Ethylene, styrene, and alpha-methylstyrene polymerization by mono(pentamethylcyclopentadienyl)(Cp(*)) complexes of titanium, zirconium, and hafnium: Roles of cationic complexes of the type [Cp*MR(2)](+) (R=alkyl) as both coordination polymerization catalysts and carbocationic polymerization initiators

A variety of monocyclopentadienyl and mono(pentamethylcyclopentadienyl) complexes of titanium, zirconium, and hafnium are assessed for abilities to initiate polymerization of ethylene, styrene, and, in part, a-methylstyrene. In general, little or no activity was found for either neutral species of the types CpMMe(3) and CpMMe(2)OR or for cationic 12- and 14-electron species of the types [CpMR(2)L](+) and [CpMR(2)L(2)](+), respectively (Cp = eta(5)-cyclopentadienyl; R = alkyl; L = amine, phosphine ligands). In contrast, much better olefin polymerization initiators result from abstraction of a methyl carbanion from Cp*MMe(3) (Cp* = eta(5)-pentamethylcyclopentadienyl) by B(C6F5)(3), a reaction which gives cationic, l0-electron species of the type ''[Cp*MMe(2)][BMe(C6F5)(3)]''. Of these, the complex [Cp*TiMe(2)][BMe(C6F5)(3)] (A) is an excellent initiator or initiator precursor for the polymerization of ethylene and styrene, resulting in high yields respectively of high molecular weight polyethylene and atactic (a-PS) and/or syndiotactic polystyrene (s-PS), depending on conditions; the tacticity of purified s-PS, as judged by C-13{H-1} NMR spectroscopy, approaches 100%. While the polymerization of ethylene probably involves a classical Ziegler-Natta process, polymerization of styrene to s-PS and a-PS apparently involves respectively a Ziegler-Natta process and carbocationic initiation. High yields of essentially syndiotactic poly(alpha-methylstyrene) are obtained by utilizing the same initiator system, also apparently via a carbocationic process.