2), Ru-N1-P-N2 171.8(5), Σ(N1) 357.7, Σ(N2) 359.5.

2 .Figure 3 .Scheme 3 .

 233 Scheme 2. General scheme for generation of hydride complexes 2a-e.

Scheme 5 .

 5 Scheme 5. Generation of the hydrides 2 via dissociation of O i Pr -from 3 followed by rotation and hydride transfer.

  (2 mmol) and 226 µL of internal standard (dodecane) in 5 mL of solvent ([PhCOMe] = 3.66•10 -1 M), 0.02 mmol of Ru catalyst ([Ru] = 4•10 -3 M)); 15 μL NaHMDS (2M in THF), NaHMDS/Ru = 1.5; tincthe conditioning time @40°C before the substrate addition, kobs -the pseudo-first order rate constant. [b] 0.01 mmol of Ru catalyst ([Ru] = 2•10 -3 M)). [c] Results from ref.25 .

Figure 4 .

 4 Figure 4. Eyring analysis of the second order rate constant ks of the acetophenone transfer hydrogenation in isopropanol catalyzed by 1a, 1b, 1d, 1e in the presence of base at different temperatures. Reaction conditions as in Table1.

Scheme 6 .

 6 Scheme 6. Two possible mechanisms of acetophenone hydrogenation catalyzed by ruthenium iminophosphonamides depending on the basicity of the N-atoms.

Figure 5 .

 5 Figure 5. First order kinetics analyses of the acetophenone transfer hydrogenation in neat isopropanol catalysed by 1c, 1d at different temperatures. Reaction conditions: complex 1c (0.01 mmol, [Ru] = 2•10 -3 M), complex 1d (0.02 mmol, [Ru] = 4•10 -3 M), acetophenone (233 μl, 2 mmol), isopropanol (5 ml). [PhCOMe] = 3.7*10 -1 M. (insert) Eyring analysis of the second order rate constant ks.

Figure 6 .

 6 Figure 6. Free energy profile for the dissociation of the model [(C6H6)Ru(NPN)Cl] complex in iPrOH and for the generation of the catalytically active [(C6H6)RuH(NPNH)] + complex (NPN = MeNPPh2NMe).

Table 1 .

 1 Results of the catalytic transfer hydrogenation of acetophenone by 1a-f.[a]

	Precat alyst	tinc, min	0.5 h	1 h	Conversion, % 1.5 h 2 h	3 h	4 h	kobs, h -1
	1a	120	4.9	8.6	12.8	15	22.6	26.5	0.082
	1b	60	2.7	4.4	6.1	9.8	13.3	17.3	0.048
	1c [b]	5	16.3	27.7	35.5	40.1	44.5	45.4	0.354
	1d	5	6.4	13.3	18.4	22.3	28.4	30.5	0.133
	1e	30	5.1	9.2	11.1	16.3	24.1	31.7	0.100
	1f [c]	15	15.9	26.8	38.2	48	64.3	74.9	0.336
	[a] Standard conditions : 233 µL of acetophenone

Table 2 .

 2 The activation parameters of acetophenone hydrogenation with complexes 1a-f in the presence of base.

		1а	1b	1c	1d	1е	1f*
	ΔН ≠ , kсal•mol -1	11.7±1	10.9±1	-	19.1±0.8	11.6±1	9.7±1
	ΔS

Acknowledgements

This work was supported by Russian Science Foundation (grant No. 19-13-00459). Spectral characterization, elemental analysis were performed with the financial support from Ministry of Science and Higher Education of the Russian Federation using the equipment of Center for molecular composition studies of INEOS RAS. We also thank the French "Ministère des Affaires Etrangères" for a Ph.D. fellowship to ISS and the CALMIP mesocenter of the University of Toulouse for a grant of computational time.

Experimental

General procedures.

All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry argon. Solvents were purified by standard methods and distilled prior to use. The assignment of the resonances for the complexes 1 and 2 was based on the analysis of 31 P, 1 H, 13 C{ 1 H}, 1 H-1 H COSY and 1 H- 13 C HSQC NMR spectra. 1 H, 31 P and 13 C NMR spectra were obtained on Bruker Avance 300 and Bruker AvanceIII 400 spectrometers and referenced to the residual signals of deuterated solvent ( 1 H and 13 C), and to 85% H3PO4 ( 31 P, external standard). Complexes [(η 6 -C6Me6)RuCl2]2, 30 [(η 6 -Cym)RuCl2]2, 31 the iminophosphonamines A-C, 27 D 29 and [Ph2P(NH-p-C6H4COOEt)2]Br 28 were prepared according to described procedures. Synthesis of Ph2P(NHAr*)(NAr*) (Ar* = p-C6H4COOEt) (E). To a suspension of [Ph2P(NHAr*)2]Br (1.25 g, 2.10 mmol) in benzene (40 mL), neat Et2NH (0.24 mL, 2.2 mmol) was added and the mixture was stirred for 2 h. The precipitate was filtered off and the filtrate was evaporated to dryness. The residue was dissolved in diethyl ether (20 mL) and the product was precipitated with hexane (40 mL). The white crystalline precipitate was dried in vacuo (yield = 1.06 g, 98%). Anal. calcd for C30H29N2O4P: C, 70.30; H, 5.70; N, 5.47%. Found: C, 70.59; H, 5.87; N, 5.34%. 31 P{ 1 H} NMR (C6D6): δ -5.3. 1 Н NMR (C6D6): δ 8.01 (d, 3 JНН = 8.4, 4H, C6H4(CO2Et)), 7.91 (ddd, 3 JHP = 12.8, 3 JHH = 8.0, 4 JHH = 1.8, 4H, o-HPh), 7.22 (d, 3 JНН = 8.4, 4H, C6H4(CO2Et)), 6.97 (m, 6H, (m+p)-HPh), 6.29 (s, 1H, NH), 4.07 (q, 3 JHH = 7.2, 4H, CH2CH3), 0.99 (t, 3 JHH = 7.2, 6H, CH2CH3). 13 C{ 1 H} NMR (C6D6): δ 166.7 (s, CO2Et), 145.4 (s, i-C6H4(CO2Et)), 131.9 (d, 2 JСР = 9.8, о-CPh), 131.8 (d, 4 JСР = 2.8, p-CPh), 131.2 (s, m-C6H4(CO2Et)), 130.9 (d, 1 JСР = 131.9, i-CPh), 128.6 (d, 3 JСР = 12.3, m-CPh), 122.1 (br. s, p-C6H4(CO2Et)), 120.2 (d, 3 JСР = 18.3, o-C6H4(CO2Et)), 59.9 (s, CH2CH3), 13.9 (s, CH2CH3).

Synthesis of [(η 6 -C6Me6)RuCl{Ph2P(N-p-Tol)(NMe)}] (1d).

To a solution of iminophosphonamine D (0.32 g, 1.00 mmol) in benzene (50 mL) a 0.6 M solution of NaHMDS in toluene (1.8 mL, 1.08 mmol) was added and the resulting solution was stirred for 2 h at room temperature. Then solid [( 6 -C6Me6)RuCl2]2 (0.34 g, 0.50 mmol) was added, and the reaction mixture was stirred overnight. The resulting solution was diluted with hexane (50 mL), and the precipitate was filtered off. The filtrate was concentrated to 5 mL and then hexane was added with small portions within 0.5 h (20 mL in total) to induce crystallization. The precipitated orange-red crystals were filtered off, washed with hexane (2 x 5 mL) and dried in vacuum (yield = 0.47 g, 77%). Anal. calcd for C32H38ClN2PRu: C, 62.18; H, 6.20%. Found: C, 62.54; H, 6.58%. 31

Synthesis of [(η 6 -p-cymene)RuCl{Ph2P(N-p-C6H4COOEt)2}] (1e).

To a suspension of E (0.49 g, 0.96 mmol) in benzene (40 mL) a 0.6 M solution of NaHMDS in toluene (1.7 mL, 1.02 mmol) was added and the resulting solution was stirred for 1 h at room temperature. Then solid [(η 6 2). General procedure. The hydride complexes 2 were generated in situ in a NMR tube. To a solution of the chloride complex 1 (0.020 mmol) in C6D6 (0.6 mL) and isopropanol (0.20 mmol, 10 μL), a NaHMDS solution (2.0 M in THF, 15 μL, 0.03 mmol, 1.5 equiv.) was added. The resulting suspension was vigorously stirred for 5 min at room temperature to complete the dissolution of 1 and then the reaction was followed by NMR. Complex 2а was generated from 1a (0.014 g, 0.020 mmol) and NaHMDS (0.030 mmol) in 95% after 2 h. 31 Alternative method of generation of 2c. To a solution of 1c (0.011 g, 0.020 mmol) in C6D6 (0.6 mL), NaEt3BH (1.0 M in toluene, 20 μL, 0.020 mmol) was added, the resulting mixture was shaken for 5 min at room temperature and analyzed by 31 Р{ 1 H} NMR. According to 31 P{ 1 H} NMR (C6D6), the content of 2c (δP 49.8, δRuH -4.12) after 20 min was ca. 70% and it gradually decreased with time.

