Enantiopure Isotactic PCHC Synthesized by Ring-Opening

Article pubs.acs.org/Macromolecules

Enantiopure Isotactic PCHC Synthesized by Ring-Opening Polymerization of Cyclohexene Carbonate William Guerin,† Abdou Khadri Diallo,† Evgueni Kirilov,† Marion Helou,‡ Martine Slawinski,‡ Jean-Michel Brusson,§ Jean-François Carpentier,† and Sophie M. Guillaume*,† †

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, F-35042 Rennes, Cedex, France ‡ Zone Industrielle Feluy C, Total Raffinage Chimie Feluy, B-7181 Seneffe, Belgium § Corporate Science, Total S.A., Tour Michelet A, 24 Cours Michelet − La Défense 10, 92069 Paris La Défense, Cedex, France S Supporting Information *

ABSTRACT: The ring-opening polymerization (ROP) of racemic trans-cyclohexene carbonate (rac-CHC) and enantiopure trans-(R,R)-CHC is successfully carried out with various catalyst systems. Poly(cyclohexene carbonate) (PCHC) with a slight isotactic bias (Pm = ca. 60−76%) is obtained by ROP of rac-CHC catalyzed by zinc diaminophenolate, zinc βdiketiminate, yttrium bis(phenolate), or 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) in combination with an alcohol as a coinitiator. Purely isotactic PCHC is synthesized for the first time via ROP of enantiopure (R,R)-CHC with a zinc/benzyl alcohol catalyst system. All reactions proceed without decarboxylation, affording well-defined PCHCs with Mn,NMR up to 17 000 g mol−1 and ĐM = ca. 1.2. Purely isotactic PCHC is semicrystalline, with Tg = 130 °C, Tc = 162 °C, and Tm = 248 °C. DFT computations further highlight the significant favorable impact of the trans-cyclohexane ring-strain to enable the ROP of CHC, as opposed to meso-CHC which is unreactive.

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INTRODUCTION Both aromatic and aliphatic polycarbonates (PCs) have gained major industrial and academic interest owing to the diversity in their properties, affording both commodity plastics and engineering plastics.1,2 Three distinct synthetic routes have been developed to prepare PCs. The polycondensation reaction between a dialkyl/diaryl carbonate and an α,ω-diol is rather limited in terms of molar mass, often affording oligomers.3,4 The copolymerization of epoxides with carbon dioxide is another approach which has been quite intensively explored and which is still motivating many studies.4−13 This strategy commonly involves incomplete alternating selectivities and/or partial decarboxylation of the PC, resulting in the formation of ether units along the PC chain. Also, epoxide/CO 2 copolymerization frequently leads to the formation, alongside the PC, of the corresponding five-membered ring cyclic carbonate monomer (5CC). Recent advances aim at the selective formation of either the 5CC monomer or the poly(5CC). Finally, the ring-opening polymerization (ROP) of a cyclic carbonate monomer is nowadays a privileged procedure to access well-defined high molar mass PCs. Yet, whereas six-membered and, to a much lesser extent, sevenmembered ring carbonates have been successfully polymerized by ROP under mild operating conditions, the related 5CCs have hardly been ring-opened polymerized,14−17 as the result of their poor polymerizability linked to their small ring strain.18,19 Hence, the ROP of 5CCs generally requires harsher conditions (in particular, high reaction temperatures) and often proceeds © 2014 American Chemical Society

