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

Physical properties

MAO is pyrophoric, and is violently reactive with any chemical bearing an acidic proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

. However, MAO is generally used as a solution in an (aromatic) hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 due to its relatively high solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

 in such hydrocarbons. Most commonly, MAO is sold as a solution in toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

 but it is also soluble in similar solvents such as xylene
Xylene
Xylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...

, cumene
Cumene
Cumene is the common name for isopropylbenzene, an organic compound that is an aromatic hydrocarbon. It is a constituent of crude oil and refined fuels. It is a flammable colorless liquid that has a boiling point of 152 °C...

, or mesitylene
Mesitylene
Mesitylene or 1,3,5-trimethylbenzene is an aromatic hydrocarbon with three methyl substituents attached to the benzene ring. It is prepared by distillation of acetone with sulfuric acid or by trimerization of propyne in sulfuric acid, which, in both cases, acts as a catalyst and dehydrating agent....

. Its solubility is largely dependent on the content of trimethylaluminium
Trimethylaluminium
Trimethylaluminium is the chemical compound with the formula Al26, abbreviated as Al2Me6, 2 or the abbreviation TMA. This pyrophoric, colorless liquid is an industrially important organoaluminium compound...

, 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.

Uses

MAO is most well known for being a co-catalyst for olefin polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

s of the Ziegler-Natta
Ziegler-Natta catalyst
A Ziegler–Natta catalyst is a catalyst used in the synthesis of polymers of 1-alkenes . Three types of Ziegler–Natta catalysts are currently employed:* Solid and supported catalysts based on titanium compounds...

 type. Natta and Ziegler utilised trimethylaluminium (TMA) as a co-catalyst, and it was not until the mid 1970’s that Kaminsky
Walter Kaminsky
Walter Kaminsky is a German chemist. His research dwells in olefin polymerization, and also in plastic recycling. He discovered the high activity of Group 4 metallocene/methylaluminoxane mixtures as catalysts for olefin polymerization in 1980.He was awarded, among other prizes, the 1999 Benjamin...

 discovered the utility of MAO for catalysis (see Kaminsky catalyst
Kaminsky catalyst
A Kaminsky catalyst is a catalytic system for alkene polymerization discovered in 1980. Kaminsky catalysts are based on metallocenes of group 4 transition metals with methylaluminoxane . These catalysts are a type of Ziegler-Natta catalyst, but they are homogeneous and show extremely high activity...

). 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
Scavenger (chemistry)
A scavenger in chemistry is a chemical substance added to a mixture in order to remove or inactivate impurities or unwanted reaction products. Their use is wide-ranged:...

.

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.

Further reading

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