Catalytic studies. In a typical experiment, the precatalyst 1 (0.02 mmol) was suspended in iPrOH (5 mL) and then treated (if appropriate) with a 0.6 M solution of NaHMDS in toluene (1-1.5 equiv.). In the presence of base, the mixture was preliminary stirred for 5-120 minutes (Table 1), then it was warmed at the desired temperature followed by the addition of dodecane (1 mmol, 0.225 ml) and acetophenone (2 mmol, 0.233 ml). Aliquots (0.1 mL) were diluted with diethyl ether (0.8 mL) and filtered through a silica bed. The reaction samples were analysed by GC equipped with J&W GC Column with a DB-1MS stationary phase (program: 25 min at 50 ˚C, with further heating rate 20 ˚C / min to 170 ˚C; retention times: τ(acetophenone) = 24 min, τ(1-phenylethanol) = 25 min, τ(dodecane) = 31 min). X-ray crystallography. Single crystals of 1d, 1e were obtained by slow diffusion of hexane into benzene solutions. The data collections were performed on a Bruker APEX DUO diffractometer equipped with an Apex II CCD detector, using CoKα radiation (λ = 1.54178 Å) for 1e and MoKα radiation (λ = 0.71073 Å) for 1d. Frames were integrated using the Bruker SAINT software package 32 by a narrow-frame algorithm. A semiempirical absorption correction was applied with the TWINABS (1e) or the SADABS [START_REF] Krause | [END_REF] (1d) programs using the intensity data of equivalent reflections. The structures were solved with the dual-space method with SHELXT 34 and refined by the full-matrix least-squares technique against F 2 hkl in anisotropic approximation with the SHELXL 35 software package. The positions of the hydrogen atoms were calculated, and all hydrogen atoms were refined using the riding model with 1.5Ueq(Cm) and 1.2Ueq(Ci), where Ueq(Cm) and 1.2Ueq(Ci) are respectively the equivalent thermal parameters of methyl and all other carbon atoms to which corresponding H atoms are bonded. The crystals of 1e were twins of relatively poor quality; the maximal achieved data completeness was only 83.2% up to the resolution of 0.833 Å. However, due to the high data/parameters ratio (> 10) and relatively low residuals we do

Conflicts of interest

There are no conflicts to declare.


Zhuoqun Wang

Rinaldo Poli

Christophe Detrembleur

Antoine Debuigne
email: adebuigne@uliege.be

Organometallic-Mediated Radical (Co)Polymerization of -Methylene--Butyrolactone: Access to pH-Responsive Poly(vinyl alcohol) Derivatives

Keywords: Vinyl lactone, reversible deactivation radical polymerization, organometallicmediated radical polymerization, poly(vinyl alcohol), pH-responsive

Conjugated vinyl lactones commonly serve as precursors of polymers with pendant cyclic esters, which can undergo several chemical modifications. They have notably been copolymerized with "more activated monomers" (MAMs) like (meth)acrylates. By contrast, the radical polymerization of the non-conjugated methylene lactone analogs has been disregarded as well as their copolymerization with "less activated monomers" (LAMs) such as acyclic vinyl esters.

 .

Copolymerization conditions were optimized to produce poly(MγBL-co-VAc) with predictable molar mass, low dispersity and precise composition. Finally, some post-polymerization modifications of these poly(MγBL-co-VAc) were carried out in order to produce unprecedented carboxylic acid-functionalized poly(vinyl alcohol)s (PVA) via hydrolysis of the pendant acyclic and cyclic esters. A preliminary study of the solution properties of these potentially pH-responsive polymers was also performed.

EXPERIMENTAL SECTION

Materials.

Cobalt(II) acetylacetonate (Co(acac)2) (97%, Acros), 4-pentynoic acid (98%, Fluorochem), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) (98%, Aldrich), 2,2'-azobis(4-methoxy-2,4dimethyl valeronitrile) (V-70, t½ = 10 h at 30 °C) (> 98 %, Wako), 2,2'-azobisisobutyronitrile (AIBN) (AIBN, t½ = 10 h at 65 °C) (98 %, Aldrich) were used as received. Sodium hydroxide (NaOH, ≥ 97%, Acros), silica gel for column chromatography (60 Å, ROCC S.A.), tetrahydrofuran (THF, ≥ 99.9 %, VWR), chloroform (> 99%, VWR), methanol (MeOH, ≥ 99.8 %, VWR), nheptane ( > 99.6 %, VWR), n-hexane ( > 99%, VWR), ethyl acetate ( ≥ 99.9%, VWR), acetonitrile ( ≥ 99.99 %, Acros) were used as received. N,N-dimethylformamide (DMF, > 99%, VWR) were

INTRODUCTION

Vinylidene lactones have received great interest in the last decade as building blocks for the production of bio-based specialty polymers. [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] Among them, α-methylene-γ-butyrolactone (αMγBL), [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] or tulipalin A, can be isolated from tulips whereas -methyl-α-methylene-γbutyrolactone (MαMγBL) [START_REF] Gowda | Synthesis of β-Methyl-α-Methylene-γ-Butyrolactonefrom Biorenewable Itaconic Acid[END_REF] and γ-methyl-α-methylene-γ-butyrolactone (γMαMγBL) [START_REF] Manzer | Catalytic Synthesis of α-Methylene-γ-Valerolactone : a Biomass-Derived Acrylic Monomer[END_REF] are prepared from biomass-derived itaconic acid and levulinic acid, respectively (Scheme 1). Several approaches have been considered for the conversion of these methylene lactones into (co)polymers including ring opening polymerization [START_REF] Tang | The Quest for Converting Biorenewable Bifunctional α -Methylene-γ -butyrolactone into Degradable and Recyclable Polyester: Controlling Vinyl-Addition/Ring-Opening/Cross-Linking Pathways[END_REF][START_REF] Hong | Coordination Ring-Opening Copolymerization of Naturally Renewable α -Methylene-γ -butyrolactone into Unsaturated Polyesters[END_REF][START_REF] Danko | Functional Polyesters with Pendant Double Bonds Prepared by Coordination-Insertion and Cationic Ring-Opening Copolymerizations of ε-Caprolactone with Renewable Tulipalin A[END_REF] leading to polyesters but also group transfer, [START_REF] Miyake | Living Polymerization of Naturally Renewable Butyrolactone-Based Vinylidene Monomers by Ambiphilic Silicon Propagators[END_REF][START_REF] Hu | Living Group Transfer Polymerization of Renewable α -Methylene-γ-Butyrolactones Using Al(C6F5)3 Catalyst[END_REF] anionic, [START_REF] Hu | Anionic Polymerization of MMA and Renewable Methylene Butyrolactones by Resorbable Potassium Salts[END_REF] zwitterionic, [START_REF] Zhang | Alane-Based Classical and Frustrated Lewis Pairs in Polymer Synthesis : Rapid Polymerization of MMA and Naturally Renewable Methylene Butyrolactones into High-Molecular-Weight Polymers[END_REF][START_REF] Yuetao; Miyake | Lewis Pair Polymerization by Classical and Frustrated Lewis Pairs: Acid, Base and Monomer Scope and Polymerization Mechanism[END_REF][START_REF] Chen | Reactivity of Amine/E(C6F5)3 (E = B, Al) Lewis Pairs toward Linear and Cyclic Acrylic Monomers: Hydrogenation vs[END_REF][START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF][14] coordination [15][16][17][18] and radical [19][20][21][START_REF] Akkapeddi | The Free Radical Copolymerization Characteristics of α-Methylene-γ-Butyrolactone[END_REF][START_REF] Yoon | Novel 193nm Photoresist Based on Olefin-Containing Lactones[END_REF][START_REF] Cockburn | Free Radical Copolymerization Kinetics of γ-Methyl-α-Methylene-γ-Butyrolactone (MeMBL)[END_REF][START_REF] Miyake | Polymerizability of Exo-Methylene-Lactide Toward Vinyl Addition and Ring Opening[END_REF] polymerizations providing polymers with pendant five-membered ring.

