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 degrees 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( 1)H, C-13 APT, 2D gradient HSQC, and 2D HMBC NMR. tert-Chloride chain ends formed via PIB+Ti2Cl9- or PIB-AMB(+)Ti(2)Cl(9)(-) ion pair collapse were not observed. Olefinic chain ends formed via beta-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 of the 4-allyloxyphenyl ring at C2, via a five-membered cyclic intermediate (indanyl ring formation). MALDI-TOF-MS indicated that no more than two comonomer units were added to each chain end, as the mass spectrum consisted of only three major distinguishable distributions, namely, difunctional PIB chains possessing one AMB unit per chain end, chains possessing one AMB unit at one end and two at the other end, and chains possessing two units at each chain end. MALDI-TOF-MS confirmed the absence of tent-chloride and olefinic end groups. GPC analysis indicated that no chain coupling or fragmentation occurred during the end-capping reaction.