Abdou Khadri Diallo

Evgueni Kirillov

Martine Slawinski

Jean-Michel Brusson

Sophie M Guillaume

Jean-François Carpentier
email: jean-francois.carpentier@univ-rennes1.fr

Syndioselective Ring-Opening Polymerization and Copolymerization of Trans-1,4-Cyclohexadiene Carbonate Mediated by Achiral Metal-and Organo-Catalysts

Keywords: Cyclic Carbonate, Ring-Opening Polymerization, Stereoselectivity, DFT

The ring-opening polymerization (ROP) of trans-1,4-cyclohexadiene carbonate (CHDC) has been investigated computationally and experimentally. DFT computations indicate that ring-opening of CHDC is thermodynamically possible, yet to a lesser extent than that of trans-cyclohexene carbonate (CHC). Effective homopolymerizations of rac-CHDC and simultaneous or sequential copolymerizations of rac-CHDC with rac-CHC and L-LA were achieved with a diaminophenolate zinc-based complex ([(NNO)ZnEt]) or a guanidine (TBD) associated with an alcohol. These ROP reactions, which confirmed the lower reactivity of rac-CHDC vs. rac-CHC, especially in homopolymerization, proceeded without any decarboxylation. Quite uniquely, highly syndiotactic PCHDC was obtained from ROP of rac-CHDC with both the zinc-and TBD-based catalysts, as revealed by 13 C{ 1 H} NMR studies.

The prepared homopolymers and block or random copolymers were characterized by 1 H, 13 C{ 1 H} NMR, MALDI-ToF-MS, SEC and DSC techniques.

Introduction

Aromatic and aliphatic polycarbonates (PCs) are important commodity and engineering materials used in a variety of applications. Classic aromatic PCs derived from bisphenol A (BPA) show valuable thermo-mechanical properties; however, they have raised major concerns related to the toxicity of the latter BPA monomer and of phosgene used as the coreagent. Although aliphatic PCs do not have properties that match yet those of aromatic PCs derived from BPA, this class of polymers has attracted much interest in recent years. Indeed, depending on the chemical repeat units and the microstructure, some of these polymers show promising thermal properties (e.g., high Tg and Tm). 1 Also, thanks to their functional termini, they can serve as valuable pre-polymers for the production of other important materials (e.g., polyurethanes from dihydroxy-telechelic low molar mass pre-polymers 2 ). Moreover, besides the traditional, yet poorly controlled polycondensation reaction between a dialkyl carbonate and an α,ω-diol, 3 aliphatic PCs can be more effectively produced by two alternative routes: a) the metal-catalyzed alternating ring-opening copolymerization (ROCOP) of epoxides/CO2, and b) the ring-opening polymerization (ROP) of cyclic carbonate monomers (Scheme 1).

Scheme 1. ROCOP and ROP routes toward polycarbonates, illustrated with cyclohexadiene oxide (CHDO) and cyclohexadiene carbonate (CHDC) for the formation of poly(cyclohexadiene carbonate) (PCHDC).

ROCOP's attractive features include the use of carbon dioxide, the possibility to reach high molar mass polymers, and also to access sometimes high stereoselectivities via desymmetrization of achiral meso epoxides with chiral catalysts. 4 On the other hand, ROCOP faces the issue of incomplete alternating selectivities, which may result in the formation of ether units along the PC chain, and also the formation, alongside the PC, of the corresponding five-membered ring cyclic carbonate (5CC) monomer. A large number of ROCOP studies involved cyclohexene oxide (CHO) and hence led to the production of poly(cyclohexene carbonate) (PCHC), along with variable amounts of its corresponding 5CC monomer (i.e., cyclohexene carbonate, CHC). 5 Whereas the vast majority of catalysts for CHO/CO2 ROCOP promote the formation of atactic PCHC, some of them, essentially Zn, Cr or Co complexes based on chiral salen or -diketiminate derivatives, enable the synthesis of syndiotactic-(up to 81% of r-centered tetrads) 6 or isotactic-(up to (R,R)/(S,S) = 98:2) 7 enriched PCHCs.

ROP of cyclic carbonates (CCs) is another privileged procedure to access welldefined, high molar mass aliphatic PCs. [START_REF]Handbook of Ring-Opening Polymerization[END_REF] Given the ready availability of a large number of 5CCs from epoxide carboxylation (vide supra) or from biosourced 1,2-diols, 9 the valorization of these monomers into PCs certainly deserves consideration. Yet, whereas six-and, to a lesser extent, seven-membered ring CCs have been quite selectively polymerized by ROP under mild operating conditions, the related 5CCs have hardly been ring-opened (co)polymerized. 10 This is obviously related to their small ring-strain and poor polymerizability, 11 which is intimately associated to their abovementioned easy formation.

Hence, although several ROP catalyst systems have been identified for the copolymerization of ubiquitous 5CCs such as ethylene and propylene carbonates, the reactions proceed only at high temperatures with poor-to-modest 5CC conversion, and they are always accompanied by a partial or total decarboxylation of the latter monomer. 10 On the other hand, there are a few examples of strained 5CCs which can undergo selective ROP. Endo and coworkers first performed the successful ROP of the sugar-derived 5CC, methyl 4,6-O-benzylidene-2,3-Ocarbonyl-R-D-glucopyranoside, without decarboxylation using n-BuLi, MOtBu (M = Li, Na, K) or DBU as initiators (Mn,SEC up to 20 200 g.mol 1 , ÐM = ca. 1.8). 12 Also, we have reported recently the easy and efficient ROP of trans-cyclohexene carbonate (CHC). 13 PCHCs with controlled molar masses (Mn,NMR up to 17 000 g.mol 1 , ÐM = ca. 1.2) and a slight isotactic bias (Pm up to 76%) were obtained by ROP of the racemic monomer (rac-CHC) catalyzed by zinc diaminophenolate, zinc -diketiminate, yttrium bis(phenolate) complexes, or 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD), in combination with an alcohol as a co-initiator. Purely isotactic PCHC was synthesized for the first time via ROP of enantiopure (R,R)-CHC with a zinc/benzyl alcohol catalyst system. Parallel to the work of Haba et al. who used tBuOLi to initiate the ROP of CHC, 14 we unveiled the determining impact of the fused trans-cyclohexyl ring-strain to enable the ROP of CHC, as opposed to meso-CHC which is unreactive.

In this contribution, we now report that the ROP of trans-cyclohexadiene carbonate (trans-1,2-cyclohex-4-enyl cyclocarbonate, trans-CHDC, Scheme 1) can be performed also by various metal-based or organo-catalysts. The corresponding poly(cyclohexadiene carbonate) (PCHDC) has just been recently reported by the ROCOP route from meso-CHDO (cyclohexadiene oxide, that is 1,2-epoxy-4-cyclohexene, Scheme 1) 15 and was obtained in a non-stereoselective manner. Quite uniquely, we evidence that the ROP of rac-CHDC, despite the use of non-chiral catalysts, affords pure syndiotactic PCHDCs; besides; highly isotactic PCHDC has been prepared independently from enantio-enriched trans-CHDC. The effective preparation of copolymers of CHDC with CHC and L-lactide is presented as well.

Experimental section

Computational Details. All DFT calculations were carried out using the Gaussian09 program suite, [START_REF] Gaussian | Gaussian[END_REF] employing the M06 functional, [START_REF] Zhao | [END_REF] and using a standard double- polarized basis set, namely the 6-31() set. 18 All stationary points were fully characterized via analytical frequency calculations as true minima (all positive eigen values). Enthalpies were obtained at T = 298 K within the harmonic approximation.

Methods and Instrumentation

NMR spectra were recorded on a AM-400 Bruker spectrometers at 298 K in CDCl3. A relaxation delay of 3 s was used during the acquisition to yield quantitative 1 H NMR spectra.

The chemical shifts were referenced to tetramethylsilane (TMS) using the residual protiosolvent ( 1 H) or the carbon ( 13 C) resonance of the deuterated solvent.

Monomer conversions were determined from 1 H NMR spectra of the crude polymer samples, from the integration (Int.) ratio Int.Polym/[Int.Polym+Int.Monom], typically using the methine and methylene hydrogens.

Molar masses (Mn and Mw) and dispersities (ÐM = Mw/Mn) of polymers and copolymers were determined by size exclusion chromatography (SEC) using a Polymer Laboratories PL-GPC 50 instrument equipped with a set of two ResiPore Mixed E 300 × 7.5 mm columns and a refractive index detector. The sample was eluted with CHCl3 (PCHDC) or THF (copolymers) at 30 °C at 1.0 mL•min -1 and the calibration was carried out using 11 monodisperse polystyrene standards (Mn range from 580 to 380,000 g•mol -1 ).

Differential scanning calorimetry (DSC) analyses were performed on a Setaram DSC 131 apparatus calibrated with indium at a rate of 10 °C.min 1 , under continuous flow of helium (25 mL.min 1 ), using aluminum capsules (typically 10 mg of polymer).

