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 organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

)

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

s, commonly

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...

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 su ...

, cumene, or mesitylene, 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 reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

trimethylaluminium Trimethylaluminium or TMA is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula (abbreviated as , where Me stands for methyl), as it exists as a dimer. This colorless liquid is pyrophoric. It is an ...

, as indicated by this idealized equation: :''n'' Al(CH₃)₃ + ''n'' H₂O → (Al(CH₃)O)_''n'' + 2''n'' CH₄ After many years of study, single crystals of an active MAO were analyzed by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. The molecule adopts a ruffled sheet of tetrahedral Al centers linked by triply bridging oxides.

Uses

MAO is well known as catalyst activator for

olefin In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as α-olefins. The International Union of Pu ...

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 fo ...

s by

homogeneous catalysis In chemistry, homogeneous catalysis is catalysis where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast, heterogeneous catalysis describes processes where the catalysts and substrate are in d ...

. In traditional Ziegler–Natta catalysis, supported titanium trichloride is activated by treatment with

(TMA). TMA only weakly activates homogeneous precatalysts, such as

zirconocene dichloride Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air. Preparation and structure Zircono ...

. In the mid-1970s Kaminsky 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 ...

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

scavenger Scavengers are animals that consume Corpse decomposition, 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 he ...

for protic impurities.

Previous studies

Diverse mechanisms have been proposed for the formation of MAO and many structures as well. * * * * *{{cite journal , title = Theoretical studies of the structure and function of MAO (methylaluminoxane) , author1=Ziegler, T. , author2=Zurek, E. , journal = Progress in Polymer Science , volume = 29 , issue = 2 , year = 2004 , pages = 107–198 , doi = 10.1016/j.progpolymsci.2003.10.003

References

Polymer chemistry Aluminium compounds Catalysts Pyrophoric materials Methyl compounds

Preparation and structure

Uses

Previous studies

See also

References