Chloromaleic anhydride was found to copolymerize with divinyl ether to form soluble copolymers of 1:1 composition, devoid of residual unsaturation. A bicyclic structure is proposed in which the polymer backbone consists only of divinyl ether units. The ease with which the copolymers underwent dehydrohalogenation suggests that the hydrogen and chlorine atoms on the anhydride unit are in a trans configuration as a result of a stepwise cycli-zation process.