Essentially quantitative allylation of linear and three-arm star tert-chloro capped polyisobutylenes [tCl–PIB–Clt and PIB(Clt)3] has been achieved by the use of allyltrimethylsilane (AllylSiMe3) and Friedel–Crafts acids. Quantitative allylation occurs under suitable conditions, e.g., slight molar excess of TiCl4 and AllySiMe3 polar media, −70°C. These conditions have been developed from quantitative model allylation experiments using 2,4,4-trimethyl-2-chloropentane. Essentially quantitative allylation occurs with long-chain or “once fired” tert-chloro-end groups. Complete allylation can also be effected in a “one-pot two-step” synthesis, i.e., by first preparing the tert-chloro-ended PIB by the inifer method and without isolating the product, completing the synthesis by allylation in the same reactor. Extensive but not quantitative allylation can also be obtained in situ, i. e., during polymerization, by the use of the AllylSiMe3/cumyl chloride/BCI3/isobutylene combination. While allylation proceeds with great ease under the mildest conditions, e.g., −70°C, vinylation with vinyltrimethylsilane could not be achieved even under more forcing conditions.