Syntheses

All manipulations were performed under inert atmosphere (argon, <3 ppm O2) using standard Schlenk, vacuum line, and glovebox techniques. Benzyl alcohol and isopropanol were distilled over Mg turnings under argon atmosphere and kept over activated 3-4 Å molecular sieves. Complex [(NNO)ZnEt] was synthesized following the literature procedure. 21 1,5,.0]dec-5-ene (TBD) (98%, Aldrich) and Y[N(SiMe3)2]3 (98%, Strem Chem.) were used as received. Racemic trans-cyclohexene carbonate (rac-CHC) was prepared as previously reported. Technical grade L-lactide (L-LA) was provided by Total Raffinage Chimie and purified by recrystallization from a hot (80 °C), concentrated isopropanol solution, followed by two subsequent recrystallizations in hot (105 °C) toluene. After purification, L-LA was stored at all times at a temperature of -30 °C in the inert atmosphere of the glove-box. Trans-racemic-1,2-cyclohex-4-enediol was prepared from cyclohexadiene following the reported literature procedure. 19 Trans-(R,R)-1,2-cyclohex-4-ene-diol (enantio-enriched, 83% ee) was obtained by resolution of the racemic product (Scheme 3) following the same procedure used for 1,2cyclohexanediol. 20 A round-bottom flask was charged with dicyclohexylcarbodiimide (DCC; 2.00 g, 9.64 mmol), (±)-trans-cyclohex-4-ene-1,2-diol (1.00 g, 8.76 mmol) and N,Ndimethylaminopyridine (DMAP) (0.107 g, 8.76 mmol). Freshly distilled THF (20 mL) was introduced into the flask under an argon atmosphere, and the solution was cooled at 10 °C. A solution of (S)-(+)-O-acetylmandelic acid (1.70 g, 8.76 mmol) in THF (10 mL) was slowly added in. Stirring was maintained over 6 h until the completion of the reaction (TLC monitoring). The reaction mixture was filtered to remove the urea derivative; the filtrate was evaporated under vacuum to obtain a colorless liquid, which was chromatographed using silica gel and 20% EtOAc/petroleum ether as eluent. The first fraction (minor fraction, the other one was discarded) was collected and the solvent was removed under vacuum (0.30 g, 30%). This product was stirred in a 1N methanolic NaOH solution (20 mL) at room temperature for 4 h. After disappearance of the ester (TLC monitoring), MeOH was evaporated, the crude product was purified through Kugelrohr distillation (200 °C, 4 mm Hg) and re-crystallized from benzene. Trans-(R,R)-1,2-cyclohex-4-ene-diol was obtained as a white solid (0.276 g, 92% for this step). Enantiomeric excess (83% ee) was measured by chiral GC using a Beta-dex column, helium (40.0 cm.s 1 ), 90 °C to 160 °C at 2 °C.min 1 , Rt(S,S) = 19.6 min, Rt(R,R) = 20.1 min (Figure S1).

(±)-Trans-1,2-cyclohex-4-enyl cyclocarbonate (rac-CHDC). A solution of triethylamine (10.8 mL, 79 mmol) in THF (30 mL) was added dropwise at 0 °C to a solution of (±)-trans-cyclohex-4-ene-1,2-diol (4.00 g, 35 mmol) and ethyl chloroformate (7.5 mL, 79 mmol) in THF (60 mL). Once the addition completed, the reaction mixture was stirred at 23 ± 2 °C over 6 h. The mixture was next filtered to remove the formed ammonium salt, and THF was removed under vacuum. The crude product was purified by flash column chromatography (pentane/ethyl acetate: 100:0 to 80:20), and then recrystallized three times from diethyl ether to give (±)-trans-CHDC as thin white crystalline needles (3.43 g, 70%). Mp = 128 °C; 1 H NMR (CDCl3, 400 MHz, 23 °C, Figure S2): δ (ppm) 5.67 (d, 2H, CH2CH=CHCH2) A similar procedure was followed for the synthesis of PCHDC with Y[N(SiMe3)2]3 or TBD as catalyst. Typically, [(NNO)ZnEt] (4.0 mg, 9.3 µmol, 1 equiv.) was added to BnOH (1 equiv., as a stock solution in toluene) and stirred at room temperature over 15 min, just prior to the addition of rac-CHDC (64 mg, 0.46 mmol, 50 equiv) and the quantity of toluene necessary to reach a concentration of 4M (Table 1, entry 1). The mixture was then stirred at 60 °C over the appropriate time period (reaction times were not optimized). The polymerization was then stopped upon addition of an acetic acid solution (ca. 0.05 mL of a 1.6 mol.L 1 solution in toluene) and the volatiles removed under vacuum. After determination of the CHDC conversion by 1 H NMR analysis of this crude product, the resulting mixture was dissolved in CH2Cl2, and the polymer was purified upon precipitation (when a significant conversion could be achieved) in cold methanol, filtered and dried under vacuum. The isolated PCHC was then analysed by NMR, SEC, and DSC. 1 H and 13 C NMR spectra of PCHDC homopolymers are available in the Supporting Information (Figures S6-S11).

Trans-(R,R)-1,2-cyclohex-4-enyl cyclocarbonate ((R,R)-CHDC

Simultaneous Copolymerization of rac-CHDC with rac-CHC or L-LA with [(NNO)

ZnEt] / BnOH. The polymerizations were conducted as described above using a mixture of the two monomers. The P(CHC-co-PCHDC) and P(LLA-co-PCHDC) copolymers were isolated by re-precipitation with methanol from a CH2Cl2 solution and then analysed by 1 H NMR, 13 C NMR, SEC and DSC (see the Supporting Information).

Sequential Copolymerization of rac-CHC with rac-CHDC with [(NNO)ZnEt] /

BnOH. The homopolymerization was conducted as described above for rac-CHDC. Rac-under controlled atmosphere to determine the monomer conversion and the molecular mass of the formed PCHC, and rac-CHDC was then introduced in the reaction mixture. The reaction was continued for 3 h. The same workup as above was then applied. The P(CHC-co-PCHDC) and P(LLA-co-PCHDC) copolymers were isolated by re-precipitation with methanol from a CH2Cl2 solution and then analysed by 1 H NMR, 13 C NMR, SEC and DSC (see the Supporting information).

Results and Discussion

Preliminary DFT computations. In order to get a first hint, we assessed the polymerizability of the trans and meso diastereoisomers of CHDC via DFT computations. As for our prior work on the ROP of CHC, 13a the ring-opening of these two molecules by methanol was used as a simple model (Scheme 2). Given the generally accepted uncertainty of ±2 kcal.mol 1 for DFT computations, the calculated G° value of +0.8 kcal.mol 1 suggests that trans-CHDC may be thermodynamically ring-opened. In contrast, for meso-CHDC which is obviously less strained than the trans diastereomer, the clearly endergonic G° value of +6.4 kcal.mol 1 indicates its non-polymerizability. These ring-opening energy values are unfavorably shifted from the corresponding ones determined for trans-CHC (G° = 3.6 kcal.mol 1 ) and meso-CHC (G° = 2.6 kcal.mol 1 ), suggesting a lower polymerizabilty of CHDC as compared to CHC; 13a of note, the G°trans/G°meso energy difference is similar for both CHDC and CHC (5.56.2 kcal.mol 1 ). As we previously confirmed experimentally that meso-CHC is indeed not polymerizable in contrast to trans-CHC (in line with DFT computations), 13a we thus decided to focus our experimental investigations on trans-CHDC. Scheme 2. Model reactions used for DFT computations of ring-opening of meso-and trans-CHDC.

Experimental investigations on the ROP of trans-CHDC

Homopolymerization. Homopolymerization of trans-CHDC was first investigated with a series of ROP catalyst systems that previously proved to be quite efficient towards cyclic esters and carbonates, and notably trans-CHC. These include the binary systems based on the zinc diaminophenolate complex [(NNO)ZnEt], 21 the simple homoleptic Y(N(SiMe3)2)3, 22 or guanidine TBD, all combined to benzyl alcohol (BnOH) or isopropyl alcohol (iPrOH) as coinitiator (Scheme 3). In order to assess the stereoselectivity of the polymerizations, the ROP of trans-(R,R)-CHDC was investigated in parallel to that of the trans-racemic monomer.

Trans-(R,R)-CHDC was first prepared by reaction of ethyl chloroformate with 1,2-cyclohex-4-enediol enantio-enriched (83% ee) through a resolution using (S)-(+)-O-acetyl-mandelic acid (refer to the Experimental section). Scheme 3. ROP of trans-CHDC (racemic or (R,R)) towards highly isotactic PCHDC mediated by metal-or organo-based catalyst systems. Polymerizations were run in toluene, at high monomer concentration ([CHDC]0 = 4 M), in the temperature range 60100 °C (Table 1). As anticipated from the DFT computations, trans-CHDC is significantly less reactive than trans-CHC, whichever the catalyst system used. With [(NNO)ZnEt]/BnOH, which proved the most active among the three systems investigated, the reaction proceeded at 60 °C but required 5 days to convert 43 equiv. (apparent turnover frequency, TOFapp = 0.36 h 1 ; entry 1). This is ca. 2 orders of magnitude lower than for trans-CHC under comparable conditions (TOFapp = 30150 h 1 ). 13a

Raising the temperature to 80 or 100 °C hardly increased the activity (TOFapp = 0.51 h 1 ; entry 3 and TOFapp = 1.3 h 1 ; entry 6, respectively). This is most likely due to the proximity to the ceiling temperature of PCHDC. 23 Although the detailed kinetics were not investigated, the ROP of (R,R)-CHDC appeared twice slower than that of the racemic monomer (TOFapp = 0.17 vs. 0.36 h 1 ; entries 1 vs. 7). The commercially available, simple organic guanidine TBD catalyst combined with BnOH showed a lower activity than the zinc system (entry 4); this reactivity trend is in line with the results for trans-CHC. 13a On the other hand, the yttriumbased system showed very poor productivity (entry 5).

When poor monomer conversions were reached, such as with the latter sluggish catalyst, only oily materials which could not be isolated were produced. Higher molar mass P(rac-CHDC)s (entries 1, 3 and 4) and P(R,R-CHDC) (entry 7) were isolated as white powders by precipitation from CH2Cl2/MeOH (1:15 v/v). These polymers are soluble in chlorinated solvents (CH2Cl2, CHCl3) at room temperature and are insoluble in hydrocarbons (toluene, pentane) and ethers (THF, Et2O). They were characterized by 1 H and 13 C NMR spectroscopy, MALDI-ToF mass spectrometry and DSC analysis.

