Methylaluminoxane, commonly called MAO, is a mixture of

organoaluminium compound Organoaluminium chemistry is the study of compounds containing bonds between carbon and aluminium. It is one of the major themes within organometallic chemistry. Illustrative organoaluminium compounds are the dimer trimethylaluminium, the monomer ...

s with the approximate formula (Al(CH₃)O)_''n''. It is usually encountered as a solution in (

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

)

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

s, commonly

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...

but also

xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are s ...

cumene Cumene (isopropylbenzene) is an organic compound that contains a benzene ring with an isopropyl substituent. It is a constituent of crude oil and refined fuels. It is a flammable colorless liquid that has a boiling point of 152 °C. Near ...

, or

mesitylene Mesitylene or 1,3,5-trimethylbenzene is a derivative of benzene with three methyl substituents positioned symmetrically around the ring. The other two isomeric trimethylbenzenes are 1,2,4-trimethylbenzene (pseudocumene) and 1,2,3-trimethylbenze ...

, Used in large excess, it activates precatalysts for alkene polymerization.

Preparation and structure

MAO is prepared by the incomplete

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...

trimethylaluminium Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( CH3)6 (abbreviated as Al2Me6 or TMA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industriall ...

, as indicated by this idealized equation :''n'' Al(CH₃)₃ + ''n'' H₂O → (Al(CH₃)O)_''n'' + 2''n'' CH₄ Diverse mechanisms have been proposed for the formation of MAO. Well defined analogues of MAO can be generated with tert-butyl substituents. Al6oxaneBarron

Uses

MAO is well known as catalyst activator for

olefin In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, a ...

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many f ...

s by

homogeneous catalysis In chemistry, homogeneous catalysis is catalysis by a soluble catalyst in a solution. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysi ...

. In traditional Ziegler–Natta catalysis, supported

titanium trichloride Titanium(III) chloride is the inorganic compound with the formula TiCl3. At least four distinct species have this formula; additionally hydrated derivatives are known. TiCl3 is one of the most common halides of titanium and is an important cataly ...

is activated by treatment with

(TMA). TMA only weakly activates homogeneous precatalysts, such as zirconacene dichloride. In the mid-1970s

Kaminsky Kaminsky is a surname with various origins. It may be derived from Czech/Slovak Kaminský, uk, Камінський, russian: Каминский, be, Камінскі, or Polish Kamiński. Feminine forms include Kaminská (Czech and Slovak), K ...

discovered that metallocene dichlorides can be activated by MAO (see

Kaminsky catalyst A Kaminsky catalyst is a catalytic system for alkene polymerization. Kaminsky catalysts are based on metallocenes of group 4 transition metals (Ti, Zr, Hf) activated with methylaluminoxane (MAO). These and other innovations have inspired develop ...

). The effect was discovered when a small amount of water was found to enhance the activity in the Ziegler–Natta system. MAO serves multiple functions in the activation process. First it alkylates the metal-chloride pre-catalyst species giving Ti/Zr-methyl intermediates. Second, it abstracts a ligand from the methylated precatalysts, forming an electrophilic, coordinatively unsaturated catalysts that can undergo ethylene insertion. This activated catalyst is an

ion pair In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each o ...

between a cationic catalyst and an weakly basic MAO-derived anion. MAO also functions as

scavenger Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feedin ...

for protic impurities.

Preparation and structure

Uses

See also

References

Further reading