Methylaluminoxanes

Most commercially available methylaluminoxanes are produced by careful reaction of water with trimethylaluminum (TMAL) in toluene. Reaction must be closely controlled to avoid what renowned organometallic chemist John Eisch called a life threatening pyrotechnic spectacle (16). Unfortunately, there have been explosions and injuries reported during MAO preparations. Water must be introduced at low temperature and in forms that moderate the potentially violent reaction. For example, water has been introduced as hydrated salts, ice shavings or atomized spray. Even with these precautions, explosive reactions have occurred. The overall reaction is given in eq 6.1. 

Yields are often low in lab preps (usually 60%). The product is called methyl-aluminoxane (MAO) or, less commonly, polymethylaluminoxane (PMAO). MAO is an ill-defined, complex composition, virtually insoluble in aliphatic hydrocarbons. MAO is typically supplied as a toluene solution containing 13% Al, which corresponds to 28% concentration of MAO. 

Strides have been made in industrial-scale production of MAO. Process improvements afford greatly improved yields. This has been achieved by use of alternative reactants and/or continuous processes, employing highly dilute solutions, low ratios of water to TMAL and recycle of intermediate streams (17). 

As isolated from toluene solution, neat MAO is an amorphous, friable white solid containing 43-44% Al (theory 46.5%). Like most commercially available aluminum alkyls, it is pyrophoric and explosively reactive with water. Freshly prepared MAO solutions form gels within a few days when stored at ambient temperatures ( 20 °C). However, lower storage temperatures (0-5 °C) delay gel formation. Consequently, manufacturers store and transport MAO solutions in refrigerated containers. Commercially available MAO contains residual TMAL (15-30%), called free TMAL or active aluminum. The literature is contradictory on the influence of free TMAL on activity of single site catalysts both reductions and increases have been reported (18-20). Perhaps the most important drawback of methylaluminoxane is its cost, which is substantially higher than conventional aluminum alkyls. Despite these untoward aspects, methylaluminoxane remains the most widely used cocatalyst for industrial single site catalysts. 

Other alkylaluminoxanes, e.g., isobutylaluminoxane (IBAO), are also available, more easily produced and significantly less costly than methylaluminoxane. 

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EinaHy, in 1976, Kaminsky and Sinn in Germany discovered a new family of catalysts for ethylene polymerization. These catalysts (ie, Kaminsky catalysts) contain two components a metallocene complex, usually a zkconocene, and an organoaluminum compound, methylaluminoxane (8,9). These catalysts and thek various later modifications enable the synthesis of ethylene copolymers with a high degree of branching uniformity. Formally classified as MDPE, LLDPE, or VLDPE, the resins thus produced have a number of properties that set them apart from common PE resins in terms of performance... 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Acetone is the best solvent for NBR hydrogenation in the presence of palladium carboxylates. No hydrogenation is achieved when chloroform or chlorobenzene are the solvents. Since it is understood that palladium is reduced to colloidal metal in the presence of hydrogen, attempts have also been made to reduce the palladium by hydrazine [76], methylaluminoxane [84], and trialky] aluminum [85] to improve the catalytic activity. 

Styrene monomer was purified by vacuum distillation over CaHi. Inhibitor in styrene was removed using activated alumina. N-heptane was purified by distillation over sodium to remove tire trace of residual moisture. The purified slyreaie and n-heptane were stored over activated alumina under nitrogen blanket. Et[Ind]2ZrCl2 Strem Chem.), MAO (modified methylaluminoxane, type 3A, Akzo Novel) wee used without fiirtha" purification. 

In a typical polymerization, the immobilized precatalyst, toluene, and methylaluminoxane (MAO) at a ratio of 800 1 Al Ti were added to the reactor in a drybox. The mixture was allowed to stir for 20 minutes to allow for sufficient activation of the catalyst. The reactor was then removed from the glovebox, placed in a 25 C water bath, then ethylene at 60 psi was introduced. The polymerization was allowed to continue for 10 minutes, then terminated by adding acidic ethanol. The precipitated polymers were washed with ethanol, then dried at 70 C. 

In order to enhance the activity of coordination catalysts we typically add a cocatalyst. The cocatalyst works synergistically with the catalyst to allow us to tailor the tacticity and molecular weight of the product while also enhancing the rate of the reaction. An example of a commercially used cocatalyst is methylaluminoxane used in conjunction with metallocene catalysts. 

Thus, in the presence of methylaluminoxane (MAO) at 23°C, (C5H5B-N(/-Pr)2)2 ZrCl2 polymerizes ethylene with an activity of 105 kg ofpolyethylene/(h [Zr] mol), similar to that observed with well-studied Cp2ZrCl2 as the catalyst. It is believed that MAO is functioning in its usual role in these Ziegler-Natta polymerizations (methylation of Zr and abstraction of methyl to form a highly reactive Zr cation).41... 

Figure 29 Structures of typical metallocenes and proposed structure of methylaluminoxane (MAO).

Figure 21 Structure of methylaluminoxane (extreme left), and some metallocene catalyst systems. Reproduced from Kaminsky [128], with permission of The Royal Society of Chemistry.

The key to highly active metallocene catalysts is the use of cocatalysts. In an activation step, the cocatalyst creates out of the metallocene a polymerization-active species. At first, methylaluminoxane (MAO) was usually used to activate metallocenes. Nowadays an alternative activation via borane and borate is becoming more and more important [20, 24, 25]. 

Sivaram S, Srinivasa RS (1995) Homogeneous metallocene-methylaluminoxane catalyst systems for ethylene polymerization. Prog Polym Sci 20 309-367... 

It has recently been shown that some of the sluggish reactions, such as those with styrene and w-alkenol derivatives, can be significantly accelerated by the addition of H20, MAO (methylaluminoxane) [167], and IBAO (isobutylaluminoxane) [168], and that the ee values can be improved by several % (Generalization 17). Some of the earlier results discussed above have also been reviewed recently [169,170]. 

Manufacturing systems, molecular, 17 58 MAO/metallocene ratio, 16 94. See also Methylaluminoxane (MAO)... 

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