with loss of carbon dioxide, resulting in the formation of poly(ether−carbonate)s rather than pure PCs. Nonetheless, given the availability of a large number of 5CCs obtained from epoxide/CO2 copolymerization (vide supra) or from bioresources,20−23 the valorization of these monomers into PCs certainly deserves consideration. Among 5CCs, the ROP of ethylene carbonate and propylene carbonate are the most documented. Several catalyst systems have been identified, but the ROP is again always accompanied by a partial or total decarboxylation of the poly(5CC). Remarkably, Endo and co-workers were able to perform the successful ROP of a sugar-derived 5CC, namely methyl 4,6-Obenzylidene-2,3-O-carbonyl-R-D-glucopyranoside (MBCG), without decarboxylation using n-BuLi, MOtBu (M = Li, Na, K), and 1,8-diazabicyclo[5.4.0]undec-7-en (DBU) initiators (Mn,SEC up to 20 200 g mol−1 vs polystyrene standards, ĐM = 1.82).24,25 This represents, to our knowledge and parallel to our own work,26 the first successful example of an anionic coordination−insertion ROP of a 5CC. Other PCs derived from 5CCs have been synthesized by epoxide/CO 2 copolymerization. 4−12 In particular, poly(cyclohexene carbonate) (PCHC) has been obtained along with variable amounts of its corresponding 5CC monomer, namely cyclohexene carbonate (CHC), from the cyclohexene Received: May 7, 2014 Revised: June 12, 2014 Published: June 24, 2014 4230

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Scheme 1. ROP of CHC toward Isotactic PCHC Mediated by Metallic or Organic Catalysts

Table 1. ROP of rac-CHC and (R,R)-CHC Mediated by Various Catalyst/Alcohol Systems in Solutiona or in Bulk at Different Temperatures entry

CHC

catalyst

alcohol

[CHC]0:[cat.]0:[BnOH]0

temp (°C)

1 2 3 4 5 6 7 8 9 10 11h 12h 13 14 15 16 17 18 19 20 21 22 23i,42 24h,42 25h,42

(R,R)-CHC (R,R)-CHC (R,R)-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC

Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-1 Zn-2 Zn-2 Zn-2 Y-1 Y-1 Y-2 Y-2 TBD TBD TBD KOtBu KOtBu KOtBu

BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH BnOH i PrOH i PrOH i PrOH i PrOH BnOH BnOH BnOH

100:1:1 200:1:1 500:1:2.5 50:1:1 50:1:1 100:1:1 100:1:1 100:1:1 250:1:5 500:1:5 100:1:1 100:1:1 50:1:1 100:1:1 250:1:1 100:1:1 100:1:1 50:1:1 100:1:1 100:1:1 100:1:1 250:1:5 25:1 25:1 25:1

60 60 60 100 100 100 80 60 60 60 70 130 100 80 60 40 60 40 60 60 60 60 60 60 150

reaction timeb (h)

CHC convc (%)

Mn,theod (g mol−1)

Mn,NMRe (g mol−1)

1.5 0.8 4 18 24 24 3.75 3 4 4 15 10 76 5 4 16 16 16 16 1 19 4 3 3 3

94 68 23 72 86 63 89 91 92 87 57 79 82 84 81 56 70 83 67 75 93 85 44 69 35

13360 19330 6500 5120 6110 8950 12650 13100 6540 12400 8100 11200 5800 12000 5760 8010 10100 6000 9600 10700 13200 6000 1560 2500 1250

14300 19700 5500 6750 7600 9360 9800 14200 6540 12100 6900 9150 7500 11600 6700 7800 10600 6700 12000 8200 18100 6100

Mn,secf (g mol−1)

ĐM g

isotacticity (Pm, %)h

21000 21000 4800 7320 7800 7850 11000 11400 6200 10850 7200 9900 6100 8850 5500 ndk 13100 8600 11400 7800 10000 6100 400 11000 4300

1.34 1.30 1.09 1.28 1.43 1.28 1.64 1.33 1.15 1.07 1.05 1.19 1.27 1.27 1.21 ndk 1.21 1.14 1.19 1.13 1.54 1.16 1.0 3.9 1.4

