Cossee’s mechanism

Figure 2.19 illustrates Cossee s mechanism for polymerization with coordination catalysts. The active site is depicted as having a coordination vacancy that attracts the electrons in the olefin rr-bond. Coordination is followed by insertion into the polymer chain (R) and the re-establishment of the coordination vacancy for further monomer insertion. This figure also shows an important characteristic of coordination polymerization that makes it very different from free-radical polymerization the monomer is inserted between the carbon-metal bond. As a consequence, the electronic and steric environment surrounding the transition metal has a huge influence on the kinetics of polymerization. This is why... 

Figure 2.19 Cossee s mechanism X are ligands, generally Cl atoms and R is the growing poiymer chain.

However, the possibility that the type of conformation of the monomer which can be coordinated on the catalytic complex in a cisoid or transoid conformation plays a role, cannot be excluded. This role is decisive in Cossee s mechanism (70), according to which a monodentate-transoid or bidentate-cisoid coordination of the diolefin is responsible for the formation of either trans or cis 1,4-units respectively in the polymer (Scheme 14). As for the mode of the addition of metal and growing chain to the entering unit, the results obtained by Porri and Aglietto (71) in the study of the stereospecific polymerization of cis,cis-l,4-di-... 

The actual mechanism by which the propagation occurs and the factors governing the formation of stereoregular polymers is state debatable. Among the several mechanisms proposed, the bimetallic mechanism of Natta and the monometallic mechanism of Cossee have received much attention. Cossee s... 

A related mechanism for the tetramerization of etfiylene has been deduced from studies on Sasol s chromium-catalyzed process that forms 1-octene. The catalyst is, again, thought to operate by a Cr(I)/Cr(III) cycle such as the one in Scheme 22.17. This mechanism was supported by several studies. For example, Cr(0) complexes of the PNP ligand [Cr(CO) (PNP)] were shown to be catalytically inactive, but the complexes formed from these Cr(0) species and AgX species were found to be active. Moreover, labeling studies showed that the side products were formed from a metallacyclic intermediate, rather than a Cossee enchainment mechanism. This labeling study was analogous to that published by Bercaw discussed above. The octene presumably forms from an unusual nine-membered metallacyclic intermediate. 

This mechanism was replaced by Cossee s monometallic mechanism (Fig. 1.9) the Ti (III) species has a vacancy in its coordination sphere the alkene molecule coordinates (in the first time) through the n bond, before reacting with the anionic... 

In Natta s catalyst, the steric constraints near the active sites due to the presence of the chloride anions make the coordination of propylene stereospecific. In other words, coordination of propylene through one particular enantioface is energetically favored. This mechanism for the stereospecific propylene polymerization is referred to as the Cossee-Arlman mechanism. 

On the basis of the above observations, an insertion mechanism has been proposed by Natta s group, and more formally discussed by P. Cossee [2]. Its main features, and further sophistication, are summarized here after. 

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