The living cationic polymerization of isobutyl vinyl ether (IBVE) was investigated in the presence of various cyclic and acyclic ethers with 1-(isobutoxy)ethyl acetate [CH3CH (OiBu) OCOCH3, 1]/EtAlCl2 initiating system in hexane at 0-degrees-C. In particular, the effect of the basicity and steric hindrance of the ethers on the living nature and the polymerization rate was studied. The polymerization in the presence of a wide variety of cyclic ethers [tetrahydrofuran (THF), tetrahydropyran (THP), oxepane, 1,4-dioxane] and cyclic formals (1,3-dioxolane, 1,3-dioxane) gave living polymers with a very narrow molecular weight distribution (MWD) (M(w)BAR/M(n)BAR less-than-or-equal-to 1.1). On the other hand, propylene oxide and oxetane additives resulted in no polymerization, whereas 1,3,5-trioxane gave the nonliving polymer with a broader MWD. The polymerization rates were dependent on the number of oxygen and ring sizes, which were related to the basicity and the steric hindrance. The order of the apparent polymerization rates in the presence of cyclic ether and formal additives was as follows : nonadditive approximately 1,3,5-trioxane much greater than 1,3-dioxane > 1,3-dioxolane much greater than 1,4-dioxane much greater than THP > oxepane much greater than THF much greater than oxetane, propylene oxide approximately 0. The polymerization in the presence of the cyclic formals was much faster than that of the cyclic ethers : for example, the apparent propagation rate constant k in the presence of 1,3-dioxolane was 10(3) times larger than that in the presence of THF. Another series of experiments showed that acyclic ethers with oxyethylene units were effective as additives for the living polymerization with 1/EtAlCl2 initiating system in hexane at 0-degrees-C. The polymers obtained in the presence of ethylene glycol diethyl ether and diethylene glycol diethyl ether had very narrow molecular weight distribution (M(w)BAR/M(n)BAR less-than-or-equal-to 1.1), and the M(n)BAR was directly proportional to the monomer conversion. The polymerization behavior was quite different in the polymerization rates and the MWD of the obtained polymers from that in the presence of diethyl ether. These results suggested the polydentate-type interaction or the alternate interaction of two or three ether oxygens in oxyethylene units with the propagating carbocation, to permit the living polymerization of IBVE.