Methylaluminoxane Explained

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.^{[2] [3]}

Preparation and structure

MAO is prepared by the incomplete hydrolysis of trimethylaluminium, as indicated by this idealized equation:^[4]

n Al(CH₃)₃ + n H₂O → (Al(CH₃)O)_n + 2n CH₄After many years of study, single crystals of an active MAO were analyzed by X-ray crystallography. The molecule adopts a ruffled sheet of tetrahedral Al centers linked by triply bridging oxides.^[5]

Uses

MAO is well known as 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).^[6] The effect was discovered when a small amount of water was found to enhance the activity in the Ziegler–Natta system.

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. ^[7] MAO also functions as scavenger for protic impurities.

Previous studies

Diverse mechanisms have been proposed for the formation of MAO and many structures as well.

• Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure-Activity Relationships . Chen, E. Y.-X. . Marks, T. J. . . 2000 . 100 . 4 . 1391–1434 . 10.1021/cr980462j . 11749269.
• Methyaluminoxane (MAO) Polymerization Mechanism and Kinetic Model from Ab Initio Molecular Dynamics and Electronic Structure Calculations . Lacramioara Negureanu . Randall W. Hall . Leslie G. Butler . Larry A. Simeral . amp . . 2006 . 128 . 16816–16826 . 10.1021/ja064545q . 17177432 . 51.
• 10.1021/om00020a011. Tert-Butylaluminum Hydroxides and Oxides: Structural Relationship between Alkylalumoxanes and Alumina Gels. 1994. Harlan. C. Jeff. Mason. Mark R.. Barron. Andrew R.. Organometallics. 13. 8. 2957–2969.
• 10.1021/ja00065a005. Hydrolysis of tri-tert-Butylaluminum: The First Structural Characterization of Alkylalumoxanes [(R2Al)2O]n and (RAlO)n. 1993. Mason. Mark R.. Smith. Janna M.. Bott. Simon G.. Barron. Andrew R.. Journal of the American Chemical Society. 115. 12. 4971–4984.
• Theoretical studies of the structure and function of MAO (methylaluminoxane) . Ziegler, T. . Zurek, E. . . 29 . 2 . 2004 . 107–198 . 10.1016/j.progpolymsci.2003.10.003.

See also

• Aluminoxane

References

1. Web site: MAO Datasheet. Albemarle. https://web.archive.org/web/20040411145640/http://www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf. 2004-04-11.
2. 10.1016/S0926-860X(01)00829-8. Metallocene catalysis. 2001. Kaminsky. W.. Laban. A.. Applied Catalysis A: General. 222. 1–2. 47–61.
3. 10.1039/A800056E. Highly active metallocene catalysts for olefin polymerization. 1998. Kaminsky. Walter. Journal of the Chemical Society, Dalton Transactions. 9. 1413–1418.
4. http://www.freepatentsonline.com/EP0623624.html Process for the preparation of aluminoxanes – Patent EP0623624
5. 10.1126/science.adm7305 . Structure of Methylaluminoxane (MAO): Extractable [Al(CH₃)₂]⁺ for Precatalyst Activation . 2024 . Luo . Lubin . Younker . Jarod M. . Zabula . Alexander V. . Science . 384 . 6703 . 1424–1428 . 2024Sci...384.1424L .
6. A. Andresen . H.G. Cordes . J. Herwig . W. Kaminsky . A. Merck . R. Mottweiler . J. Pein . H. Sinn . H.J. Vollmer . Halogen-free Soluble Ziegler-Catalysts for the Polymerization of Ethylene . . 15 . 1976 . 630 . 10.1002/anie.197606301 . 10.
7. 'Living Polymers' on Polymerization with Extremely Productive Ziegler Catalysts . . 19 . 5 . 1980 . 390–392 . Hansjörg Sinn . Walter Kaminsky . Hans-Jürgen Vollmer . Rüdiger Woldt . 10.1002/anie.198003901 .