Ethylene and Propylene Polymerization by a Series of Highly Electrophilic, Chiral Monocyclopentadienyltitanium Catalysts

The compounds Cp*TiMe2C6F5, Cp*TiMe2OC6F5, and Cp*TiMe2Cl (Cp* = η5-C5Me5) react with the borane B(C6F5)3 to form the thermally unstable, chiral complexes Cp*TiMe(C6F5)(μ-Me)B(C6F5)3, Cp*TiMe(OC6F5)(μ-Me)B(C6F5)3, and Cp*TiMeCl(μ-Me)B(C6F5)3, respectively, which are similar to the known Cp*TiMe2(μ-Me)B(C6F5)3. All four μ-Me compounds behave as sources of the highly electrophilic species [Cp*TiMeE]+ (E = Me, Cl, C6F5, OC6F5) when treated with the borane, the last three being chiral, and all four systems exhibit catalytic activities for the polymerization of ethylene to high-molecular-weight polyethylene. Despite the chirality at titanium of three of the compounds, polymerization of propylene by all of them results in the formation of atactic, elastomeric polypropylene. NMR analyses of the propylene polymers formed show that initiation involves 1,2-insertion into a Ti−Me bond, and while propagation involves primarily head-to-tail 1,2-insertions, an unusually high (by metallocene standards) proportion of the insertions also involves 2,1-misinsertions but essentially no 1,3-enchainment. The major olefinic end groups are vinylidene (CH2CMe−), resulting from β-hydrogen transfer following a 1,2-insertion, and vinyl (CH2CH−), resulting from β-hydrogen transfer from the methyl group following a 2,1-insertion or, more likely, β-methyl transfer following a 1,2-insertion. Small amounts of internal olefins are also formed via β-hydrogen transfer following a 2,1-insertion. An EPR study of the Cp*TiMe3/B(C6F5)3 system in toluene indicates that