Methyl alumoxanes

The metallocene catalyst is used m combination with a promoter usually methyl alumoxane (MAO)... 

MacKenzie (4) prepared poly(ethylcne-co-carbon monoxide) at low pressure using a nickel-based catalyst, (IV), with methyl alumoxane. 

Recently, Ewen 1291 has found that the soluble Cp2Ti(Ph)2 (Ph = phenyl)/methyl-alumoxane catalyst produces isotactic polypropylene ([m] = 0.83-0.85) at temperatures below —30 °C. The polymerization was carried out in the temperature range of —85 to 50 °C. The highest activity was achieved at —45 °C, and the isotactic (meso) dyad fraction [m] of the produced polypropylene decreased from 0.85 to 0.50 with an increase in the polymerization temperature. Figure 28 shows the time dependence of polymer yield, Mn, number of polymer chain produced per titanium atom, [N], and mjmn, obtained at —60 °C. The yield of polymer is almost proportional to time, but Rln increases to a constant value. The number of polymer chains [N] increases with time, and the value of Mw/Mn increases toward 2.0, indicating that chain transfer... 

In Nd-carboxylate systems A1R3 and A1R2H cocatalysts are also replaced by alumoxanes, which were described by Sinn and Kaminsky et al. in 1980 [194-196]. Most of these studies focus on methyl alumoxane (MAO) [175,197-200]. Also tetraisobutyl dialumoxane (TIBAO) was investigated [175]. Differences between aluminum alkyl- and alumoxane-based cocatalysts are also addressed in Sect. 2.1.2. 

The titanium complexes 29 and 30 were found to polymerize ethylene after the addition of the Lewis add AlEtCl2- The reaction of 10 with methyl alumoxane resulted in the m-methyl bridged bimetallic compound 31, Eq. (31), which was able to polymerize ethylene in the absence of a Lewis acid cocatalyst, when dichloromethane was used as solvent... 

The Step 2 product was added to a flame-dried Schlenk flask with a stirrer and then backfilled three times with ethylene and 50 ml of toluene added. Stirring was begun, and methyl alumoxane dissolved in heptane added by syringe. After 30 minutes the reaction was quenched with acidified ethanol, filtered, dried in vacuum at 40° C for 10 hours, and the product was isolated. 

Erker, G Temme, B. Use of cholestanyhndene-derived nonbridged group 4 bent metaUocene/methyl alumoxane catalysts for stereoselective propene polymerization. J. Am. Chem. Soc. 1992, 114, 4004-4006. 

The research has varied the metal, the non-coordinating methyl ligands, and the coordinating anionic pi systems. The combination of trimethyl aluminum and water is considered to be an in-situ source of methyl alumoxane (MAO), which can both methylate the metallocene and form a metal cation. Common metallocene catalyst systems now use a zirconium dichloride and MAO. MAO is often represented by the formula (CH3A10) but this is an oversimplification and it is likely that MAO has a cage-Uke structure [10] and that the ratio of methyl to aluminum is greater than 1.0. 

Structural features depicting the evolution of metallocene catalysts from the 1950s to 1990s are illustrated [68] in Table 13. Bis-Cp structure when activated with methyl alumoxane could be commercialized in a limited way. Activity and MW capability of substituted bis-Cp metallocenes were substantially higher than that of the unsubstituted structure. The mono-Cp structure was further developed [68] which produced high MW ethylene copolymers with higher comonomer content. Bridged, substituted bis-Cp structures could polymerize propylene to an isotactic or syndio-... 

This single-component cationic metallocene catalysis is undoubtedly an extraordinary new development which eliminates the problems associated with the methyl alumoxane cocatalyst usage and has been represented as follows for its synthesis ... 

The many questions of the role of methyl alumoxane (MAO) to activate metallocene complexes and increase the magnitude of their ability to increase the polymerization of olefins by several orders have... 

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