The radical polymerization of the methylene lactones has been largely investigated and occurs via repeated radical addition onto their exo-cyclic double bond (Scheme 1). In this respect, αMγBL derivatives can be considered as cyclic analogs of the petroleum-based methyl methacrylate (MMA). As a matter of fact, it belongs to the class of more activated monomers (MAMs), which consist in conjugated monomers bearing a radical stabilizing group onto the double bond.

Homopolymers with pendant butyrolactone moieties were notably prepared via conventional radical polymerization of αMγBL 19,20 and MαMγBL 21 . Compared to PMMA, these Scheme 1. Radical polymerization of conjugated and non-conjugated methylene-γbutyrolactones.

Considering the higher reactivity of αMγBL-based monomers compared to MMA, 21 their copolymerization with unconjugated Less Activated Monomers (LAMs) including vinyl acetate (VAc) or α-olefins is clearly jeopardized. However, -methylene-γ-butyrolactoneMγBL), a cyclic structural analog of VAc, appears as an ideal candidate for incorporating pendant lactones within LAMs-based copolymers (Scheme 1). Surprisingly, MγBL has almost been completely disregarded in the polymerization field. To our knowledge, only one example relative to the tentative conventional radical copolymerization of MγBL and MMA is succinctly mentioned in the literature [START_REF] Yoon | Novel 193nm Photoresist Based on Olefin-Containing Lactones[END_REF] and the reversible deactivation radical (co)polymerization of MγBL has never been reported. In the present work, we explored for the first time the conventional radical homopolymerization of MγBL as well as its copolymerization with VAc. Reactivity ratios were dried over molecular sieves prior to use. Dichloromethane (CH2Cl2) was degassed and dried over 4 Å molecular sieves. Vinyl acetate (VAc) (> 99%, Aldrich) was dried under calcium hydride, purified by distillation under reduced pressure and degassed by freeze-drying cycle under vacuum.

The alkyl-cobalt (III) adduct initiator (PVAc<4-Co(acac)2, [Co(acac)2-(CH(OAc)-CH2)<4R0]; R0 being the primary radical generated by 2,2'-azo-bis(4-methoxy-2,4-dimethyl valeronitrile) (V-70, Wako) was prepared as described previously [START_REF] Debuigne | Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators[END_REF] and stored as a CH2Cl2 solution at -20 °C under argon. Dialyses were carried out with Spectra/Por Dialysis Membrane (Pre-treated RC Tubing 1 kDa).

Characterization. Size exclusion chromatography (SEC) of PMBL and of P(MBL-co-VAc)

with FMBL > 0.5 were carried out in DMF containing 0.025 M of LiBr at 55 C with a Waters chromatograph equipped with three columns (Waters Styragel PSS GRAM 1000Ǻ (2), 30Ǻ), a dual λ absorbance detector (Waters 2487) and a refractive index detector (Waters 2414). The system was operated at a flow rate of 1 mL/min. A polystyrene calibration was used. SEC analyses of P(MBL-co-VAc) with FMBL < 0.5 were carried out in THF at 45 C at a flow rate of 1 mL/min with a Viscotek 305 TDA liquid chromatograph equipped with 2 PSS SDV linear M columns calibrated with PS. [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR, HSQC and COSY spectra were recorded at 298 K with a Bruker Avance III HD spectrometer (B0 = 9.04 T) (400 MHz) and treated with MestreNova software. IR spectra were recorded on ThermoFischer Scientic Nicolet IS5 equipped with an ATR ID5 module using a diamond crystal (650 cm -1 -4000 cm -1 ). Differential scanning calorimetry (DSC) was performed on a TA Instruments Q1000 DSC, using hermetic aluminium pans, indium standard for calibration, nitrogen as the purge gas, a sample weight of about 5 mg. The sample was cooled down to -40 °C at 40 °C/min cooling rate, followed by an isotherm at -40°C for 2 min and heating up to 120 °C at 10 °C/min heating rate. This cycle was repeated twice. TGA analyses were carried out with a Hi-Res TGA Q500 from TA Instruments under nitrogen at a heating rate of 20 °C/min from ambient temperature to 600 °C at a heating rate of 20 °C/min.

Computational details.

The computational work was carried out using the Gaussian09 suite of programs. [START_REF] Frisch | Revision D.01[END_REF] Gas-phase geometry optimizations were performed without any symmetry constraint using the BPW91* functional, which is a reparametrized version of B3PW91 with the same parameters previously optimized for B3LYP, [START_REF] Reiher | Theoretical Study of the Fe(phen)2(NCS)2 Spin-Crossover Complex with Reparametrized Density Functionals[END_REF] and the 6-311G(d,p) basis functions for all light atoms (H, C, O, Si), whereas the Co atom was treated with the SDD basis set augmented by an f polarization function (α = 2.780) [START_REF] Ehlers | A set of f-polarization functions for pseudopotential basis sets of the transition metals Sc-Cu, Y-Ag and La-Au[END_REF] A correction for dispersion was also included during the geometry optimizations using Grimme's D3 empirical method (BPW91*-D3), using SR6 and S8 parameters identical to those optimized for B3PW91. [START_REF] Grimme | A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[END_REF] The unrestricted formulation was used for open-shell molecules, yielding only minor spin contamination (<S 2 > at convergence was very close to the expected values (0.75 for doublet states and 3.75 for the quartet state). All final geometries were characterized as local minima by verifying that all second derivatives of the energy were positive. Thermochemical corrections were obtained at 298.15 K on the basis of frequency calculations, using the standard approximations (ideal gas, rigid rotor and harmonic oscillator). A further correction of 1.95

Kcal/mol was applied to bring the G values from the gas phase (1 atm) to the solution (1 mol/L) standard state. [START_REF] Bryantsev | Calculation of Solvation Free Energies of Charged Solutes Using Mixed Cluster/Continuum Models[END_REF] The Cartesian coordinates and energies of all optimized structures are available in the Supporting Information (Table S5). Copolymerizations were stopped at low conversion in order to avoid significant composition drift.

Synthesis of -methylene--butyrolactone (MBL

Conversions were determined by [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR in CDCl3. The copolymers were purified by repeated precipitation in heptane (three times) and dried at 50 °C under reduced pressure. The molar fractions of the comonomers in the copolymer (FMBL and FVAc) were determined by [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR in CDCl3. These data, provided in Table S1, were then used for the determination of the reactivity ratios via three methods described below.

(i) The Fineman-Ross linearization method [START_REF] Fineman | Linear method for determining monomer reactivity ratios in copolymerization[END_REF] which generates a straight line whose slope and intercept with the ordinate (Y-axis r) correspond, respectively, to r1 and r2 (Eq. 1).

f(F-1)/F = r1 (f²/F) -r2 (Eq. 1)

, where f = f1/f2 and F = F1/F2

(ii) The Kelen-Tüdős method [START_REF] Kelen | Analysis of the Linear Methods for Determining Copolymerization Reactivity Ratios. I. A New Improved Linear Graphic Method[END_REF] which involves parameters η and ζ, mathematical functions of the mole ratios in the monomer feed (f) and in the copolymer (F) and of a parameter α calculated on the basis of the lowest and highest values of (f²/F). The determination of r1 and r2 is made possible by the extrapolation and the interception at ξ=1 and ξ=0, giving respectively r1 and (-r2/α) (Eq. 2).

η = (r1 + (r2/α)) ζ -(r2/ α) (Eq. 2)
,where η = (f(F-1))/(F(α+(f²/F)); ζ = (f²/F)/(α+(f²/F)) ; α = ((f²/F)max(f²/F)min) 0.5

(iii) The non-linear least squares method [START_REF] Wamsley | Synthesis of random terpolymers and determination of reactivity ratios ofN-carboxyanhydrides of leucine, ?-benzyl aspartate, and valine[END_REF][START_REF] Ting | Precise compositional control and systematic preparation of multimonomeric statistical copolymers[END_REF] based on the Mayo-Lewis equation (Eq. 3).

F1 = (r1 f1² + f1 f2) / (r1 f1² + 2 f1 f2 + r2 f2²) (Eq. 3)
Based on the reactivity ratio, the Skeist's model [START_REF] Skeist | Copolymerization: the Composition Distribution Curve[END_REF] (Eq. 4 and 5) was used to predict the cumulative and instantaneous copolymer composition (Fcumul and Finst) as a function of the monomer conversion.