99 99 99 ndk nd ndk ndk 66 66 67 ndk ndk ndk 66 67 ndk 76 ndk 64 ndk 60 64 nrl nrl nrl

a

All reactions were performed in toluene with [CHC]0 = 4.0 M unless otherwise stated. bReaction times were not optimized. cCHC conversion as determined by 1H NMR spectroscopy of the crude reaction mixture. dTheoretical molar mass calculated from the relation: Mn,theo = ([CHC]0/ [BnOH]0) × conversionCHC × MCHC + MROH, with MCHC = 142 g mol−1, MBnOH = 108 g mol−1, and MiPrOH = 60 g mol−1. eMolar mass values determined using the resonances of the methylene (BnOH, δ = ca. 5.14 ppm) or methine (iPrOH, δ = ca. 4.85 ppm) alkoxide signal in the 1H NMR spectrum of the isolated PCHC in CDCl3 at 23 °C. fNumber-average molar mass value (uncorrected; refer to the Experimental Section in Supporting Information) determined by SEC in THF at 30 °C versus polystyrene standards. gDispersity values determined by SEC in THF at 30 °C. h Pm = probability of meso linkage as determined by 13C NMR analysis of the methylene region. iReaction run in bulk. jReaction run in THF. knd: not determined. lnr: not reported.

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oxide (CHO)/CO2 copolymerization (Scheme S1).12,13,27−30 This route, however, cannot provide access to all possible PCHC microstructures. Indeed, CHC units within a PCHC prepared from a meso-(S,R)-CHO/CO2 copolymerization using a (stereoselective) catalyst can only feature (R,R) and/or (S,S) configurations (but no (R,S); Scheme S2).31 This results from the favorable anti nucleophilic attack inducing the inversion of the configuration of one of the asymmetric carbon atoms (Scheme S3).27 Whereas the large majority of catalysts for CHO/CO2 copolymerization promote the formation of atactic PCHC, some of them, essentially derived from zinc, chromium or cobalt salen derivatives, enable the synthesis of syndiotactic (up to 81% of r-centered tetrads)32 or isotactic (up to (R,R)/ (S,S) = 99:1)27,33−41 enriched PCHCs. However, purely isotactic PCHC has not been reported to date, even less from the ROP of CHC. Provided the CHC enantiomers can be obtained, ROP of CHC may, in principle, enable to access to all microstructures of PCHC, including those having (R,S)/(S,R)configurated CHC repeating units, unlike from the CHO/CO2 copolymerization. Commercially available racemic- (trans(R,R)/(S,S)−), enantiopure trans-(R,R)- or trans-(S,S)-, and meso-(R,S)-1,2-cyclohexanediols provide access to the corresponding four CHCs upon transesterification with e.g. an alkyl chloroformate (refer to the Experimental Section in the Supporting Information). Ultimately, one may thus prepare, provided a suitable (stereoselective) catalyst is available, atactic, (R,R) or (S,S) isotactic-, and meso-syndiotactic PCHCs (Scheme S4). In this contribution, we report that the ROP of trans-CHC by various metallic or organic catalyst systems enables the preparation of isotactic PCHCs; highly isotactic PCHC has been prepared from enantiopure trans-CHC (Scheme 1).26 We also explored via DFT thermodynamic calculations some of the reasons that allow this bicyclic monomer to polymerize without decarboxylation, unlike the typical behavior of 5CCs monomers.

