Ethylene and propylene polymerization by cationic monocyclopentadienyl titanium catalysts containing the weakly coordinating anion [B(C6F5)(4)](-)

The compounds Cp*TiMe2E (Cp* = eta(5)-C5Me5; E = Me, C6F5, OC6F5, Cl) react with trityl tetrakis(perfluorophenyl)borate, [Ph3C][B(C6F5)(4)], to form t he thermally unstable dititanium complexes [(Cp*TiMeE)(2)(mu-Me)] [B(C6F5)( 4)], all of which behave as sources of the highly electrophilic species [Cp *TiMeE](+). An investigation of the activities of these [B(C6F5)(4)] salts as ethylene and propylene polymerization catalysts shows that they are more active than the analogous compounds Cp*TiMeE(mu-Me)B(C6F5)(3), as anticipa ted since [B(C6F5)(4)](-) is a poorer ligand than is [BMe(C6F5)(3)](-). How ever, contrary to current perceived wisdom, substitution of a methyl ligand of [Cp*TiMe2](+) by the more electron withdrawing C6F5, OC6F5 and Cl ligan ds in these monocyclopentadienyl systems does not generally result in catal ysts exhibiting lower activities and producing lower molecular weight polym ers. An EPR study of the Cp*TiMe3/[Ph3C][B(C6F5)(4)] system in chlorobenzen e at room temperature indicates that <0.01% of the titanium is present occa sionally during polymerization as a complex of titanium(III), suggesting th at a contribution to the catalytic processes by titanium(III) species is un likely. (C) 1999 Elsevier Science S.A. All rights reserved.