Methylalumoxane cocatalysts

Recently the authors invented another efficient, immobilized methylalumoxane cocatalyst partially hydrolyzed trimethylaluminum (PHT) that proved to be different from common methylalumoxanes. As a partial crystalline solid, PHT exhibits an octahedral environment for the aluminum atoms. PHT can be... 

Coates and coworkers Tian et al., 2001 have developed the fiuorinated phenoxy-imine complexes (e.g., 50) with MAO (methylalumoxane) cocatalyst. At O C this system produced highly syndiotactic polypropylene ([rrrr] = 0.96) with a... 

The above Cp2MQ2 species are immediate precursors for highly active Ziegler-Natta catalysts for die preparation of isotactic polypropylene with methylalumoxane as a cocatalyst die racemic mixtures can be directly employed in stereospecific propylene polymerizations (see Section IV,D,2) (225,227,228) the enantiomers are employed in asymmetric hydrooligomerizations ofa-olefins... 

Almost all group 4 metal complexes require a cocatalyst to generate an active metal-alkyl cationic species. Ordinary alkylaluminums - used in conventional Ziegler Natta catalysts - are insufiicient to activate these compounds on their own. The principal activator nsed is methylalumoxane (MAO), a structurally enigmatic material with a mixture of nuclearities. Its purpose is to alkylate the metal dichloride and to abstract one of the reactive hgands to form the ion pair active catalyst. The interaction is dynamic and a large excess of MAO is needed for effective catalyst performance, thus inhibiting a comprehensive characterization of these catalysts. 

Since the discovery of methylalumoxane as effective cocatalyst for the activation of metallocenes, many studies have been devoted to a better understanding of the nature of the active species and of... 

To understand the nearly unlimited versatility of metallocene complexes, it is necessary to take a closer look at the catalyst precursor and its activation process with the cocatalyst methylalumoxane (Scheme 1). 

Metallocene catalysts, for example dicyclopentadienyl zirconium dichloride, in combination with the cocatalyst methylalumoxane, helped establish a regulated set of properties, making it possible to customize molar mass, molar mass distribution, tacticity, heat resistance, rigidity, hardness, cold impact strength, and transparency. Added to these advantageous physical properties is the reactivity of this catalyst... 

Kaminsky, W., Kiilper, K., Brintzinger, H. H., and Wild, F. R. W. P. 1985. Polymerization of propene and butene with a chiral zirconocene and methylalumoxane as cocatalyst. Angewandte Chemie International Edition 24 507-508. 

The particular choice for 140 as a catalyst precursor was determined by its increased air stability and solubility in organic solvents. Initial studies were carried out using 1-dodecene with stoichiometric methylalumoxane (MAO) as a cocatalyst and carbometallating reagent, simultaneously showing almost complete conversion after 30 h. In order to clarify the role of McjAl as the methyl... 

The use of metallocenes and alumoxane as cocatalyst results in extremely high polymerization activities (see Tables 4 and 5). This system can easily be used on a laboratory scale. The methylalumoxane (MAO) is prepared by careful treatment of trimethylalumi-num with water [148] ... 

As shown by Tait under these conditions every zirconium atom forms an active complex and produces about 20000 polymer chains per hour. At temperatures above 50 °C, the zirconium catalyst is more active than the hafnium or titanium system the latter is decomposed by such temperatures. Transition metal compounds containing some halogene show a higher activity than systems that are totally free of halogen. Of the cocatalysts, methylalumoxane is much more effective than the ethylaluminoxane or isobutylalumoxane. 

Figure 4 Mechanism of the polymerization of olefins by zirconocenes. Step 1 The cocatalyst (MAO methylalumoxane) converst the catalyst after complexation into the active species that has a free coordination position for the monomer and stabilizes the latter. Step 2 The monomer (alkene) is allocated to the complex. Step 3 Insertion of the alkene into the zirconium alkyl bond and provision of a new free coordination position. Step 4 Repetition of Step 3 in a very short period of time (about 2000 propene molecules per catalyst molecule per second), thus rendering a polymer chain.

Thus, we demonstrate that norbomene-ethylene copolymers can be synthesized using comparatively simple nickel phosphorylide complexes. Although these copolymerization catalysts are less active than the systems based on zirconocene and methylalumoxane, they do not require any cocatalyst (e.g. alumoxane, which is an expensive chemical). Moreover, the lower oxophility of nickel in the ylide catalysts allows ethylene to be copolymerized with functionalized norbomenes. 

Herwig J, Kaminsky W (1983) Halogen free soluble Ziegler catalysts with methylalumoxane as cocatalyst. Polym Bull 9 464... 

The role of methylalumoxane (MAO) as a cocatalyst to activate zirconocene compounds such as Cp ZrCl to create a single-site ethylene polymerization catalyst is similar, in some respects, to the role simple aluminum alkyls (AlRj) play in activating Ziegler catalysts. For example, MAO acts as an alkylating agent to form the initial Zr-carbon bond (Zr-CH ) necessary to initiate the polymerization process. However, experimental evidence obtained by a variety of methods clearly has shown that the MAO reacts with the zirconium center to form a zirconium cation of the type [CpjZr-CHS] in which the zirconium is not reduced to a lower oxidation state, but remains as a d° metal and Zr(IV) oxidation state. The MAO, therefore, forms an anion moiety to complete the ion pair necessary to create the active species, as illustrated in Equation 4.1. 

Kamljord, Wester and Rytter [48] reported a method in which only the methylalumoxane material is reacted with silica that was previously calcined at 450°C for six hours. For example, 7 ml of 30 wt% MAO in toluene solution was added to 2.0 g of calcined silica at 20 C using an incipient wetness method in which the volume of the MAO solution used corresponds to the total pore volume of the dried silica. Next, the toluene was removed by evaporation to isolate a free-flowing silica/MAO intermediate reaction product. Then a solution of (BuCpl ZrCl (0.0076 g/0.019 mmol) in 3 ml of toluene is added to 2 g of the silica/MAO intermediate. The finished dry catalyst is obtained after the toluene is removed by evaporation. Analytical data found that the finished catalyst contains 0.09 wt% Zr and 22.0 wt% Al to provide an Al/Zr molar ratio of 870. The activity of this catalyst was 5,025 kg PE/mol Zr/hr using triethylaluminum as an external cocatalyst during a slurry ethylene homopolymerization process carried out at 70°C and 4 bar ethylene pressure. 

This silica/zirconium intermediate was then activated with modified methylalumoxane (MMAO) as cocatalyst. This approach produced various catalysts with an activity of 300-1300 kg PE/mol Zr/h/atm. The polyethylene had a relatively high molecular weight, i.e., > 200,000, and exhibited MyM values of 3-4.8, suggesting a somewhat broader molecular weight distribution than one single-site catalyst. This catalyst probably contained two or more similar single-site catalysts. 

EtAlCl or EtAlcyEt AlCl as cocatalyst. The melting point data of the four examples shown above suggest that the polyethylene with a homogeneous branching distribution is consistent with a single-site polymerization catalyst similar to catalysts discovered by Kaminsky in the late 1970s based on a zirconocene compound and methylalumoxane as cocatalyst. 

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