Green-Rooney proposal

Many questions remain about the initiation, propagation, and termination steps of the ethene polymerization mechanism. The most important models proposed to date are the Cossee model, which requires a vacant coordination site on the metal center in the position adjacent to the growing alkyl chain, where ethene is coordinated before insertion into the chain (628), and the Green-Rooney model, which requires the presence of a metal-carbene species and a vacant site where ethene is coordinated prior to insertion (629). 

Two major mechanisms have been proposed for alkene polymerization. These are the Cossee-Arlman mechanism and the Green-Rooney mechanism. A modified version of the latter has also been considered to explain the behavior of homogeneous, metallocene catalysts. The original Cossee-Arlman mechanism was proposed for the TiCl3 based heterogeneous catalyst. In the following sections we discuss these different mechanisms in some detail. In the following discussion in accordance with the results obtained from the metallocene systems, the oxidation states of the active surface sites are assumed to be 4+. 

Green and Rooney81 proposed an alternative mechanism (Scheme 11.13b) that also accounted for Z-N catalysis. The mechanism resembles a metathesis-like pathway by starting with a-elimination to give a metal-carbene hydride followed by cycloaddition with the alkene monomer to form a metallacyclobutane. Reductive elimination finally yields a new metal alkyl with two more carbon atoms in the growing chain. The Green-Rooney mechanism, although plausible overall, requires an a-elimination, a process that is difficult to demonstrate. 

As we saw in Section 2.2.2, an interaction between metal and the a-hydro-gen of an alkyl group is called an agostic interaction. In an extreme situation, where the interaction between the metal atom and the a-hydrogen leads to formal cleavage of the carbon-hydrogen bond, a mechanism involving a metal-carbene intermediate may be invoked. This proposal is known as the Green and Rooney mechanism, and two of the proposed catalytic intermediates are shown by 6.9 and 6.10. 

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