The Zn-1/BnOH system successfully ring-open polymerized enantiopure (R,R)-CHC (Table 1, entries 1−3), thus confirming the favorable polymerizability predicted by DFT computations. The corresponding enantiomerically pure isotactic (vide inf ra) PCHCs were thus synthesized with an apparent turnover frequency (TOF) of 170 h−1. The ROP of rac-CHC was next investigated in detail, in solution or in bulk at 60−130 °C, in the presence of a bicomponent catalytic system composed of a zinc or yttrium complex as Zn-1, [(BDI)Zn{N(SiMe3)2}] (Zn-2; BDI = CH(CMeNC 6 H 3 -2,6-iPr 2 ) 2 ), 4 5 , 4 6 [(ONO-OMe)Y{N(SiMe2H)2}THF] (Y-1), or [(ONNO)Y{N(SiMe2H)2}] (Y2),47−50 or an organocatalyst (1,5,7-triazabicyclo[4.4.0]dec-5ene, TBD) (Scheme 1), combined with an alcohol (BnOH or iPrOH) as a co-initiator/chain-transfer agent. These catalysts were selected from our own library for their efficiency in the ROP of cyclic esters including six- or seven-membered ring carbonates and, for some of them, for their established stereoselectivity in the ROP of related chiral cyclic esters.47−52 Indeed, using a potentially stereoselective catalyst may ideally enable the synthesis of a PCHC with a specific microstructure (i.e., tacticity) upon differentiating the two enantiomers from the ROP of rac-CHC. Such an ultimate behavior is highly desirable considering the larger/cheaper availability of racemic vs other enantiopure monomers. The ROP of rac-CHC performed in toluene with Zn-1/ BnOH at [rac-CHC]0/[Zn-1]0/[BnOH]0 = 50 or 100:1:1, proceeded with a lower conversion at 100 °C than at 80 or 60 °C (Table 1, entries 4−8). This is most likely due to the proximity to the ceiling temperature of PCHC reported around 100 °C.53 Larger monomer loadings (250−500 equiv) similarly afforded at 60 °C, under immortal conditions (i.e., with an excess of BnOH as chain transfer agent),47,48 PCHC in rather good yields (Table 1, entries 9 and 10). Attempts to carry out the ROP in bulk at 70 and 130 °C afforded, within shorter reaction times, PCHCs with a lower dispersity (Table 1, entries 11 and 12). However, the rac-CHC conversion from such a melt remained lower than that observed in solution polymerization, most likely as the result of the PCHC crystallinity at 70 °C (hence inducing monomer diffusion problems) and of an operating temperature of 130 °C (i.e., above the ceiling temperature of PCHC, thereby favoring the depolymerization).27,53 The temperature thus appeared as an essential parameter to adjust for the suitable ROP of rac-CHC, as next further demonstrated with Zn-2 (Table 1, entries 13−15). Consequently, the reaction temperature was maintained at 60 °C, and the following polymerizations were run in toluene. The yttrium catalysts Y-1 and Y-2generated in situ from the reaction of the bis(phenol) proligand with [Y{N(SiMe2H)2}3]in combination with isopropanol were found similarly more active at 60 °C than at 40 °C (Table 1, entries 16−19). These bisphenolate yttrium catalysts remained, under the same operating conditions, however less active than the zinc ones (Table 1, e.g., entries 8 vs 17, 19). Finally, the commercially available, simple organic guanidine TBD catalyst showed, in solution at 60 °C, a fairly good efficiency (75−85% conversion in 1−4 h) and control; yet, prolonged reaction time proved detrimental, inducing broader dispersities, likely diagnostic of transesterification or related side reactions (entries 20−22). In line with the above DFT computations, the attempted ROP of meso-CHC mediated by Zn-1/BnOH at 60 °C in toluene or at 125 °C in bulk failed to give any polymer (Table

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RESULTS AND DISCUSSION DFT Computations. In order to get a first hint on the factors governing the ROP of CHC, we first assessed the polymerizability of enantiopure (R,R)-CHC (trans-CHC) and (R,S)-CHC (meso-CHC) from DFT computations. The opening of the CHCs was evaluated using a simple methanol model molecule (Scheme S5). In the case of (R,R)-CHC, the calculated ΔGp° is negative (−15 kJ mol−1), indicating that this molecule can be thermodynamically ring-opened. This computed free energy value fits well the values of enthalpy (ΔHp° = −23 kJ mol−1) and entropy (ΔSp° = −63 J mol−1 K−1) previously evaluated from experimental monomer concentration at equilibrium for the ROP of trans-CHC.42 Conversely, in the case of meso-CHC, which is obviously less strained than the trans diastereomer, the positive value computed for ΔGp° (+11 kJ mol−1) suggested its nonpolymerization (Scheme S6). Catalytic ROP Reactions. The ROP of these CHC monomers was next evaluated experimentally. A model catalytic system formed by the zinc diaminophenolate complex [(NNO)ZnEt] (Zn-1; (NNO)− = 2,4-di-tert-butyl-6-{[(2′dimethylaminoethyl)methylamino]methyl}phenolate)) 43 (Scheme 1) combined with benzyl alcohol (BnOH) as a coinitiator and chain transfer agent was selected, based on its efficiency in the ROP of related cyclic esters.43,44 Experiments were typically run in toluene at 60 °C (Table 1). 4232

