Ziegler-Natta catalysts chiral active centers

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. 

It is in the stereospecific polymerization of propylene that metallocene complexes display their astonishing versatility. Commercial Ziegler-Natta catalysts for isotactic polypropylene - based on combinations of TiCU, MgCl2, Lewis bases and aluminum alkyls - depend on a metal-centered chirality which exists at specific edge and defect sites on the crystal lattice to direct the incoming monomer in a particular orientation. These catalysts produce small amounts of undesirable atactic material due to the presence of achiral active sites. 

Thus, the experimental data show that the composition of catalytic systems does not influence the stereoregularity of the corresponding polymer fractions but only their relative content. Hence, the stereospecificity of the active centers of these catalysts is the same including the one-component catalyst TiCl2. This confirms the concepts on the monometallic character of AQ in the Ziegler-Natta catalysts. The possible existence of chiral titanium atoms on the titanium chloride surface was studied by Cossee and Arlman ... 

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. 

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