Isobutylene polymerization is initiated by Cp*TiMe2(mu-Me)B(C6F5)(3), formed by combining Cp*TiMe3 and B(C6F5)(3) in a 1:1 ratio. The polymerization process exhibits the signature of a carbocationic mechanism, as molecular weights generally increase with decreasing temperature, polydispersities are similar to 2, and polymers contain vinylidene end groups. Initiation probably occurs via eta(1)-coordination of a molecule of monomer to the cationic species [Cp*TiMe2](+), while propagation and chain transfer proceed as with conventional Lewis acid initiators. Although addition of the proton trap 2,6-di-tert-butylpyridine does have an adverse effect on polymerization, this is not because traces of possible protic initiators are being scavenged but rather because the 2,6-di-tert-butylpyridine coordinates to the titanium cation and inhibits activation of monomer. Further evidence that [Cp*TiMe2](+) behaves as a carbocationic initiator is that the system induces-dimerization of 1,1-diphenylethylene to 1,3,3-triphenyl-3-methylindan and also the formation of isobutylene-isoprene copolymers (butyl rubber) in which the isoprene is incorporated via trans-1,4-addition, again typical of conventional initiators. This initiator system is the first metallocene-based carbocationic initiator system to be discovered, and it also appears to be one of the more active.