Copolymerization of CO2 and meso epoxides using enantioselective β-diiminate catalysts: a route to highly isotactic polycarbonates

We report a new class of catalysts for the enantioselective copolymerization of CO2 and alicyclic meso epoxides. C-1-Symmetric beta-diiminate zinc catalysts were designed on the basis of mechanistic information and subsequently optimized using structure-activity relationships and iterative ligand design. The optimized catalysts yield highly isotactic poly(cyclohexene carbonate) and poly(cyclopentene carbonate) with units of up to 94% ee under mild conditions. A diblock copolymer of CO2/cyclohexene oxide and CO2/cyclopentene oxide was synthesized, demonstrating the controlled behaviour of the catalyst. Solid-state structures and solution-state dynamics of the catalyst were studied to explain trends in enantioselectivities and turnover frequencies. A strong correlation between the isotacticity of the polymer and its melting temperature was observed. X-ray powder diffraction was used to measure crystallinity and study the changes in morphology observed upon annealing.