Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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 an (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
Prepration and structure
- n Al(CH3)3 + n H2O → (Al(CH3)O)n + 2n CH4
Mechanisms have been proposed for the formation of MAO.
MAO is most well known for being a co-catalyst 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 that 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 two functions in the activation process. First it alkylate 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 tetrakis[3,5-bis(trifluoromethyl)phenyl]borate 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.
- Chen, E. Y.-X.; Marks, T. J. (2000). "Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure-Activity Relationships". Chem. Rev. 100 (4): 1391–1434. doi:10.1021/cr980462j. PMID 11749269.
- Process for the preparation of aluminoxanes - Patent EP0623624
- Lacramioara Negureanu; Randall W. Hall; Leslie G. Butler & Larry A. Simeral (2006). "Methyaluminoxane (MAO) Polymerization Mechanism and Kinetic Model from Ab Initio Molecular Dynamics and Electronic Structure Calculations". J. Am. Chem. Soc. 128 (51): 16816–16826. doi:10.1021/ja064545q. PMID 17177432.
- A. Andresen; H.G. Cordes; J. Herwig; W. Kaminsky; A. Merck; R. Mottweiler; J. Pein; H. Sinn; H.J. Vollmer (1976). "Halogen-free Soluble Ziegler-Catalysts for the Polymerization of Ethylene". Angew. Chem. Int. Ed. 15 (10): 630. doi:10.1002/anie.197606301.
- Hansjörg Sinn; Walter Kaminsky; Hans-Jürgen Vollmer; Rüdiger Woldt (1980). "'Living Polymers' on Polymerization with Extremely Productive Ziegler Catalysts". Angewandte Chemie International Edition in English. 19 (5): 390–392. doi:10.1002/anie.198003901.