Methylaluminoxane, commonly called MAO, is a mixture of organoaluminium compounds with the approximate formula (Al(CH3)O)n. It is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene,[1] Used in large excess, it activates precatalysts for alkene polymerization.

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

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

[6] The effect was discovered when a small amount of water was found to enhance the activity in the Ziegler–Natta system.

Second, it abstracts a ligand from the methylated precatalysts, forming an electrophilic, coordinatively unsaturated catalysts that can undergo ethylene insertion.