The negative role of chloride counter-anion in the activation process of zirconocene dichloride by methylaluminoxane

Abstract

The different elementary steps in the racEt(Ind) 2ZrCl 2 activation process by commercial methylaluminoxane (MAO) are studied by UV–VIS spectroscopy technique 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, [racEt(Ind) 2ZrMe] +, [MAOCl] − with an absorption band centered at λ=440 nm, inactive towards hexene polymerization. Addition of MAO in large excess (Al:Zr=2000) is therefore required to form active species identified as TMA separated ion pairs, [racEt(Ind) 2Zr +(μ-Me) 2AlMe 2], [MAO-Cl] − ( λ=470 nm). The activation of racEt(Ind) 2ZrMe 2 by MAO reveals much easier and is complete at low Al:Zr ratios (Al:Zr=150), yielding directly active ion pairs [racEt(Ind) 2ZrMe] +, [MAOMe] −, absorbing at λ=439 nm. These data underline the negative role of [MAOCl] − as a counter-anion in the activation process of zirconocene dichloride precursor for olefin polymerization. In the same conditions, the use of TMA-depleted MAO allows the direct formation, at low Al:Zr ratio, of active tight ion pairs ( λ=440 nm).