The cationic polymerizations of isobutylene (IB) initiated by the H2O/TiCl4 in dichloromethane (CH2Cl2) at -30 degreesC were carried out in the absence and presence of various external electron pair donors (EDs). Controlled polymerization with a slow polymerization rate and a narrow molecular weight distribution (MWD, M-w/M-n = 1.11-1.28) of the polymer was achieved by using certain appropriate H2O/TiCl4/ED systems. The kinetics of the IB polymerization with the H2O/TiCl4/ED initiating system was investigated. It indicated that the polymerization rate was first-order with respect both to monomer and to initiator concentrations in the presence of strong or weak EDs. Polymerizations exhibited a second-order dependence on TiCl4 concentration in those cases where weak EDs, such as methyl acetate (MAC), methyl acrylate (MA), sulfolane (HDF), or methyl benzoate (NIB), were used. On the other hand, first order in TiCl4 concentration was observed when strong EDs, such as dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), pyridine (Py), or triethylamine (TEA), were used.