Initiation of Polymerization at the Active Center

Another unique feature of the Phillips catalyst is the mechanism responsible for the onset of the polymerization reaction. 

Unlike Ziegler-type catalysts, based mostly on titanium, where an aluminum alkyl cocatalyst is responsible for the formation of a Ti-alkyl bond and initiation of the ethylene polymerization process, chromium-based catalysts do not require such a cocatalyst. 

The chromium center may be initiated by formation of a chromium-hydrogen bond (Cr + H-source — Cr-H) to provide a Cr-hydride, which then reacts with ethylene (Cr-H + CH2=CH2- Cr-CH -CHj) to form a Cr-alkyl to initiate the polymerization process. One possible source of the hydrogen atom is a surface hydroxyl group (Si-OH) known to be present on the silica surface. 

As pointed out by Groppo and coworkers [9], if one assumes that a Cr-alkyl species is required in order to initiate the polymerization process, then ethylene plays three important roles as (i) a reducing agent to create a Cr species in a lower oxidation state as a coordinatively unsaturated active chromium precursor, (ii) as an alkylation agent, and (iii) as a monomer for chain propagation. 

Intermediate 1 results from the oxidative addition of two ethylene molecules to the coordinately unsaturated Cr(II) center. Formation of Intermediate 2 involves a beta-hydride transfer to the chromium center from 

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