Metallocene-initiated polymerization coinitiator

Other Lewis acids have been considered as alternatives to MAO for two reasons (1) one might avoid the cost of the large excess of MAO required to activate the metallocene and (2) simpler systems, which allow isolation of the product(s) from reaction of metallocene and coinitiator, would be useful to obtain a better understanding of metallocene-initiated polymerizations. 

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... 

The rate of polymerization is derived in a straightforward manner. The following derivation describes polymerization with a zirconocene (Zr) and MAO, but is general and applies to any metallocene coupled with any coinitiator. Reaction between the initiator zirconocene and MAO forms the active initiating species I, which subsequently adds monomer to yield the initial propagating species M ... 

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