Conv = 1 -(M/M0) = 1-[(f1/f1°) α (f2/f2°) β [(f1°-δ)/ (f1-δ)] γ ] (Eq. 4)
,where M0 and M are the initial and the instantaneous monomer concentration, f° and f correspond to the initial and the instantaneous mole fraction in the feed and α, β, δ, γ are defined as follows:

α = r2 /(1-r2) ; β = r1/(1-r1) ; γ = (1-r1r2)/ (1-r1) (1-r2); δ = (1-r2)/(2-r1- r 2 ) F1 cumul = [f1°-f1(1-conv)]/conv (Eq. 5)
Organometallic-mediated radical polymerization of MBL via reversible termination (RT).

MBL (0.98 g, 10 mmol) was placed in a 15 mL Schlenk tube, degassed by three freeze-pumpthaw cycles and placed under argon. A solution of alkyl-cobalt(III) initiator (PVAc<4-Co(acac)2) in CH2Cl2 (0.18 mL of a 0.111 M stock solution, 0.020 mmol) was added to MBL followed by evaporation of CH2Cl2 under reduced pressure at room temperature. The reaction mixture ([MBL]0/[PVAc<4-Co(acac)2]0 = 500/1,) was heated at 40 °C for 24 h. After 24 h, one sample was taken and added with TEMPO (1 mg/0.1 mL of sample) in order to quench the polymerization before determining the conversion by gravimetry. Another sample was taken and dissolved in DMF (containing 10 mg of TEMPO per mL of DMF) before determining the molecular parameters (Mn, Đ) by SEC calibrated with polystyrene in DMF. Finally, the polymerization mixture was quenched by addition of TEMPO (0.20 g) dissolved in DMF (1 mL). The final copolymer was purified by precipitation in methanol (three times) and dried overnight under vacuum at 80 °C before characterization via 1 H NMR, [START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR, HSQC and COSY in CD3CN.

Organometallic

Base hydrolysis of poly(MBL-co-VAc).

Poly(MBL-co-VAc) (1 g, 13800 g/mol, FMBL = 0.32) was dissolved in THF (10 mL). NaOH (1 g, 25 mmol) in deionized water (10 mL) was added to the reaction mixture followed by stirring at room temperature for 48 h. The solution was then dialyzed (membrane 1 kDa) against distilled water for 48 h followed by lyophilisation. The copolymer, recovered as a powder, was characterized via 1 H NMR, [START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR, HSQC and COSY in D2O and by FT-IR.

RESULTS AND DISCUSION

Synthesis and conventional radical polymerization of MBL.

A straightforward synthesis of methylene lactones consists in the cycloisomerization of -alkynoic acids catalyzed by palladium, [START_REF] García-Álvarez | Green Chemistry Pd ( II ) -catalyzed cycloisomerisation of γalkynoic acids and one-pot tandem cycloisomerisation / CuAAC reactions in water †[END_REF][START_REF] Saavedra | Impregnated Palladium on Magnetite as Water Compatible Catalyst for the Cycloisomerization of Alkynoic Acid Derivatives[END_REF] gold, 68-69 copper [START_REF] Botelho | Cu ( I ) Camphor Coordination Polymers : Synthesis and Study of the Catalytic Activity for Cyclization of 4-Pentyn-1-oic Acid[END_REF][START_REF] Naidu | z Sustainable Chemistry A Green and Recyclable Copper and Ionic Liquid Catalytic System for the Construction of Poly-heterocyclic Compounds via One-pot Tandem Coupling Reaction[END_REF] or cobalt [START_REF] Leconte | A Highly Active Diradical Cobalt(III) Catalyst for the Cycloisomerization of Alkynoic Acids[END_REF] complexes. Inspired by a recent report, [START_REF] Saavedra | Impregnated Palladium on Magnetite as Water Compatible Catalyst for the Cycloisomerization of Alkynoic Acid Derivatives[END_REF] we prepared the five membered ring MBL through cycloisomerization of 4-pentyn-1oic acid using 0.5 mol% of Pd(OAc) 2 as catalyst (Scheme 2A). Compared to the original work, [START_REF] Saavedra | Impregnated Palladium on Magnetite as Water Compatible Catalyst for the Cycloisomerization of Alkynoic Acid Derivatives[END_REF] water was replaced by chloroform as solvent to avoid the hydrolysis of the targeted lactone into 4-oxo-pentynoic acid. The conversion of 4-pentyn-1-oic acid reached 77 % after 45 h at room temperature but was almost complete after 24 h when performed at 50 °C, as indicated by the 1 H NMR spectrum in Figure S1b. The vinyl lactone was then isolated in a 70 % yield by silica gel chromatography. [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H and [START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR spectra (Figure S1c and S1d, respectively) confirmed the structure and the purity of MBL which was stored under nitrogen at -6 °C before use in polymerization. Note that neither γMγBL nor γ-alkynoic acid can be extracted from the biomass.

Scheme 2. Synthesis and conventional radical (co)polymerization of MBL.

The conventional radical homopolymerization of MBL was first tested in bulk using a low temperature azo-initiator, namely 2,2'-azobis(4-methoxy-2,4-dimethyl valeronitrile) (V70), at 40 °C (Scheme 2b), but no polymerization occurred under these conditions (Table 1, entry 1).

However, the conversion of MBL reached 41 % when the polymerization was conducted at 70 °C using 5 mol % of azobis(isobutyronitrile) (AIBN) as initiator (Table 1, entry 2). A poly(methylene-γ-butyrolactone) (PMBL) with a Mn SEC of 4200 g/mol and a dispersity of 1.89 was obtained. The 1 H NMR (Figure 1a) as well as the COSY and HSQC (Figure S2) experiments confirmed the structure of the PMBL. Next, we considered the conventional radical copolymerization MBL with its acyclic analog, i.e.

vinyl acetate (Scheme 2c). The MBL/VAc copolymerization was carried out in bulk for 24 h at 40 °C in the presence of 5 mol% of V70. Two initial molar fractions of MBL (f°MBL) were considered, i.e. 0.2 and 0.4 (Table 1, entries 3 and 4). In both cases, the comonomer conversion exceeded 50 %. After isolation by precipitation in heptane, polymers were analyzed by [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR (Figure 1b) but also by COSY and HSQC (Figure S3). The latter confirmed the presence of MBL and VAc units in both structures. The incorporation of MBL within the copolymer was quantified by [START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR by comparing the relative intensity of the signal corresponding to the methylene protons e of MBL units at 2.6 ppm and the signal b assigned to the methine protons of VAc units around 5 ppm. Accordingly, we found that 19 and 32 mol% of MBL were respectively incorporated in the P(MBL-co-VAc)s (Table 1 entries 3 and 4, respectively). Interestingly, the molar fraction of MBL in the copolymer (FγMBL) was only slightly lower than its molar fraction in the initial feed (f°γMBL). This observation was further corroborated by the reactivity ratios measured hereafter. In order to determine the MBL/VAc reactivity ratios and get insight into the distribution of the MBL and VAc units, a series of copolymerizations with different initial feed compositions (f°γ MBL ranging from 0.1 to 0.7) were initiated by V70 in bulk at 40 °C (Table S1). Polymerizations were stopped at low conversion to prevent significant composition drift. The copolymers were purified by precipitation in heptane before measuring their composition by 1 H NMR as described above. These experimental data were then used to determine the reactivity ratios by the Fineman-Ross 61 (FR, Figure 2a) and Kelen-Tüdős 62 (KT, Figure 2b) linearization methods and by the nonlinear least square fitting method based on the Mayo Lewis equation [START_REF] Wamsley | Synthesis of random terpolymers and determination of reactivity ratios ofN-carboxyanhydrides of leucine, ?-benzyl aspartate, and valine[END_REF][START_REF] Ting | Precise compositional control and systematic preparation of multimonomeric statistical copolymers[END_REF] (NL, Figure 2c). Note that the reactivity ratios obtained by these methods are highly consistent (rγMBL = 0.2, rVAc = 1.05).

The low value of rγMBL is in agreement with its low tendency to homopolymerize at 40 °C. On the other hand, the rVAc close to 1 indicates that radical chains with a VAc terminal unit react almost randomly with MBL and VAc. Based on these reactivity ratio values, we also evaluated the possible composition drift along the P(MBL-co-VAc) copolymer using the Skeist's model. [START_REF] Skeist | Copolymerization: the Composition Distribution Curve[END_REF] Figure 2d represents the evolution of the cumulative (FγMBL cumul) and instantaneous (FγMBL Inst) molar fraction of γMBL in the P(MBL-co-VAc) copolymer as a function of the overall molar monomer conversion when using 40 mol% of γMBL in the initial feed (f°γMBL = 0.4). As a confirmation of the validity of this model, the cumulative composition of the P(MBL-co-VAc) copolymer (FγMBL = 0.32) formed at 54 % of overall conversion (Table 1, entry 4) is in agreement with the cumulative molar fraction of MBL predicted at this conversion by the Skeist's model (see Figure 2d, FγMBL cumul = 0.33). Interestingly, the instantaneous composition (FγMBL inst)

remains relatively constant at least until 60 % conversion and suggests a low drift of the composition along the polymerization. At higher conversion, the Skeist's curve indicates a significant increase of the MBL content in the copolymer. Nevertheless, one must keep in mind that incorporating more than 50 % of this monomer within the copolymer is difficult given its low propensity to homopolymerize. The knowledge of the insertion mode of the comonomers within P(MBL-co-VAc) copolymers is a major asset for addressing their precision synthesis via reversible deactivation radical polymerization.

