Methylaluminoxane
Methylaluminoxane or MAO is a cocatalyst introduced by Walter Kaminsky and Hansjörg Sinn for highly active Ziegler-Natta catalysts ( Kaminsky catalysts ) based on metallocene .
Manufacturing
MAO is produced by the partial hydrolysis of trimethylaluminum with hydrated salts (e.g. Al 2 (SO 4 ) 3 · 18 H 2 O) or by the controlled addition of ice.
A defined structure for MAO cannot be given, it is rather a mixture of linear, cyclic and complex polycyclic units. These oligomers have an average molar mass of approx. 1000 g · mol −1 .
properties
MAO is pyrophoric and also reacts violently with any substance that carries acidic protons. MAO is used in solution of aromatic hydrocarbons due to its good solubility. It is usually sold as a solution in toluene , but xylene , cumene or mesitylene are also good solvents. The solubility of MAO depends heavily on the trimethylaluminum content , which is usually present as a precursor in proportions of up to five percent by weight of the solution. An MAO-toluene solution is clear to cloudy and reacts with air to form thick smoke.
The structure of MAO is little known; it is believed that it forms a large number of different oligomers in solution.
application
MAO is used as cocatalyst for olefin - polymerization with metallocene complexes used. It is assumed that MAO converts the metallocene dichloride into a highly active, cationic metallocene methyl species by dimethylation and subsequent abstraction of a methyl anion .
In the Ziegler-Natta process, methylaluminoxane has three functions:
- It alkylates the transition metal
- It acts as a Lewis acid and creates an unoccupied coordination site by splitting off a halide or alkyl anion
- It acts as a cleaning agent by reacting with impurities (H 2 O, CO 2 )
Individual evidence
- ↑ Datasheet of the Albemarle Corporation ( Memento of the original from September 27, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf ( Memento of the original from September 27, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Chen, EY-X .; Marks, TJ: Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure-Activity Relationships . In: Chem Rev.. . 100, No. 4, 2000, pp. 1391-1434. doi : 10.1021 / cr980462j . PMID 11749269 .
- ↑ Entry on methylaluminoxane. In: Römpp Online . Georg Thieme Verlag, accessed on January 2, 2015.
