ENANTIOSELECTIVITY IN THE REGIOIRREGULAR PLACEMENTS AND REGIOSPECIFICITY IN THE ISOSPECIFIC POLYMERIZATION OF PROPENE WITH HOMOGENEOUS ZIEGLER-NATTA CATALYSTS

With the aim of a possible rationalization of the probability distribu tions of stereochemical configurations of the regioirregular units in isotactic polymer samples (prepared in the presence of ethylenebis(4,5 ,6,7-tetrahydro-1-indenyl)zirconium dichloride or rac-ethylenebis(l-in denyl)zirconium dichloride and methylaluminoxane), a detailed molecula r mechanics analysis on previously proposed model catalytic sites is r eported. In particular, catalytic intermediates suitable for primary a nd secondary insertions of propene on primary and secondary polypropen e chains, for both systems with rac-ethylenebis(4,5,6,7-tetrahydro-1-i ndenyl) and rac-ethylenebis(1-indenyl) ligands, are compared. The mode ls are able to rationalize the observed enantioselectivities, not only for the regioregular placements but also for the regioirregular place ments (secondary insertion on a primary chain and primary insertion on a secondary chain). Moreover, the nonbonded interactions in the model s give a contribution in favor of the monomer coordination suitable fo r the primary insertion, even after an occasional secondary insertion, and are able to account for the higher regiospecificity observed for titanocene-based, with respect to zirconocene-based, catalytic systems .