1H NMR study of the catalytic system (rac-Me2Si(2-Me,4-PhInd)2ZrCl2— polymethylalumoxane)—triisobutylaluminum

Abstract

The products of the reactions of polymethylalumoxane (MAO) with triisobutylaluminum (TIBA), rac Me 2 Si(2 Me,4 PhInd) 2 ZrCl 2 (1) with MAO (1 + MAO), and (1 + MAO) + TIBA were studied by 1 H NMR at different molar ratios of the components. When the ratio Al TIBA /Al MAO is ~6, the reaction between MAO and TIBA involves the replacement of the methyl group of MAO by isobutyl groups and the formation of isobutylmethylalumoxane or mixed isobutylmethylalumoxane structures. When the TIBA content in the system increases to 30 mol.%, these structures are rearranged to form products with a low degree of association. With the equimolar ratio of the reactants, the main reaction products are tetraisobutylalumoxane and polyisobutylalumoxane. The 1 + MAO system with the molar ratio Al MAO /Zr = 50 affords a MAO bonded monomethyl monochloride derivative [L 2 ZrCl µ Me] δ+ [MAO] δ-. An in crease in this ratio to 150 produces intermediate binuclear complexes [L 2 ZrCl µ Me MeZrL 2 ] + [MAO] -and [Me 2 Al (µ Me) 2 ZrL 2 ] + [MAO] -. The addition of TIBA induces the replacement of the ZrMe groups by isobutyl groups at the first step of the interaction and formation of nonidentified reaction products at the subsequent steps.Metallocene cationic complexes of Group IVB ele ments are highly efficient catalysts for olefin homo and copolymerization. A catalyst system is formed by the reaction of a metallocene complex with an activator (cocatalyst). The concept that catalytic intermediates (precursors of the active site) are alkylmetal cationic complexes bonded to a counterion 1-4 was confirmed by numerous spectroscopic data. 5-7 Polymethylalum oxane (MAO) ( O-Al(Me) ) n (n = 4-30) contain ing a considerable amount of free or partially bound trimethylaluminum (TMA) (up to 30 mol.% with re spect to aluminum) is most frequently used as a co catalyst. 8 When activating the starting metallocene dichlo ride L 2 ZrCl 2 (L are hapto bonded cyclopentadienyl, indenyl, or fluorenyl ligands), MAO bears numerous func tions: it acts as an alkylating agent (Scheme 1, reac tion (1)), cation generating agent (reaction (2)), and, in addition, purifies the reaction medium from moisture ad mixtures. Scheme 1 L 2 ZrCl 2 + MAO L 2 ZrMeCl (1) L 2 ZrMeCl + MAO L 2 Zr + Me...ClMAO -The MAO activated catalysts are most active at low metallocene concentrations (1•10 -5 -1•10 -7 mol L -1 ) and high cocatalyst/catalyst molar ratios (1•10 3 -1•10 4 ). Disadvantages of MAO are its high cost and instability during storage. Other most popular activators are fluoaryl boranes and fluoaryl borates. 8 However, their use as acti vators is restricted, because they have no components interacting with humidity admixtures in the reaction me dium and can activate only dimethylated metallocenes. Nevertheless, there are examples for the efficient activa tion of metallocene dichlorides by a combined activator including borates and triisobutylaluminum (TIBA). 9-13 Some workers 14-16 propose an approach to metallo cene activation based on the treatment of the metallocene dichloride ...

Effect of Different Aluminum Alkyls on the Metallocene/Methylaluminoxane Catalyzed Polymerization of Higher α‐Olefins and Styrene

“…[20] It should be also noted, that TIBA can react with MAO to form a mixed aluminoxane containing isobutyl groups. [11,57,59] The increase in the size of alkyl groups in aluminoxane makes the active cationic ion pair (L 2 ZrR þ ÁÁÁMAO À ) looser, this can lead to an increase of the catalyst activity, [11] which correlates well with our experimental results. However, the formation of a looser ion pair can also give rise to some loss in the stereospecificity of the catalyst, as in the case of 1-hexene polymerization with rac-EtInd 2 ZrCl 2 /MAO/TIBA (see Figure 2).…”

“…Moreover, the mixed aluminoxanes containing isobutyl groups are characterized by higher Lewis acidity than MAO that can be also influenced positively the catalyst activity. [57,59] A great difference between linear and bulky aluminum alkyls in their ability to act as a chain transfer agent was also observed: among different aluminum alkyls tested, the ability to a chain transfer increased with decreasing the bulkiness of the corresponding aluminum alkyls. Indeed, small size aluminum alkyl (TEA) diminished the molar mass strongly, medium size (THA) almost does not influence M n , while bulky TIBA noticeably increased the number-average molecular weight of the polymers obtained (see Table 1).…”

Alkylaluminum/Urea Hybrid Cocatalysts and Their Use in Iron‐Catalyzed Oligomerization of Ethylene

“…The broadening of TiBA signals (1.01 and 0.28 ppm) may indicate the existence of TiBA/aluminoxane associates. [36,37] Since the cocatalysts have been not isolated and used directly for ethylene oligomerization, excess TiBA can form two bridging alkyl groups between the Al and Fe center, thereby consuming the vacant site that is required for ethylene coordination. This coordination of TiBA is therefore responsible for the activity loss of the catalyst.…”