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

2001; Elsevier BV; Volume: 174; Issue: 1-2 Linguagem: Inglês

10.1016/s1381-1169(01)00188-1

1873-314X

Jean‐Noël Pédeutour, Henri Cramail, Alain Deffieux,

Synthetic Organic Chemistry Methods

The different elementary steps in the racEt(Ind)2ZrCl2 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)2AlMe2], [MAO-Cl]− (λ=470 nm). The activation of racEt(Ind)2ZrMe2 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).