Modified MAO

A number of highly active ethylene polymerisation catalysts have resulted from the combination of functionalised NHC ligands with Ti, the first of these was the bis(phenolate)carbene ligated complex 3 [8], Upon activation with modified MAO (MMAO), this species gave an activity of 290 kg-mol bar h in the one test reported, making it one of the most active carbene-based olefin polymerisation catalysts known. In later work the same complex was evaluated with straight MAO activation, and activities of up to ca. 100 kg mol -bar" -h" were reported for linear polyethylene production [9],... 

The structure of MAO is poorly defined and varies with preparation conditions, but it performs well in activating the metallocene initiator. The use of MAO is complicated by its lack of long-term storage stability. It is usually supplied by manufacturers as a cloudy solution of MAO in toluene MAO has very low solubility in aliphatic solvents. Precipitation is often observed on long standing, especially if the container is frequently opened and exposed to moisture and oxygen. This precipitation, if not too extensive, may not affect the utility of the MAO as a coinitiator. A modified MAO, known as MMAO, offers some improvement in storage stability and improved solubility in aliphatics. MMAO is prepared by controlled hydrolysis of a mixture of trimethylaluminum and triisobutylaluminum. 

Godward Univ. of California, Berkeley) and a team of researchers used a laser-heated diamond cel) (modified Mao-Bel type) in their research on the ullrahigh-pressure melting of lead. This study is pan of a project to characterize materials at ultrahigh pressures. Such data can be useful for a wide range of applications in (he planetary sciences and the physics of condensed matter,... 

Modified MAOs (MMAO) that incorporate Bu and other alkyl groups to improve stability and optimize catalytic activity are commercially available. The use of additional modifiers such as boranes, boroxines, and pentafiuorophenyl derivatives has been investigated in recent years. MAO has also been supported on silica and other materials for the preparation of heterogeneous aUcene polymerization catalysts. These developments are detailed in several reviews. " ... 

A series of bis(/3-ketoiminate)zirconium complexes with a general structure of 111 was synthesized via either salt metathesis or amine elimination.111-113 The molecular structures of both dichloride complexes (R = Ph, />-tolyl) show that the O, N, and Cl atoms are trans, cis, and as, respectively. Moderate activity is observed for the complexes of this type for ethylene oligomerization when activated with an alkylaluminum chloride114 and for polymerization of ethylene when activated with modified MAO.1... 

Treating MAO with a small quantity of />hydro-quinone results in formation of another modified MAO which is insoluble in toluene. This modified MAO, however, cannot activate metallocenes alone, and additional TIBA is required to render the system active for olefin polymerization. 

The supported MAO may be chemically modified when supported. MAO supported on silica or alumina was taken up in tetrahydrofuran (THE) and reacted with a cross-linking agent such as bisphenol A (4,4 -isopropylidenediphenol). A one-step method reacts the MAO and modifier simultaneously in the support step. The modified MAO support, reacted with Et-(Ind)2ZrCl2, polymerizes propylene to spherical polymer particles while the unmodified support forms coarse agglomerates. Pentafluorophenol has also been used to modify the supported alumoxane, probably converting it to (—A1(0)(0C6F5)—) . ... 

Finally, stereoblock polypropylene has been prepared with living catalysts. In one case, a change in reaction medium was used to alter the stereospecificity. Shiono and co-workers synthesized syndiotactic-atactic diblock polypropylene using the living halfmetallocene catalyst precursor in Equation 22.15. In heptane solvent at 0 °C, the combination of this complex and modified MAO (MMAO) produces moderately syndiotactic polypropylene ([rr] = 0.72), and in chlorobenzene it produces atactic polypropylene ([rr] = 0.43). Thus, adding chlorobenzene to an ongoing polymerization in heptane converts the relative stereochemistry of the inserted monomers from syndiotactic to atactic, and a diblock polymer is formed. 

Similar safety considerations apply to modified MAO, although when solvent has been removed these materials are often glasses or viscous oils. Modified MAO solutions in light alkanes such as isopentane may form dangerous mixtures with air because of the volatility of the solvent and the presence of an intrinsic source of ignition. 

Modified MAO, containing methyl and f-butyl groups Number average molecular weight Weight average molecular weight Polydispersity index (M v/Mn)... 

BI°"P FeClj, 2 BP P FeCl2, MMAO modified MAO, dibenzhydryl-6-methyl phenyl is one of the aryl entities... 

On the other hand, much research has been carried out not only on finding excellent second components but also on modifying MAO as a cocatalyst. Exxon has discovered that anionic reagents like B(C6F5)4 could act on metallocene as a catalyst and form real active cationic sites. 

Due to the widespread usa of alumimrm alkyls in olefin polymerization reactions (e.g., as water scavenger or for catalyst alkylation), it is often difficult to assess the influence of changes in the reaction conditions on a specific observance in the copolymetization behavior. The choice of an appropriate aluminum compound is cmcial for a satisfartory incorporation of polar monomers into polyolefins. MAO, often used for catalyst activation, is not always able to proted the catalyst from deactivation.This can be attributed to insufficient com-plexation " and a nonuniform composition of the commerdal MAO solutions. The contained trimethylalumi-num (TMA) can also lead to increased chain transfer from the catalyst to the present aluminum compoimds. Hence, isobutyl modified MAO, for example, exhibits better protective characteristics. Therefore, and due to the difficult examination of MAO containing reaction mixtures, the addition of further well-defined aluminum compounds like TMA, triethy-laluminum (TEAL), tri-n-butylaluminum (TNBA), TIBA, and tri-/i-octylaluminum (TOA) has been examined. 

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