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

Abstract

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...