Elementary mechanisms of metallocene activation by methylaluminoxane cocatalysts for olefin polymerization

Abstract

AbstractThe different elementary steps in the activation process of racEt(Ind)2ZrCl2 by commercial methylaluminoxane (MAO) have been studied by UV/visible spectroscopy and correlated with hexene polymerization kinetics. After monomethylation of the zirconocene dichloride (λ = 396 nm), abstraction of Cl ligand by MAO at low Al/Zr ratios (Al/Zr = 150) yields tight ion pairs, with an absorption band centred at λ = 440 nm, which are inactive towards hexene polymerization. Addition of MAO in large excess (Al/Zr = 2000) is required to form the active species. The latter, characterized by an absorption at λ = 470 nm, were identified as trimethylaluminium (TMA) separated ion pairs. The activation by commercial MAO of racEt(Ind)2ZrX2, with X being methyl, benzyl or dimethylamino as extractable group, instead of chloride, reveals much easier and complete activation at low Al/Zr ratios (Al:Zr = 50–150). For these systems, formation of active ion pairs of different catalytic activities and absorbing in the range λ = 430–440 nm is observed. The use of TMA‐depleted MAO instead of commercial MAO, as cocacalyst for racEt(Ind)2ZrCl2 actvation also allows the formation of active tight ion pairs absorbing at λ = 440 nm at low Al/Zr ratio. These results underline the negative role of chloride ligands when associated with TMA (contained in MAO) in the activation process of zirconocene for olefin polymerization, and also suggest interesting alternatives to reduce the amount of MAO necessary for metallocene activation.© 2002 Society of Chemical Industry