Methylaluminoxane

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Methylaluminoxane, commonly called MAO, is a white solid with the general formula (-Al(CH₃)O-)_n. 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 in solution with 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. The toluene solution is clear to cloudy and reacts with air at the surface giving off a dense smoke<math>.^1</math>

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. 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<math>.^3</math>

MAO is a poorly-defined material, and probably adopts a number of structures in solution. However, it is commonly believed that MAO alkylates and then activates the metal-chloride pre-catalyst species, forming an ion pair which should allow ethene insertion<math>.^4</math>

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 thousandfold excess.

See also

Kaminsky catalyst

References

• 1) www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf [1]
• 2) nobelprize.org/chemistry/laureates/1963/natta-lecture.pdf [2]
• 3) Sinn, H., Kaminsky, W., Vollmer, H. J. Angew Chem Int. Engl. 1980, 19, 390.
• 4) Ziegler, T., Zurek, E. Progress in Polymer Science. Vol. 29, Issue 2, Feb 2004, 107-198.