A new mechanism is proposed for the living carbocationic polymerization [LC+Pzn) of isobutylene (IB) initiated by tertiary ester/MtCln (MtCln = BC13 or TiCl4) complexes (Scheme I). According to this mechanism, the initiating systems, e.g., tertiary ester/MtCln (I), first yield tert-C+…OCORMtCln- ion pairs (II) which may initiate IB polymerization by monomer addition to the tert-C+…OCORMtCln- species (III). Species III eventually will undergo collapse with the counteranion, yielding a tertiary chloride capped PIB chain (IV) plus MtCln-1OCOR, an in situ electron pair donor (ED). In line with previous observations, excess MtCln-1 will reionize IV, yielding the active propagating species PIB+…MtCln+1 (V). Thus the temporarily dormant species IV are in dynamic equilibrium with the propagation-active species V. The function of the EOs, both in situ and external ones, is to stabilize the active carbocations, affecting beneficially the course of polymerization. The transitory formation of tertiary chlorides by collapse of the “mixed” counteranion OCORMtCln-1 (II → VI), however, may also occur prior to polymerization. The exact route leading to the final IV ⇌ V equilibrium depends on reaction conditions, i.e., the nature of initiator and MtXn, stability of the mixed counteranion, solvent polarity, temperature, etc. The latter equilibrium accounts for the presence of tertiary CI end groups (PIB-Clt) upon quenching. This is a crucial deviation from the earlier mechanistic view according to which the dormant species are tertiary ester or tertiary ether terminated PIB. These propositions have been substantiated by a variety of means: (1) by demonstrating the existence of reversible termination in the PIB-Clt/excess BCI3/IB system; (2) by analysis of literature sources dealing with the transformation of tertiary ester and tertiary ether/MtCln-1 complexes to the corresponding tertiary chlorides; (3) by observations that the equimolar 2-acetyl-2,4,4-trimethylpentane (TMPOAc)/TiCl4 system is totally inactive; (4) by polymerizations induced by equimolar mixtures of cumyl acetate (CumOAc) + cumyl methyl ether (CumOMe)/excess TiCl4 and CumCl + CumOMe/excess TiCl4 systems, both of which give living polymerizations and narrow molecular weight distribution (MWD) PIBs; (5) by polymerizations of IB induced, on the one hand, by TMPOAc/excess TiCl4 and, on the other hand, by TMPOAc/excess TiCl4/ethyl acetate (EtOAc) systems, where only the latter give narrow-MWD product; and (6) by comparing the MWDs of PIBs prepared by CumCl/excess TiCI4 and CumCl/excess TiCl4/EtOAc and demonstrating the MWD narrowing effect of the deliberately added external ED (EtOAc). The information generated in these studies has been analyzed, and a mechanistic interpretation of the observed facts is offered. © 1990, American Chemical Society. All rights reserved.
