Methylaluminoxane: Difference between revisions

| verifiedrevid = 422874719

| CASNo = 120144-90-3

| CASNo_Ref = {{Cascite|changed|CAS}}

| ChemSpiderID = 10687336

| PubChem = 16685262

| EC_number = 485-360-0

| StdInChI=1S/CH3.Al.O/h1H3;;

| StdInChIKey = CPOFMOWDMVWCLF-UHFFFAOYSA-N

| SMILES = C[Al]=O

}}

| Formula = (Al(CH₃)_xO_y)_''n''

| Appearance = White solid

}}

| MainHazards = [[Pyrophoric]]

| GHSPictograms = {{GHS02}}

| GHSSignalWord = Warning

| HPhrases = {{H-phrases|228|250|252}}

| PPhrases = {{P-phrases|210|222|235+410|240|241|280|302+334|370+378|407|413|420|422}}

}}

'''Methylaluminoxane''', commonly called '''MAO''', is a mixture of [[organoaluminium compound]]s with the approximate formula (Al(CH₃)O)_''n''.<ref name=chen>{{cite journal | title = Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure-Activity Relationships |author1=Chen, E. Y.-X. |author2=Marks, T. J. | journal = [[Chem. Rev.]] | year = 2000 | volume = 100 | issue = 4 | pages = 1391–1434 | doi = 10.1021/cr980462j | pmid=11749269}}</ref> It is usually encountered as a solution in ([[aromatic]]) [[solvent]]s, commonly [[toluene]] but also [[xylene]], [[cumene]], or [[mesitylene]],<ref name=AlbemarleDatasheet>{{cite web|url=http://www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf|title=MAO Datasheet|publisher=Albemarle|archive-url=https://web.archive.org/web/20040411145640/http://www.albemarle.com/acrofiles/sc2008f_MAO_datasheet.pdf|archive-date=2004-04-11}}</ref> Used in large excess, it activates precatalysts for alkene polymerization.<ref>{{cite journal |doi=10.1016/S0926-860X(01)00829-8|title=Metallocene catalysis|year=2001|last1=Kaminsky|first1=W.|last2=Laban|first2=A.|journal=Applied Catalysis A: General|volume=222|issue=1–2|pages=47–61}}</ref><ref>{{cite journal |doi=10.1039/A800056E|title=Highly active metallocene catalysts for olefin polymerization|year=1998|last1=Kaminsky|first1=Walter|journal=Journal of the Chemical Society, Dalton Transactions|issue=9|pages=1413–1418}}</ref>

==Preparation and structure==

MAO is prepared by the incomplete [[hydrolysis]] of [[trimethylaluminium]], as indicated by this idealized equation<ref>[http://www.freepatentsonline.com/EP0623624.html Process for the preparation of aluminoxanes – Patent EP0623624<!-- Bot generated title -->]</ref>

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

Diverse mechanisms have been proposed for the formation of MAO.<ref>{{cite journal | title = Methyaluminoxane (MAO) Polymerization Mechanism and Kinetic Model from Ab Initio Molecular Dynamics and Electronic Structure Calculations |author1=Lacramioara Negureanu |author2=Randall W. Hall |author3=Leslie G. Butler |author4=Larry A. Simeral |name-list-style=amp | journal = [[J. Am. Chem. Soc.]] | year = 2006 | volume = 128 | pages = 16816–16826 | doi = 10.1021/ja064545q | pmid = 17177432 | issue = 51}}</ref> Well defined analogues of MAO can be generated with tert-butyl substituents.<ref>{{cite journal |doi=10.1021/om00020a011|title=Tert-Butylaluminum Hydroxides and Oxides: Structural Relationship between Alkylalumoxanes and Alumina Gels|year=1994|last1=Harlan|first1=C. Jeff|last2=Mason|first2=Mark R.|last3=Barron|first3=Andrew R.|journal=Organometallics|volume=13|issue=8|pages=2957–2969}}</ref><ref>{{cite journal |doi=10.1021/ja00065a005|title=Hydrolysis of tri-''tert''-Butylaluminum: The First Structural Characterization of Alkylalumoxanes &#91;(R2Al)2O&#93;n and (RAlO)n|year=1993|last1=Mason|first1=Mark R.|last2=Smith|first2=Janna M.|last3=Bott|first3=Simon G.|last4=Barron|first4=Andrew R.|journal=Journal of the American Chemical Society|volume=115|issue=12|pages=4971–4984}}</ref>

[[File:Al6oxaneBarron.svg|thumb|132px|Aluminoxane with OH groups (R = ''tert''-Bu).]]

MAO is well known as [[catalyst activator]] for [[olefin]] [[polymerization]]s by [[homogeneous catalysis]]. In traditional [[Ziegler–Natta catalyst|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 [[Walter Kaminsky|Kaminsky]] discovered that metallocene dichlorides can be activated by MAO (see [[Kaminsky catalyst]]).<ref>{{cite journal |author1=A. Andresen |author2=H.G. Cordes |author3=J. Herwig |author4=W. Kaminsky |author5=A. Merck |author6=R. Mottweiler |author7=J. Pein |author8=H. Sinn |author9=H.J. Vollmer | title = Halogen-free Soluble Ziegler-Catalysts for the Polymerization of Ethylene | journal = [[Angew. Chem. Int. Ed.]] | volume = 15 | year = 1976 | pages = 630 | doi = 10.1002/anie.197606301 | issue = 10}}</ref> 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. <ref name=Sinn1>{{cite journal | title = 'Living Polymers' on Polymerization with Extremely Productive Ziegler Catalysts | journal = [[Angewandte Chemie International Edition in English]] | volume = 19 | issue = 5 | year = 1980 | pages = 390–392 |author1=Hansjörg Sinn |author2=Walter Kaminsky |author3=Hans-Jürgen Vollmer |author4=Rüdiger Woldt | doi = 10.1002/anie.198003901 }}</ref> MAO also functions as [[Scavenger (chemistry)|scavenger]] for protic impurities.

# {{cite journal | title = Theoretical studies of the structure and function of MAO (methylaluminoxane) |author1=Ziegler, T. |author2=Zurek, E. | journal = [[Progress in Polymer Science]] | volume = 29 | issue = 2 | year = 2004 | pages = 107–198 | doi = 10.1016/j.progpolymsci.2003.10.003}}

[[Category:Pyrophoric materials]]