Isobutylene polymerization is initiated by Cp*TiMe2(μ-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 ∼2, and polymers contain vinylidene end groups. Initiation probably occurs via η-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.

Addition of equimolar amounts of (yS-C5Me5)TiMe3 and B(C6F5)3 to solutions of isobutylene or isobutylene and isoprene (99: 1 molar ratio) in toluene a t -78 "C results in polymerization to give, respectively, isobutylene homopolymer with a high weight average molecular mass of -5 x lo5 and a narrow polydispersity (2.1), or an isobutylene-isoprene copolymer exhibiting a composition, a molecular mass distribution and 1H and 13C NMR spectra identical to those of a sample of commercial butyl rubber.