Formula (Al(CH3)xOy)n | Appearance white solid | |
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Methylaluminoxane, commonly called MAO, is a white solid with the general formula (Al(CH3)O)n.
Contents
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.
MAO is a poorly defined material, and probably adopts a number of structures in solution.
However, recently a comprehensive and believable description of the mechanism of formation of MAO has been proposed. The proposed mechanism and structural models for MAO explain fundamental experimental evidence such as chemical composition and co-catalytic activity for this multicomponent material.
Preparation
MAO is prepared by a (controlled) hydrolysis of trimethylaluminium.
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-1970s that Kaminsky discovered the utility of MAO for catalysis (see Kaminsky catalyst). 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 It is believed that MAO alkylates and then activates the metal-chloride pre-catalyst species, forming an ion pair which should allow ethene insertion
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 (BArF4−). Such well-defined activators may be used stoichiometrically, whereas MAO is typically present in a reaction mixture in approximately hundredfold to thousandfold excess.