Olefin alkene polymerization and dismutation on metals

Although the addition of olefins to aluminium hydrides is not readily reversible, exchange occurs between aluminium alkyl groups and olefins. Olefin insertions into the metal-alkyl bond extend the alkyl chain, followed by displacement by the reactant olefin to give relatively low molecular weight polymers (up to c, C20 linear alpha-olefins from ethylene). 

Phillips supported chromium (II) catalyst, the most commonly used for high density polyethylene (HDPE) manufacture, possibly behaves in a similar manner, but the olefin insertion reaction is faster by several orders of magnitude. In the original Zeigler catalyst systems for HDPE, an aluminium alkyl is used to reductively alkylate the primary component, most frequently a titanium compound, to give the true catalytic species. 

A similar scheme, albeit by activating supported complexes of titanium, zirconium, etc., appears to describe the behaviour of the new metallocene catalysts , claimed to allow greater control and a potentially exciting new range of polymer properties.  

Aluminium and other metal alkyls also activate tungsten and molybdenum compounds (particularly oxychlorides), to generate homogeneous or supported olefin dismutation catalysts. It is now believed that an initial M-alkyl (M = W, Mo) group is converted to a metal alkylidene group by a-hydrogen abstraction. Coordinated olefin now gives a metallocyclobutane (isolable in some cases)  

However, some oxides, such as supported Re207 (at 50-120°) or molybdenum oxide (at higher temperatures), show dismutation activity without alkyl treatment. Presumably, partial reduction of the surface by olefin exposes suitably liganded (M—O—M=CR2) centres. Further reduction (slow anion migration to the surface) leads to deactivation, but activity is restored by reoxidation. 

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