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

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Abstract

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).

Introduction

The activation pathway of metallocenes by methylaluminoxane (MAO) is still a question that puzzles the scientific community involved in olefin polymerization. Although the main elementary steps yielding “cationic” metallocene active species upon addition of MAO onto metallocene derivatives are now quite well-established (Scheme 1), several pending questions remain: the need of large amounts of MAO (generally Al:Met≥2000) to obtain high polymerization activity, the exact nature of the species (ion pairs, solvated ion pairs, loose ion pairs, free ions, etc.) including the nature and role of the [MAO] counter anion that may compete with the incoming monomer during polymerization, are important aspects of the activation process still under debate.

A series of spectroscopic studies have already been performed to further understand the activation mechanism of zirconocene derivatives by MAO but it has to be admitted that a present limitation to the understanding still comes from the high amount and complex structure of MAO making quite difficult to precisely characterize the active catalytic species. NMR spectroscopy was found a useful technique to investigate these systems. Tritto and co-workers were able to detect by 13C NMR several intermediate species formed by the reaction of Cp2ZrMe2 with MAO. However, the necessity to use relatively high zirconium concentrations ([Zr]>0.01 M) and low MAO:Zr ratios (Al:Zr≤40) [1] makes that the observed species are not representative of those present in highly active systems. The high concentration of MAO required to form such polymerization active species remains a strong limitation for their direct characterization by NMR and evidences for the generally admitted cation-like alkyl zirconocene structure have been obtained mainly from analogies found with boron-type activator/metallocene systems that work at stoichiometric ratio [2], [3], [4]. Recently, limits of the NMR technique could be shifted to lower zirconium concentrations ([Zr]≥4.5×10−4 M) and higher Al:Zr ratios (Al:Zr=4000) close to those used for highly active polymerization systems [5]. This was achieved at very low temperature (≤−25°C) by using Cp2ZrMe2 with 13C-labeled methyl groups. Interestingly, the formation of a hetero-bimetallic complex between Cp2ZrMe2 and TMA has been identified as the active species in such catalytic system.

UV–VIS spectroscopy was found to be a very useful and complementary technique, since it is possible to follow the different transformation steps of metallocene species at very low zirconium concentration (≈10−5 M) and high Al:Met ratio (up to 5000), much closer to the polymerization conditions [6], [7], [8], [9]. Indeed, aromatic ligands directly linked to the transition metal yield to the formation of absorption bands in the range 300–800 nm, characteristics of ligands to metal charge transfer (LMCT) and which express the electronic change on the metal. In the meantime, MAO does not show any absorption in this domain and is totally transparent allowing direct zirconocene observation even at very high Al:Zr ratios. We already reported a series of UV–VIS investigations focused on the activation process of zirconocene dichloride by MAO [6], [7], [8]. Depending on the Al:Zr ratio, the formation of several species could be readily shown by the UV–VIS absorption band changes. However, the exact reason for the need of very large amount of MAO to get the final polymerization active species could not be clearly elucidated.

In this paper, we report our last understandings on the MAO activation process of zirconocene dichloride with a special focus on the role of the extracted chlorine ligands and of TMA; spectroscopic and kinetic observations lead to evidences for their direct influence on the retarded formation of active species, and on the necessary huge amount of MAO required for activation.

Section snippets

Materials

racEt(Ind)2ZrCl2 and trimethylaluminium (TMA) (97% or 2 M in toluene) were purchased from Sigma Aldrich Chimie, Saint Quentin Fallavier, France. B(C6F5)3 (minimum 97%) and [C6H5N(CH3)2H]+[B(C6F5)4] were purchased from Strem Chemicals, Inc., Bischheim, France. All zirconocenes and boron derivatives were kept in a glove box under nitrogen. MAO (10% w/w in toluene, containing 30–35% TMA, CK Witco GmbH, Bergkam, Germany) and TMA were used as received, following conventional safety procedures.

Results and discussion

Activation of racEt(Ind)2ZrCl2 by MAO has been already extensively studied by UV–VIS spectroscopy [6]. The formation of different zirconocene species when varying the Al:Zr ratio was successively observed from the changes in the UV–VIS absorption spectra. Briefly, addition of small amounts of MAO (Al:Zr ratios up to 30) onto racEt(Ind)2ZrCl2 (λmax=427 nm, Fig. 1A, curve a), leads to the formation of a new species with an absorption band located at 396 nm (Fig. 1A, curve e). On the basis of the

Conclusions

This paper gives clear evidences for the negative role of extracted chloride ligands in the activation process of racEt(Ind)2ZrCl2 by MAO for olefin polymerization. This is attributed to the formation of a tight ion pair in which strong interaction between chloride anion and zirconium vacant site still takes place and impedes olefin coordination and insertion.

The replacement of chloride ligands by methyl groups allows a much easier activation of zirconocene derivatives by MAO.

The possibility to

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