The living polymerization of isobutylene (IB) coinitiated by dimethylaluminum chloride (Me2AlCl) was reinvestigated in hexanes/CH3Cl 60/40 (v/v) solvent mixture at -80 degreesC in the presence and absence of a proton trap, 2,6-di-tert-butylpyridine (DTBP). Termination and chain transfer were not detected in either case. The polymerization, however, was much slower in the presence of DTBP. Experiments carried out at varied DTBP concentrations confirmed that DTBP does not interact with the Lewis acid or with carbocations and its role is to scavenge protic impurities. The large difference in the polymerization rate in the presence and absence of proton trap was attributed to the different nature of the true coinitiator, Me2AlCl, in the presence of DTBP and MeAlCl2 (or more likely methylaluminum sesquichloride), a stronger Lewis acid that could be present as an impurity or may form by the reaction of Me2AlCl with protic impurities (e.g., HCl), in the absence of DTBP. Polymerizations employing Me2AlCl or MeAlCl2 separately or the mixture of the two Lewis acids confirmed that MeAlCl2 is at least 200 000 times stronger than Me2AlCl. Therefore, even in the presence of traces of MeAlCl2 in Me2AlCl, the polymerization is coinitiated by MeAlCl2. Separate experiments confirmed that the living polymerization of IB can be accomplished with methylaluminum sesquichloride or MeAlCl2 as Lewis acid in hexanes/ CH3Cl 60/40 (v/v) at -80 degreesC. Controlled polymerization of IB coinitiated by MeAlCl2 or methylaluminum sesquichloride that yields polymers with theoretical molecular weights and narrow molecular weight distributions, however, was possible only at [Lewis acid] less than or equal to 3 x 10(-3) M. Under these conditions the M-n was directly proportional to the monomer/initiator ratio up to [monomer]/[initiator] = 1000, and in chain extension experiments the M(n)s doubled and tripled upon addition of a second and third monomer increment, respectively. The loss of control at higher Lewis acid concentration was attributed to the extremely rapid nature of the polymerization. The new highly active catalysts are promising candidates to substitute TiCl4, the only Lewis acid presently employed commercially, in the living polymerization of IB.