The MALDI-ToF mass spectrum of a PCHDC prepared with the [(NNO)ZnEt]/BnOH catalytic system showed a single major population of macromolecules with the expected architecture of regular CHDC repeat units with benzyloxy and hydroxy end-groups (Figure 1); no evidence of decarboxylation products was observed. 24 Detailed microstructural analysis of the recovered PCHDCs was performed by 13 C{ 1 H} NMR spectroscopy (Figures 2 and S7, S9, S11 and S13). For comparison purposes, we also prepared a PCHD by copolymerization of CHDO and CO2 in the presence of a catalyst which is known to provide slightly syndiotactic PCHCs from CHO and CO2, that is rac-(Salen)CoBr (Scheme 4). 15a,26 The carbonyl region of the NMR spectra is indicative of the tacticity of the PCHDCs. The polymer recovered from ROP of trans-(R,R)-CHDC features a unique sharp resonance at  154.0 ppm assigned to pure isotactic PCHDC (Figures 2c andS11). This signal is also observed as the less intense one in the 13 C{ 1 H} NMR spectrum of the PCHDC prepared via ROCOP of CHDO and CO2 with rac-(Salen)CoBr, in addition to a major, broad signal centered at  153.6 ppm, assigned to the syndiotactic sequences (rr, likely overlapping with mr) (Figures 2d andS13). Most interestingly, the latter resonance at  153.6 ppm is the only one observed, but then as a quite sharp signal, in the spectra of the PCHDCs prepared from trans-rac-CHDC, either with the [(NNO)ZnEt]/BnOH or TBD/BnOH catalyst systems (Figures 2a,b). This indicates that these polymers are highly syndiotactic PCHDCs. This is in striking contrast with the ROP of trans-rac-CHC which provided with the same catalyst systems PCHCs with 6476% isoselectivity. 13a The high syndioselectivity implies a type of "enantiomer-differentiating polymerization". Its origin is still unclear but the striking difference observed between the behavior of rac-CHDC and that of rac-CHC indicates that there are subtle effects imparted by the six-membered ring fused with the 5-membered cyclocarbonate. The PCHDCs which could be recovered as solids were analyzed by SEC in CHCl3 (as they are completely insoluble in THF). These analyses proved however erratic as the polymers were not always eluted, and the few chromatograms obtained often appeared bi/multimodal (Figures S14 andS15) for reasons we could not explain, possibly linked to the difficult elution and/or absorption of PCHDC on the columns. Similar problems were encountered previously with highly syndiotactic poly(3-hydroxybutyrate)s; 27 note that PCHDCs obtained by ROCOP of CHDO with CO2, which are just slightly syndiotacticenriched (Figure 2d) could be analyzed by SEC in THF. 15 Nevertheless, the SEC molar masses thus determined were roughly in agreement with the calculated ones. We assume that these SEC results do not reflect the real control achieved in the ROP of CHDC, as further evidenced by convincing SEC data obtained on CHDC copolymers (vide infra).

The thermal characteristics of syndiotactic PCHDCs prepared from rac-CHDC were determined by DSC (Figure 3). Independently of the catalyst used for their preparation ([(NNO)ZnEt]/BnOH or TBD/BnOH), these materials featured a crystallization at Tc = ca.

267 °C and a melting transition at Tm = ca. 318 °C; however, they already start to decompose at this temperature (Figure 3). 28 Slightly syndiotactic PCHDCs produced via ROCOP of CHDO and CO2 exhibited no melting transition, only a glass transition at Tg = 115123 °C. 15 This glass transition was not clearly observed in the syndiotactic PCHDCs prepared by ROP of rac-CHDC. 1, entry 3).

Copolymerization of rac-CHDC.

In order to explore the ability of rac-CHDC to copolymerize and eventually characterize the resulting materials, both simultaneous and sequential copolymerizations were attempted. Two cyclic ester comonomers were selected for this purpose: trans-cyclohexene carbonate (rac-CHC), for its obvious relationship with rac-CHDC, and L-lactide (L-LA), a monomer of topical interest.

Simultaneous Copolymerizations of rac-CHC and rac-CHDC: Preparation of Random P(CHC-co-CHDC)

Copolymers. The simultaneous copolymerization of rac-CHC and rac-CHDC was performed in toluene at 100 °C with the [(NNO)ZnEt]/BnOH system (Scheme 5, Table 2). Rac-CHDC proved to be less reactive than rac-CHC, in line with the results of the DFT computations and homopolymerization experiments. The resulting P(CHC-co-CHDC) copolymers are insoluble in methanol but quite soluble in THF, CHCl3, CH2Cl2 and were readily analyzed by SEC in THF. In contrast to the erratic SEC analyses of PCHDC homopolymers run in CHCl3, the P(CHC-co-CHDC) copolymers all featured a monomodal and symmetric trace (Figure S16). The number-average molar masses determined vs. polystyrene standards (uncorrected for a possible difference in hydrodynamic radii) were close to the theoretical values and the dispersity remained quite narrow, even after 16 h of reaction at this temperature (Table 2). The 1 H NMR spectra of P(CHC-co-CHDC) copolymers show readily distinguishable signals for PCHC and PCHDC units, notably those corresponding to the main chain methine hydrogens at δ 4.63 and 4.93 ppm, respectively, and the olefinic hydrogens of PCHDC at δ 5.56 ppm (Figure 4). No signal that could reflect decarboxylation was observed. Besides, the signals of the benzyloxy end-groups were clearly identified at δ 5.13 and 7.35 ppm. This enabled to determine the molar masses by NMR, which were found in good agreement with the SEC and theoretical values (Table 2). The carbonyl region of the 13 C{ 1 H} NMR spectrum showed several broadened signals at δ 153.4153.9 ppm (Figure 5), suggesting a random enchainment of CHDC and CHC units (Figure 5). The DSC trace (Figure S17) of these P(CHDC-co-CHC) copolymers showed a single glass transition at Tg = 102 °C. This value is lower than that reported for PCHDC and PCHC homopolymers (Tg = ca. 115123 °C15 and 121 °C, 13 respectively), but similar to that observed in polycarbonates derived from the terpolymerization of CHO, CHDO and CO2. 15a No melting transition was detected, which is consistent with the above mentioned random rather than blocky structure of these copolymers. was attempted via the sequential polymerization of rac-CHC followed by that of rac-CHDC in the presence of [(NNO)ZnEt]/BnOH system at 60 °C in toluene (Scheme 6; Table 3). The reaction proceeded with almost complete conversion of rac-CHC and ca. 50% conversion of rac-CHDC; even though this would lead to a larger conversion of rac-CHDC, the reaction time for the second stage was not further extended to avoid side processes (transcarbonatation, etc). Thanks to the 1 H NMR signals of the benzyloxy end-groups (δ 5.06 and 7.28 ppm), the molar mass could be monitored and was shown to increase expectedly from the first (Mn,NMR = 6600 g.mol 1 ) to the second block (Mn,NMR = 10 800 g.mol 1 ). In fact, the 1 H and 13 C NMR spectra of this copolymer (Figures S18 andS19), isolated after precipitation in methanol, resemble much that of the random copolymer obtained by simultaneous copolymerization (vide supra). However, the materials could be differentiated by their thermal properties. The DSC trace of the P(CHC-b-PCHDC) copolymer features, in addition to a glass transition at Tg = 109 °C similar to that of the random copolymer, a melting transition at Tm = 262 °C (Hm = 22.1 J.g 1 ) assignable to the syndiotactic PCHDC block (Figure S20). Scheme 6. Sequential copolymerization of rac-CHC followed by rac-CHDC. 4, entry 2).

Sequential

The 1 H and 13 C{ 1 H} NMR spectra of these P(CHDC-co-LLA) copolymers show the expected signals for the two monomers (Figures S21 andS22). The 1 H signals for the phenyl hydrogens of the benzyloxy end-groups can be discerned and then used to determine the molar mass. The Mn,NMR values thus determined agreed well with the theoretical Mn,theo ones.

The (uncorrected) Mn,SEC values differ much according to the relative amount of CHDC-LLA units, suggesting quite different hydrodynamic volumes.

The DSC traces of the copolymers feature only a glass transition; no melting transition was observed. The copolymers with a significant fraction of CHDC have a higher Tg (74 °C, entry 2; 87 °C, entry 3; Figures S23 andS24) than that of the PLLA homopolymer (ca.

6065 °C).

Conclusions

We have shown for the first time that trans-cyclohexadiene carbonate (CHDC) can be polymerized with regular ROP catalyst systems made of a zinc-based complex or a guanidine, associated with an alcohol. As anticipated by DFT computations, its reactivity is however significantly lower than that of trans-cyclohexene carbonate (CHC). This is exemplified both in homopolymerizations of CHDC and copolymerizations of CHDC with CHC and L-LA.

The [(NNO)ZnEt]/BnOH system proved to be a more active catalyst than TBD for these ROP reactions, which proceeded without decarboxylation. Quite interestingly, even though no chiral catalyst was used, ROP of rac-CHDC proceeded in a stereoselective fashion: highly syndiotactic PCHDC was obtained with both the zinc and TBD-based catalysts, as revealed by 13 C NMR studies. ROP of rac-CHDC thus provides a new entry toward engineering polycarbonates that is much complementary to ROCOP of cyclohexadiene oxide with CO2.

Indeed, the latter route thus far only yielded atactic or slightly syndiotactic PCHDC. 15 The high syndioselectivity observed in the ROP of rac-CHDC contrasts with the modest isoselectivity (Pm = ca. 75%) for the ROP of rac-CHC with the same catalyst systems. 13a This evidences subtle effects induced by the adjacent ring fused to the 5-membered cyclocarbonate moiety.

  , 4.27 (m, 2H, CH2CHOC(=O)OCHCH2), 2.642.70 (m, 2H, CHHCH=CHCHH), 2.402.48 (m, 2H, CHHCH=CHCHH); 13 C{ 1 H} NMR (CDCl3, 100 MHz, 23 °C, Figure S3): δ (ppm) 154.9 (OC(=O)O), 124.1 (CH2CH=CHCH2), 79.8 (CH2CH(CH)(OC(=O))), 29.6 (CH2-CH=CHCH2).

  ) was obtained similarly. The product was recrystallized twice before use in ROP. Mp = 128 °C, []D 20 = 81.5 (c 0.36, CH2Cl2); 1 H NMR (Figure S4) and 13 C NMR (Figure S5) spectra are identical to those of rac-CHDC. Typical procedure for the ROP of rac-CHDC promoted by [(NNO)ZnEt] / BnOH.

Figure 1 .

 1 Figure 1. MALDI-ToF mass spectrum of a PCHDC prepared by ROP of trans-rac-CHDC

Scheme 4 .

 4 Scheme 4. Preparation of slightly syndiotactic-enriched PCHDC via ROCOP of CHDO/CO2. 15a

Figure 2 .

 2 Figure 2. of the carbonyl region of the 13 C{ 1 H} NMR spectra (100 MHz, CDCl3,

Figure 3 .

 3 Figure 3. DSC thermogram (first heating cycle; heating rate = 10 °C.min 1 ; argon flow) of a

Scheme 5 . 2 .a

 52 Scheme 5. Simultaneous copolymerization of rac-CHDC and rac-CHC

Figure 4 .

