Article pubs.acs.org/Macromolecules
Block and Random Copolymers of 1,2-Cyclohexyl Cyclocarbonate and L‑Lactide or Trimethylene Carbonate Synthesized by RingOpening Polymerization Abdou Khadri Diallo,† William Guerin,† Martine Slawinski,‡ Jean-Michel Brusson,§ Jean-François Carpentier,† and Sophie M. Guillaume*,† †
Institut des Sciences Chimiques de Rennes, Organometallics, Materials and Catalysis Laboratories, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, F-35042 Rennes, Cedex, France ‡ Total Raffinage Chimie Feluy, Zone Industrielle Feluy C, B-7181 Seneffe, Belgium § Total S.A., Corporate Science, Tour Michelet A, 24 Cours Michelet - La Défense 10, 92069 Paris La Défense, Cedex, France S Supporting Information *
ABSTRACT: The sequential and random ring-opening polymerizations (ROP) of racemic-trans-cyclohexene carbonate (rac-CHC) or enantiopure trans-(R,R)-cyclohexene carbonate ((R,R)-CHC) with L-lactide (LLA) or trimethylene carbonate (TMC) have been performed. Catalytic systems based on zinc diaminophenolate [(NNO)ZnEt] ((NNO)− = 2,4-di-tertbutyl-6-{[(2′-dimethylaminoethyl)methylamino]methyl}phenolate)) or tris[N,N-bis(trimethysilyl)amide]yttrium (Y[N(SiMe3)2]3) complexes, or a guanidine-type organocatalyst (1,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD), combined to an alcohol (BnOH or iPrOH) as initiator/chain-transfer agent were used. Well-defined diblock P(rac-CHC)-b-PLLA and P((R,R)CHC)-b-PLLA and random P(rac-CHC)-co-PLLA and P(rac-CHC)-co-PTMC copolymers were thus synthesized with molar mass values up to Mn,NMR = ca. 34 000 g mol−1 and rather narrow dispersity values (ĐM = 1.2−1.7). 1H and 13C{1H} NMR characterizations of the copolymers revealed the presence of −OBn or −OiPr chain-end groups, thereby supporting the active role of exogenous alcohol as initiator. No decarboxylation reaction was ever observed during any copolymerization, thus providing PCHC/PLLA and PCHC/PTMC copolymers void of ether defects. Thermal analysis of the copolymers assessed by DSC and TGA confirmed their block or random structure. The block PCHC-b-PLA and the random PCHC-co-PLLA and PCHC-co-PTMC copolymers represent the first examples of such copolymers synthesized by ring-opening copolymerization of the two comonomers. The latter PCHC-co-PTMC copolymers randomly combining CHC and TMC units are the first examples ever reported.
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INTRODUCTION Aliphatic polyesters and polycarbonates are important classes of polymers gaining increasing attention both in academia and in industry as potential substitutes to polymers derived from fossil feedstock. Indeed, a large number of the corresponding monomers, namely cyclic esters and cyclic carbonates, respectively, are derived from renewable bioresources. For instance, isotactic poly(L-lactide) (PLLA) is a thermoplastic polymer derived from starch-containing supplies, and it is nowadays considered as a degradable alternative polymer of commodity plastics for packaging, textile, electronics, or other biomedical applications.1−6 Similarly, poly(trimethylene carbonate) (PTMC) can be derived from glycerol, a byproduct of the production of biodiesel from triglycerides, and is, thanks to its biocompatibility and biodegradability used as a biomaterial, considered as potential “green” commodity polycarbonate.7−13 Polyesters and polycarbonates are often industrially produced by the polycondensation of α,ω-diols with diacids and of α,ωdiols with phosgene or dialkyl carbonates, respectively. The major drawbacks of this route include harsh operating conditions © XXXX American Chemical Society
or noxious reagents with the necessity to remove side products, a poor control of the reaction, and the access to only limited polymer architectures. Alternatively, polyesters and polycarbonates can be more effectively synthesized by ring-opening polymerization (ROP) of the corresponding cyclic monomers.14−16 Polycarbonates can also be prepared by metalcatalyzed alternating ring-opening copolymerization (ROCOP) of epoxides with anhydrides or carbon dioxide, respectively.17−24 These ROP and ROCOP processes are usually well-controlled, allowing to obtain well-defined polymers in particular in terms of molar mass predictability, narrow dispersity, chain-end fidelity, and tunable microstructure and tacticity. To that end, a large array of active metal-based or organic catalytic systems have been developed, achieving in some cases high productivity and stereoselectivity. Received: March 14, 2015 Revised: April 27, 2015
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DOI: 10.1021/acs.macromol.5b00548 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Tandem Strategy for the Synthesis of PCHC Block Copolymers from the Ring-Opening Copolymerization of Cyclohexene Oxide (CHO) with CO2 and Subsequent Block Copolymerization with L-Lactide38,39
