ETHYLENE AND PROPYLENE POLYMERIZATION BY A SERIES OF HIGHLY ELECTROPHILIC, CHIRAL MONOCYCLOPENTADIENYLTITANIUM CATALYSTS

The compounds CpTiMe2C6F5, Cp*TiMe2OC6F5, and Cp*TiMe2Cl(Cp* = eta(5) -C5Me5) react with the borane B(C6F5)(3) to form the thermally unstabl e, chiral complexes CpTiMe(C6F5)-(mu-Me)B(C6F5)(3), Cp*TiMe(OC6F5)(mu -Me)B(C6F5)(3), and CpTiMeCl(mu-Me)B(C6F5)(3), respectively, which ar e similar to the known CpTiMe2(mu-Me)B(C6F5)(3). All four mu-Me compo unds behave as sources of the highly electrophilic species [CpTiMeE]( +) (E = Me, Cl, C6F5, OC6F5) when treated with the borane, the last th ree being chiral, and all four systems exhibit catalytic activities fo r the polymerization of ethylene to high-molecular-weight polyethylene . Despite the chirality at titanium of three of the compounds, polymer ization of propylene by all of them results in the formation of atacti c, elastomeric polypropylene. NMR analyses of the propylene polymers f ormed show that initiation involves 1,2-insertion into a Ti-Me bond, a nd while propagation involves primarily head-to-tail 1,2-insertions, a n unusually high (by metallocene standards) proportion of the insertio ns also involves 2,1-misinsertions but essentially no 1,3-enchainment. The major olefinic end groups are vinylidene (CH2=CMe-), resulting fr om beta-hydrogen transfer following a 1,2-insertion, and vinyl (CH2=CH -), resulting from beta-hydrogen transfer from the methyl group follow ing a 2,1-insertion or, more Likely, beta-methyl transfer following a 1,2-insertion. Small amounts of internal olefins are also formed via b eta-hydrogen transfer following a 2,1-insertion. An EPR study of the C pTiMe3/ B(C6F5)(3) system in toluene indicates that <0.01% of the tit anium is occasionally present during polymerization as a complex of ti tanium(III), suggesting that a contribution to catalysis by titanium(I II) species is unlikely.