The fast living cationic polymerization of isobutyl vinyl ether was demonstrated with FeCl3 as a Lewis acid catalyst in the presence of a cyclic ether. The reaction was completed in 15 s even under mild conditions and was 5 or 1000 times faster than the reaction with SnCl4 or EtAlCl2, respectively, in the presence of 1,4-dioxane. The faster reaction was attributed to the superior ability of FeCl3 to extract Cl− in comparison with the other two acids. In addition, the molecular weight distributions of the product polymers were invariably very narrow (weight-average molecular weight/number-average molecular weight < 1.1). Through the reduction of the basicity of a cyclic ether (e.g., 1,3-dioxolane), ultrafast living polymerization (97% in 3 s) was achieved.