Methylaluminoxane commercial

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. 

The Cp2ZrMe2—[Al(t-Bu)0]6 catalyst exhibits lower activity in ethylene polymerisation than the Cp2ZrMe2— [Al(Me)0]x catalyst. Reasons for decreased activity of the former catalyst compared with the latter are difficult to determine, since the exact speciation of methylaluminoxane is unknown however, commercial methylaluminoxane contains a large amount of trimethylalu-minium [359, 360],... 

Metallocenes (Fig. 2) are sandwich structures, typically incorporating a transition metal such as titanium, zirconium, or hafnium in the center. The metal atom is linked to two aromatic rings with five carbon atoms and to two other groups—often chlorine or alkyl. The rings play a key role in the polymerization activity (23-27). Electrons associated with the rings influence the metal, modifying its propensity to attack carbon-carbon double bonds of the olefins. The activities of these metallocenes combined by aluminum alkyls, however, are too low to be of commercial interest. Activation with methylaluminoxane, however, causes them to become 10-100 times more active than Ziegler-Natta catalysts. 

An enormous increase (factor up to 1 million) in activity was found in 1975 at the University of Hamburg when water was added in a ratio of A1(CH3)3 H20 = 1 2 and, in 1977, using the isolated reaction product of methylaluminoxane (MAO) together with titanocenes and zirconocenes (Cp2Ti(CH3)2, Cp2Zr(CH3)2, Cp2ZrCl2) as catalysts for ethene polymerization [26,27]. In these combinations, metallocenes become more active than commercially used Ziegler catalysts. 

Though metallocenes have been known since 1951 (5), it was not until the work of Kaminsky, Sinn and coworkers (6, 7) in the mid- and late-1970s that the enormous potential of metallocene single site catalysts was realized. The key discovery was the dramatic increase in activity resulting from use of methylaluminoxanes in place of diethylaluminum chloride and other conventional cocatalysts. Commercial use of metallocene single site catalysts began in the early 1990s. 

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. 

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. 

Published data on methylaluminoxane isolated from toluene have shown a wide range of molecular weights (300-3000 amu, primarily using cryoscopic methods). Possible reasons for the irreproducibility were proposed by Beard, et al. (23), who showed that cryoscopic molecular weight measurements of commercially available methylaluminoxane are influenced by several variables, such as process oils, residual toluene (solvent) and TMAL content. Beard reported "corrected" cryoscopic molecular weights of 850, suggesting x in eq 16 to be 15. 

Modified methylaluminoxanes (MMAO) have also been offered commercially since the early 1990s. MMAO (32) is a generic term encompassing all products wherein some of the methyl groups are replaced by other alkyl groups. The most commonly used modifiers are isobutyl and n-octyl groups. 

Most modified methylaluminoxanes are prepared by reaction with water (eq 6.3). There are several formulations of MMAO (differentiated by a suffix, e.g., "MMAO-3A"), each with different composition and properties. One commercially available MMAO is produced by the nonhydrolytic... 

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

Conventional MAO has very low solubility in aliphatic solvents as well as poor storage stability in solution, which considerably limits its utility. Other more soluble and commonly used aluminoxanes are ethylaluminoxane and isobutylaluminoxane, which are synthesized by the partial hydrolysis of triethyl-aluminum (TEA) and triisobutylaluminum (TIBA), respectively. However, these alkylaluminoxanes do not perform as well as MAO in metallocene-mediated olefin polymerization. " It was reported, however, that tetrakis(isooctyl) alumoxane [(i-octyl)2—O—Al-(i-octyl)2], prepared by reaction of Al(i-octyl)3 with 0.5 equiv of water, exhibits remarkable cocatalytic activity, comparable to or even greater than that obtained with MAO, for ethylene polymerization catalyzed by racemic an5a-bis(indenyl)-type zir-conocene dichlorides. Furthermore, commercial modified methylaluminoxanes (MMAO) available from... 

Ishihara et al. reported in 1986 that syndiotactic polystyrene can be prepared with the aid of organic or inorganic titanium compounds activated with methylaluminoxane [177]. There is much greater incentive to commercialize syndiotactic polystyrene than the isotactic one. This is because isotactic polystyrene crystallizes at a slow rate. That makes it impractical for many industrial applications. Syndiotactic polystyrene, on the other hand, crystallizes at a fast rate, has a melting point of 275°C, compared to 240°C for the isotactic one, and is suitable for use as a strong structural material. 

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