Mechanism of Polymerization with Supported Chromium Catalysts

5 Mechanism of Polymerization with Supported Chromium Catalysts 

The initial step in the mechanism of ethylene polymerization using Phillips catalysts is believed to occur by way of an oxidation-reduction reaction between Cr (VI) and ethylene as depicted in eq 5.1. This generates Cr (II) and vacant coordination sites. As mentioned above, polymerization may be initially slow because of sluggish reduction or desorption of the oxidation by-products which can coordinate with (and block) active centers. 

It is speculated that a chromium hydride or alkyl species is formed, but the exact mechanism for its formation is not known, though approximately six decades have elapsed since the basic discoveries of Hogan and Banks. The hydride (or alkyl) initially forms a n-complex with ethylene. The ii-complex collapses to an alkylchromium moiety that serves as the active center for the polymerization (eq 5.2). 

Chain termination occurs primarily by j8-elimination with hydride transfer to chromium and by /3-elimination with hydride transfer to monomer. These terminations are analogous to those previously shown for Ziegler-Natta polymerizations (see eq 3.8 and 3.9 in Chapter 3). In some cases, supported chromium catalysts, e.g., chromocene on SiO and Cr on AlPO, are responsive to hydrogen 

Polymerizations of ethylene using chromium catalysts have been reviewed in 

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