Methylaluminoxane

Methylaluminoxane, commonly called MAO, is a white pyrophoric solid. This means that it bursts into flames when exposed to air or water. It is violently reactive with any chemical bearing an active hydrogen. However, MAO is rarely dealt with as a solid. Generally it is placed in solution with an aromatic hydrocarbon (due to its high solubility in such hydrocarbons). Most commonly, MAO is sold in solution with toluene but it is also soluble in similar chemicals such as xylene, cumene , and mesitylene. Its solubility is largely dependant on the content of trimethyl aluminium. The toluene solution is clear to cloudy and reacts with air at the surface giving off a dense smoke.¹

The table shown below is data from a specific Albemarle Corporation solution. Albemarle is the top producer of MAO in the world.¹

MAO is most well known for being a co-catalyst for olefin polymerizations of the Ziegler-Natta type. Natta and Ziegler came up with the method of using trimethyl aluminum and a metallocene for single site catalysis of ethylene polymerization in 1957. They later received a Nobel Prize for their work.² It was not until the middle of the 1970’s that Kaminsky recognized the importance of MAO during the reaction. This was noticed when a small amount of water caused strong polymerizing activity in the system.³ For this reason, MAO is called a Ziegler-Natta catalyst as well as a Kaminsky catalyst.

It is important to note that the structure and action of MAO is not understood by the chemistry community. This is strongly due to the difficulty in isolating stable crystals. However, a common explanation shows an ion pair being formed by MAO and the metallocene.⁴

L¹L²MRMe + MAO - - > [L1L2MR] ⁺ [MeMAO] ^-

A model of the polymerization procedure is shown below.⁵

Due to the unknown mechanism of MAO, alternatives have been found in tetraarylborate salts such as [[tetrakis[3,5-bis(trifluoromethyl)phenyl]borate]] anion (BArF4- ). However, degradation of the borate anions has been a concern.

references

1) www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf [www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf]

2) nobelprize.org/chemistry/laureates/1963/natta-lecture.pdf [1]

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.

5) jordan-group.uchicago.edu/current_research.html [2]