Group I—III Metal Component

The group I III metal component increases activity and stereoselectivity by alkylation (often together with reduction) (Eqs. 8-28 through 8-32) of the transition metal component to form more active and stereoselective reaction sites. The group I—III metal component may also be involved in stabilizing the active transition metal sites by complexation and may be... 

If the group I-III metal component does not compete with monomer for the active sites, then... 

The derivations of Eqs. 8-48 through 8-50 assume that transfer to hydrogen does not involve adsorption of hydrogen at the active sites prior to transfer. If hydrogen competes with monomer and the group I-III metal component for adsorption at the active sites, the treatment described above requires modification of 0A and m and the introduction of H2. 

The decay type behavior observed in some cases may be due to active site destruction. This can be due to thermal deactivation or further reduction of the transition metal by the group I-III metal component. The decay type kinetics is explained later while discussing the mechanism of polymerization in terms of the deactivation of some of the active sites. 

None of the above reactions terminates the kinetic chain. All are treated as chain transfer reactions since there is reinitiation of new propagating chains. The relative extents of the various termination reactions depend on the monomer, identity and concentrations of the initiator components, temperature, and other reaction conditions. There are considerable differences in the efficiencies of chain transfer to different Group I-III metal components for example, diethylzinc is much more effective in chain transfer compared to triethylaluminum. Molecular hydrogen is a highly effective chain-transfer agent and is commonly used for molecular weight control in the industrial production of polypropylene. 

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