Crystalline CO2 Copolymer from Epichlorohydrin via Co(III)-Complex-Mediated Stereospecific Polymerization

Abstract

As a cheap and easily obtainable raw material, epichlorohydrin is an attractive candidate for copolymerization with CO2 to produce degradable polycarbonate. However, the poor polymer selectivity as well as the concomitant production of ether linkage units in the previous studies hindered further research on this topic, such as asymmetric, stereo- and regioselective ring-opening of epichlorohydrin during its copolymerizaton with CO2. Herein, we report highly stereospecific alternating copolymerization of CO2 and epichlorohydrin for the first time by utilizing chiral bifunctional cobalt–salen catalysts. It was found that the substituents on the phenonate groups around the metal center had a notable effect on the regioselectivity of the ring-opening step for epichlorohydrin. Using an enantiopure salenCo(III) complex bearing an adamantane group and an appended bulky dicyclohexyl ionic ammonium salt, a highly regioregular ring-opening step was observed with a concomitant 97% retention of configuration at the methine carbon center. The isotactic poly(chloropropylene carbonate) is a typical semicrystalline polymer with an enhanced Tg of 42 °C and a Tm of 108 °C. The test of mechanical properties shows that the yield strength and tensile strength of the crystalline copolymer are about 10 and 30 times that of its amorphous counterpart, respectively.