Methylaluminoxane

Methylaluminoxane, commonly called MAO, is a white solid with the general formula (Al(CH₃)O)_n.

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Physical properties

MAO is pyrophoric, and is violently reactive with any chemical bearing an acidic proton. However, MAO is generally used as a solution in an (aromatic) hydrocarbon due to its relatively high solubility in such hydrocarbons. Most commonly, MAO is sold as a solution in toluene but it is also soluble in similar solvents such as xylene, cumene, or mesitylene. Its solubility is largely dependent on the content of trimethylaluminium, a precursor of MAO that is typically present as about five percent (by weight) of the solution. The toluene solution is clear to cloudy and reacts with air at the surface giving off a dense smoke.^[1]

MAO is a poorly-defined material, and probably adopts a number of structures in solution.^[2]

Preparation

MAO is prepared by a (controlled) hydrolysis of trimethylaluminium.^{[citation needed]}

Uses

MAO is most well known for being a co-catalyst for olefin polymerizations of the Ziegler-Natta type. Natta and Ziegler utilised trimethylaluminium (TMA) as a co-catalyst, and it was not until the mid 1970’s that Kaminsky discovered the utility of MAO for catalysis (see Kaminsky catalyst).^{[citation needed]} He noticed that a small amount of water enhanced the polymerizing activity in the Ziegler-Natta system and deduced that water must react with trimethylaluminum to give MAO^[3] It is believed that MAO alkylates and then activates the metal-chloride pre-catalyst species, forming an ion pair which should allow ethene insertion^[4]

In polymerisations MAO also functions as a scavenger.

Alternatives

Due to the unknown structure and mechanism of MAO, alternatives have been found in tetrakisperfluoroarylborate salts such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion (BAr^F₄⁻). Such well-defined activators may be used stoichiometrically, whereas MAO is typically present in a reaction mixture in approximately hundredfold to thousandfold excess.

See also

• Aluminoxane

References

1. ^ ^{a b} www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf [1]
2. ^ Chen, E. Y.-X., Marks, T. J., Chem. Rev., 2000, 100, 1391-1434.
3. ^ nobelprize.org/chemistry/laureates/1963/natta-lecture.pdf [2]
4. ^ Sinn, H., Kaminsky, W., Vollmer, H. J. Angew Chem Int. Engl. 1980, 19, 390.

Further reading

1. Ziegler, T., Zurek, E. Progress in Polymer Science. Vol. 29, Issue 2, Feb 2004, 107-198.

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