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 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 = η 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* = η 5 -pentamethylcyclopentadienyl) by B(C 6 F 5 ) 3, a reaction which gives cationic, 10-electron species of the type “[Cp*MMe 2 ][BMe(C 6 F 5 ) 3 ]”. Of these, the complex [Cp*TiMe 2 ][BMe(C 6 F 5 ) 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 13 C{ 1 H} 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(α-methylstyrene) are obtained by utilizing the same initiator system, also apparently via a carbocationic process.