AbstractIn order to clarify the propagation reaction, vinyl ether was copolymerized with the corresponding alkenyl ether under various conditions.cis‐Propenyl ether (cis‐PE) was several times more reactive thantrans‐PE and the corresponding vinyl ether in the copolymerization catalyzed by BF3· O(C2H5)2in toluene. However, the reactivity ofcis‐PE relative totrans‐PE and the vinyl ether was found to be greatly decreased with increasing polarity of the solvent and to be very close to unity in such polar solvents as nitroethane. On the other hand, the reactivity oftrans‐IBPE relative to IBVE was scarcely changed by polymerization conditions. Also, the nature of the initiator and polymerization temperature affect the reactivity ofcis‐PE relative to the vinyl ether. These phenomena were explained by the relative stability of the bridged and open car bonium ions based on the polarity of the solvent and steric hindrance due to substituents in