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 the 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 anticipated since [B(C6F5)(4)](-) is a poorer ligand than is [BMe(C6F5)(3)](-). However, contrary to current perceived wisdom, substitution of a methyl ligand of [Cp*TiMe2](+) by the more electron withdrawing C6F5, OC6F5 and Cl ligands in these monocyclopentadienyl systems does not generally result in catalysts exhibiting lower activities and producing lower molecular weight polymers. An EPR study of the Cp*TiMe3/[Ph3C][B(C6F5)(4)] system in chlorobenzene at room temperature indicates that <0.01% of the titanium is present occasionally during polymerization as a complex of titanium(III), suggesting that a contribution to the catalytic processes by titanium(III) species is unlikely. (C) 1999 Elsevier Science S.A. All rights reserved.