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S1 and Scheme S6). A control ROP experiment using an equimolar mixture of rac-CHC and meso-CHC successfully afforded PCHC from the former monomer, but left meso-CHC unreacted, thereby ruling out the potential presence of catalystinhibiting impurities within the meso-CHC batch (Table S1, entry 4). KOtBu was similarly reported ineffective in polymerizing meso-CHC, whatever the conditions (diglyme, THF, bulk, 60 °C, 150 °C).42 Regardless of the presently studied catalytic system, under the optimized conditions (toluene, 60 °C), a rather good control of the PCHCs molecular features was generally achieved (Table 1, entries 1−22). A good agreement between the theoretical molar mass values (Mn,theo, determined from the monomer conversion and the initial [CHC]/[catalyst]/[ROH] ratio) and the experimental molar mass data determined from the 1H NMR analysis of the spectra of the precipitated PCHCs (Mn,NMR, refer to the Supporting Information) was observed. Despite the absence of correction for the possible difference in hydrodynamic volume of the PCHCs and the polystyrene standards used for calibration, the experimental molar mass values measured by SEC (Mn,sec) were also in fair agreement with these data. The PCHCs synthesized in the present work were better defined in terms of theoretical/experimental molar mass values agreement and of narrower dispersities (ĐM = ca. 1.2) than the ones reported from KOtBu (Table 1, entries 1−22 vs 23−25).42 Also, higher molar mass PCHCs were isolated from the present ROP catalytic systems (Mn,NMR ≤ 17 000 g mol−1, Mn,SEC ≤ 21 000 g mol−1, vs Mn,SEC ≤ 11 000 g mol−1,42 Table 1). The typical 1H NMR spectrum of PCHCs, synthesized by ROP of rac- or (R,R)-CHC by any catalytic system, and isolated after precipitation in cold methanol, showed the expected signals corresponding to the main chain methine and methylene hydrogens (Figures S1−S6). The signals of the benzyloxy or isopropoxy methylene and methine end-groups were also clearly identified. These data confirmed that the ROP is initiated by benzyl alcohol/alkoxide or isopropanol/ isopropoxide. Noteworthy, the absence of signals typical of ether units (δ1H = 3.3−3.1 ppm, δ13C = 66.5−67.7 ppm) indicated the absence of decarboxylation during the polymerization. Microstructural analysis of the recovered PCHCs was performed by 13C{1H} NMR spectroscopy to assess the stereoselectivity of the different catalytic systems. The assignment of the carbonyl and methylene regions was based on previous work describing PCHCs of various stereoregularities prepared from CHO/CO2 copolymerization.27,32−34 Distinct tetrads can thus be observed in the carbonyl region as illustrated Figure 1. Isotactic-enriched PCHCs reported in the literature were characterized by an intense (up to 98%) downfield carbonyl resonance for the mmm and mmr tetrads at δ = 153.7 ppm, with a series of lower intensity resonances at δ = 153.3−153.1 ppm assigned to stereoerrors.27,32 The observation of a sole contribution at δ = 153.9 ppm in the spectra of PCHCs prepared from (R,R)-CHC supports the pure isotacticity (Pm = 99%, as determined by integration of the 13 C resonances) of the PCHC thus formed (Figure 1a and Figure S7). This is further supported by the two well-resolved resonances for the nonequivalent methylene carbons in the PCHC backbone at δ = 29.8 and 23.2 ppm, corresponding to the mm triad, as previously assigned in the literature (Figure 1a′ and Figure S7).32 Comparatively, the spectra of PCHCs prepared from rac-CHC all displayed series of carbonyl and

