Extensive studies on the mechanism of carbocationic polymerization initiation and particularly termination yielded information useful for the synthesis of a variety of new mono- and difunctional (telechelic) polymers. Unraveling aspects of initiation has opened new avenues toward the synthesis of novel cationic graft copolymers. Understanding details of termination has led among other products to cyclopentadiene-ended polyisobutylene and cyclopentadienylated rubbers. The latters crosslink by Diels-Alder addition and produce thermally reversible networks. Combination of controlled initiation and termination yielded telechelic products e. g., α, ω-diene-polyisobutylene. The use of Cl2/BCl3combinations led to the synthesis of α, ω-dichloropolyisobutylenes.