Asymmetric Alternating Copolymerization of CO2 with meso-Epoxides: Ring Size Effects of Epoxides on Reactivity, Enantioselectivity, Crystallization, and Degradation

In this study, we report a systematical investigation regarding the ring size effects of meso-epoxides on reactivity and enantioselectivity as well as crystallization and degradation of the polycarbonates resulting from the asymmetric alternating copolymerization of various alicyclic epoxides (cyclopentene oxide, cyclohexene oxide, cycloheptene oxide, and cyclooctene oxide) with CO2 using the privileged chiral catalyst system of biphenol-linked bimetallic salcyCo(III)DNP complex and PPN-DNP (PPN = bis-(triphenylphosphine)iminium and DNP = 2,4-dinitrophenoxide). The ring size of meso-epoxides has a strong impact on both reactivity and enantioselectivity: epoxide with a six-membered ring is highly reactive, five- or seven-membered-ring fused epoxide is less reactive, and especially, no copolymerization reactivity is observed for cyclooctene oxide with an eight-membered ring due to the inherent high steric hindrance of the crowded catalytic active site for binding the epoxide and growing copolymer chain. Moreover, a reverse relationship was discovered between copolymerization reactivity and enantioselectivity. In addition, further investigation has demonstrated that the ring size of mesoepoxides also impacts significantly crystallization, stereocomplexation, and degradation of the resultant polycarbonates.