Reversible deactivation radical (co)polymerization of MBL.

As a reminder, high level of control was achieved for the OMRP of VAc, the acyclic analog of MBL, when Co(acac)2 was used as controlling agent. [START_REF] Debuigne | Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators[END_REF][START_REF] Morin | Effect of head-to-head addition in vinyl acetate controlled radical polymerization: why is Co(acac)2mediated polymerization so much better?[END_REF] The equilibrium between the alkylcobalt(III) dormant species and the active radical species occurred via reversible cleavage of the Co-C bond at moderate temperature. Typically, the polymerization of VAc proceeded in a controlled manner when carried out in bulk at 40 °C using a preformed PVAc~4-Co(acac)2 species as initiator (Scheme 3). Under these conditions, PVAc with low dispersity (Đ ~1.1) and predictable molar mass was formed. [START_REF] Morin | Effect of head-to-head addition in vinyl acetate controlled radical polymerization: why is Co(acac)2mediated polymerization so much better?[END_REF] Considering the structural similarities between VAc and MBL, the homopolymerization of the vinyl lactone was tested first under the same conditions, namely 40 °C, in bulk, with PVAc~4-Co(acac)2 species as initiator [MBL]/[PVAc~4-Co(acac)2] = 500 (Table S2, entry 1). However, the polymerization stopped after 6 h (conversion = 9 %) and no evolution of the molar mass was observed along the polymerization (Mn SEC ~ 7000 g/mol, Đ ~ 1.6-1.7), suggesting an uncontrolled process. Note that very similar results were obtained when repeating the same experiment at 65 °C (Table S2, entry 2). Such a low tendency of MBL to propagate via OMRP most probably results from premature irreversible termination reactions.

Scheme 3. Organometallic-mediated radical copolymerization of MBL and VAc.

In order to gain insight in the OMRP of MBL and to prepare novel polymer structures, we considered the reversible deactivation radical copolymerization of MBL and VAc (Scheme 3).

A series of organometallic-mediated radical copolymerizations of MBL and VAc was first carried out in bulk at 40 °C with the PVAc~4-Co(acac)2 initiator (Table 2). Three initial composition feeds were considered with MBL molar fraction (f°MBL) ranging from 0.2 to 0.7. Molecular parameters (Mn and Đ) of the copolymer and the monomer conversion were monitored throughout the polymerization via SEC and 1 H NMR, respectively. At the desired time/conversion, TEMPO was added to the reaction medium in order to trap irreversibly the radical chains and stop the polymerization. The content of MBL in the final copolymer (FMBL), measured by 1 H NMR, is indicated in Table 2. A representative spectrum is provided in Figure S4a. The structure of the P(MBL-co-VAc) copolymer is also confirmed by the COSY and HSQC spectra (Figures S4b andS4c).

Regardless of the initial composition feed, PVAc~4-Co(acac)2 successfully initiated the MBL/VAc copolymerization at 40 °C. Moreover, the copolymerization rate was clearly lower when the content of MBL in the feed was higher. After 12 h, the conversion reached 36 %, 20 % and 11 % for f°MBL equal to 0.2, 0.4 and 0.7, respectively (Table 2). This trend is obvious when considering the time dependence of the ln[M]0/[M] function (Figure 3a) for the three compositions.

The molar fraction of MBL was systematically lower in the copolymer compared to the feed as suggested by the reactivity ratios (rγMBL = 0.2, rVAc =1.05). Note that the experimental FMBL (FMBL exp = 0.19, 0.33 and 0.53) were close to the prediction based on these reactivity ratios (FMBL th = 0.17, 0.31 and 0.50). As an additional proof of the incorporation of MBL in the copolymers, the thermal degradation profile of the copolymers and their glass transition temperature (Tg) differ from those of PVAc (Figure S5). Due to the incorporation of rigid lactones along the backbone of the P(MBL-co-VAc) copolymers, Tg increased from 31 °C for a homo-PVAc prepared by OMRP to 64 °C, 77 °C and 83 °C for copolymers with FMBL equal to 0.19, 0.33 and 0.53, respectively. As illustrated in Figure 3b, Mn of P(MBL-co-VAc) regularly increases with monomer conversion, suggesting the controlled character of the copolymerization. The shift of the SEC chromatograms of the samples withdrawn at different polymerization times is represented in Figure 3c. Moreover, the dispersities remained low (Đ ~ 1.1-1.3) until 30 % conversion but tended to broaden at higher conversion, especially for the MBL-rich copolymers. The theoretical molar masses of the final copolymers are also mentioned in Table 2. The latter are calculated based on the monomer/initiator ratio, the conversion and the composition of the copolymers. Note that Mn exp is determined by SEC using a PS calibration which might also account for the deviation from Mn th. As expected for a controlled process, the P(MBL-co-VAc) molar mass also strongly depends on the comonomer/initiator ratios. For a similar MBL/VAc molar ratio (2/8), P(MBL-co-VAc) with higher molar masses were obtained when increasing the [comonomers]/[PVAc-Co] ratio as illustrated in Figure S6 and Table S3.

(■) (see Table 2 for detailed conditions). (c)

Overlay of SEC traces for the OMRP of MBL and VAc (f°MBL = 0.2, Table 2 entry 1).

Next, we took advantage of the controlled copolymerization of MBL and VAc for macromolecular engineering, in particular for the one-pot synthesis of PVAc-b-P(MBL-co-VAc) block copolymers. The general strategy consists in the sequential OMRP of VAc and MBL/VAc copolymerization (Figure 4). In practice, a well-defined PVAc-Co(acac)2 precursor (Mn SEC = 10000 g/mol, Đ = 1.06) prepared at 40 °C in bulk from PVAc4-Co(acac)2 served as macroinitiator for the MBL/VAc copolymerization (f°MBL = 0.2) performed under the same experimental conditions. The overlay of the SEC chromatograms in Figure 4 confirms the efficiency of the chain extension as well as the controlled character of the synthesis of the statistical sequence. Indeed, the chromatogram corresponding the PVAc macroinitiator was clearly shifted towards higher molar masses. It should be noted, however, that a tailing appeared on the low molar mass side of the peak probably due to bimolecular radical termination or chain transfer reactions causing a broadening of the peaks beyond 30 % of monomer conversion. At this stage, we cannot exclude that the broadening of the SEC peaks also results from difficulty to reactivate some inverted head-to-head MBL adducts. Nevertheless, MBL-containing block copolymers were successfully prepared for the first time. Some interesting mechanistic conclusions can be drawn at this stage on the OMRP of MBL. All the above mentioned experiments were initiated with a preformed alkylcobalt(III) species in the absence of additional radical source and therefore necessarily followed a reversible termination (RT) pathway, [START_REF] Debuigne | Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators[END_REF][START_REF] Maria | Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac)2]: Degenerative Transfer versus Reversible Homolytic Cleavage of an Organocobalt(III) Complex[END_REF][START_REF] Debuigne | Key role of metal-coordination in cobalt-mediated radical polymerization of vinyl acetate[END_REF] which consists in the reversible homolytic rupture of the cobalt-carbon bond at the polymer chain-end (Figure 5). In the case of the OMRP of VAc, the bond dissociation enthalpy of the PVAc-Co(acac)2 model (species 2) was estimated at 9.2 Kcal/mol by DFT calculations. Moreover, in absence of coordinating molecules, the PVAc-Co(acac)2 dormant species also benefits from extra-stabilization of 3.9 Kcal/mol upon intramolecular chelation via the terminal ester unit of the PVAc chain (species 3). [START_REF] Debuigne | Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators[END_REF][START_REF] Maria | Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac)2]: Degenerative Transfer versus Reversible Homolytic Cleavage of an Organocobalt(III) Complex[END_REF][START_REF] Debuigne | Key role of metal-coordination in cobalt-mediated radical polymerization of vinyl acetate[END_REF] For the sake of the comparison, we In addition to the reversible termination (RT) pathway, OMRP is also known to proceed via a degenerative chain transfer (DT) mechanism, in which an active radical chain displaces the dormant chain bound to the metal via an associative exchange reaction. [START_REF] Debuigne | Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators[END_REF][START_REF] Maria | Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac)2]: Degenerative Transfer versus Reversible Homolytic Cleavage of an Organocobalt(III) Complex[END_REF][START_REF] Debuigne | Key role of metal-coordination in cobalt-mediated radical polymerization of vinyl acetate[END_REF] This mode notably prevails in the absence of coordinating molecules and when an excess of radicals relative to cobalt is generated in the polymerization medium. In this case, the polymerization rate and radical concentration are close to those observed for conventional radical polymerizations. In practice, the DT polymerizations are initiated from commercially available conventional radical initiators used in excess compared to the cobalt complex, preventing the synthesis of sensitive alkylcobalt(III) initiators. Both the homopolymerization of MBL and the MBL/VAc copolymerization were tested in the DT mode (scheme 4). S4, entry 1). Under these conditions, the conversion increased from 6 to 28 % between 3 h and 24 h of reaction without significant evolution of the polymer molar mass (Mn ~ 7000 g/mol, Đ ~ 1.6-1.8), indicating an uncontrolled polymerization process. On the other hand, inspired by previous reports on the OMRP of VAc by DT, the controlled copolymerization of MBL/VAc (f°MBL = 0.2) was examined in the DT regime at 30 °C using three equivalents of V70 relative to Co(acac)2 (Table S4, entry 2). Kinetics data and the molecular parameters (Mn, Đ)

