Base initiated depolymerization of polycarbonates to epoxide and carbon dioxide co-monomers: a computational study†

High-accuracy CBS-QB3(+) calculations were used to obtain the free energy barriers for several polycarbonates of interest to undergo alkoxide back-biting to give the corresponding epoxide and carbon dioxide. Free energy barriers to epoxide formation were modest for most polymeric alkoxides (12.7–17.4 kcal mol−1), and they were higher than for the same starting material to give cyclic carbonate (10.7–14.6 kcal mol−1). Poly(cyclopentene carbonate) differs: epoxide formation has a lower free energy barrier (13.3 kcal mol−1) than cyclic carbonate formation (19.9 kcal mol−1). These results explain why poly(cyclopentene carbonate) depolymerizes to cyclopentene oxide when treated with a strong base, whereas propylene and styrene polycarbonates depolymerize to their respective cyclic carbonates. Recycling via regeneration of the monomer represents the ideal method for producing material of the highest quality.