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

Abstract

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 polystyrene (s-PS), depending on conditions; the tacticity of purified s-PS, as judged by 13C{1H} 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.

Syndiospecific living polymerization of 4-methylstyrene with (trimethyl)pentamethylcyclopentadienyltitanium/tris(pentafluorophenyl)borane/trioctylaluminium catalytic system

“…A single sharp resonance for the phenyl C-1 carbon was observed at 142.9 ppm, assignable to syndiotactic sequence. Wang et al reported that a significant amount of atactic polystyrene (extractable by fractionation with 2-butanone) was formed with a similar catalytic system at low polymerization temperatures 16) . As shown in Fig.…”

Synthesis and characterization of elastoplastic poly(styrene-co-ethylene) using novel mono(?5-cyclopentadienyl) titanium and modified methylaluminoxane catalysts

“…Currently, mono( 5 -cyclopentadienyl)titanium complexes activated by MAO for syndiospecific polymerization of styrene are well known, [1][2][3][4][5] as well as for the polymerization of ethylene 6,7 and propylene. 8,9 The active species of mono( 5 -cyclopentadienyl)titanium/MAO catalyzing different monomers have been analyzed by various methods.…”

Catalytic systems Me2SiCp*NtBuMX2/(CPh3)(B(C6F5)4) (M?Ti, X?CH3, M?Zr, X?iBu) in copolymerization of ethylene with styrene

“…Whenever the primary styrene incorporation occurs, the system proposed should bear carbocationic active sites that are responsible for the initial stage of atactic PS design. Eventually, carbocationic active sites yielding atactic PS at low polymerization temperature (0°C) were assigned by Baird et al 40 to switching properties (sPS 3 atactic product) of Cp*TiMe 3 /B(C 6 F 5 ) 3 catalyst. Shaffer and Ashbaugh 41 demonstrated that NCA (RB Ϫ (C 6 F 5 ) 3 and B Ϫ (C 6 F 5 ) 4 ) can bear active sites for carbocationic styrene polymerization in the presence of either metallocene or nonmetallocene complexes under specified conditions.…”