Cossee stereospecific polymerizations

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

Arlman, E.J. Cossee, P. Ziegler-Natta catalysis III. Stereospecific polymerization of propene with the catalyst system TiCls-AlEts. J. Catal. 1964, 3, 99-104. 

Figure 9.4 Monometallic mechanism for stereospecific polymerization. (After Cossee, 1967.)...

FIGURE 1.7 Monometallic mechanism for stereospecific polymerization. (After Cossee, P. 1967. The Stereochemistry of Macromolecules, A. D. Ketley, ed., Vol. 6. Marcel Dekker, New York.)... 

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-... 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

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. 

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