Alternating copolymerization of CO2 and styrene oxide with Co(III)-based catalyst systems: differences between styrene oxide and propylene oxide

A detailed study of the difference in reactivity of the copolymerization reactions of styrene oxide vs. propylene oxide with carbon dioxide utilizing binary (salen) cobalt(III) catalyst systems to provide perfectly alternating copolymers is reported. This investigation focuses on the discrepancy exhibited by these two terminal epoxides for the preference for C-O bond cleavage during the ring-opening process. It was found that the nucleophilic ring-opening of styrene oxide occurs predominantly at the methine C-alpha-O bond which leads to an inversion of configuration at the methine carbon center. This tendency results in a significantly lower reactivity as well as a deterrent for synthesizing stereoregular poly(styrene carbonate) when compared to the propylene oxide/CO2 process. The chiral environment about the metal center had a notable effect on the regioselectivity of the ring-opening step for styrene oxide, with the methylene C-beta-O bond being preferentially cleaved. Using a binary catalyst system composed of an unsymmetrical (S, S, S)-salenCo(III) complex in conjunction with the onium salt PPNY (PPN = bis(triphenylphosphine) iminium, and Y = 2,4-dinitrophenoxy), a highly regioregular ring-opening step was observed with a concomitant 96% retention of configuration at the methine carbon center.