Aspects of the living polymerization of isobutylene (IB) initiated by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4combination have been investigated in the presence and absence of di-tert-butylpyridine (DtBP) and various electron donors (ED), such as pyridine, tri-n-butylamine, di-n-butylamine,N,N-dimethylacetamide,N,N-dimethylformamide, urea, nitromethane, acetonitrile, acetone, acetyl chloride, tetrahydrofuran, tri-n-butyl phosphine, triethylphosphite, dimethyl sulfide, and dimethylsulfoxide. All these EDs, commonly regarded as “poisons” of cationic polymerizations, exert, in fact, beneficial effects; for example, they cause the narrowing of molecular weight distributions (MWD). The active agents are TiCl4-ED complexes which in some systems may be partially insoluble and precipitate from the charges. A detailed series of experiments with the TMPCl/TiCl4/IB/pyridine system demonstrates the living nature of these polymerizations.