Methylalumoxane

Methylalumoxane (MAO) is a class of oligomeric aluminoxanes widely used as a cocatalyst and activator in olefin polymerization catalysis, particularly with metallocene and related catalysts. It is typically prepared by partial hydrolysis of trimethylaluminium in hydrocarbon solvents, yielding a complex, highly reactive mixture of aluminoxane species containing methyl groups.

Structure and composition of MAO are not those of a single defined compound but of a dynamic distribution of oligomeric units. The prevailing view describes the species as polymeric Al–O–Me networks with varying degrees of polymerization and methylation. Consequently, the general formulation is often simplified as a repeating Al(O–Me) unit, but MAO realmente consists of a mixture of methylalumoxane fragments whose exact size and structure depend on preparation and conditions.

Function and mechanism: MAO serves two roles in polymerization. It acts as a scavenger for impurities such as water and oxygen, which can deactivate catalysts, and it acts as a chemical activator for metallocene catalysts by generating highly electrophilic cationic metal centers accompanied by a non-coordinating aluminoxane counteranion. In practice, MAO helps convert precatalysts into active catalytic species, though the precise nature of the active species remains a matter of research due to MAO’s complex speciation.

Industrial context and handling: MAO is widely used in the production of polyolefins, often in combination with zirconocene or hafnocene catalysts. It is expensive and moisture-sensitive, typically handled under inert atmosphere in hydrocarbon solvents, and it is highly reactive and pyrophoric, requiring appropriate safety precautions.