Cationic polymerization of n-butyl propenyl ether (BuPE; CH 3 CHA CHOBu, cis/trans ฯญ 64/36) was examined with the HCl-IBVE (isobutyl vinyl ether) adduct/ZnCl 2 initiating system at ฯช15 ฯณ ฯช78 ยฐC in nonpolar (hexane, toluene) and polar (dichloromethane) solvents, specifically focusing on the feasibility of its living polymerization. In contrast to alkyl vinyl ethers, the living nature of the growing species in the BuPE polymerization was sensitive to polymerization temperature and solvent. For example, living cationic polymerization of IBVE can be achieved even at 0 ยฐC with HCl-IBVE/ZnCl 2 , whereas for BuPE whose โค-methyl group may cause steric hindrance ideal living polymerization occurred only at ฯช78 ยฐC. Another interesting feature of this polymerization is that the polymerization rate in hexane is as large as in dichloromethane, much larger than in toluene. A new method in determining the ratio of the living growing ends to the deactivated ones was developed with a devised monomer-addition experiments, in which IBVE that can be polymerized in a living fashion below 0 ยฐC was added to the almost completely polymerized solution of BuPE. The amount of the deactivated chain ends became small in hexane even at ฯช40 ยฐC in contrast to other solvents. Thus hexane turned out an excellent solvent for living cationic polymerization of BuPE.