F. Barsan et al., Polymerization of isobutylene and the copolymerization of isobutylene and isoprene initiated by the metallocene derivative Cp*TiMe2(mu-Me)B(C6F5)(3), MACROMOLEC, 31(24), 1998, pp. 8439-8447
Isobutylene polymerization is initiated by Cp*TiMe2(mu-Me)B(C6F5)(3), forme
d by combining Cp*TiMe3 and B(C6F5)(3) in a 1:1 ratio. The polymerization p
rocess exhibits the signature of a carbocationic mechanism, as molecular we
ights generally increase with decreasing temperature, polydispersities are
similar to 2, and polymers contain vinylidene end groups. Initiation probab
ly 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, thi
s is not because traces of possible protic initiators are being scavenged b
ut 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-dimeri
zation of 1,1-diphenylethylene to 1,3,3-triphenyl-3-methylindan and also th
e formation of isobutylene-isoprene copolymers (butyl rubber) in which the
isoprene is incorporated via trans-1,4-addition, again typical of conventio
nal initiators. This initiator system is the first metallocene-based carboc
ationic initiator system to be discovered, and it also appears to be one of
the more active.