Figure 1. Carbonyl (a−f) and methylene (a′−f′) regions of the 13 C{1H} NMR spectra (100 MHz, CDCl3, 23 °C) of PCHC samples synthesized by ROP of [a, a′] (R,R)-CHC from Zn-1/BnOH (Pm = 99%; Table 1, entry 2), [b, b′] rac-CHC from Zn-1/BnOH (Pm = 66%; Table 1, entry 8), [c, c′] Zn-2/BnOH (Pm = 66%; Table 1, entry 14), [d, d′] Y-1/iPrOH (Pm = 76%; Table 1, entry 17), [e, e′] Y-2/ iPrOH (Pm = 64%; Table 1, entry 19), and [f, f′] TBD (Pm = 60%; Table 1, entry 21) (Figures S7−S12).

methylene signals with significant intensities, indicating a poorer stereoregularity, with a slight isotactic bias (Pm = ca. 67% for zinc and yttirum catalysts and ca. 62% for TBD; Figure 1b−f and Figures S8−S12, respectively). The thermal characteristics of PCHC, as determined by DSC and TGA analyses, depend much on its microstructure, as expected. While atactic PCHC is hardly crystalline to noncrystalline with a glass transition temperature Tg < ca. 120 °C, syndiotactic-enriched PCHC has a slightly higher Tg (130 °C), and a melting temperature (Tm) has only been observed with isotacticity content >90%.41,54 Highly isotactic PCHC prepared from CHO/CO2 copolymerization ((R,R)/ (S,S) ratio = 98:2) was reported as a typical semicrystalline polymer with Tm = 216 °C and a decomposition temperature of 310 °C.27 The DSC thermograms of the ca. 60% and 99% isotactic-enriched PCHCs, herein prepared from rac-CHC and (R,R)-CHC, respectively, using Zn-1/BnOH as catalytic system (Table 1, entries 8 and 2, respectively), are compared in Figure 2. The less isotactic PCHC samples only showed a Tg = ca. 121 °C. The absence of a melting transition confirmed the amorphous character of this PCHC, which was found to decompose at Td = 279 °C (Figure 2a,b, top black trace). The purely isotactic PCHC gave a Tg of 130 °C, a value 10 °C higher than that reported for a 92% isotactic-enriched PCHC55 and showed an exothermic crystallization peak Tc at 162 °C 4233

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Figure 2. (a) DSC thermograms (first heating cycle) and (b) TGA thermograms of a 60% (black trace) and 99% (red trace) isotactic-enriched PCHC synthesized from rac-CHC and (R,R)-CHC, respectively (Table 1, entries 8 and 2, respectively).

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along with an endothermic Tm at 248 °C.27 While such a crystallization transition was previously reported for a 99% isotactic PCHC with Mn,SEC = 35600 g mol−1,41 the melting temperature of 92% and 98% isotactic-enriched PCHCs were found at Tm = 207 and 216 °C, respectively.27 However, isotactic (R,R)-PCHC prepared in the present study reproducibly started degrading at Td = ca. 240 °C (Figure 2).