dependence on the monomer conversion are plotted in Figure 6. In contrast to the MBL/VAc copolymerization initiated with the PVAc-Co(acac)2 initiator and carried out under the RT regime, an induction period of several hours was observed in the DT mode (Figure 6b). As described elsewhere for the OMRP of VAc, [START_REF] Debuigne | Mechanistic insights into the cobalt-mediated radical polymerization (CMRP) of vinyl acetate with cobalt(III) adducts as initiators[END_REF] the induction period corresponds to the time necessary to generate a sufficient amount of radicals for converting most of the Co II (acac)2 into the alkylcobalt(III) dormant species. After this period, a controlled polymerization of MBL and VAc was observed as suggested by the pseudo-first order kinetics and the linear increase of the molar mass with the monomer conversion (Figure 6b) while maintaining a low dispersity (Đ ~1.1-1.3) (Figure 6a). Overall, this approach represents a straightforward route for the controlled synthesis of P(MBL-co-VAc)s involving readily available initiator and controlling agent. Poly(vinyl alcohol) (PVA) is a major industrial polymer commonly prepared by hydrolysis of poly(vinyl acetate). [START_REF] Debuigne | Synthesis of poly(vinyl acetate) and poly(vinyl alcohol) containing block copolymers by combination of cobalt-mediated radical polymerization and ATRP[END_REF][START_REF] Detrembleur | Preparation of well-defined PVOH/C60 nanohybrids by cobalt-mediated radical polymerization of vinyl acetate[END_REF][START_REF] Bryaskova | Synthesis of Poly(vinyl acetate)-b-Polystyrene and Poly(vinyl alcohol)-b-Polystyrene copolymers by Cobalt-Mediated Radical Polymerization[END_REF] The presence of one hydroxyl group per repeating unit makes it highly hydrophilic although PVA with high level of hydrolysis exhibit limited water solubility due to the formation of regular hydrogen-bonding networks. [START_REF] Dréan | Controlled synthesis of poly(vinylamine)-based copolymers by organometallic-mediated radical polymerization[END_REF][START_REF] Debuigne | Synthesis of poly(vinyl acetate) and poly(vinyl alcohol) containing block copolymers by combination of cobalt-mediated radical polymerization and ATRP[END_REF][START_REF] Park | Acid-Catalyzed Hydrolysis Reaction of Poly(vinyl acetate)[END_REF][START_REF] Hassan | Structure and Applications of Poly (vinyl alcohol) Hydrogels Produced by Conventional Crosslinking or by Freezing/Thawing Methods[END_REF] On the other hand, the literature is replete with examples of modified-PVA produced by radical copolymerization of VAc with comonomers prior to hydrolysis or partial hydrolysis. [START_REF] Bryaskova | Synthesis of Poly(vinyl acetate)-b-Polystyrene and Poly(vinyl alcohol)-b-Polystyrene copolymers by Cobalt-Mediated Radical Polymerization[END_REF][START_REF] Debuigne | Amphiphilic poly(vinyl acetate)-bpoly(N-vinylpyrrolidone) and novel double hydrophilic poly(vinyl alcohol)-b-poly(Nvinylpyrrolidone) block copolymers prepared by cobalt-mediated radical polymerization[END_REF][START_REF] Bernard | Poly(vinyl ester) Star Polymers via Xanthate-Mediated Living Radical Polymerization : From Poly(vinyl alcohol) to Glycopolymer Stars[END_REF][START_REF] Harrisson | RAFT Polymerization of Vinyl Esters: Synthesis and Applications[END_REF][START_REF] Arai | Reason for the High Solubility of Chemically Modified Poly(vinyl alcohol)s in Aqueous Solution[END_REF] Both approaches disrupt the regular sequence of hydroxyl groups typical of PVA. In this respect, the hydrolysis of the MBLcontaining (co)polymers prepared above should offer new functionalities to PVA without altering its characteristic hydroxyl sequence. Indeed, the hydrolysis of the pendant lactone ring should release one alcohol moiety and one carboxylic acid group within the same repeating unit (scheme 5). This equals to the modification of the PVA backbone at the methine position. Interestingly, decorating PVAs with carboxylic acid moieties should impart to them unique pH-responsiveness.

These were strong incentives to investigate the hydrolysis of both PMBL and P(MBL-co-VAc)

and to carry out a preliminary study of its water solution properties.

Scheme 5. Hydrolysis of PMBL and P(MBL-co-VAc).

A homo-PMBL (4200 g/mol, Đ = 1.89) and a P(MBL-co-VAc) (13800 g/mol, Đ = 1.65, FMBL = 0.32), prepared by conventional radical polymerization and OMRP respectively, were subjected to basic hydrolysis. Typically, PMBL was treated with sodium hydroxide in an acetonitrile/water

(1/1 : v/v) mixture at room temperature for 48 h followed by dialysis against distilled water. Note that the water solubility of the treated polymer was a first indication of significant modification of the polymer structure. The successful hydrolysis of most of the pendant lactone ring of PMBL was further confirmed by IR (Figure S7b). Indeed, the IR spectrum shows a significant decrease of the characteristic signals of the esters units (C=O ester) at 1759 cm -1 and the appearance of a large band at 3300 cm -1 , typical of the hydroxyl moieties, as well as an intense peak at 1556 cm -1 assigned to the carboxylate groups. The 1 H NMR in D2O of the treated PMBL, namely the poly(4hydroxypent-4-enoic acid) (PHPEA), also supports the successful hydrolysis of lactones (Figure 7a). Indeed, a signal corresponding to methylene protons b at the alpha position of the carboxyl group appeared at 2.20 ppm. Note that some residual lactone (10 %) was detected at 2.65 ppm. The structure of the hydrolyzed PMBL was also corroborated by COSY and HSQC (Figure S8). The P(MBL-co-VAc) copolymer (13800 g/mol, Đ = 1.65, FMBL = 0.32) was treated with sodium hydroxide in a THF/water (1/1 : v/v) mixture at room temperature for 48 h, leading to the corresponding modified PVA copolymer via hydrolysis of both lactones and acyclic pendant esters as confirmed by IR (Figure S7d) and 1 H NMR (Figure 7b). In addition to the peak at 2.20 ppm corresponding to the methylene protons e, namely the alpha position of the carboxyl group in the hydrolyzed MBL units, we also observed characteristic signals of the vinyl alcohol units, namely the methine protons b around 4 ppm. In this case, 3 % of unmodified lactone was detected at 2.65 ppm. COSY and HSQC spectra of the P(HPEA-co-VA) copolymer is provided as supporting information (Figure S9). Finally, we carried out a preliminary study of the aqueous solution properties of the PHPEA and P(HPEA-co-VA) (FHPEA = 0.32) (co)polymers prepared above. Given the presence of carboxylic group along their backbone, special care was devoted to their possible pH-responsiveness. As illustrated in Figure 8, both PHPEA and P(HPEA-co-VA) were fully soluble in water at pH 7 (at 5 mg/mL). Under these conditions, the pendant carboxylic acid groups are deprotonated, which favors the (co)polymer solubilization. However, drastic changes occurred upon acidification of the solution to pH 1. Indeed, the PHPEA clearly precipitates after protonation of its carboxylic acid groups leading to the formation of rather large particles as emphasized by DLS (mean diam > 2 µm). In contrast, the aqueous solution of P(HPEA-co-VA) did not turn opaque when acidified to pH 1, rather it became slightly turbid suggesting the formation of smaller objects. In addition to few large aggregates, DLS emphasized the formation of objects with an average diameter of 550 nm. Compared to PHPEA, which exhibits one pH sensitive carboxylic acid function per repeating unit, the P(HPEA-co-VA) presents higher hydroxyl/carboxylic acid molar ratio which favored the stabilization of the submicron objects at low pH. Note that the transition occurred around pH 2.2 and was fully reversible for more than 7 acidification/basification cycles as illustrated in Figure S10.

