A method for chain end functionalization of polyisobutylene (PIB) has been developed whereby living PIB is end-capped with the comonomer, 4-(4-allyloxyphenyl)-2-methyl-1-butene (AMB), at full isobutylene conversion. Addition is readily limited to just over one comonomer unit per chain end through the use of a low [AMB]/[chain end] ratio. When difunctional (telechelic) living PIB was end-capped using [AMB]/[chain end] = 2 at −70 °C, NMR showed that between 1.08 and 1.22 comonomer units on average were added per PIB chain end, depending on solvent polarity. Chain end structure and mechanism of formation were investigated by 1H, 13C APT, 2D gradient HSQC, and 2D HMBC NMR. tert-Chloride chain ends formed via PIB+Ti2Cl9– or PIB-AMB+Ti2Cl9– ion pair collapse were not observed. Olefinic chain ends formed via β-proton loss also were not detected; instead, a cyclic end group structure was observed. Upon addition of AMB the resulting carbocation tends to undergo terminative chain transfer consisting of alkylation o...