Homogeneous polymerization Cossee mechanism

Figure 4. Schematic representation of Cossee-Arlman type reaction mechanism for homogeneous polymerization with metallocene-based catalysts.(Reprinted with permission from ref 51. Copyright 1994.)...

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+. 

Of these four, the majority of the research tends to support the transition metal-carbon center model, as the mechanistic scheme of choice. Using a homogeneous catalyst (Cp2TiEt2) as a model, Breslow and Newburg proposed that polymerization growth occurred at the Ti—C center. Somewhat later, Cossee extended this concept into a more elaborate mechanism for supported catalysts, which he substantiated with molecular orbital calculations (see Ref. 179, Chapt. 13). 

Three mechanisms have been proposed to explain metallocene-based homogeneous and Ziegler-Natta polymerization schemes. The Cossee-Arlman mechanism... 

In the mid-1980s NMR-based direct evidence for Cossee t rpe of mechanism came from NMR monitoring of a homogeneous catalytic reaction. The lanthanide complex 2.64 was used as the precatalyst for these studies. All these findings taken together validated the basic polymerization mechanism as outlined in Figure 6.4. 

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