 4 Figure 4. 1 H NMR spectrum (400 MHz, CDCl3, 23 °C) of a P(CHDC-co-CHC) prepared by

Figure 5 .

 5 Figure 5. 13 C{ 1 H} NMR spectrum (100 MHz, CDCl3, 23 °C) of a P(CHC-co-CHDC)

  Copolymerization of rac-CHC and rac-CHDC: Preparation of Block P(CHC-b-CHDC) Copolymers. The preparation of a diblock P(CHC-b-PCHDC) copolymer

a

  Calculated from the 1 H NMR of the crude product. b Theoretical molar mass value of P(CHDC-co-LLA) calculated from the relation: Mn,theo = Mrac-CHDC × [rac-CHDC]0/[BnOH]0 × conv.rac-CHDC + MLLA × [LLA]0/[BnOH]0× conv.LLA + MBnOH with Mrac-CHDC = 140 g.mol 1 , MLLA = 144 g.mol 1 , and MBnOH= 108 g.mol 1 . c NMR molar mass value of P(CHDC-co-LLA) calculated from the integral value ratio of the signals of OBn end-group hydrogens to internal methine or methylene hydrogens. d Experimental number average molar mass (uncorrected) and dispersity values determined by SEC in THF at 30 °C using polystyrene standards. e Glass transition temperature determined by DSC (2 nd heating cycle, 10 °C.min 1 ).

Figure 6 .

 6 Figure 6. SEC trace (THF, 30 °C) of a P(CHDC-co-LLA) prepared by simultaneous

Table 1 .

 1 ROP of trans-CHDC (racemic or (R,R)) mediated by metal-or organo-based catalyst systems. Monomer conversion as determined by 1 H NMR analysis of the crude reaction mixture. d Theoretical molar mass calculated from the relation: Mn,theo= ([CHDC]0/[ROH]0) × ConversionCHDC × MCHDC + MROH, with MCHDC = 140 g.mol 1 , MBnOH = 108 g.mol 1 and MiPrOH = 60 g.mol 1.e Mn,SEC as determined by SEC in CHCl3 calibrated vs. polystyrene standards with peak mass values of main signals in (multimodal) distribution(s), unless otherwise stated; see the Supporting Information. f Oily material which could not be isolated. g Mn,NMR as determined from terminal groups.

	Entry	Catalyst	ROH	[CHDC]0/	Temp.	Time	Conv. c	Mn,theo d	Mn e
				[Cat.]0/[ROH]0	(°C)	(h)	(%)	(g.mol 1 )	(g.mol 1 )
	1 a	[(NNO)ZnEt] BnOH	50:1:1	60	120	87	6200	4000/6300
	2 a	[(NNO)ZnEt] BnOH	50:1:1	80	6	26 f	1900	nd f
	3 a	[(NNO)ZnEt] BnOH	100:1:1	80	120	61	8600	6100
	4 a	TBD	BnOH	100:1:1	80	120	37	5200	3800
	5 a	Y[N(SiMe3)2]3 iPrOH	100:1:1	80	120	5 f	800	nd f
	6 a	[(NNO)ZnEt] BnOH	100:1:1	100	16	21 f	3000	nd f
	7 b	[(NNO)ZnEt] BnOH	50:1:1	60	72	25	1900	3500 g

a Trans-racemic monomer.

b

Trans-(R,R) (83% ee) monomer.

c

Table 1 ,

 1 entry 3),[b] ROP of trans-rac-CHDC with TBD/BnOH (Table 1, entry 4), [c] ROP of trans-(R,R)-CHDC with [(NNO)ZnEt]/BnOH (Table 1, entry 7), and [d] ROCOP of CHDO/CO2 with rac-(Salen)CoBr.

Table 3 .

 3 Sequential Calculated from the 1 H NMR of the crude product at the intermediary and final stages. b Theoretical molar mass calculated from the relation: Mn,theo= ([CHC]0/[BnOH]0) × Conv.CHC × MCHC + MBnOH, with MCHC = 142 g.mol 1 and MBnOH = 108 g.mol 1 . c NMR molar mass of the copolymer calculated from the integral value ratio of the signals of OBn end-group hydrogens to methine / methylene hydrogens of the monomers. d Theoretical molar mass value of P(CHC-co-CHDC) calculated from the relation: Mn,theo = MCHC × [CHC]0/[BnOH]0× conv.CHC + MCHDC × [CHDC]0/[BnOH]0× conv.CHDC + MBnOH, with MCHDC = 140 g.mol 1 . e Experimental number average molar mass (uncorrected) and dispersity values determined by SEC in THF at 30 °C using polystyrene standards.

	PCHC	PCHC-b-PCHDC

copolymerization of rac-CHC followed by rac-CHDC mediated by the [(NNO)ZnEt]/BnOH system at 60 °C (Scheme 6). a P(CHDC-co-LLA) Copolymers. Random copolymerizations of rac-CHDC and L-LA in various ratios were carried out with the [(NNO)ZnEt]/BnOH system at 80 °C in toluene (Scheme 7, Table

4

). Figure

6

shows the representative SEC trace of a P(CHDC-co-LLA) copolymer with a Mn,SEC of 17 100 g.mol 1 and a dispersity of 1.39 (Table

4

, entry 2). The monomodal, symmetric SEC trace reveals that both monomers copolymerize quite effectively, although monitoring of monomers' conversion indicate that, as anticipated, L-LA is much more reactive than rac-CHDC. Scheme 7. Simultaneous copolymerization of rac-CHDC and L-LA

Table 4 .

 4 Simultaneous copolymerization of rac-CHDC and L-LA mediated by the [(NNO)ZnEt]/BnOH system at 80 °C.

	Entry [rac-CHDC]0/	Time	rac-CHDC	L-LA	Mn,theo b	Mn,NMR c	Mn,SEC d	ÐM	Tg
		[L-LA]0/	(h)	Conv. a	Conv. a	(g.mol 1 )	(g.mol 1 )	(g.mol 1 )		(°C)
		[Zn]0/[BnOH]0		(%)	(%)
	1	50:150:1:1	6	77	100	27 100	23 200	8800	1.35	nd
	2	100:100:1:1	24	74	100	24 900	18 600	17 100	1.39	74
	3	150:50:1:1	6	56	100	19 100	17 400	6300	1.43	87

Acknowledgements

Financial support of this research (Post-Doctoral grant to A.K.D.) from Total Raffinage Chimie is gratefully acknowledged.

Supporting information includes NMR, SEC and DSC data for monomer and polymers.

This information is available free of charge via the Internet.

A. Chatterjee, M. Sasikumar and N. N. Joshi, Synth. Commun., 2007, 37, 1727-1733. 21 C. K. Williams, L. E. Breyfogle, S. K. Choi, W. Nam, V. G. Young, M. A. Hillmyer and W. B. Tolman, J. Am. Chem. Soc., 2003, 125, 11350-11359.


William Guerin

Marion Helou

Martine Slawinski

Jean-Michel Brusson

Jean-François Carpentier

Sophie M Guillaume
email: sophie.guillaume@univ-rennes1.fr

Ethylene Carbonate/Cyclic Ester Random Copolymers Synthesized by Ring-Opening Polymerization

The diaminophenolate and -diketiminate zinc complexes [(NNO)ZnEt] ((NNO)  = 2,4-di-

tert-butyl-6-{[(2'-dimethylaminoethyl)-methylamino]methyl}phenolate))

and

respectively, the Lewis acidic triflate salt Al(OTf)3, and the organic guanidine TBD (= 1,5,7-triazabicyclo[4.4.0]dec-5-ene), combined to a protic source as initiator, typically benzyl alcohol (BnOH), enabled the successful copolymerization of ethylene carbonate (EC) with various cyclic esters such as -butyrolactone (BL), -valerolactone (VL), -caprolactone (CL) or L-lactide (LLA). The random copolymerizations proceeded smoothly under mild operating conditions, preferentially from [(NNO)ZnEt]/BnOH at 60 °C in toluene within a few hours, affording the corresponding copolymers void of ether units, with Mn,SEC values in the range ca. 600093 350 g.mol 1 and with unimodal, moderately broad dispersity values (ÐM = 1.32.1).

Under the same experimental conditions, the homopolymerization of EC did not proceed. The first EC/BL random copolymers were thus synthesized with up to 26 mol% of EC inserted within the polyester, while the second example of P(EC-co-VL) was isolated. P(EC-co-VL), P(EC-co-CL), and P(EC-co-LLA) copolymers were prepared with higher than previously reported EC content, namely 23, 37, and 17 mol% vs. 10, 31, and 4 mol%, respectively. In contrast to other catalyst systems, the Al(OTf)3/BnOH system promoted CO2 elimination from the copolymers, thereby leading to ether defaults. Microstructural analysis of the copolymers by 13 C{ 1 H} NMR spectroscopy revealed the presence of signals previously never described and possibly arising from consecutive EC units within the random copolymers.

Thermal transition temperatures measured by DSC further supported the random nature of these copolymers.

Table of content Ethylene Carbonate/Cyclic Ester Random Copolymers by ROP Introduction

In the past decade, polycarbonates (PCs) have gained growing industrial and academic interest owing to their properties. Their range of thermal, mechanical and optical characteristics as well as their non-toxicity, biocompatibility, (bio)degradability, and bioresorbability have contributed to promote such polymers at the forefront of both commodity and engineered materials alongside traditional polyesters. Also, recent advances in PCs synthesis have evidenced sustainable and valuable green approaches in compliance with environmental considerations. Chemically speaking, PCs can be rather easily functionalized thus providing tunable polymer materials with further valuable opportunities.