  Conventional radical copolymerization of MBL and VAc. V-70 (0.31 g, 1.0 mmol) and MBL (0.78 g, 8.0 mmol) were placed in a 15 mL Schlenk tube, degassed by three freeze-pumpthaw cycles and placed under argon. After addition of degassed VAc (1.10 mL, 1.03 g, 12.0 mmol), the polymerization medium (f°MBL = 0.4) was heated at 40 °C. After 24 h, samples were withdrawn for determining the conversion by[START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR in CDCl3 and the molecular parameters (Mn, Đ) by SEC in THF. The final copolymer was purified via repeated precipitations (three times) in n-heptane and dried under vacuum overnight at 80 °C before characterization by 1 H NMR,[START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR, HSQC and COSY in CDCl3.A similar experiment was carried out with a different initial comonomer feed ratio, namely f°MBL = 0.2, keeping constant all other parameters(40 °C, [comonomers]0/ [V-70] 0 = 100/5).Determination of reactivity ratios for radical copolymerization of MBL and VAc. A series of MBL/VAc copolymerizations were carried out in bulk at 40 °C with different comonomer ratios (0.1 < f°MBL < 0.7) using V-70 as initiator ([comonomers]0/[V-70]0=100/3).

  -mediated radical copolymerization of MBL and VAc via reversible termination (RT). MBL (0.49 g, 5.0 mmol) was placed in a 15 mL Schlenk tube, degassed by three freeze-pump-thaw cycles and placed under argon. A solution of alkyl-cobalt(III) initiator (PVAc<4-Co(acac)2) in CH2Cl2 (0.78 mL of a 0.064 M stock solution, 0.05 mmol) was added to MBL followed by evaporation of CH2Cl2 under reduced pressure at room temperature. Finally, degassed VAc (1.84 mL, 1.72 g, 20 mmol) was injected under argon. The reaction mixture (f°MBL = 0.2 and f°VAc = 0.8, [VAc]0/[MBL]0/[PVAc<4-Co(acac)2] = 400/100/1) was heated at 40 °C for 12 h. Samples were regularly taken and added with of TEMPO in order to quench the polymerization. The monomer conversion was determined by[START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR in CDCl3 (1 mg of TEMPO was added per mL of CDCl3). The molecular parameters (Mn, Đ) of the polymer were determined by SEC in THF using a polystyrene calibration (samples were dissolved in THF containing 10 mg of TEMPO/mL of THF). After 12 h, the polymerization mixture was quenched by addition of TEMPO (0.20 g) dissolved in DMF (1 mL). The final copolymer was purified by precipitation in n-heptane (three times) and dried overnight under vacuum at 80 °C before characterization via 1 H NMR,[START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR, HSQC and COSY in CDCl3. Similar experiments were carried out with a different monomer initiator ratios keeping constant all other parameters (f°MBL = 0.2, 40 °C, bulk): [VAc]0/[MBL]0/[PVAc<4-Co(acac)2] 0 = 800/200/1 and 200/50/1. The initial comonomer feed ratio was also varied (f°MBL = 0.4 and 0.7) keeping constant all other parameters (40 °C, bulk, [comonomers]0/ [PVAc<4-Co(acac)2] 0 = 500). Note that the copolymer produced from f°MBL = 0.7 was recovered by precipitation in methanol instead of heptane.Organometallic-mediated radical polymerization of MBL via degenerative chain transfer (DT). MBL (0.98 g, 10 mmol) was placed in a 15 mL Schlenk tube. After degassing by three freeze-pump-thaw cycles, the degassed MBL was transferred to a 15 mL Schlenk tube with Co(acac)2 (0.005 g, 0.02 mmol) and AIBN (0.0099 g, 0.06 mmol) under argon. The reaction mixture ([MBL]0/ [AIBN] 0/[Co(acac)2] 0 = 500/3/1) was heated at 70 °C for 24 h and samples were regularly taken from the reaction mixture and added with TEMPO in order to quench the polymerization. The monomer conversion was determined by gravimetry after addition of TEMPO (1 mg/0.1 mL of sample). The molecular parameters (Mn, Đ) of the polymer were determined by SEC in DMF using a polystyrene calibration (samples were dissolved in DMF containing 10 mg of TEMPO/mL of DMF). After 24 h, the polymerization mixture was quenched by addition of TEMPO (0.20 g) dissolved in DMF (1 mL).The final copolymer was purified by precipitation in methanol (three times) and dried overnight under vacuum at 80 °C before characterization via 1 H NMR,[START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR, HSQC and COSY in CD3CN. A similar experiment was conducted at 40 °C using V-70 instead of AIBN ([MBL]0/[V-70] 0/[Co(acac)2] 0 = 500/3/1). Organometallic-mediated radical copolymerization of MBL and VAc via degenerative chain transfer (DT). MBL (0.49 g, 5.0 mmol) were degassed by three freeze-pump-thaw cycles and transferred to a 15 mL Schlenk tube with Co(acac)2 (0.013 g, 0.05 mmol) and V-70 (0.046 g, 0.15 mmol) under argon. Degassed VAc (1.84 mL, 1.72 g, 20 mmol) was injected under argon. The reaction mixture (f°MBL = 0.2, [VAc]0/[MBL]0/[V-70]0/[Co(acac)2] 0 = 400/100/3/1) was heated at 40 °C for 28 h. Samples were regularly picked out of the tube and added with traces ofTEMPO in order to quench the polymerization. The monomer conversion and the molecular parameters (Mn, Đ) of the polymer were determined by[START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR in CDCl3 and SEC calibrated with polystyrene in THF, respectively. After 28 h, the polymerization mixture was quenched by addition of 1 mL TEMPO in THF (0.20 g/mL). The final copolymer was purified by precipitation in heptane (three times) and dried overnight under vacuum at 80 °C before characterization via 1 H NMR,[START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR, HSQC and COSY in CDCl3.Synthesis of PVAc-b-P(MBL-co-VAc) block copolymer by OMRP.A solution of alkylcobalt(III) initiator (PVAc<4-Co(acac)2) in CH2Cl2 (0.23 mL of a 0.111 M stock solution, 0.025 mmol) was added into a 15 mL Schlenk tube followed by evaporation of CH2Cl2 under reduced pressure at room temperature. Degassed VAc (1.20 mL, 1.08 g, 12.5 mmol) was injected under argon at 40 °C. After 3 h, a sample was picked out of the tube and added with traces of TEMPO (~10 mg/mL) in order to quench the polymerization for determining the monomer conversion by[START_REF] Gowda | Sustainable Polymers from Biomass-Derived α-Methyleneγ-Butyrolactones[END_REF] H NMR in CDCl3 (conversion = 27 %) and the molecular parameters of the PVAc precursor (10000 g/mol, Đ = 1.06) by SECTHF cal PS. Then, unreacted VAc was evaporated under reduced pressure at room temperature. The degassed mixture of MBL (0.25 g, 2.5 mmol) and VAc (0.96 mL, 0.86 g, 10 mmol) were injected under argon. The polymerization medium was heated at 40 °C for 15 h and samples were regularly taken and added with traces of TEMPO in order to quench the polymerization. The monomer conversion and the molecular parameters (Mn, Đ) of the polymer were determined by 1 H NMR in CDCl3 and SECTHF cal PS, respectively. After 15 h, the polymerization mixture was quenched by addition of 1 mL TEMPO in THF (0.20 g/mL) (global conv = 59 %, Mn SEC THF cal PS 19000 g/mol, Đ = 1.43). The final copolymer was purified by precipitation in n-heptane (three times) and dried overnight under vacuum at 80 °C before characterization via 1 H NMR,[START_REF] Zhang | Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N -Heterocyclic Carbenes: Mechanisms of Chain Initiation[END_REF] C NMR, HSQC and COSY in CDCl3.

