aluminoxanes

Aluminoxanes are a class of organoaluminium compounds defined by aluminum–oxygen frameworks bearing alkyl substituents. They are formed by partial hydrolysis or controlled oxidation of trialkylaluminium reagents and exist in solution as complex mixtures of oligomeric species with Al–O–Al linkages. The most widely used member is methylaluminoxane (MAO). The precise molecular structures of aluminoxanes are not fully defined; they are described as a distribution of cage- and chain-like oligomers with repeating Al–O units and methyl ligands on aluminium.

Preparation and composition: MAO is typically prepared by controlled hydrolysis of trimethylaluminium in an inert organic solvent such as toluene, yielding a concentrated, highly reactive aluminoxane solution that contains MAO oligomers and residual AlMe3. Other aluminoxanes such as ethylaluminoxane (EAO) or tert-butyl aluminoxane (TBAO) are prepared in a similar manner from corresponding trialkylaluminium precursors. The exact stoichiometry and size of the oligomers depend on preparation conditions and solvent; hence aluminoxanes are treated as a generic cocatalyst with a broad distribution of species rather than a single defined compound.

Use and mechanism: In olefin polymerization, aluminoxanes act as cocatalysts for metallocene and other single-site catalysts. They generate cationic metal centers by abstracting an alkyl group from the precatalyst, forming a metal–cation paired with a non-coordinating aluminoxane-derived anion. They also scavenge impurities that would deactivate catalysts. The efficiency and activity of aluminoxanes depend on the aluminum content and the solvent; MAO is widely used but can be expensive and moisture-sensitive.

Handling and safety: Aluminoxanes are highly reactive to air and moisture, and most commercial MAO is supplied as a hydrocarbon solution under inert atmosphere. They are pyrophoric and must be handled with appropriate precautions.