A new design perspective on initiating systems for living cationic polymerization was gained by thorough examination of various metal chlorides as catalysts in conjunction with a weak Lewis base in the cationic polymerization of p-methoxystyrene. The Lewis acids clearly differed in controllability, in contrast to the findings of a previous report on the polymerization of isobutyl vinyl ether (IBVE) using various catalysts (all the metal chlorides used in the present study induced the controlled polymerization of IBVE, although the reaction rates depended on the chlorophilic and oxophilic nature of the central metals: Macromolecules 2009, 42, 3965). Metal tetrachlorides and dichlorides such as SnCl4, TiCl4, ZrCl4, HfCl4, and ZnCl2 induced controlled polymerizations to produce polymers with predetermined molecular weights and very narrow molecular weight distributions (MWDs). In contrast, frequent side reactions (beta-proton elimination and the Friedel-Crafts reaction) occurred with the trichlorides FeCl3 and GaCl3, yielding polymers with molecular weights lower than the theoretical values and with broad MWDs. Another trichloride, AlCl3, produced polymers with very high molecular weights owing to its very low initiation efficiency. NbCl5, a pentachloride, was also unable to control the polymerization. The structures of the counteranions with or without a coordinating weak Lewis base were shown to be responsible for the difference in the controllability between the metal chlorides.