Figure 1 . 1 H

 11 Figure 1. 1 H NMR of PMγBL (a) and P(MBL-co-VAc) (FMγBL = 0.32) (b) in CD3CN and

Figure 2 . 4 (

 24 Figure 2. Determination of the reactivity ratios for the radical copolymerization of MBL and

Figure 3 .

 3 Figure 3. (a) Time dependence of ln[M]0/[M] and (b) dependence of Mn (full symbols) and Đ

Figure 4 .

 4 Figure 4. General strategy for the synthesis of MBL-containing block copolymers by OMRP and

  performed similar DFT calculations for MBL. The bond dissociation enthalpy of the model PMBL-Co(acac)2 (species 2') was slightly higher compared to the PVAc-Co(acac)2 one, namely 12.0 Kcal/mol. However, the stabilization of the dormant PMBL-Co(acac)2 by intramolecular chelation of the cobalt by the ester function unit is hampered by the geometrical constraint of the lactone. The reorganization of the coordination sphere from square pyramidal with apical alkyl group to square pyramidal with the apical position occupied by one O atom of one acac ligand, which is required to generate a cis vacant site for monomer chelation in species 3', is energetically costly (+15.1 kcal/mol), but cannot be compensated by donation of the carbonyl oxygen electron density as in species 3. Instead, only a slight interaction with the endocyclic oxygen atom may occur, as shown by the distortion imposed on the cobalt-bonded C atom configuration (Co-C-O = 84.1°; Co-C-C(H3) = 117.1°; Co-C-C(H2) = 117.6°). This interaction, however, is very weak (Co•••O = 2.27 Å; cf. with the Co-O distance of 1.93 Å in the chelated VAc monomer in species 3). Overall, the chelated PVAc-Co and non-chelated PMBL-Co (dormant species 3 and 2', respectively) exhibit quite similar global bond dissociation enthalpies (13.1 and 12.0 Kcal/mol, respectively) and should efficiently reactivate under similar conditions, for example in bulk at 40 °C. Consequently, the extremely low conversion reported earlier for the homopolymerization ofMBL by OMRP at 40 °C cannot result from a high PMBL-Co(acac)2 bond strength. It can rather be attributed to a low kp of MBL at this temperature. As a confirmation, no conversion was observed for the conventional radical homopolymerization of MBL at 40 °C (Table1, entry 1). Concerning the OMRP of MBL at 70 °C, it is reasonable to assume that the PMBL-Co(acac)2 bond is too weak to establish an appropriate equilibrium between dormant and active species at this temperature, leading to massive and premature termination of the radical chains and ultimately inhibition of the polymerization by accumulation of the cobalt(II) deactivator in the polymerization medium. However, the successful organometallic-mediated radical copolymerization of VAc and MBL at 40 °C is fully consistent with similar C-Co bond strength for the VAc and MBL model dormant species. Under these conditions, both dormant species can reactivate and the terminal MBL radicals can further cross propagate with VAc leading to welldefined P(MBL-co-VAc) copolymers.

Figure 5 .

 5 Figure 5. DFT calculation of the bond dissociation enthalpies for the MBL-Co(acac)2 and

Scheme 4 .

 4 Scheme 4. Mechanism of the homopolymerization of MBL and the MBL/VAc

Figure 6 .

 6 Figure 6. (a) Time dependence of ln[M]0/[M] and (b) dependence of Mn and Đ on the monomer

Figure 7 . 1 H

 71 Figure 7. 1 H NMR spectra in D2O after hydrolysis of (a) PHPEA and (b) P(HPEA-co-VA) obtained

Figure 8 .

 8 Figure 8. Illustration of the aqueous solution behavior and pH-responsiveness of (a) PHPEA and

Table 1 .

 1 Conventional radical (co)polymerization of γMBL.

	Entry	Initiator / Temp	f 	γMBL	f 	VAc	Conv. (%) γMBL VAc	Mn SEC (g/mol)	Ð	F γMBL
	1	V70 / 40 °C		1		0	<1 a	NA	/	/	/
	2	AIBN / 70 °C		1		0	41 a	NA	4200 c	1.89 c	1.00
	3	V70 / 40 °C	0.20	0.80	66 b	75	12000 d	2.24 d	0.19 e
	4	V70 / 40 °C	0.40	0.60	54 b	57	8100 d	2.01 d	0.32

e Conditions: bulk polymerization, 24 h, 5 mol% of azoinitiator vs monomer(s). a Determined by gravimetry. b Determined by 1 H NMR in CDCl3. c Determined by SECDMF cal PS. d Determined by SECTHF cal PS. e Determined by 1 H NMR in CDCl3.

Table 2 .

 2 Organometallic-mediated radical (co)polymerization of MBL and VAc.

	Entry f 	MBL	f  VAc	Time (h)	Conv (%) a	Mn SEC (g/mol) b	Ð b	FMBL a	Mn th (g/mol) d
					2	7	5200	1.10
					4	15	6600	1.17
					6	21	7900	1.22
	1	0.20	0.80
					8	27	8500	1.28
					10	32	9400	1.32
					12	36	10200	1.33	0.19	15900
					3	6	4000	1.13
					6	11	5700	1.16
					9	15	6900	1.22
	2	0.40	0.60	12	20	8000	1.28
					15	25	8400	1.36
					30	35	12000	1.56	0.33	15800
					3	4	4400	1.19
					6	6	5900	1.25
	3	0.70	0.30	9	9	6900	1.36
					12	11	7900	1.43
					30	20	9500 c	1.63 c	0.53	9200

Conditions: 40 C, [monomer]/[RCo] =500. a Total monomer conversion determined by 1 H NMR in CDCl3. b Determined by SEC in THF using a PS calibration, c Determined by SEC in DMF/LiBr using a PS calibration due to the low solubility of this sample in THF. d Theoretical molar mass (Mn th) calculated based on the [monomer]/[RCo] ratio, the conversion and the composition of the copolymer.

  Am. Chem. Soc. 2013, 1792517942. (14) He, J.; Zhang, Y.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y. Chain Propagation and Termination Mechanisms for Polymerization of Conjugated Polar Alkenes by [Al]-Based Frustrated Lewis Pairs. Macromolecules 2014, 47, 7765-7774. (15) Chen, X.; Caporaso, L.; Cavallo, L.; Chen, E. Y. Stereoselectivity in Metallocene-Catalyzed Coordination Polymerization of Renewable Methylene Butyrolactones: From Stereorandom to Stereo-perfect Polymers. J. Am. Chem. Soc. 2012, 7278-7281. (16) Miyake, G. M.; Newton, S. E.; Mariott, W. R.; Chen, E. Y. Coordination Polymerization of Renewable Butyrolactone-Based Vinyl Monomers by Lanthanide and Early Metal Catalysts. DaltonTrans. 2010, 39, 6710-6718. No. 43, 9328-9336.(19) Akkapeddi, M. K. Poly (α-Methylene-γ-Butyrolactone) Synthesis, Configurational Structure, and Properties. Macromolecules 1979, 505 (1979), 546-551. (20) Vobecka, Z.; Wei, C.; Tauer, K.; Esposito, D. Poly(α-Methylene-γ-Valerolactone) 1.

	Sustainable Monomer Synthesis and Radical Polymerization Studies. Polymer 2015, 74,
	262-271.

(17) Hu, Y.; Wang, X.; Chen, Y.; Caporaso, L.; Cavallo, L.; Chen, E. Y. Rare-Earth Half-Sandwich Dialkyl and Homoleptic Trialkyl Complexes for Rapid and Stereoselective Polymerization of a Conjugated Polar Olefin. Organometallics 2013, No. 32, 1459-1465. (18) Hu, Y.; Xu, X.; Zhang, Y.; Chen, Y.; Chen, E. Y. Polymerization of Naturally Renewable Methylene Butyrolactones by Half-Sandwich Indenyl Rare Earth Metal Dialkyls with Exceptional Activity. Macromolecules 2010,

ACKNOWLEDGMENTS

The authors from Liege thank the Fonds de la Recherche Scientifique (FNRS) and the Fonds Wetenschappelijk Onderzoek -Vlaanderen (FWO) for funding the EOS project n°O019618F (ID EOS: 30902231) and the NECOPOL project (convention U.N054.17). A.D. and C.D. are Research Associate and Research Director by F.R.S.-FNRS, respectively. We are grateful to the CALMIP mesocenter of the University of Toulouse for the allocation of computational resources.

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website.

Additional data, DFT calculations as well as SEC, NMR, DSC and TGA characterizations.