The polymerization of olefin by transition-metallic compounds activated by methylaluminoxane (MAO) shows a higher polymerization activity and production increase than the classical Ziegler-Natta catalysts . MAO as activator has different functions. In a first step it alkylates the metallocene dichloride, than it forms a polymerization active ion pair by abstraction of a chloride or methyl anion from the metallocene, and it reactivates polymerization inactive (sleeping) transition metal carbon bridged aluminum complexes. MAO is more active than ethylaluminoxane or other aluminum oxygen compounds. A high excess of MAO is necessary mainly to destroy impurities.<br/>Metallocene/MAO-catalysts are soluble in hydrocarbons, show only one type of active sites (single site catalysts), and can easily be modified in their chemical structure. These properties make it possible to predict the properties of the resulting polyolefin very accurately from the knowledge of the structure of the catalyst, and thus to control molecular weight and distribution, comonomer content, and tacticity by careful selection of the appropriate conditions. The single site character of metallocene-based catalysts leads to a better understanding of the mechanism of the olefin polymerization. Important for the polymerization activity is the formation of bulky anions, which have a weak bonding to the metallocene cation. Knowing that the bulky structure of MAO is necessary for the activation of metallocene catalysts, other bulky and weakly coordinating activators such as pentafluorophenylborate were introduced.<br/>In solution the activator MAO can be absorbed perfectly on the surface of particles or fibers especially on nanomaterials. It can react with the OH-groups of silica, or other supports. After adding the metallocene and ethene or propene polyolefin nanocomposites are formed (in-situ polymerization).<br/>The development and commercialization of MAO activated catalysts have just started and have already expanded the product range of polyolefin. In the future polyolefin nanocomposites and copolymers with polar groups open up the approach to new classes of materials with great property combinations. These materials show exciting physical and chemical properties such as improved stiffness, high gas barrier properties, significant flame retardancy, and high crystallization rates. The search to find other cheaper, bulky activators for polymerization active catalysts will be an important task for the future.<br/>

Methylaluminoxane as activator for olefin polymerization catalysts

253rd National Meeting (2017)

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