AbstractEfforts have been made to prepare perfect terminally functional linear telechelic polyisobutylenes with various Lewis acid coinitiators, BCl3, AlCl3, TiCl4, SnCl4, VCl4, and SbCl5. However, the expected desirable results (F̄n= 2.0,M̄w/M̄n= 1.5) were obtained only with BCl3. Moreover, the concentration of BCl3affects the molecular weight of telechelic polyisobutylenes prepared by the inifer technique. The Kennedy–Smith inifer mechanism is insufficient to explain these observations. Extending this model by the concept of counteranion stability, of Heublein et al.,5and assuming the participation of free ions and ion pairs in propagation and termination, led to a satisfactory mechanistic model. The exceptional utility of the BCl3/BCl⊖system is due to the high Lewis acid strength of BCl3and low counteranion stability o