Thus, PCs nowadays appear as topical leading candidates for added-value applications in automotive, aircraft, construction, electronic and biomedical materials. 1,2,3,4 PCs can be prepared according to three distinct synthetic methods, namely i) polycondensation of phosgene, triphosgene, or a dialkyl or diaryl carbonate with an ,-diol, 5,6,7,8 ii) copolymerization of epoxides with carbon dioxide, 9,10,11,12,13,14,15,16,17 or iii) ring-opening polymerization (ROP) of a cyclic carbonate monomer. 2,18,19,20,21,22,23 While the former route does not meet all safety and health requirements for those in PC industry and for the consumers, the more friendly "greener" CO2/epoxide approach sometimes suffers from incomplete selectivity, resulting in (often detrimental) ether units within the recovered PC and/or formation of cyclic carbonate as co-product. On the other hand, ROP of cyclic carbonates provides a good method towards the synthesis of well-defined PCs with adjustable macromolecular parameters. PCs have thus been produced without CO2 loss through a controlled ROP mediated by (organo)metallic or organic (i.e., metal-free) catalytic systems, enabling fine tuning of the PC molar mass and dispersity (ÐM = Mw/Mn) values, as well as end-group fidelity, upon limitation of the undesirable transcarbonatation and other sidereactions. 2,[18][19][20][21][22][23] In this general ROP context, five-membered cyclic carbonates (5CCs) clearly stand apart from their larger (≥ 6) congeners. Indeed, thermodynamics, with ring-opening enthalpies values (ΔH°) most often positive, does not favor their polymerizability. 22,24,25,26 While increasing the reaction temperature can shift the monomer/polymer equilibrium by raising the entropy, thus making the free energy (ΔG°) negative, decarboxylation is then generally observed. The homopolymerization of 5CCs is thus rarely controlled and remains difficult to optimize.

Few 5CCs have been ring-open polymerized. To our knowledge, these are limited to ethylene carbonate (EC), 27,28,29,30,31,32,33,34,35 propylene carbonate (PPC), 26,33,36,37,38 methyl 4,6-

O-benzylidene-2,3-O-carbonyl-R-D-glucopyranoside

(MBCG), 39,40 and cyclohexene carbonate (CHC). 22,23 In the literature, the ROP of EC and PPC are the most documented ones, certainly because of their commercial availability. Although the ceiling temperature of EC and PPC is below 25 °C, these 5CCs have been polymerized above 100 °C, most likely as the result of CO2 abstraction which makes ΔS° positive thereby overcoming the positive enthalpy, so that the polymerization then becomes thermodynamically possible. [26][27] Various initiators 41 have been used in the ROP of EC and PPC, but these reactions always led to partial or complete decarboxylation. [26][27][28][29][30][31][32][33][34][35][36][37][38] On the other hand, the homopolymerization of MBCG and CHC proceeded without elimination of CO2 and afforded better-defined polymers. 22,23,39,40,42 For the latter two monomers, ROP actually occurs as the result of the favorable ring-strain of the glucopyranosyl and cyclohexyl moiety fused onto the 5CC moiety, respectively, which sufficiently destabilizes the monomer. The PCs thus obtained (poly(MBCG): Mn,SEC up to 20°200 g.mol 1 , ÐM = 1.8; poly(CHC): Mn,NMR up to 17°000 g.mol 1 , ÐM = ca. 1.2) generally featured higher molar mass values as compared to those derived from the ROP of EC or PPC (Mn,SEC commonly in the range 300up to22 100 g.mol 1 , ÐM = 1.143.1).

Also, EC has been successfully copolymerized by ROP with a few other cyclic monomers, namely -caprolactone (CL), -valerolactone (VL), L-lactide (LLA), and 2,2-dimethyltrimethylene carbonate (DTC). [START_REF] Williams | [END_REF]44,45,46,47,48,49,50,51 The initiating systems always 52 involved rare earth complexes: for LLA (in bulk at 50 °C), over a few hours-to-several days. 4350 The extent of decarboxylation was observed to depend on the initial EC/comonomer ratio. For instance, no CO2 abstraction was detected in the synthesis of random EC/CL copolymers starting from a EC/CL molar ratio of 60:40, while partial (14%) decarboxylation occurred at 70:30. 44 The amount of EC incorporated in these various EC copolymers remained in the range 431 mol% and affected the thermo-mechanical properties of the copolymers. 47 Degradability of poly(ECco-CL) and poly(EC-co-VL) was also shown to be much improved upon incorporation of EC units along the polyester chain. 44 Thus, although 5CCs such as EC and glycerol carbonate are industrially available in large volumes, their valorization by ROP still remains challenging. Very few examples of the successful homopolymerization or copolymerization by ring-opening approach have been established, even less with optimized controlled activity and productivity. 22,26,33,[36][37][38] EC random copolymers remain limited and are, so far, only accessible from rare earth-based catalysts.

[(C5Me5)2SmMe(THF)], [ (
In this contribution, we report the synthesis of EC/cyclic esters copolymers from the controlled ROP of EC and -butyrolactone (BL), VL, CL or LLA, using the 55,56 respectively, the Lewis acidic salt Al(OTf)3 (OTf = O3SCF3), or the organocatalyst TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene), combined to a protic source as initiator, typically benzyl alcohol (BnOH) (Scheme 1). Well-defined random copolymers featuring as much as 38 mol% of EC were thus prepared with the zinc-based systems, and thoroughly characterized by 1 H, 13 C NMR, SEC and DSC analyses.

diaminophenolate or -diketiminate zinc complex, [(NNO)ZnEt] ((NNO)  = 2,4-di-tert-butyl- 6-{[(2'-dimethylaminoethyl)-methylamino]methyl}phenolate)), 54 or [(BDI iPr )Zn{N(SiMe3)2}] (BDI iPr = CH(CMeNC6H3-2,6-iPr2)2),
Scheme 1. Synthesis of PEC-co-polyester by ring-opening copolymerization of EC with BL, VL, CL or LLA.

Experimental section

Materials

All polymerizations were performed under inert atmosphere (argon) using standard were added to a solution of [(NNO)ZnEt] (4.0 mg, 9.3 µmol, 1 equiv.) in toluene (0.1 mL).

The mixture was then stirred at 60 °C in the case of BL, VL or CL, and at 100 °C in the case of LLA, over the appropriate period of time (reaction times were not systematically optimized). The polymerization was then quenched upon addition of an acetic acid solution (ca. 0.05 mL of a 1.6 mol.L -1 solution in toluene). In the case of BL, the conversion was determined from this crude reaction mixture. In the case of VL and CL comonomers, the reaction mixture was concentrated to dryness under vacuum, and the conversion of both monomers was determined by 1 H NMR analysis of the residue in CDCl3. The crude polymer was next dissolved in CH2Cl2 (ca. 2 mL) and precipitated in cold methanol (ca. 10 mL), In a typical experiment (Table 1, entry 18), [(NNO)ZnEt] (5.0 mg, 11.7 µmol, 1 equiv.)

was added to BnOH (1.2 µL, 11.7 µmol, 1 equiv.) placed in toluene (ca. 0.1 mL) and stirred over 15 min just prior to the addition of both monomers, CL (267 mg, 2.34 mmol, 200 equiv.)

and EC (1.03 g, 11.7 mmol, 1000 equiv.). The remaining of the procedure was identical to that described above for the simultaneous copolymerization of EC and BL, VL, CL or LLA using only [(NNO)ZnEt] or [(BDI iPr )Zn{N(SiMe3)2}].

Results and Discussion

Previous studies on the ROP of cyclic esters such as BL, LLA, cyclohexene carbonate, or trimethylene carbonate, have unveiled the catalytic efficiency, both in terms of activity and productivity, of the -diketiminate and diaminophenolate zinc complexes, [(BDI iPr )Zn{N(SiMe3)2}] and [(NNO)ZnEt] respectively, as well as of metal triflates (M(OTf)3 with M = Ca, Sc, Zn, Al, Bi) and organic bases such as TBD. 22,57,58,59 These catalyst systems were, in particular, effective even in the ROP of rather reluctant-to-(co)polymerize four-membered ring lactones such as BL or benzyl -malolactonate, 55,56,60,61,62,63,64,65 and of similarly hard-to-homopolymerize five-membered ring cyclohexene carbonate. 22 We have therefore selected these systems for evaluating their performances in the simultaneous ringopening copolymerization of EC with BL, VL, CL and LLA. Taking into account the reported examples mentioned above of the successful copolymerization of EC with CL and with the closely related one carbon-smaller lactone displaying similar ROP affinity, namely VL, [START_REF] Williams | [END_REF][44][45] both CL and VL looked promising for their copolymerization with EC. Finally, given the current academic and industrial interests in lactides, LLA was also evaluated as a comonomer. The EC/BL and EC/CL simultaneous copolymerizations were first attempted with the robust Al(OTf)3/BnOH system, known to withstand high temperatures in related carbonate polymerizations, 68,69,74,75 at 110 °C (Table 1, entries 1, 12, respectively). In these copolymerizations, the lactone was (nearly) completely consumed while EC was converted to some extent (27100%). However, complete decarboxylation was observed, as evidenced by the absence of the typical carbonate signal in the 13 C NMR spectra (13C155.455.0 ppm, vide infra) 23,45 of the isolated copolymers. Also, the characteristic methylene resonances of ethylenoxy units were unambiguously identified at 1H 3.73.3 ppm and13C71.1 ppm, in agreement with literature data. 45,70,71,72 This is exemplified with the spectra recorded on a sample recovered from EC/CL copolymerization (Figures S3S4). Based on these initial results, no such EC/comonomer copolymerization using Al(OTf)3 was further investigated.

Synthesis of EC/lactones and EC

Mn,theo= {([EC]0/[X]0) × ConversionEC × MEC} + {([lactone]0/[X]0) × ConversionLactone × MLactone} + MBnOH, with [X]0 = [BnOH]0,
Alternatively, catalytic systems operating under milder conditions, and ideally without concomitant decarboxylation, were then sought.

