MODELS FOR THE STEREOSPECIFICITY IN HOMOGENEOUS AND HETEROGENEOUS ZIEGLER-NATTA POLYMERIZATIONS

The model initiating sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler-Natta polymerizations of olefins, are reviewed. A comparative analysis of the involved elements of chirality and their relevance for the stereospecific behaviour of the models is reported. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non-bonded interactions. This mechanism of enantioselectivity, which involves the chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems.