Titanium(TV)-based new initiating systems for living cationic polymerization of isobutyl vinyl ether (IBVE) have been developed by using a series of isopropoxytitanium chlorides [TiCl4-n(OiPr)(n); n = 0-4] in conjunction with the IBVE-HCl adduct in CH2Cl2 at -15 degrees C. The H-1 and C-13 NMR analysis of the titanium compounds showed that their Lewis acidity decreased with the increasing number (n) of isopropoxyl groups and affected the polymerizations accordingly. For example, TiCl4 and TiCl3(OiPr), the strongest Lewis acids among the five, induced very rapid polymerizations to give polymers with broad molecular weight distributions (MWDs) (($) over bar M(w)/($) over bar M(n) similar to 1.6). In contrast, the reaction rate with TiCl2(OiPr)(2), a weaker Lewis acid, was moderate. The MWDs of the polymers were narrow throughout the reaction (($) over bar M(w)/($) over bar M(n) < 1.2), and the number average molecular weights increased in direct proportion to monomer conversion. The polymerization with Ticl(OiPr)(3) was slower, and the MWDs became slightly broader. No polymer was obtained with a much weaker Lewis acid, Ti(OiPr)(4). Another modified titanium chloride, TiCl2(OPh)(2), also led to a controlled polymerization of IBVE which is similar to the one with TiCl3(OiPr)(2). These results indicate that living cationic polymerizations of IBVE can be achieved by controlling the Lewis acidity of the titanium(IV) activators with electron-donating groups.