Selective transformations of carbon dioxide and epoxides into biodegradable polycarbonates by the alternating copolymerization of the two monomers represent some of the most well-studied and innovative technologies for potential large-scale utilization of carbon dioxide in chemical synthesis. For the most part, previous studies of these processes have focused on the use of aliphatic terminal epoxides or cyclohexene oxide derivatives, with only rare reports concerning the synthesis of CO(2) copolymers from epoxides containing electron-withdrawing groups such as styrene oxide. Herein we report the production of the CO(2) copolymer with more than 99% carbonate linkages from the coupling of CO(2) with epichlorohydrin, employing binary and bifunctional (salen)cobalt(III)-based catalyst systems. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving two electronically different epoxides: epichlorohydrin and propylene oxide. The relative small activation energy difference between copolymer versus cyclic carbonate formation for the epichlorohydrin/CO(2) process (45.4 kJ/mol) accounts in part for the selective synthesis of copolymer to be more difficult in comparison with the propylene oxide/CO(2) case (53.5 kJ/mol). Direct observation of the propagating polymer-chain species from the binary (salen)CoX/MTBD (X = 2,4-dinitrophenoxide and MTBD = 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) catalyst system by means of electrospray ionization mass spectrometry confirmed the perfectly alternating nature of the copolymerization process. This observation in combination with control experiments suggests possible intermediates involving MTBD in the CO(2)/epichlorohydrin copolymerization process.

The depolymerization reactions of several polycarbonates produced from the completely alternating copolymerization of epoxides and carbon dioxide have been investigated. The aliphatic polycarbonates derived from styrene oxide, epichlorohydrin, or propylene oxide and CO 2 were found to undergo quantitative conversion to the corresponding cyclic carbonate following deprotonation of their −OH end group by azide ion. The process was shown to involve the unzipping of the copolymer in a backbiting fashion leading to a steady decrease in the copolymer's molecular weight while maintaining its narrow molecular weight distribution. This pathway for depolymerization was further supported by the observation that upon endcapping the copolymer with an acetate group, it was stabilized. Temperature-dependent kinetic studies provided energy of activation (E a ) barriers for cyclic carbonate formation which increased in the order: poly(styrene carbonate) (46.7 kJ/mol) < poly(CO 2 -alt-epichlorohydrin) (76.2 kJ/mol) ≤ poly(propylene carbonate) (80.5 kJ/mol). On the other hand, upon addition of the (salen)CrCl copolymerization catalyst, the depolymerization process was greatly suppressed, e.g., the E a determined for poly(styrene carbonate) in this instance was 141.2 kJ/mol. By way of contrast, the copolymer produced from the alicyclic epoxide, cyclohexene oxide, was only found to undergo depolymerization to trans-cyclohexene carbonate in the presence of (salen)CrCl plus nBu 4 NN 3 , albeit extremely slowly.

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The hydroxyl-terminated copolymer, poly(cyclopentene carbonate), derived from carbon dioxide and cyclopentene oxide was deprotonated by the strong base sodium bis(trimethylsilyl)amide (NaHMDS) in toluene and shown to undergo depolymerization to cyclopentene oxide and cis-cyclopentene carbonate. The degradation process was demonstrated to be retarded under a 0.7 MPa pressure of CO 2 , with the product distribution being enhanced in favor of cis-cyclopentene carbonate. Unlike the related copolymer, poly(indene carbonate), the degradation pathway was found to be insensitive to either light or the radical trap, TEMPO. The depolymerization process was further shown to be catalyzed by (salen)CrCl/n-Bu 4 NN 3 , with the major product being cyclopentene oxide. Although the reaction rate in the presence of metal catalyst was inhibited by an added pressure of CO 2 , the product distribution still highly favored epoxide production. On the other hand, upon reducing the pressure above the polymer solution during the depolymerization reaction, the rate of reaction was accelerated and was more selective for cyclopentene oxide formation. Preliminary studies were investigated to optimize the efficient recycling of poly(cyclopentene carbonate) to its monomers, cyclopentene oxide and CO 2 . Computational studies have been performed on depolymerization reaction of polycarbonates derived from CO 2 and epoxides which strongly support these experimental findings.

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