Pinwheel-Shaped Tetranuclear Organoboron Catalysts for Perfectly Alternating Copolymerization of CO2 and Epichlorohydrin.

Abstract

The copolymerization of carbon dioxide (CO2) and epoxides to produce aliphatic polycarbonates is a burgeoning technology for the large-scale utilization of CO2 and degradable polymeric materials. Even with the wealth of advancements achieved over the past 50 years on this green technology, many challenges remain, including the use of metal-containing catalysts for polymerization, the removal of the chromatic metal residue after polymerization, and the limited practicable epoxides, especially for those containing electron-withdrawing groups. Herein, we provide kinds of pinwheel-shaped tetranuclear organoboron catalysts for epichlorohydrin/CO2 copolymerization with >99% polymer selectivity and quantitative CO2 uptake (>99% carbonate linkages) under mild conditions (25-40 °C, 25 bar of CO2). The produced poly(chloropropylene carbonate) has the highest molecular weight of 36.5 kg/mol and glass transition temperature of 45.4 °C reported to date. The energy difference (ΔEa = 60.7 kJ/mol) between the cyclic carbonate and polycarbonate sheds light on the robust performance of our metal-free catalyst. Control experiments and density functional theory (DFT) calculations revealed a cyclically sequential copolymerization mechanism. The metal-free feature, high catalytic performance under mild conditions, and no trouble with chromaticity for the produced polymers imply that our catalysts are practical candidates to advance the CO2-based polycarbonates.