Journal of Molecular Catalysis A-Chemical, Vol.174, No.1-2, 81-87, 2001
The negative role of chloride counter-anion in the activation process of zirconocene dichloride by methylaluminoxane
The different elementary steps in the racEt(Ind)(2)ZrCl2 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 (lambda = 396 nm), abstraction of Cl ligand by MAO at low Al:Zr ratios (Al:Zr = 150) yields tight ion pairs, [racEt(Ind)(2)ZrMe](+), [MAOCl](-) with an absorption band centered at lambda = 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)(2)Zr+(mu -Me)(2)AlMe2], [MAO-Cl](-) (lambda = 470 nm). The activation of racEt(Ind)(2)ZrMe2 by MAO reveals much easier and is complete at low Al:Zr ratios (Al:Zr = 150), yielding directly active ion pairs [racEt(Ind)(2)ZrMe](+), [MAOMe](-), absorbing at lambda = 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 (lambda = 440 nm).