Scheme 2. Synthesis of PCHC-b/co-Polyester Copolymers by Ring-Opening Copolymerization of CHC with LLA or TMC
ring cyclic carbonate (5CC), commonly known as difficult to ring-open polymerize,33,34 was shown to arise from the favorable impact of the trans-cyclohexylene adjacent ring. Isotactic PCHC is a semicrystalline polycarbonate featuring a high glass transition temperature Tg of 130 °C, a crystallization temperature Tc of 162 °C, and a high melting temperature Tm of 248 °C.31 These attractive thermal characteristics of PCHC thus prompted us to investigate the introduction of CHC units within PLLA or PTMC, through the ring-opening copolymerization of both monomers, LLA and TMC, so as to improve the final polyester and polycarbonate material properties. A recent approach allowing to prepare block copolymers of a poly(5CC) with another segment based on a cyclic ester or cyclic carbonate is the sequential copolymerization involving the use of a macroinitiator, first prepared from epoxide/CO2 ROCOP, in the subsequent ROP of a distinct monomer such as LLA or εcaprolactone (CL) (Scheme 1).35−39 According to this strategy,
Polyesters and polycarbonates have also been combined into copolymers so as to improve the thermal and mechanical properties of the polymer material by taking advantage of the synergy of the various constituting segments. For instance, one approach to improve the ductility, brittleness, and low melting temperature (Tm = 170−180 °C) of semicrystalline isotactic PLLA, and thereby to broaden its applications, has been to copolymerize L-lactide (LLA) with other comonomers such as trimethylene carbonate (TMC) which provides amorphous and flexible PTMC segment.25−30 Recently, we reported the synthesis of purely (Pm = 100%) to slightly (Pm = ca. 60−76%) isotactic poly(cyclohexene carbonate) (PCHC) from the ROP of enantiopure (R,R)-trans- or racemic-trans-cyclohexene carbonate ((R,R)-CHC or rac-CHC), respectively, catalyzed by a zinc diaminophenolate complex, [(NNO)ZnEt] ((NNO)− = 2,4-ditert-butyl-6-{[(2′-dimethylaminoethyl)methylamino]methyl}phenolate)).31,32 The successful ROP of such a five-membered B
DOI: 10.1021/acs.macromol.5b00548 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Table 1. Sequential Copolymerization of rac-CHC or (R,R)-CHC (60 °C) Followed by LLA (100 °C, 6 h) Mediated by Metallic or Organic Catalysts in Toluene PCHC-b-PLLA
entry
CHC
catalyst
ROH
[CHC]0: [LLA]0: [Cat.]0: [ROH]0
1 2 3 4 5 6 7 8 9 10 11 12
rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC rac-CHC (R,R)-CHC (R,R)-CHC (R,R)-CHC (R,R)-CHC
[(NNO)ZnEt] [(NNO)ZnEt] [(NNO)ZnEt] TBDg TBDg TBDg Y[N(SiMe3)2]3 Y[N(SiMe3)2]3 Y[N(SiMe3)2]3 [(NNO)ZnEt] [(NNO)ZnEt] [(NNO)ZnEt]
BnOH BnOH BnOH BnOH BnOH BnOH iPrOH iPrOH iPrOH BnOH BnOH BnOH
50:150:1:1 100:100:1:1 150:50:1:1 50:150:1:1 100:100:1:1 150:50:1:1 50:150:1:1 100:100:1:1 150:50:1:1 50:150:1:1 100:100:1:1 150:50:1:1
CHC ROP reaction timea (h)
CHC convb (%)
PCHC Mn,theoc (g mol−1)
PCHC Mn,NMRd (g mol−1)
LLA convb (%)
Mn,theoe (g mol−1)
Mn,NMRf (g mol−1)
Mn,SECg (g mol−1)
ĐM g
6 6 6 24 24 24 16 16 16 6 6 6
85 86 77 91 91 91 87 77 71 97 97 97
6100 12300 16500 6600 13000 19500 6200 11000 15200 7000 13900 20800
4400 12300 17200 6800 16900 17900 7700 n.d. n.d. 8100 14000 16900
98 97 91 96 94 100 97 97 100 100 100 100
27300 26300 23100 27300 26500 26700 27100 25000 22400 28600 28300 28000
30400 27000 22500 34000 28700 29300 26300 13000 23600 23500 23300 26600
28000 25300 18100 22600 17000 17100 28600 22900 20300 27500 45600 37000
1.7 1.5 1.2 1.5 1.4 1.4 1.7 1.6 1.6 1.5 1.4 1.4
a
Reaction times were not optimized. bMonomer conversion calculated from the 1H NMR analysis of the crude product (refer to the Experimental Section). cTheoretical molar mass value of PCHC calculated from the relation: Mn,theo = MCHC × [CHC]0/[ROH]0 × conversionCHC + MROH, with MCHC = 142 g mol−1, MBnOH = 108 g mol−1, and MiPrOH = 60 g mol−1. dExperimental molar mass value of PCHC determined by 1H NMR (refer to the Experimental Section). eTheoretical molar mass value of PCHC-b-PLLA calculated from the relation Mn,theo = {MCHC × [CHC]0/[ROH]0 × conversionCHC}+ {MLLA × [LLA]0/[ROH]0 × conversionLLA}+ MROH, with MLLA = 144 g mol−1. fExperimental molar mass value of PCHC-b-PLLA determined by 1H NMR from the integral value ratio of the signals of benzyl (BnOH) or methyl (iPrOH) end-group hydrogens to internal methine hydrogens (refer to the Experimental Section). gExperimental number-average molar mass and dispersity values determined by SEC in THF at 30 °C using polystyrene standards. n.d. = not determined.
