Cossee mechanism reactions

Figure 6.6 a Propagation cycle in the synthesis of polyethylene according to the Cossee mechanism b calculated energy/reaction coordinate diagram for the ethene insertion step. 

According to the Cossee mechanism, the two key steps in Ziegler—Natta polymerizations are monomer coordination and migratory insertion into the metal-polymer chain bond (Scheme 10). In analogy to heterogeneous Ziegler—Natta catalysis, a first-order reaction rate with respect to monomer concentration is generally assumed also for metallocene-based catalysts, that is. 

Equation 3, however, requires that monomer coordination is the rate-determining step. However, there are many observations which are very difficult to explain by a simple single-site Cossee mechanism, such as a reaction rate order higher than one reported for propene,ethene, styrene,and diene ° ° polymerizations. 

Cossee, P. Reaction mechanism of ethylene polymerization with heterogeneous Ziegler-Natta... 

All three reactions rely on repeated alkene insertion into an M—C bond to fomn new C—C bonds via the Cossee mechanism. The three types differ in the relative rates of chain growth (kg) by insertion to termination (k,), normally by elimination. If chain termination is very efficient, alkene dimerization may be seen. If it is very inefficient, a polymer will result, as in Ziegler-Natta and metallocene catalysis. In the intermediate case, oligomeric a olefins can be formed (Fig. 12.1), as in the SHOP process. Even though we discnss these reactions separately, they are nevertheless closely related. 

FIGURE 12.1 Relative rates of insertion and elimination determine the value of n in the products of di-, oligo-, and polymerization reactions in the Cossee mechanism. Slower elimination implies higher n. 

In the study on the olefin insertion, the most important step in the polymerization reaction, various methods are discussed and all of them support the Cossee mechanism and the presence of a-agostic effects [112, 113],... 

The basic mechanism of propylene polymerization is the same as that of ethylene polymerization. Here also the cocatalyst initiates polymerization by chain transfer reactions, which are then followed by propagation steps according to the Cossee mechanism. Finally, chain terminations occur either by j3-elimination or by deliberate addition of hydrogen. 

The catalytic cycle for oligomerization is basically the same as that for polymerization reaction by the Cossee mechanism (see Figure 6.4). The additional details shown here are the conversion of 6.51 to 6.48, and a chain transfer reaction (6.8.2.1). In the chain transfer reaction, 6.52 is converted to 6.53 by a concerted /3-hydride transfer, without the involvement of a hydride intermediate. The release of the a-alkene product from 6.53 regenerates 6.49. 

In the Ni-catalyzed oligomerization reaction, chain growth therefore takes place by the Cossee mechanism. However, there is another... 

Explain why (a) Cossee mechanism is not the only possible mechanism for chain propagation in ethylene oligomerization reaction (b) selectivity of the oligomerization of ethylene is expected to be a function of the electronic and steric properties of the P, O ligands (c) conversion of 6.46 to 6.47 is accompanied by the formation of styrene (d) the reaction of Ni(COD)j with appropriate phosphonium ylide and PPhj gives 6.46 (e) in ethylene oligomerization, product formation may take place by chain transfer (f) on replacement of PPhj in 6.46 by PMOj, there is an almost complete inhibition of the catalytic reaction. 

As shown by reactions 7.2.1.1 and 7.2.1.2, selective dimerization of ethylene may take place either by the Cossee mechanism or by the metallacycle mechanism. With the Ni catalysts, the Cossee mechanism operates. The experimental evidence for this is discussed later. 

The theoretical problems of the coordination mechanism of the propagation reaction were considered by Cossee et al. 170,186, 187). 

A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113), Arlman (114), and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers at which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. 

A generally accepted mechanism for the polymerization of ethylene by typical Ziegler-Natta catalysts has been proposed by Cossee (81). More recently a mechanism for transition metal hydrocarbyl catalyzed polymerization of olefins has been discussed (41, 42, 43) which is essentially similar to that shown in Reaction 5. 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

Polymerization takes place at the edges or corners of crystallites where metal atoms are necessarily coordinatively unsaturated. The reaction steps are those expected for a migratory alkyl transfer mechanism (Section 21-6) and has become known as the Cossee-Arlman mechanism ... 

The hydrovinylation reaction is suggested to proceed by an extension of the conventional Cossee-type mechanism addition of a Ni-H species to the alkene, insertion of a second alkene molecule into the resulting Ni-alkyl bond followed by P-W transfer with elimination of the product and regeneration of the hydride. 

The mechanism generally accepted for the chain-growth reaction of Scheme 2 is reported in Figure 4. Cossee... 

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