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

摘要

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 polystyrene carbonate) with 98% isotacticity is a typical semicrystalline material, possessing a melting temperature (T-m) of 137.3 degrees C and an enhanced glass transition temperature (T-g) of 101.2 degrees C. The isotactic 3-methoxystyrene oxide/CO2 copolymer is also a semicrystalline polymer with a T-m of around 90 degrees C, while the corresponding isotactic 3-chlorostyrene oxide/CO2 copolymer appears amorphous feature, possessing a T-g of 82.1 degrees 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 T-m of 164.1 degrees C, about 27 degrees 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 T-m of 147.3 degrees C.