AbstractThe ternary copolymerization of carbon dioxide with 1,2‐epoxybutane and other epoxybutane isomers using the diethylzinc/water system as catalyst was carried out to investigate the relative reactivity between epoxides.cis‐2,3‐Epoxybutane was much more reactive thantrans‐2,3‐epoxybutane in the copolymerization with carbon dioxide. The copolymer obtained from the reaction of carbon dioxide andcis‐2,3‐epoxybutane upon hydrolysis gavethreo‐2,3‐butanediol, indicating the exclusive inversion of configuration in the ring‐opening of the epoxide. In the formation of cyclic carbonate as a byproduct of the copolymerization,cis‐2,3‐epoxybutane andtrans‐2,3‐epoxybutane exh