Regioselective copolymerization of epoxides and phthalic thioanhydride to produce isotacticity-rich semiaromatic polythioesters

The copolymerization of epoxides with phthalic thioanhydride (PTA) constitutes a straightforward route to diverse semiaromatic polythioesters. However, the obtained polythioesters, especially those derived from the terminal epoxides, are amorphous with low glass transition temperature (Tg, <66 degrees C), which largely limits their application. Herein, we propose to improve the thermal performance through increasing the isotacticity of the copolymers in a regioselective copolymerization manner. Detailly, the unsymmetrical Salcy-based Cr-complexes with the modification of the steric hindrance around the catalytic center was applied to achieve the highly regioselective copolymerization of epoxides and PTA. The regioselectivities of the copolymerization of epoxides with PTA reach over 92%, with some epoxides the regioselectivity even over 99%, thus affording the isotacticity-rich polythioesters. The density functional theory (DFT) calculation revealed that the steric hindrance from the -OiPr group together with the matched chiral configuration of binaphthyl and cyclohexanediamine is the key to achieving the high regioselectivity. Moreover, the semiaromatic polythioesters stereocomplex are accessible via blending the equal-mass (R)-and (S)-isotactic PGE/PTA copolymers. Comparing to the amorphous isotactic parent PGE/PTA copolymer, the stereocomplex exhibited crystalline nature with a melting temperature (Tm) of 126.8 degrees C.

Automated summary

The thermal performance of semiaromatic polythioesters obtained from the copolymerization of epoxides with phthalic thioanhydride (PTA) has been improved by increasing the isotacticity of the copolymers in a regioselective manner. This was achieved through the use of unsymmetrical Salcy-based Cr-complexes with modified steric hindrance around the catalytic center, resulting in highly regioselective copolymerization and the formation of isotacticity-rich polythioesters. Furthermore, semiaromatic polythioester stereocomplexes were obtained by blending equal-mass (R)- and (S)-isotactic PGE/PTA copolymers, which exhibited crystalline nature with a higher melting temperature compared to the amorphous isotactic parent copolymer.