Under similar bulk conditions, yet at a lower temperature (6090 °C), the zinc complexes [(BDI iPr )Zn{N(SiMe3)2}] and [(NNO)ZnEt], with or without BnOH, were then evaluated (Table 1, entries 26, 711, 1320, respectively). These copolymerizations proceeded without noticeable decarboxylation of EC. This can be ascribed to the nature of the catalyst itself but also likely as well to the relatively lower operating temperature that such systems allow. The zinc -diketiminate complex associated to BnOH catalyzed the simultaneous copolymerization of EC with BL, affording 1518 mol% of EC inserted into the final P(EC-co-BL) copolymer ( the higher the EC content in the initial mixture of the two comonomers, the lower the overall catalytic activity (i.e., the lower the conversion of both VL and EC), and eventually the more difficult the EC insertion. Overall, the [(NNO)ZnEt] catalytic system allows to insert up to 23 mol% and 3337 mol% of EC in the VL and CL copolymer, respectively. These contents are unmatched as, so far, the maximum EC incorporated into a PVL and PCL backbone was reported at 10 mol% and 31 mol%, respectively, from [(C5Me5)2SmMe(THF)] (with Organocatalysts, known as active in the ROP of cyclic carbonates, 22,39,57,59,73,74,75 were also examined in the copolymerization of EC with CL, selected as model comonomer (Table 1, entries 2123). TBD combined to BnOH was found active, yet to a lesser extent than the [(NNO)ZnEt]/BnOH catalytic system. Only 17 mol% and 36 mol% of EC and CL, respectively, were consumed within 1 h at 60 °C (vs. 20 mol% and 85 mol%, respectively, with the zinc system; Table 1, entries 21,15). The amount of EC ultimately inserted in the copolymer produced with TBD was as high as 38 mol%, thus supporting the consumption of EC at the earlier stage of the polymerization, as observed above with the diaminophenolate zinc system. A similar behavior has been reported for the copolymerization of CL with -butyrolactone, a monomer similarly known to be non-easily polymerizable. 76 Further attempts with various [EC]0/[CL]0 and extended reactions time did not reveal more successful, and TBD was thus not further considered.

Incorporation of carbonate units such as ECs along the poly(lactide) backbone to improve its physical properties is also one of our current topical endeavor. 57,58,59 To our knowledge, the sole EC/LLA copolymerization reported prior to this work was carried out from SmI2/Sm, and ultimately afforded at best 4 mol% of EC randomly inserted into the PLLA chain. 45 however somewhat lower than those reported in the literature for these same copolymers (ÐM = ca. 1.522.12). [START_REF] Williams | [END_REF][44][45][46] On the other hand, the EC/LLA copolymers herein synthesized displayed slightly larger dispersity values (ÐM = 1.502.16) as compared to those measured on alike copolymers prepared from SmI2/Sm (ÐM = 1.301.67) from yet lower EC initial loadings ([EC]0:[LLA]0 = 25:75 to 62:38). 45 The EC copolymers were characterized by 1 H and 13 C NMR spectroscopy and DSC analysis. Structural identification was based on the detailed ( 1 H, 13 C{ 1 H}, 1 H-13 C HMQC and HMBC NMR spectroscopy) characterization of EC/CL copolymers (Figures S3S9,19, respectively), as reported by Agarwal et al. 45 and Evans et al. [START_REF] Williams | [END_REF] The integrals of the EC/lactone or lactide NMR signals indeed matched the conversion of each monomer, respectively.  173.7, 64.2, 34.3, 28.3, 25.5, and 24.5 ppm). Some additional resonances were clearly observed as compared to the spectrum of the alike copolymer prepared from SmI2/Sm, 45 thus suggesting a distinct microstructure in the copolymer synthesized in the present study. The carbonyl region of the 13 C{ 1 H} NMR spectrum shows three resonances assigned to EC units inserted into the polyester within three distinct chemical environments (Figure 3). The most downfield carbonyl signal (155.40 ppm) may be assigned to an isolated EC unit within several consecutive CL units. Of the two other most upfield signals (155.06, 155.11 ppm), one may correspond to two adjacent EC units or to two close by EC units separated by only one CL unit. So far, only isolated EC units giving a unique signal in the carbonyl region ( ca.

{([EC]0/[X]0) × ConversionEC × MEC} + {([LLA]0/[X]0) × ConversionLLA × MLLA} + MBnOH, with X = [BnOH

P(EC-co

155 ppm) have been claimed for P(EC-co-CL) in the literature. [START_REF] Williams | [END_REF]45 Agarwal et al. reported that the absence of several signals assigned to the carbonyl of EC suggested its occurrence as isolated EC units. Furthermore, the absence in the present copolymer spectrum of three (or more) consecutive EC units was suggested by the absence of signal at  154.69 ppm, as recorded in the spectrum of a PEC homopolymer (Figure S2; prepared from CO2/ethylene oxide copolymerization) (Figures 2,S6). The assignment of the signal at  155.11 ppm to two adjacent EC units or to two close by EC units separated by only one CL unit, was suggested by the relative intensity of the signals at  155.40, 155.11 and 155.06 ppm according to the amount of EC inserted within the polyester: the larger the insertion of EC (i.e., the more likely two EC units are to be next to each other), the larger the intensity of the signal at  155.11 ppm vs. that at  155.40 ppm (Figure 3). The carbonyl signal at 155.06 ppm may be assigned to a EC unit in a slightly different environment or, taking into account its small intensity, 78 to a chain-end EC-containing unit; this could not be clarified yet. synthesized from [(NNO)ZnEt] and featuring 33 mol% of inserted EC (Table 1, entry 15) and proposed assignments. Thermal analysis. Thermal characterization of the copolymers was performed by DSC (Table 3). A P(EC-co-BL) featuring 26 mol% of EC (Table 1, entry 4) displayed a unique glass transition temperature Tg at 1.6 °C (Figure S7), a temperature typical of a PHB commonly ranging from 4 to +10 °C. 80 Similarly, the DSC analysis of a P(EC-co-VL) and P(EC-co-CL) featuring 1314 mol% of EC (Table 1, entries 7, 13), showed a Tg at 49.1 °C and 9.1 °C, along with a melting temperature Tm of +30.2 °C and +38.5 °C, and a crystallization temperature of 9.2 °C and 2.2 °C, respectively (Figures S8,9 and melting temperatures of a P(EC-co-LLA) featuring 9 mol% of EC (Table 2, entry 5) were measured at Tg = 53 °C and Tm = 149 °C, respectively (Figure S9). These temperature values were lower than those of a PLLA homopolymer (Tg = 65 ° C and Tm = 175 °C). 57,59 The Tg of all the copolymers was found to dependent on the molar ratio of EC:cyclic ester units, in agreement with the Fox theory. All these DSC results supported that the copolymerization of EC with either BL, VL, CL or LLA resulted in random copolymers, with the amount of EC inserted affecting, as expected, the thermal behavior of the resulting random copolymers (Table 3).

Table 3. Thermal characteristics of the PEC-polyester random copolymers.

Conclusion

Zinc-based and organic catalyst systems, distinct from and less air-and moisture-sensitive than the previously established rare earth-based ones, [START_REF] Williams | [END_REF][44][45][46][47][48][49][50] also ineffective in the EC homopolymerization, have thus been unveiled for the successful controlled copolymerization of EC with lactones or lactide. The copolymerization proceeded without decarboxylation under mild operating conditions (preferentially 60 °C, up to 100 °C; toluene; 0.56 h) affording copolymers with Mn,SEC values in the range ca. 600093 350 g.mol -1 and rather moderate dispersity values (1.24 < ÐM < 2.15). The first examples of EC/BL random copolymers, featuring up to 26 mol% of EC inserted within the polyester, have been prepared

from [(NNO)ZnEt]/BnOH. This same zinc-based system allowed the successful synthesis of P(EC-co-VL) and P(EC-co-CL) copolymers by copolymerization of EC with the larger lactones. Whereas the latter copolymers were already exemplified in several works, 4346 the former P(EC-co-VL) copolymers represent, to our knowledge, the second example to date. 44 In these latter two cases, the largest EC content recovered in the copolymer could be improved from the previously reported limit of 10 mol% and 31 mol% obtained from [(C5Me5)2SmMe(THF)], 44 to 23 and 37 mol%, respectively. The organic guanidine TBD in the presence of BnOH afforded up to 38 mol% of EC inserted within the PCL chains. The zinc diaminophenolate complex associated to BnOH also enabled to increase the EC extent within a PLA chain, from the previously established amount of 4% reached from SmI2/Sm, 45 up to 17 mol%. In comparison, the [(BDI iPr )Zn{N(SiMe3)2}] system remained less efficient in inserting carbonate units along the polyester chains, with 18 mol% and 5 mol% of EC incorporated within the ultimate P(EC-co-BL) and P(EC-co-LLA), respectively. On the other hand, the aluminum triflate/BnOH system evaluated in the EC/BL and EC/CL copolymerizations significantly promoted decarboxylation. The [(NNO)ZnEt]/BnOH catalytic system thus turned out as the most active and productive one so far disclosed in the successful simultaneous ring-opening copolymerization of EC with VL or CL, without decarboxylation.

The microstructural analyses of these EC/BL, VL, CL, LLA copolymers assessed by 1 H, and in particular 13 C{ 1 H}NMR revealed the presence of signals previously never described and assigned to the presence of consecutive EC units within the random copolymers. Thermal transition temperatures measured by DSC further supported the random nature of these copolymers. 2, entry 5).

  C9H7)2Sm(THF)1.5], [(C13H9)2Sm(THF)2], [{(Me3Si)2N}2Sm(THF)2], [SmI2(THF)], SmI2/Sm, or Ln(DBMP)3 (Ln = La, Nd, Sm, Dy; DBMP = 2,6-di-tert-butyl-4-methylphenolate). Simultaneous 53 and more rarely sequential 43 copolymerizations were carried out in toluene at room temperature except

  filtered and dried under vacuum. The final P(EC-co-BL,-VL, -CL, or -LLA) copolymers were then analyzed by NMR, SEC and DSC. Typical procedure for the simultaneous copolymerization of EC and BL, VL, CL or LLA using [(NNO)ZnEt]/BnOH, [(BDI iPr )Zn{N(SiMe3)2}]/BnOH, Al(OTf)3/BnOH, or TBD.

  or [X]0 = [Catalyst]0 when [BnOH]0 = 0, MEC = 88 g.mol 1 , MBL = 86 g.mol 1 , MVL = 100 g.mol 1 , MCL = 114 g.mol 1 , MBnOH = 108 g.mol 1 . f Number-average molar mass value (uncorrected; refer to the Experimental section) determined by SEC in THF at 30 °C versus polystyrene standards. g Dispersity values determined by SEC in THF at 30 °C. h Complete decarboxylation was observed by NMR analyses. i Reaction ran in toluene at [EC]0 = 4.0 M. j Not determined because the polymer could not be precipitated. k Mn value determined by NMR analysis = 16 400 g.mol 1 .

