Methylaluminoxane

Methylaluminoxane, commonly called MAO, is an organoaluminium compound with the approximate formula (Al(CH3)O)n. Although it is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene, it can be isolated as a white pyrophoric solid. It is used to activate precatalysts for alkene polymerization. Methylaluminoxane, commonly called MAO, is an organoaluminium compound with the approximate formula (Al(CH3)O)n. Although it is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene, it can be isolated as a white pyrophoric solid. It is used to activate precatalysts for alkene polymerization. MAO is prepared by the incomplete hydrolysis of trimethylaluminium, as indicated by this idealized equation Mechanisms have been proposed for the formation of MAO. MAO is most well known for being a catalyst activator for olefin polymerizations by homogeneous catalysis. In traditional Ziegler–Natta catalysis, supported titanium trichloride is activated by treatment with trimethylaluminium (TMA). TMA only weakly activates homogeneous precatalysts, such as zirconacene dichloride. In the mid-1970s Kaminsky discovered that metallocene dichlorides can be activated by MAO (see Kaminsky catalyst). The effect was discovered when he noticed that a small amount of water enhanced the polymerizing activity in the Ziegler–Natta system and deduced that water must react with trimethylaluminum to give MAO. 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 between a cationic catalyst and an weakly basic MAO-derived anion. MAO also functions as a scavenger for protic impurities. Due to the unknown structure and mechanism of MAO, alternatives have been found in tetrakisperfluoroarylborate salts such as tetrakisborate anion (BArF4−). Such well-defined activators may be used stoichiometrically, whereas MAO is typically present in a reaction mixture in approximately hundredfold to thousandfold excess.