Asymmetric alternating copolymerization of cyclohexene oxide and carbon dioxide

Abstract

Asymmetric alternating copolymerization of cyclohexene oxide and carbon dioxide is reported. The asymmetric alternating copolymerization has been achieved using a dimeric zinc complex initiator, which was synthesized by the reaction of diethylzinc with chiral amino alcohol. The degree of asymmetric induction could be unambiguously evaluated based on the enantiomeric excess of trans-cyclohexane-1,2-diol which was yielded by hydrolysis of the copolymer. The chain-end analysis of the copolymer by MALDI-TOF mass spectrometry showed that the initiation reaction should be CO2 insertion into Zn-O bond. The addition of ethanol to the dimeric zinc complex improved the yield and enantioselectivity to give the copolymer of 80% ee, which is the highest selectivity ever reported. A plausible bimetallic enchainment mechanism is proposed. The asymmetric amplification that is shown to operate in this copolymerization system, is the first such example in the asymmetric synthesis polymerization. The 13C NMR spectral assignments of the copolymer are also demonstrated by synthesizing model oligomers.