Structure of Alumoxanes

As pointed out by Pasynkiewicz [27], because of the complex nature of these MAO solutions, and the wide range of structures possible, relatively few structures have been determined, and the structures that have been established usually involve bulky alkyl groups or are anionic species that are more readily isolated due to their propensity to precipitate from solution. 

The role of methylalumoxane (MAO) as a cocatalyst to activate zirconocene compounds such as Cp ZrCl to create a single-site ethylene polymerization catalyst is similar, in some respects, to the role simple aluminum alkyls (AlRj) play in activating Ziegler catalysts. For example, MAO acts as an alkylating agent to form the initial Zr-carbon bond (Zr-CH ) necessary to initiate the polymerization process. However, experimental evidence obtained by a variety of methods clearly has shown that the MAO reacts with the zirconium center to form a zirconium cation of the type [CpjZr-CHS] in which the zirconium is not reduced to a lower oxidation state, but remains as a d° metal and Zr(IV) oxidation state. The MAO, therefore, forms an anion moiety to complete the ion pair necessary to create the active species, as illustrated in Equation 4.1. 

The ion pair shown above suggests that the more important role of MAO in activating the metallocene compound as an ethylene polymerization center is to stabilize the zirconium cation by providing a bulky non-coordinating moiety similar to the well-established bulky anion tet-raphenyl borate [(CgHg) B]. 

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Probably the best description of the actual structure of alumoxane solutions, as pointed out by Barron [31], is to consider the Al-0-(Al-0) -Al oligomeric backbones to stack as three-dimensional clusters or cages [32], as illustrated in Figure 4.7. 

The structure of alumoxanes have (despite contradicting spectroscopic data) traditionally been proposed to consist of linear or cyclic chains (9). 

If, as we have previously proposed, the core structure of alumoxanes is analogous to that of boehmite then one would expect the aluminum to retain exclusively six-fold coordination. This is clearly demonstrated to be the case by the... 

The first step in the hydrolysis reaction is the formation of the water adduct [Eq. (23)] [69], which subsequently eliminates alkane to yield the hydroxide R2A10H. At elevated temperatures, further alkane elimination is observed with the formation of alkyl-alumoxane [67, 68]. Structurally characterized alumoxanes are listed in... 

The most frequent structures of organo-alumoxanes are hexameric cage structures (41-45) [67] (Figure 3.5-11) (Table 3.5-3). The Al O core can be described as a drum-like hexagonal prism with alternating A1 and O atoms. The AI3O3 hexago-... 

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. 

Figure 2.41. Comparison of the core structure of (a) siloxy- substituted alumoxane gels with (b) diaspore, and (c) boehmite minerals. The aluminum and oxygen atoms are shown in blue and red, respectively. Reproduced with permission from Chem. Mater. 1992, 4, 167. Copyright 1992 American Chemical Society.

Figure 5.18. Molecular structure of [Me2A10]5, and schematic of the cage-opening mechanism of the alumoxane co-catalyst during metallocene-catalyzed polymerization.

Figure 2. The structure of the [Al2( J.2-0H)20Ac] (aq) dimeric complex suggested by Ohman (1991) and discussed by Persson et al. (1998). Although no aluminum-acetate solids have been crystallized with this structure, other aluminum carboxylates have been found to have the carboxylate moiety bridging two aluminums, such as dawsonite (Fmeh and Golightly 1967) and alumoxanes (Bethley et al. 1997).

Polyethylenes synthesized by metallocene/MAO catalysts have a molecular weight distribution of M /M = 2. The molecular weight can easily be lowered by increasing the temperature, increasing the metallocene concentration, or decreasing the ethene concentration. The narrow molecular weight distribution is characteristic for a single site catalyst. Nearly every zirconocene forms an active site of a cationic metallocene - and an anionic MAO compound or a complex of both. The nature of the active site would be clearer if more details were known about the structure of the alumoxane. 

FIGURE 40.2 Chemical structures of modified alumoxanes (a) diacryloyl lysine-alumoxane (activated), (b) stearic acid-alumoxane (surfactant), and (c) acryloyl undecanoic amino acid-alumoxane (hybrid). 

While early examples of alumoxane precursors for alumina fiber S3mthesis had processing characteristics superior to alumina gels, they were found to be unstable and decomposed during spinning. In addition their structures were a complete mystery, making further developments difficult... 

It should be obvious, however, that whereas the mineral boehmite essentially has an infinite structure, carboxylate-alumoxanes must have a finite size. Thus there must be either end or edge groups encapsulating the boehmite-like core. We have shown previously that low molecular weight oxide- and siloxide-alumoxanes contain a significant proportion of four-coordinate aluminum centers, and that these four-coordinate aluminum atoms comprise the end groups. In the case of the... 

As already pointed out, recent advances in the synthesis and structural characterisation of discrete molecular alumoxanes have lately been the subject of review [8]. While developments in the crystallisation and structural... 

Koide, Y., Bott, S. G. and Barron, A. R., Alumoxanes as Cocatalysts in the Palladium-catalyzed Copolymerization of Carbon Monoxide and Ethylene Genesis of a Structure-Activity Relationship , Organometallics, 15, 2213-2226 (1996). 

Using bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2 Structure 6) and MAO, up to 40 000 000 g polyethylene/g Zr per h are obtained. Every zirconium atom forms an active complex and produces about 46 000 polymer chains per h. The time of insertion of one ethylene unit is only 3 X 10 s. Table 2 shows the polymerization behavior of different metallocene/alumoxane catalysts. 

Partial hydrolysis of Me2AlCl,Et20 and Me2AlCl,NCCH2Ph complexes produces derivatives of bis(chloromethyl)alumoxane.405 The spectroscopic properties of these suggest the structure (70). 

The siloxides of aluminum can be prepared in the same manner as the alkoxides and exhibit similar structural features. Interest in these compounds arose from a desire for sol-gel or polymeric precursors for aluminosilicates. Unfortunately, these compounds extensively hydrolyze, losing most of the silicon as the silanol. However, the investigation of the hydrolysis of (Et3SiO)3Al has generated considerable insight into the structure and composition of alkoxy- and siloxy-substituted alumoxane polymers, (ROAIO) . These macromolecules are now believed to have a structure related to that of boehmite. 

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