[

  EC]0/[VL]0 = 50:50 and [EC]0/[CL]0 = 60:40 in 72 h at 25 °C). 44 Also, the present [(NNO)ZnEt]/BnOH catalytic system enables effective EC/VL and EC/CL copolymerizations, without decarboxylation, with [EC]0/[VL]0 and [EC]0/[CL]0 ratios still operative up to 50:50 and 83:17, respectively. These initial loadings of comonomers allowing copolymerization compare favorably with the 50:50 and 60:40 ones, respectively, previously optimized for the Sm III methyl metallocene, as in the latter case, decarboxylation was observed at [EC]0/[VL]0 = 60:40 and [EC]0/[CL]0 = 70:30. 44

  ]0 or X = [Catalyst]0 when [BnOH]0 = 0, MEC = 88 g.mol 1 , MLLA = 144 g.mol 1 , MBnOH = 108 g.mol 1 . f Number-average molar mass value (uncorrected; refer to the Experimental section) determined by SEC in THF at 30 °C versus polystyrene standards. g Dispersity values determined by SEC in THF at 30 °C. h Not determined because the polymer could not be precipitated. Characterization of EC/lactones and EC/lactide random copolymers. All these EC lactones/lactide copolymerizations afforded copolymers with molar mass values determined by SEC (MnSEC; values uncorrected for the possible difference in the hydrodynamic volume of the copolymers vs. polystyrene standards used for calibration) varying from 6000 to 96 350 g.mol 1 , and dispersity values generally lower than ÐM = 1.85. Although the molar mass values of these EC/lactones and EC/lactide random copolymers measured by SEC did not always matched with the theoretical data based on the conversion of both conomomers, 77 these data suggested fairly controlled copolymerizations with yet some undesirable side reactions (typical inter-and intra-molecular  reshuffling and backbiting  transesterification/transcarbonatation and/or chain transfer reactions). The dispersity values measured in the present study for EC/VL and EC/CL copolymers (ÐM = 1.361.79), were

  -CL) copolymers. The typical 1 H NMR spectrum of a P(EC-co-CL) prepared by Zn catalysis displays the characteristic signals of both PCL ( 4.04, OCH2CH2; 2.30, C(O)CH2CH2; 1.65, C(O)CH2CH2, OCH2CH2; 1.39 ppm, CH2CH2CH2) and PEC ( 4.264.35, C(O)OCH2, OCH2CH2) (Figure 1). As mentioned above, the absence of the signals of ether units (1H ca. 3.73.3 ppm and13Cca.71.1 ppm) 45,7072 indicated the absence of concomitant decarboxylation during the copolymerization. The PCL resonances of P(EC-co-CL) samples were split into a set of two distinct signals assigned to two types of CL chemical environments in the copolymer (i.e., CLCLCL and ECCLCL) as previously reported. 43 As illustrated in Figure 2, the 13 C NMR spectra of the P(EC-co-CL) samples synthesized from [(NNO)ZnEt] distinctively exhibited the PCL characteristic set of signals (at

Figure 1 .Figure 2 .

 12 Figure 1. 1 H NMR spectrum (400 MHz, CDCl3, 23 °C) of a P(EC-co-CL) synthesized from

Figure 3 .

 3 Figure 3. Carbonyl region of the 13 C{ 1 H} NMR spectra (100 MHz, CDCl3, 23 °C) of P(EC-

  Figure4). Besides, another set of distinctly shifted methine, methylene and methyl hydrogens

Figure 4 .Figure 5 .

 45 Figure 4. 1 H NMR spectrum (400 MHz, CDCl3, 23 °C) of a P(EC-co-BL) synthesized from [(NNO)ZnEt] and featuring 26 mol% of inserted EC (Table1, entry 4) (* and # stand for residual EC and H2O, respectively) and proposed assignments.

Figure 6 .Figure 7

 67 Figure 6. 13 C{ 1 H} NMR spectrum (100 MHz, CDCl3, 23 °C) of a P(EC-co-VL) synthesized

Figure 8 .

 8 Figure 8. 13 C{ 1 H} NMR spectrum (100 MHz, CDCl3, 23 °C) of a P(EC-co-LLA) synthesized

  ). Incorporation of EC (PEC: Tg = +1028 °C) thus influenced similarly the thermal temperatures of PCL (Tg = 60 °C, Tm = +65 °C) and those of PVL (Tg = 63 °C, Tm = +60 °C). The glass transition

a

  Amount of EC inserted as determined from the crude reaction mixture (refer to Tables12). b Glass transition (Tg), melting (Tm), and crystallization (Tc) temperatures as measured by DSC. c Glass transition temperature (Tg) estimated from the Fox equation, using the following Tg values for homopolymers: PEC: +10 °C; PBL: +3 °C; PVL: 63 °C; PCL: 60 °C; PLLA: +65 °C.

Figure 9 .

 9 Figure 9. DSC trace of a P(EC-co-CL) featuring 14 mol% of EC (Table 1, entry 13).

Figure S1. 1 H

 1 Figure S1. 1 H NMR spectrum (400 MHz, CDCl3, 23 °C) of a commercial PEC (Mn = 250 000

Figure S2 .

 S2 Figure S2. 13 C NMR spectrum (400 MHz, CDCl3, 23 °C) of a commercial PEC (Mn =

Figure

  Figure S3. 1 H NMR spectrum (400 MHz, CDCl3, 23 °C) of a copolymer synthesized by

Figure S4 .

 S4 Figure S4. 13 C{ 1 H} NMR spectrum (100 MHz, CDCl3, 23 °C) of a copolymer synthesized by

  Figure S5. 13 C NMR spectrum (100 MHz, CDCl3, 23 °C) of a P(EC-co-BL) synthesized from [(NNO)ZnEt] and featuring 26 mol% of inserted EC (Table 1, entry 4) (* stands for residual EC).

Figure S6 .

 S6 Figure S6. Carbonyl region of the 13 C{ 1 H} NMR spectrum (100 MHz, CDCl3, 23 °C) of a

Figure S7 .

 S7 Figure S7. DSC trace of a P(EC-co-BL) featuring 26 mol% of EC (Table 1, entry 4).

Figure S8 .

 S8 Figure S8. DSC trace of a P(EC-co-VL) featuring 13 mol% of EC (Table 1, entry 7).

Figure S9 .

 S9 Figure S9. DSC trace of a P(EC-co-LLA) featuring 9 mol% of EC (Table 2, entry 5).

min 1 . Typical procedure for the simultaneous copolymerization of EC and BL, VL, CL or LLA using [(NNO)ZnEt] or [(BDI iPr )Zn{N(SiMe3)2}]. In

	hydrogens (-OCH2CH2O,PEC 3.66 ppm, EC 4.35 ppm) of EC, Int.PBL/[Int.PBL + Int.BL] of the
	methine hydrogen (-OCH(CH3)CH2,PBL 5.25 ppm, BL 4.66 ppm) of BL, Int.PVL/[Int.PVL +
	Int.VL] of the methylene hydrogens (-OCH2,PVL 4.05 ppm, VL 4.20 ppm) of VL,
	hot (80 °C) concentrated solution in 2-propanol, followed by two subsequent crystallizations Int.PCL/[Int.PCL + Int.CL] of the methylene hydrogens (-OCH2,PCL 4.04 ppm, CL 4.19 ppm)
	in toluene at 105 °C, and finally stored under argon at room temperature. The commercial of CL, and Int.PLLA/[Int.PLLA+Int.LLA] of the methine hydrogen (OCHCH3C(O), δPLLA 5.18
	PEC sample (QPAC ® 25; Mn = 250 000 g.mol 1 , ÐM = 1.9; Figures S1S2) was obtained from ppm, δLLA 5.05 ppm) of LLA. Note that the actual chemical shifts varied slightly with the
	Empower Materials. Benzyl alcohol (Acros) was distilled over Mg turnings under argon nature of the copolymer. The amount of EC inserted was determined by 1 H NMR analysis of
	atmosphere and kept over 3-4 Å activated molecular sieves. CDCl3 was dried over a mixture the precipitated polymer samples, using the intensity ratio of the signals corresponding to the
	of 3 and 4 Å molecular sieves. [(BDI iPr )Zn{N(SiMe3)2}] 55,56 and [(NNO)ZnEt], 54 were EC (C(O)OCH2CH2O) and to the PHB ((C(O)OCH2CH), PVL (C(O)OCH2CH2),
	synthesized following the literature procedures. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) PCL(C(O)OCH2CH2), or PLLA (OCH(CH3)C(O)) hydrogens.
	(98%, Aldrich), Al(OTf)3 (Aldrich) and all other reagents were used as received. Molar mass values were not determined by 1 H NMR analysis because the chain-end
	Instrumentation and measurements signal was not always observed/identified, especially in the absence of added BnOH chain
	1 H (500 and 400 MHz) and 13 C{ 1 H} (125 MHz) NMR spectra were recorded on transfer agent.
	Bruker Avance AM 500 and Ascend 400 spectrometers at 25 °C and were referenced Differential scanning calorimetry (DSC) analyses were performed on a Setaram DSC
	internally relative to SiMe4 ( 0 ppm) using the residual solvent resonances. 131 apparatus calibrated with indium at a rate of 10 °C.min 1 , under continuous flow of
	Size-exclusion chromatography (SEC) giving number-average molar mass (Mn,SEC) helium (25 mL.min 1 ), using aluminum capsules (typically 10 mg of polymer). The
	and dispersity (ÐM = Mw/Mn) values of the polymers was carried out in THF at 30 °C (flow thermograms were recorded according to the following cycles: 40 °C to +200 °C at
	rate 1.0 mL.min 1 ) on a Polymer Laboratories PL50 apparatus equipped with a refractive index detector and a set of two ResiPore Mixed E 300 × 7.5 mm columns. The polymer samples were dissolved in THF (2 mg.mL 1 ). All elution curves were calibrated with 10 °C.min 1 ; +200 °C to 40 °C at 10 °C.a typical experiment (Table 1,
	polystyrene standards; Mn,SEC values of the PBLs, PVLs and PEC copolymers were entry 13), EC (103 mg, 1.17 mmol, 125 equiv.) and CL (267 mg, 2.34 mmol, 250 equiv.)
	uncorrected for the potential difference in hydrodynamic radius vs. polystyrene. The SEC
	traces of the copolymers all exhibited a unimodal symmetrical peak.
	Monomer conversions were calculated from 1 H NMR spectra of the crude polymer
	samples by using the integration (Int.) ratio Int.PEC/[Int.PEC + Int.EC] of the methylene

Schlenk, vacuum line, and glovebox techniques. Racemic -butyrolactone (BL) (>95%, TCI Chemicals), -valerolactone (98%, VL, Alpha Aesar) and -caprolactone (CL, 99.5%, Aldrich) were purified (twice for BL) by distillation from CaH2 and stored under argon.

