ON THE EFFECTS OF METHYL SUBSTITUENTS ON CHELATING LIGANDS IN MODELS FOR HOMOGENEOUS ISOSPECIFIC ZIEGLER-NATTA CATALYSIS

A model for homogeneous isospecific Ziegler-Natta polymerization, recently proposed by us, has been tested for its ability to predict the experimentally observed behaviour when the structure of the catalysts is changed by chemical substitution. In particular, conformational energy calculations have been performed on models of the catalytic site containing the ethylene-bis(1-indenyl) ligand substituted in various positions with methyl groups. In agreement with previous experiments, the analysis indicates that the presence of substituents at the 3,3' positions strongly reduces the isospecific behaviour of this catalyst. On the contrary, calculations performed on model sites containing the non-chiral ethylene-bis(cyclopentadienyl) ligand suggest that the presence of methyl groups at 3,3' positions should induce a high degree of isospecificity in this otherwise aspecific system. This theoretical prediction has been confirmed by recently announced experimental results.