Propylene oxide was polymerized by a catalyst consisting of diethylzinc and α,β -unsaturated ketones. Ten α,β -unsaturated ketones and some other related compounds were examined. All methyl ketones were effective. α,β -Unsaturated phenyl ketones were effective except for a α,β-disubstituted derivative. α,β-Unsaturated carbonyl compounds other than ketones were practically ineffective. The initial step of the catalyst formation is considered to be the reaction of diethylzinc and α,β-unsaturated ketone which is either the conjugate addition to the α,β-unsaturated group or the hydrogen abstraction from the methyl group next to the carbonyl group. The conjugate addition gives the catalyst having larger activity and better stability at high temperature.