L-Lactide (LLA; technical grade, Total Petrochemicals) was purified by crystallization from a

Table 1 .

 1 Simultaneous copolymerizations of EC with lactones mediated by various catalyst systems in bulk at different temperatures.

	Entry Lactone	Catalyst	[EC]0/[Lactone]0/ [Catalyst]0/ [BnOH]0 a	[EC]0/ [Lactone]0 molar ratio	Temp. ( °C)	Reaction Time b (h)	EC Conv. c (%)	Lactone Conv. c (%)	EC inserted d (mol%)	Mn,theo e (g.mol 1 )	Mn,sec f (g.mol 1 )	ÐM g
	1	BL	Al(OTf)3	200:400:1:5	33:67	110	5	27 h	92	nd	6500	nd	nd
	2	BL	[(BDI iPr )Zn{N(SiMe3)2}]	165:500:1:5	25:75	80	6.25	nd	nd	15	nd	5800	1.24
	3	BL	[(BDI iPr )Zn{N(SiMe3)2}]	250:500:1:5	33:67	90	3.5	25	95	18	9300	6000	1.29
	4	BL	[(NNO)ZnEt]	250:500:1:0	33:67	60	5	33	81	26	42 100	17 400	1.56
	5	BL	[(NNO)ZnEt]	250:500:1:0 i	33:67	60	6	44	97	21	51 400	34 400	1.62
	6	BL	[(NNO)ZnEt]	250:500:1:5	33:67	60	6	32	83	23	8500	7500	1.32
	7	VL	[(NNO)ZnEt]	125:250:1:0	33:67	60	1	21	78	13	21 900	19 800	1.79
	8	VL	[(NNO)ZnEt]	250:250:1:0	50:50	60	4	9	84	23	23 000	39 780	1.56
	9	VL	[(NNO)ZnEt]	500:250:1:0	67:33	60	2	0	18	0	4500	nd j	nd j
	10	VL	[(NNO)ZnEt]	500:250:1:0	67:33	60	7	0	15	0	3750	nd j	nd j
	11	VL	[(NNO)ZnEt]	250:250:1:1	50:50	60	3	9	89	22	24 300	22 100	1.39
	12	CL	Al(OTf)3	250:500:1:2.5	33:67	110	15	100 h	100	nd	37 700	nd	nd
	13	CL	[(NNO)ZnEt]	125:250:1:0	33:67	60	4	27	100	14	31 500	28 700	1.59

/lactide random copolymers. The EC/BL, VL, CL, LLA copolymerizations were run in bulk (i.e., solvent-free reactions) at 60110 °C using Al(OTf)3, [(BDI iPr )Zn{N(SiMe3)2}], [(NNO)ZnEt] or TBD, in the absence or presence (typical [catalyst]0/[BnOH]0 ratio = 1:15; "immortal" ROP conditions) 66,67 of BnOH. It is noteworthy that all these systems remain ineffective in homopolymerization of EC (typical operating conditions: toluene at 110 °C or in bulk at 180 °C over 2448 h). The most significant results for the simultaneous copolymerizations of EC with lactones and LLA, providing PEC copolymers with poly(BL) (PBL, also referred to as poly(3-hydroxybutyrate), PHB, the most common poly(hydroxyalkanoate)), poly(VL) (PVL), poly(CL) (PCL), and poly(LLA) (PLLA)) segments, are gathered in Table

1 and Table 2

, respectively. a All reactions were performed in bulk (solvent-free) unless otherwise stated. b Reaction times were not systematically optimized. c Monomer conversion as determined by 1 H NMR spectroscopy of the crude reaction mixture (refer to Experimental Section). d Amount of EC inserted as determined from the crude reaction mixture (refer to Experimental Section). e Theoretical molar mass calculated from the relation:

Table 1 ,

 1 Table1, entries 23). Since our previous studies on the homopolymerization of the related 5CC cyclohexene carbonate have shown that this zinc -diketiminate-based catalyst is slightly less active than that based on the zinc diaminophenolate one, 22 further efforts were focused on the latter [(NNO)ZnEt] system. In the presence or in the absence of BnOH, [(NNO)ZnEt] revealed indeed active in the simultaneous copolymerization of EC with lactones at a more moderate temperature of 60 °C (Table1, entries 411,1320). entry 16). This incorporation rate of 37 mol% EC within the final P(EC-co-CL) copolymer represents the optimized value under such polymerization conditions. Although doubling the polymerization time promoted the CL consumption, the EC conversion did not similarly increase, thereby resulting in an EC incorporation rate lowered to 20 mol% (Table1, entries 1617). These observations suggest that EC was most likely inserted at the earlier stage of the EC/CL copolymerization. In comparison, the EC/VL copolymerization did not allow as much

	In the case of BL comonomer, greater EC insertion rates within the PHB were
	measured from [(NNO)ZnEt] (2126 mol%) as compared to Al(OTf)3 or
	[(BDI iPr )Zn{N(SiMe3)2}] (< 18 mol%). Whether exogenous alcohol was involved or not, the
	simultaneous copolymerization of EC with VL or CL using [(NNO)ZnEt] proceeded also at
	60 °C, generally within relatively short reaction times; these conditions compare favorably to

those reported for the alike EC/VL and EC/CL copolymerizations mediated by samarium/neodymium-based systems (reaction times typically in the range 1872 h for copolymerizations ran at 2025 °C).

23,45,46

Copolymers containing a significant amount (1338 mol%) of EC units were thus recovered (Table

1

, entries 720). The presence of BnOH does not seem to improve the behavior of [(NNO)ZnEt] in the EC/VL and EC/CL copolymerization (compare entries 8 vs. 11). The EC conversion rates remain limited to ca.

22200 equiv (vs. Zn), regardless of the reaction time and of the [EC]0/[VL/CL]0 ratio (Table

1

, entries 711 and 1320). At [EC]0/[CL]0 = 80:20, up to 37 mol% of EC are inserted in the final copolymer within 1 h, with 10% and 66% of EC and CL conversion, respectively (EC and VL consumption (Table

1

, entries 79 vs. 1315). The catalytic activity and the eventual insertion of EC into PVL appears to be quite dependent on the initial loading of EC:

Table 2 .

 2 Simultaneous copolymerization of EC with LLA mediated by zinc-based catalyst systems in bulk at different temperatures. All reactions were performed in bulk (solvent-free) unless otherwise stated. b Reaction times were not systematically optimized. c Monomer conversion as determined by 1 H NMR spectroscopy of the crude reaction mixture (refer to Experimental Section).

	Investigations of the simultaneous EC/LLA copolymerization performed at
	60150 °C with [(NNO)ZnEt] without BnOH, revealed similarly successful (Table 2, entries
	15). The reaction temperature significantly influenced the EC content into the recovered
	copolymer. Raising the temperature from 60 °C to 100 °C improved the EC insertion rate
	from 2 to 7 mol% (Table 2, entries 2,5). Similarly, at a [EC]0/[LLA]0 ratio of 83:17, the EC
	content was improved from 3 to 14 mol% by increasing the temperature from 60 °C to 120 °C

(Table 2, entries 7,11)

. However, higher temperature significantly affected the control of the copolymerization by promoting undesirable transesterification/transcarbonatation and/or chain transfer reactions, resulting in the lowering of the copolymer molar mass values. The copolymer prepared at 150 °C similarly consisted of oligomers which could not be precipitated from CH2Cl2/MeOH (Table

2

, entry 15). At 100 °C, increasing the [EC]0/[LLA]0 ratio from 17:83 to 83:17 resulted as expected in a larger insertion of EC units within PLLA, ranging from 1.5 to 17 mol%, respectively (Table

2

, entries 36,10). The EC/LLA simultaneous copolymerization carried out at 100 °C using [(BDI iPr )Zn{N(SiMe3)2}] afforded high LLA conversions, but the extent of EC inserted into PLLA remained low (5 mol%; Table

2

, entries 16). a d Amount of EC inserted as determined from the crude reaction mixture (refer to experimental section). e Theoretical molar mass calculated from the relation: Mn,theo=

Table 1

 1

, entries 13,14,15, respectively).

Table 1

 1

, entry 13).

Table 1

 1

, entry 12).

Acknowledgements

The authors gratefully thank Total Petrochemicals Co. for financial support of this research (Ph.D. grant to W. G.) and the Region Bretagne and Rennes Métropole for equipment support. X. Michel is also gratefully acknowledged for completing DSC analyses.

Supporting information includes the complementing 1 H and 13 C{ 1 H} NMR spectra and DSC traces of the commercial PEC sample, EC/BL, EC/VL, EC/CL and EC/LLA copolymers.

Supporting Information

List of figures

Figure S1. 1 H NMR spectrum (400 MHz,CDCl3,23 °C) of a commercial PEC (Mn = 250 000 g.mol 1 , ÐM = 1.9) prepared from CO2/ethylene oxide copolymerization.

Figure S2

. 13 C NMR spectrum (400 MHz, CDCl3, 23 °C) of a commercial PEC (Mn = 250 000 g.mol 1 , ÐM = 1.9) prepared from CO2/ethylene oxide copolymerization.

Figure S6.

Carbonyl region of the 13 C{ 1 H} NMR spectrum (100 MHz, CDCl3, 23 °C) of a mixture of a P(EC-co-CL) featuring 28 mol% of inserted EC (Table 1, entry 14) and a commercial PEC (Mn = 250 000 g.mol 1 , ÐM = 1.9 prepared from CO2/ethylene oxide copolymerization.

Figure S7

. DSC trace of a P(EC-co-BL) featuring 26 mol% of EC (Table 1, entry 4).