Ethylene, Styrene, and α-Methylstyrene Polymerization by Mono(pentamethylcyclopentadienyl) (Cp*) Complexes of Titanium, Zirconium, and Hafnium: Roles of Cationic Complexes of the Type [Cp*MR2]+ (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, α-methylstyrene. In general, little or no activity was found for either neutral species of the types CpMMe3 and CpMMe2OR or for cationic 12- and 14-electron species of the types [CpMR2L]+ and [CpMR2L2]+, respectively (Cp = η5-cyclopentadienyl; R = alkyl; L = amine, phosphine ligands). In contrast, much better olefin polymerization initiators result from abstraction of a methyl carbanion from Cp*MMe3 (Cp* = η5-pentamethylcyclopentadienyl) by B(C6F5)3, a reaction which gives cationic, 10-electron species of the type “[Cp*MMe2][BMe(C6F5)3]”. Of these, the complex [Cp*TiMe2][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 polyst...