The polymerization of isobutylene (IB) initiated by a variety of tertiary chloride/TiCl4or BC13systems can be very beneficially influenced by deliberately added electron-pair donors (ED's). ED's whose Gutmann's donor number is larger than ∼26, e.g., dimethylsulfoxide (DMSO) and dimethylacetamide (DMA), are most suitable to mediate numerous most desirable effects. Thus, in the presence of DMSO or DMA: (1) quasi-living polymerizations initiated by cumyl chloride (CumCl)/TiCl4or 2-chloro-2,4,4-trirnethylpentane (TMPCl)/TiCl4give rise to very narrow molecular weight distribution (MWD) polyisobutylene (PIB) (Mw/Mn= 1.1–1.2); (2) the CumCl/BCl3/IB inifer system becomes living and yields PIB with very narrow MWD; (3) the polymerization-inactive TMPCl/BCl3system becomes active and induces living IB polymerization to narrow-MWD product; (4) indanyl end-group formation, which may occur in the CumCl/BCl3orp-dicumyl chloride/BCl3, inifer systems, is eliminated. The effect of reaction conditions, e.g., polarity and [TiCl4]/([CumCl] + [DMSO]) ratio, on these polymerizations has been investigated. All the observations and effects can be explained by a reduction of cationicity of the polymerization-active species by ED's.