Metallocene-trimethylaluminum system

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

Activation of a sa-metallocene dichloride precursors by methylalumoxane (MAO) - a partial-hydrolysis product of trimethylaluminum [3] - was the initial door-opener for the polymerization of propene and higher olefins by ansa-metaUocene catalysts [4, 5]. It is still being quoted as the most frequently used route to activate these catalyst systems [26]. Evidence has recently been presented that activation by MAO might involve highly reactive AlMc2 cations [27]. Yet it remains unknown which structural features of MAO might be responsible for its unique reactivity. 

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