diblock poly(styrene carbonate)-b-PLLA,35 PCHC-b-poly(εcaprolactone),37 PCHC-co-PLLA,38 and triblock PLLA-b-poly(propylene carbonate)-b-PLLA,36 PLA-b-PCHC-b-PLA,39 and PLLA-b-PCHC-b-PLLA39 copolymers have been synthesized from cobalt and mono- or dimetallic zinc complexes. Multiblock PCHC/PCL copolymers have similarly been synthesized from a zinc−cobalt double metal cyanide complex and tin octoate in a one-pot/one-step approach from the terpolymerization ROCOP of CO2, cyclohexene oxide, and CL.40 More commonly, such polycarbonate/polyester copolymers derived from 5CCs have been synthesized by ROP of the 5CC and the comonomer. In particular, the simultaneous ROP of ethylene carbonate (EC) with a few other cyclic monomers, namely CL, δ-valerolactone (VL), LLA, and 2,2-dimethyltrimethylene carbonate, successfully afforded the corresponding random copolymers.41−49 In these earlier studies, the only poly(ethylene carbonate) (PEC)-based block copolymer reported has been obtained from the sequential polymerization of EC with CL.41 Besides the p-tert-butylphenol/KHCO3 initiating system from which PEC oligomers have been reported,49 the complexes used to promote these earlier copolymerizations were all based on rare earth metals, viz. SmI2/Sm, [SmI2(THF)], [(C5Me5)2SmMe(THF)], [(C9H7)2Sm(THF) 1.5 ], [(C 13 H 9 ) 2 Sm(THF) 2 ], [{(Me 3 Si) 2 N} 2 Sm(THF)2], or Ln(2,6-di-tert-butyl-4-methylphenolate)3 (Ln = La, Nd, Sm, Dy).41−48 Decarboxylation, the most common side reaction in the ROP of 5CC leading to poly(ether carbonate)s,50 was found to depend on the couple of monomers, on the initiator, and on the initial EC/comonomer ratio as well. However, a low EC content (1−31 mol %) was reached in the resulting copolymers, yet impacting the thermomechanical properties of the copolymers.45 We have also recently established the successful synthesis of PEC copolymers incorporating βbutyrolactone, VL, CL, and LLA, without decarboxylation.51 The effective catalytic systems used were the diaminophenolate and β-diketiminate zinc complexes, [(NNO)ZnEt] and [(BDIiPr)-
Zn{N(SiMe3)2}] (BDIiPr = CH(CMeNC6H3-2,6-iPr2)2), respectively, the Lewis acidic triflate salt Al(OTf)3, or the organic TBD guanidine (TBD = 1,5,7-triazabicyclo[4.4.0]dec-5-ene), combined to a protic source as initiator, typically benzyl alcohol (BnOH).51 As much as 37 mol % of EC was thus incorporated into the polyester backbone. Finally, the same zinc diaminophenolate complex [(NNO)ZnEt], TBD, or the yttrium trisamide complex (Y[N(SiMe3)2]3), associated with an alcohol, similarly revealed active in the ROP of a related 5CC, namely trans-1,4cyclohexadiene carbonate (rac-CHDC), affording highly syndiotactic poly(cyclohexadiene carbonate), PCHDC.52 The zincbased initiator further enabled the sequential or simultaneous copolymerization of CHDC with rac-CHC and its sequential copolymerization with LLA, thereby affording block P(CHDCb-CHC) and random P(CHDC-co-CHC) and P(CHDC-coLLA) copolymers.52 Note that P(CHDC-co-CHC) have just been recently prepared from the terpolymerization of CO2, cyclohexene oxide, and cyclohexadiene oxide, yielding polycarbonates with controllable quantities of unsaturation.24 In the present contribution, we report the copolymerization of rac-trans-CHC or (R,R)-trans-CHC with LLA or TMC using catalyst systems based on [(NNO)ZnEt], Y[N(SiMe3)2]3, or TBD, combined to an alcohol (Scheme 2). Diblock and random copolymers have been synthesized from the sequential and simultaneous feeding of both monomers, respectively. The structures of these original copolymers have been investigated in details by SEC, NMR spectroscopy, and their thermal properties assessed by DSC and TGA.
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EXPERIMENTAL SECTION
Materials. All polymerizations were performed under an inert atmosphere (argon,
