Ethylene polymerization, with Lewis acid transition metal

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the first metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a], The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and A1R3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olefin polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980], The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium... 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

Recently, Baird and co-workers have reported (75) examples of polymerizations by a simple mono-Cp titanium complex, (C5(CH3)5)Ti(CH3)3 activated with a Lewis acid (B(C6F5)3) that not only copolymerizes ethylene and a-olefins but also induces polymerization of monomers normally associated with cationic polymerization such as isobutylene and vinyl ethers. Shaffer and Ashbaugh foimd (76) that for isobutylene and a-methylstyrene, the metal complex is an initiator rather than a catalyst (if it even participates at all), but that a transition from cationic to coordination polymerization occurs in styrene polymerization as temperature is raised. Even if it merely functions as an initiator, however, these investigations have revealed new polymerization systems based on anions such as [RB(C6F5)3l (R = alkyl, CeFs) that are less prone to side reactions tending to limit the MW and degree of polymerization of monomers like isobutylene at moderate temperatures (T > -80°C). 

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