The tertiary carbenium ions 2(+) and 3(+), models of the propagating species in carbocationic isobutylene polymerization, are generated from the tertiary chlorides 2c and 3c with a Lewis acid in the presence of isobutylene (1c) and one of the allylsilanes 1a or 1b, which act as terminators, Since the rate of the termination reaction is known, the ratio of the telomerization products can be evaluated to give a propagation rate constant of (6 +/- 2) x 10(8) L mol(-1) s(-1) (independent of temperature) for the carbocationic oligomerization of isobutylene in dichloromethane.