RELATIONSHIP BETWEEN REGIOSPECIFICITY AND TYPE OF STEREOSPECIFICITY IN PROPENE POLYMERIZATION WITH ZIRCONOCENE-BASED CATALYSTS

The microstructures of polypropenes produced with several zirconocene- based catalyst systems are compared, to verify the possible correlatio n between the type of stereospecificity and the amount of regioirregul arities. It is confirmed that, while syndiospecific and aspecific zirc onocenes are highly regiospecific, isospecific systems produce substan tial amounts of regioirregular monomeric units. The amount of these se condary units strongly depends on the nature of the pi-ligands and on the type of the bridge connecting them. Molecular mechanics calculatio ns are reported, indicating that the intermediates which are energetic ally suitable for the secondary and primary insertions, for isospecifi c or syndiospecific complexes, coordinate monomer enantiofaces of the opposite or the same chirality, respectively. This difference accounts for the lower regiospecificity of the isospecific catalytic complexes , assuming that the energy barrier for the rotation of the coordinated monomer around the metal-olefin bond, between the orientations suitab le for the primary and secondary insertions is lower than (or comparab le to) the activation energy for secondary monomer insertion.