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 degrees C) and at a very low catalyst loading (0.01 mol %), affording the corresponding polycarbonates with controlled molecular weights and narrow dispersities.

Automated summary

Asymmetric organocatalysis has been widely used in the synthesis of small chiral molecules, but its application in the synthesis of enantioenriched polymers is limited. This study presents a metal-free asymmetric catalytic copolymerization of meso-epoxides with CO2 using chiral organoboron-based bifunctional and binary catalysts. Although the enantioselectivities of the resulting copolymers are lower than those of metal-based catalyst systems, it is the first example of organocatalyzed synthesis of enantioenriched and perfectly alternating copolymers via asymmetric polymerization.