Methylalumoxane

Step 1 CP2Z1CI2 is converted to the active catalyst by reaction with the promoter methylalumoxane (MAO). A methyl group from MAO displaces one of the chlorine ligands of Cp2ZrCl2. The second chlorine is lost as chloride by ionization, giving a positively charged metallocene. 

Methylalumoxane as a highly Lewis-acidic reagent for organic synthesis [107]... 

The polymerization of methyl methacrylate (MMA) by Cu(ll) amidinate complexes (Scheme 222) in combination with alkyl aluminum complexes has been reported. The preferred alkylating agent is methylalumoxane (MAO) in... 

Different kinds of homogeneous catalysts based on group 4 metallocene-MAO (MAO = methylalumoxane) systems have been discovered. Depending on the kind of metallocene k-ligands, these systems present completely different... 

We have discussed the structure and synthesis of the library of molecular catalysts for polymerization in Section 11.5.1. In the present section we want to take a closer look at the performance of the catalyst library and discuss the results obtained [87], The entire catalyst library was screened in a parallel autoclave bench with exchangeable autoclave cups and stirrers so as to remove the bottleneck of the entire workflow. Ethylene was the polymerizable monomer that was introduced as a gas, the molecular catalyst was dissolved in toluene and activated by methylalumoxane (MAO), the metal to MAO ratio was 5000. All reactions were carried out at 50°C at a total pressure of 10 bar. The activity of the catalysts was determined by measuring the gas uptake during the reaction and the weight of the obtained polymer. Figure 11.40 gives an overview of the catalytic performance of the entire library of catalysts prepared. It can clearly be seen that different metals display different activities. The following order can be observed for the activity of the different metals Fe(III) > Fe(II) > Cr(II) > Co(II) > Ni(II) > Cr(III). Apparently iron catalysts are far more active than any of the other central metal... 

A new development in silsesquioxane ehemistry is the eombination of sil-sesquioxanes with cyclopentadienyl-type ligands. Reeently, several synthetie routes leading to silsesquioxane-tethered fluorene ligands have been developed. The scenario is illustrated in Seheme 47. A straightforward aeeess to the new ligand 140 involves the 1 1 reaction of 2 with 9-triethoxysilylmethylfluorene. Alternatively, the chloromethyl-substituted c/oxo-silsesquioxane derivative 141 can be prepared first and treated subsequently with lithium fluorenide to afford 140. Compound 141 has been used as starting material for the preparation of the trimethylsilyl and tri-methylstannyl derivatives 142 and 143, respeetively, as well as the novel zirconoeene complex 144. When activated with MAO (methylalumoxane), 144 yields an active ethylene polymerization system. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

Metallocenes are of low activity unless they are used with a noncoordinating anion such as a methylalumoxane or borate. This noncoordinating anion can be anchored to a silica surface. 

Methylalumoxane (MAO) (structure 5.52) is the most widely utilized counterion. MAO is an oligomeric material with the following approximate structure ... 

The second component is a special alumina-organic compound, methylalumoxane (MAO), that is prepared by partial hydrolysis of trimethylaluminum and that contains linear as well as cyclic structures in the molecules. 

A 20-liter reactor was charged with 3.11 liters of triisobutylaluminum isododecane solution (110g/l) and 800 ml of 30 wt% methylalumoxane toluene solution and then stirred at 50°C for 1 hour. This mixture was then treated with dimethylsilyl (2-methyl-l-indenyl)-7-(2,5-dimethylcyclopenta[l,2-b 4,3-b-,]-dithiophene) zirconium dichloride (13.1 mmol) suspended in isododecane (500 g) and stirred 1 hour at 50°C. The reaction mixture was then diluted with 520 ml of isododecane so that the final concentration of the catalyst mixture was 100 g/1. The catalyst was then used immediately. 

Examples of alumoxanes suitable as activating co-catalysts in the catalysts system are methylalumoxane, isobutylalumoxane, 2,4,4-trimethyl-pentylalumoxane, and 2-methyl-pentylalumoxane. Mixtures of different alumoxanes can also be used (25). Alumoxanes have a core structure analogous to boehmite, i.e., a sequence of -(Al-O)n-, either linear or also as rings. 

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