The nature of the species that gives rise to living carbocationic polymerization (LP+Pzn) of isobutylene (IB) coinitiated by Friedel-Crafts acids (FCA) in the presence of electron pair donors (ED) has been investigated. It is proposed that the key species responsible for mediating LC+Pzn of IB are FCA·ED complexes, formed by Lewis acid-Lewis base interaction, where FCA = BCl3TiCl4, and ED = ethyl acetate, tetrahydrofuran, etc. Free EDs are inhibitors of IB polymerization. These findings are in sharp contrast to those of LC+Pzn of vinyl ethers coinitiated by EtAlCl2in which excess (i.e., free) EDs arerequiredfor the living polymerizations to occur. Comparison of IB polymerizations induced by the 2-chloro-2,4,4-trimethylpentane/BCl3and 2-acetyl-2,4,4-trimethylpentane/BCl3systems shows that these reactions do not proceed by the same intermediate (i.e., bytert-chlorides) and that the mechanisms of BC13- and TiC14-coinitiated living polymerizations are different. Experiments with the proton trap 2,6-di-tert-butylpyridine indicate that proton scavenging by FCA·ED complexes may be responsible for the narrow molecular weight distribution (MWD) products obtained in BC13-coinitiated LC+Pzn of isobutylene. These findings also prove thatRi(rate of initiation) > Rp(rate of propagation), the relation required for narrow MWD in living polymerizations.