MOLECULAR MECHANICS AND MECHANISMS OF REGULATION OF THE STEREOSPECIFICITY IN ZIEGLER-NATTA CATALYSIS

The model catalytic sites, proposed in our group for the homogeneous a nd heterogeneous stereospecific Ziegler-Natta polymerizations of olefi ns, are reviewed. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantio selectivity is indicated by the study of the nonbonded interactions. T his mechanism of enantioselectivity, which involves a chiral orientati on of the growing chain, is in agreement with a large number of experi mental results for these catalytic systems. The model sites for the ho mogeneous isospecific polymerization of propene are also able to accou nt for the observed enantioselectivities in the regioirregular placeme nts. Recent calculations relative to a peculiar catalytic model site, which should present a polymerization mechanism involving a regular ba ck-skip of the chain, to the starting position after each monomer inse rtion, are shortly reviewed. The relevance of this polymerization mech anism to the comprehension of the stereospecificity of some homogeneou s and heterogeneous catalytic systems is also briefly discussed.