Chiral Organoboron-Mediated Alternating Copolymerization of meso-Epoxides with CO2

Asymmetric organocatalysis has made significant progress in recent decades as a powerful method to synthesize small chiral molecules. However, examples of organocatalytic polymerizations for the synthesis of enantioenriched polymers are rare. Herein, we present a metal-free asymmetric catalytic copolymerization of meso-epoxides with CO2 using chiral organoboron-based bifunctional and binary catalysts. Although the enantioselectivities of the resultant copolymers are significantly lower than those of previously reported chiral metal-based catalyst systems, this is the first example of an organocatalyzed synthesis of enantioenriched and perfectly alternating copolymers via asymmetric polymerization. A binary catalyst system consisting of a chiral organoboron with a rigid spirobiindane skeleton and bis-(triphenylphosphine)-iminium chloride was both highly active (up to 1033 h(-1)) and enantioselective for the copolymerization of various meso-epoxides with CO2 over a broad temperature range (0-100 ?) and at a very low catalyst loading (0.01 mol %), affording the corresponding polycarbonates with controlled molecular weights and narrow dispersities.

Automated summary

This article presents a metal-free asymmetric catalytic copolymerization method for synthesizing enantioenriched and perfectly alternating copolymers. By using a chiral organoboron and a binary catalyst system, polycarbonates with controlled molecular weights and narrow dispersities can be efficiently synthesized at low catalyst loading over a broad temperature range.