Stereoregular CO₂ Copolymers from Epoxides with an Electron-Withdrawing Group: Crystallization and Unexpected Stereocomplexation

Abstract

Highly regioselective copolymerization of CO2 and enantiopure terminal epoxides with an electron-withdrawing group, styrene oxide and its derivatives, was achieved by the use of multichiral salenCo(III) complex-based catalyst systems, in which epoxide ring-opening predominantly occurred at methylene C–O bond, affording isotactic CO2 copolymers with up to 98% head-to-tail connectivity and 97% enantioselectivity. The stereoregular poly(styrene carbonate) with 98% isotacticity is a typical semicrystalline material, possessing a melting temperature (Tm) of 137.3 °C and an enhanced glass transition temperature (Tg) of 101.2 °C. The isotactic 3-methoxystyrene oxide/CO2 copolymer is also a semicrystalline polymer with a Tm of around 90 °C, while the corresponding isotactic 3-chlorostyrene oxide/CO2 copolymer appears amorphous feature, possessing a Tg of 82.1 °C. Notably, a novel stereoselective interaction between two opposite-configuration isotactic polymers was first found in the blend of equivalent (R)- and (S)-polycarbonates from terminal epoxides. The strong interlocked interaction between isotactic (R)- and (S)-poly(styrene carbonate)s affords the crystalline stereocomplexes with a Tm of 164.1 °C, about 27 °C higher than that of the enantiopure isotactic polymers. Surprisingly, isotactic (R)- or (S)-3-chlorostyrene oxide/CO2 copolymer is typical amorphous polymeric material; however, upon mixing both enantiomers together, an interlocked interaction between polymer chains of opposite configuration occurs, affording the crystalline stereocomplexes with a Tm of 147.3 °C.