Methylaluminoxane

Methylaluminoxane, commonly called MAO, is an organoaluminium compound with the approximate formula (Al(CH3)O)n.[1] Although it is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene,[2] it can be isolated as a white pyrophoric solid. It is used to activate precatalysts for alkene polymerization.

Methylaluminoxane
Identifiers
  • 120144-90-3
Properties
(Al(CH3)xOy)n
Appearance White solid
Hazards
Main hazards Pyrophoric
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation and structure

MAO is prepared by the incomplete hydrolysis of trimethylaluminium, as indicated by this idealized equation[3]

n Al(CH3)3 + n H2O → (Al(CH3)O)n + 2n CH4

Mechanisms have been proposed for the formation of MAO.[4]

Uses

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).[5] 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. [6] MAO also functions as scavenger for protic impurities.

Alternatives

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.

See also

References
  1. 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.
  2. "MAO Datasheet" (PDF). Albemarle. Archived from the original (PDF) on 2004-04-11.
  3. Process for the preparation of aluminoxanes - Patent EP0623624
  4. 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.
  5. 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.
  6. 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.

Further reading
  1. Ziegler, T.; Zurek, E. (2004). "Theoretical studies of the structure and function of MAO (methylaluminoxane)". Progress in Polymer Science. 29 (2): 107–198. doi:10.1016/j.progpolymsci.2003.10.003.
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