Methylaluminoxane

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

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

Prepration 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 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 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 two 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 a 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. ↑ www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf
  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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