Methylalumoxane structure

We have discussed the structure and synthesis of the library of molecular catalysts for polymerization in Section 11.5.1. In the present section we want to take a closer look at the performance of the catalyst library and discuss the results obtained [87], The entire catalyst library was screened in a parallel autoclave bench with exchangeable autoclave cups and stirrers so as to remove the bottleneck of the entire workflow. Ethylene was the polymerizable monomer that was introduced as a gas, the molecular catalyst was dissolved in toluene and activated by methylalumoxane (MAO), the metal to MAO ratio was 5000. All reactions were carried out at 50°C at a total pressure of 10 bar. The activity of the catalysts was determined by measuring the gas uptake during the reaction and the weight of the obtained polymer. Figure 11.40 gives an overview of the catalytic performance of the entire library of catalysts prepared. It can clearly be seen that different metals display different activities. The following order can be observed for the activity of the different metals Fe(III) > Fe(II) > Cr(II) > Co(II) > Ni(II) > Cr(III). Apparently iron catalysts are far more active than any of the other central metal... 

Methylalumoxane (MAO) (structure 5.52) is the most widely utilized counterion. MAO is an oligomeric material with the following approximate structure ... 

The second component is a special alumina-organic compound, methylalumoxane (MAO), that is prepared by partial hydrolysis of trimethylaluminum and that contains linear as well as cyclic structures in the molecules. 

Examples of alumoxanes suitable as activating co-catalysts in the catalysts system are methylalumoxane, isobutylalumoxane, 2,4,4-trimethyl-pentylalumoxane, and 2-methyl-pentylalumoxane. Mixtures of different alumoxanes can also be used (25). Alumoxanes have a core structure analogous to boehmite, i.e., a sequence of -(Al-O)n-, either linear or also as rings. 

Miyashita, A., Nabika, M. and Susuki, T., Mechanistic Study on Syndiotactic Polymerisation of Styrene Catalysed by Titanocene-Methylalumoxane Complexes, in Proceedings of International Symposium on Synthetic, Structural and Industrial Aspects of Stereospecific Polymerisation, Milan, Italy, 1994, Abstracts, III-18. 

The isolation and structural characterization of aluminum hydroxides with terminal OH groups is a s)mthetic challenge, and efforts were made to prepare well-defined analogues of methylalumoxane (MAO). The strategy of hydrolysis of metal chlorides in the presence of a base to quench the hydrogen chloride generated during the reaction was adopted. This synthesis can be performed in two different ways (i) hydrolysis of a metal halide in a two-phase liquid NHs-toluene system in the presence of KOH and KH and (ii) hydrolysis of a metal halide in the presence of a stoichiometric amount of N-heterocyclic carbene. Both routes are reliable, but the latter one may prove to offer more versatility and be relatively easier to conceive. 

Almost all group 4 metal complexes require a cocatalyst to generate an active metal-alkyl cationic species. Ordinary alkylaluminums - used in conventional Ziegler Natta catalysts - are insufiicient to activate these compounds on their own. The principal activator nsed is methylalumoxane (MAO), a structurally enigmatic material with a mixture of nuclearities. Its purpose is to alkylate the metal dichloride and to abstract one of the reactive hgands to form the ion pair active catalyst. The interaction is dynamic and a large excess of MAO is needed for effective catalyst performance, thus inhibiting a comprehensive characterization of these catalysts. 

Methylalumoxane formed by controlled reaction of trimethylaluminum with water, under elimination of methane, is a mixture of oligomers [22]. The basic oligomer (Structure 5) forms associates and cage structures complexing additional trimethylaluminum [23,24],... 

A further breakthrough was the synthesis of enantiomeric sterorigid ansa-metallocenes by Brintzinger and co-workers [33] and the discovery by Ewen [34] that such racemic metallocene/methylalumoxane systems generate isotactic polypropylene. It was further found that the metallocene structure determines the polymer structure [35-37]. Again, with these compounds polyolefins such as syndiotactic polypropylene become available on a large scale [38]. Indeed, metallocene/methylalumoxane catalysts offer new prospects for olefin oligomers and polymers [39 2],... 

Nowadays homogeneous metallocene catalysts activated with oligomeric methylalumoxanes or other co-catalysts [16, 20, 46-54] open new prospects. These systems have an excellent activity, they have the ability to form random copolymers in combination with a narrow molecular mass and comonomer distribution. Further important advantages are that a broad variety of structures can be synthesized to obtain tailor-made catalysts [49, 53], and that zirconium compounds are scarcely reduced with the co-catalyst [54]. It is further reported that metallocenes have been used in combination with methylalumoxanes for EPDM production at temperatures below 100 °C in liquid propylene [55]. 

Probably the most important point in this list is the structure and reactivity of the counterions, namely MAO (methylalumoxane). The exact constitution and stmcture of MAO as used in technical polymerization is still not completely clear. Some structurally characterized model compounds are available. Theoretical models have therefore been used to investigate structure and stability of different MAO aggregates. However, initial investigations on possible interactions between the counterion and the catalytic site have been undertaken, but especially in the case of the technically important MAO, these attempts must be seen as only the first steps [38 0]. 

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

The analogous titanium and hafnium compounds form active catalysts, too. Especially at higher temperatures the zirconium catalysts are more stable and active than the titanium or hafnium systems. The co-catalyst has a main influence. The most used co-catalyst is methylalumoxane (MAO). At lower temperature, fluorinated borane compounds activate the catalyst, too [17,18]. The structure of MAO is complicated. Sinn characterized MAO by different analytical methods and found it to be a cage with a formula of AlisO 12(0113)24 [19]. 

FIGURE 4.4 Possible structures of methylalumoxane. (After Kaminsky, W., Sinn, H., and Woldt, R. 1983. Macrotnol. Ghent. Rapid Commun., 4, 417.)... 

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