(1) Proceedings of Symposium of the American Chemical Society, March 2003, Washington, DC; Brunelle, D. J., Korn, M. R., Eds.; ACS Symposium Series 898; American Chemical Society: Washington, DC, 2005; 281 pp. (2) Ulery, B. D.; Nair, L. S.; Laurencin, C. T. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 832−864. (3) Fukuoka, S.; Tojo, M.; Hachiya, H.; Aminaka, M.; Hasegawa, K. Polym. J. 2007, 39, 91−114. (4) Fukuoka, S.; Kawamura, M.; Komiya, K.; Tojo, M.; Hachiya, H.; Hasegawa, K.; Aminaka, M.; Okamoto, H.; Fukawa, I.; Konno, S. Green Chem. 2003, 5, 497−507. (5) Kember, M. R.; Buchard, A.; Williams, C. K. Chem. Commun. 2011, 47, 141−163. (6) Pescarmona, P. P.; Taherimehr, M. Catal. Sci. Technol. 2012, 2, 2169−2187. (7) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43, 6618−6639. (8) Klaus, S.; Lehenmeier, M. W.; Anderson, C. E.; Rieger, B. Coord. Chem. Rev. 2011, 255, 1460−1479. (9) Darensbourg, D. In Synthetic Biodegradable Polymers; Rieger, B., Künkel, A., Coates, G. W., Reichardt, R., Dinjus, E., Zevaco, T. A., Eds.; Springer: Berlin, 2012; Vol. 245, Chapter 135, pp 1−27. (10) Lu, X.-B.; Ren, W.-M.; Wu, G.-P. Acc. Chem. Res. 2012, 45, 1721−1735. (11) Lu, X.-B.; Darensbourg, D. J. Chem. Soc. Rev. 2012, 41, 1462− 1484. (12) Coates, G. W.; Jeske, R. C. Handbook of Green Chemistry; Anastas, P. T., Crabtree, R. H., Eds.; Wiley: New York, 2009; Vol. 1, pp 343−373. (13) Taherimehr, M.; Al-Amsyar, S. M.; Whiteoak, C. J.; Kleij, A. W.; Pescarmona, P. P. Green Chem. 2013, 15, 3083−3090. (14) Rokicki, G.; Parzuchowski, P. G. In Polymer Science: A Comprehensive Reference; Matyjaszewski, K., Möller, M., Eds.-inChief; Elsevier: Amsterdam, 2012; pp 247−308. (15) Keul, H. In Handbook of Ring-Opening Polymerization; Dubois, P., Coulembier, O., Raquez, J.-M., Eds.; Wiley-VCH: Weinheim, 2009; Chapter 12, pp 307−327. (16) Tempelaar, S.; Mespouille, L.; Coulembier, O.; Dubois, P.; Dove, A. P. Chem. Soc. Rev. 2013, 42, 1312−1336. (17) Endo, T.; Shibasaki, Y.; Sanda, F. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2190−2198. (18) Odian, G. Principles of Polymerization, 4th ed.; WileyInterscience: Hoboken, NJ, 2004. (19) Vogdanis, L.; Martens, B.; Uchtmann, H.; Hensel, F.; Heitz, W. Makromol. Chem. 1990, 191, 465−472. (20) North, M.; Pasquale, R.; Young, C. Green Chem. 2010, 12, 1514−1539. (21) Sakakura, T.; Kohno, K. Chem. Commun. 2009, 1312−1330. (22) Pyo, S.-H.; Persson, P.; Mollaahma, M. A.; Sörensen, K.; Lundmark, S.; Hatti-Kaul, R. Pure Appl. Chem. 2012, 84, 637−661.

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CONCLUSION In summary, we have established the possibility to polymerize CHC by an anionic coordination−insertion ROP approach with a variety of metal- and organo-catalyst systems. PCHC with a slight isotactic bias (Pm = ca. 60−76%) was obtained by ROP of rac-CHC from TBD- or various zinc- and yttriumbased catalyst systems. More outstandingly, we have realized the first synthesis of a purely isotactic PCHC (Pm = 99%) without decarboxylation via ROP of enantiopure (R,R)-CHC.26 Remarkably, isotactic PCHC is a semicrystalline polycarbonate featuring a high Tg of 130 °C. Also, the pure isotacticity imparted a crystallization temperature Tc of 162 °C observed for the first time on PCHC, while the Tm of 248 °C was, similarly to the Tg, the highest value ever reported for an isotactic PCHC. DFT computations highlighted the significant favorable impact of the trans-cyclohexyl adjacent ring to enable the ROP of the 5CC in these CHCs.

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary Schemes, the synthesis of rac-CHC, (R,R)CHC, meso-CHC, and their corresponding PCHC, and their characterizations by 1H and 13C NMR. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.M.G.). Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

The authors gratefully thank Total Raffinage Chimie for financial support of this research (Ph.D. grant to W. Guerin and Postdoc grant to A. K. Diallo) and the Region Bretagne and Rennes Métropole for equipment support. 4234

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dx.doi.org/10.1021/ma5009397 | Macromolecules 2014, 47, 4230−4235