Depolymerization of Polycarbonates Derived from Carbon Dioxide and Epoxides to Provide Cyclic Carbonates. A Kinetic Study

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 CO2 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 end-capping 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) (467 kJ/mol) < poly(CO2-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.