Ethylene and propylene polymerization by cationic monocyclopentadienyl titanium catalysts containing the weakly coordinating anion [B(C 6F 5) 4] −

Abstract

The compounds Cp*TiMe 2E (Cp*=η 5-C 5Me 5; E=Me, C 6F 5, OC 6F 5, Cl) react with trityl tetrakis(perfluorophenyl)borate, [Ph 3C][B(C 6F 5) 4], to form the thermally unstable dititanium complexes [(Cp*TiMeE) 2(μ-Me)][B(C 6F 5) 4], all of which behave as sources of the highly electrophilic species [Cp*TiMeE] +. An investigation of the activities of these [B(C 6F 5) 4] − salts as ethylene and propylene polymerization catalysts shows that they are more active than the analogous compounds Cp*TiMeE(μ-Me)B(C 6F 5) 3, as anticipated since [B(C 6F 5) 4] − is a poorer ligand than is [BMe(C 6F 5) 3] −. However, contrary to current perceived wisdom, substitution of a methyl ligand of [Cp*TiMe 2] + by the more electron withdrawing C 6F 5, OC 6F 5 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*TiMe 3/[Ph 3C][B(C 6F 5) 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.