Alkylmetallocenium ions

The alumoxane cocatalysts have at least two functions alkylation of the metallocene component, which takes place within seconds even at -60 °C (eq. (2)) and formation of the active species by abstraction of Me" (eq. (3)). The resulting active species is discussed as being a 14e (= 14 valence electron) cationic alkylmetallocenium ion formed by dissociation of the metallocene alumoxane complex [26,27]. The [alumoxane-Me]" anion is regarded as weakly or non-co-ordinating. Nearly every zirconocene atom is active, forming a single-site catalyst [28, 29]. 

The methylalumoxane may be replaced by a mixture of trialkylaluminum as an alkylating agent and dimethylaluminum difluoride as Lewis acid [31]. Dialkyl or dibenzyl metallocenes form active species when combined with the Lewis acidic tris(pentafluorophenyl)borane or organic salts of the noncoordinating tetrakis (pentafluorophenyl)borate, generating alkylmetallocenium ions [32-35]. With these co-catalysts a metallocene/co-catalysts ratio of 1 1 is used. Usually trialkylaluminum is added as a scavenger to prevent decomposition by impurities of the alkylmetallocenium ions generated in situ. 

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