Methylaluminoxane preparation

In an earlier investigation by the authors (1) an initiator mixture consisting of neodymium neodecanoate, methylaluminoxane, diisobutylaluminum, and diethylaluminum chloride was used to prepare poly(styrenc-co-butadiene) having greater than a 99% cis-1,4 bond content. 

A Ni based catalyst for addition polymerization can be prepared from nickel(2-ethylhexanoate), methylaluminoxane, and triethyl-borane (34). The polymerization is carried out in toluene as a solvent under pressure. 

The bulky methylaluminoxane anion stabilizes the coordinatively unsaturated metal cation. Stabilization by noncoordinating anions such as carbosilane dendrimers is also viable.571 Aluminoxanes, however, are required to be used in large excess to be effective. Alternatively, the active catalyst can also be prepared by reacting a metal dialkyl with fluorinated boranes, borate salts or aluminate salts. 

By changing the NHC ligands to NHCs possessing a hemilabile pyridine linkage, Jin and coworkers were able to use Ni(II) - NHC complexes as catalysts for the polymerization of norbornene and ethylene in the presence of methylaluminoxane (MAO) as a cocatalyst [47]. The Ni complexes were prepared via Scheme 7. Although the free carbenes of 16 could not be generated successfully, the desired Ni compounds (17) could be prepared via the... 

Using separate prepared methylaluminoxane (MAO) as cocatalyst together with metallocenes for olefin polymerization Kaminsky, Sinn... 

An interesting type of trivalent compound is CpM(7j4-diene)X2 (18-B-IX) X = Cl, Me, and triflate which can be prepared from CpMCLt and two equivalents of methylated allyl Grignard reagents in THF. In the presence of methylaluminoxane (MAO), these compounds polymerize ethylene85 similarly to the Ti analogues (Section 17-A-5). 

In a subsequent investigation by the author [1], high cis-content polybutadiene having a controlled molecular distribution was prepared using neodymium neodecanoate, AlH(i-C4H9)2, methylaluminoxane, and Al(i-C4H9)2C1. 

Polybutadiene and random copolymers of butadiene and isoprene both having cA-l,4-isoprene content exceeding 95% were prepared by the author [1,2], respectively, using the catalytic composition of the current invention. Polybutadiene chloride having a cw-1,4-content of not less than 90% was previously prepared by Sone [3] using methylaluminoxane, hydro genated diisobutylaluminum, neodymium tris(bis(2-ethylhexyl)phosphate), and magnesium. 

Low molecular weight high cw-butadiene content oligomers were prepared by Miller [4] using methylaluminoxane, neodymium(lll) versetate, and diisobutyl aluminum hydride. 

Poly(ethylene-co-norbomene-2,3-dicarboxylic acid anhydride) was prepared by co-polymerizing the respective monomers with the transition metal catalyst, di(3-t-butyl-2-hydroxy-l-(A-phenyhmino)benzene) titanium(IV). The polymerization was conducted at ambient temperature using methylaluminoxane as co-catalyst. After a 10 minute polymerization reaction scoping period 0.02 mol% of norbornene-2,3-dicarboxylic acid anhydride was incorporated into the co-polymer. 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

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. 

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... 

Soluble metallocene catalysts can be prepared by using hydrocarbon-soluble methylaluminoxane or organoborate activators, or their insoluble counterparts can be made by deposition of the metallocene onto silica or alumina solid acid activators [496]. In our experience, the differences in activity, and in the polymer, between soluble nonsupported and insoluble supported catalysts, are not great, which is another indication that under normal conditions the mass transport is not a major issue. 

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... 

The metallocene-ATRP route has been expanded by Matsugi etal. [164], to produce polyethylene-b-poly(methyl methacrylate). In the first step, hydroxyl-functionalized polyethylene was successfully prepared through the copolymerization of ethylene with aluminum-capped allyl alcohol, using a specific zirconium metaUocene/methylaluminoxane catalyst system. In the next step, the terminal alcohol was converted to hahde by 2-bromoisobutyryl bromide to obtain bromide-functionalized polyethylene, which could initiate the ATRP of MMA (Scheme 11.40). The block copolymers obtained exhibited unique morphological features that depended on the content of PMMA segment. Moreover, the block copolymers effectively compatibiUzed the corresponding homopolymer blend at the nanometer level. 

In addition, preparation of catalysts based on iron and cobalt [21] was also reported. These are complexes of bulky pyridine bis-imine ligands with iron or cobalt that are also activated by methylaluminoxane ... 

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. 

Zhang, Z., Guo, C.-Y., Cui, N., Ke, Y, and Hu, Y. 2004. Preparation of linear low density polyethylene by in situ copolymerization of ethylene with Zr supported on montmorillonite/ Fe/methylaluminoxane catalyst system. Journal of Applied Polymer Science 94 1690-1696. 

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