MOLECULAR MECHANICS ANALYSIS AND OSCILLATING STEREOCONTROL FOR THE PROPENE POLYMERIZATION WITH METALLOCENE-BASED CATALYSTS

A molecular mechanics analysis of model catalytic complexes for the ho mogeneous Ziegler-Natta polymerization with oscillating stereocontrol, leading to atactic-isotactic stereoblock polymers, is presented. The analysis on the unbridged bis(2-phenylindenyl)zirconium dichloride com plex, precursor of the catalyst, has allowed us to locate three minimu m-energy rotational isomers, two being close to those observed in the crystalline structure. The enantioselectivity and the stereospecificit y evaluated for models suitable for the propene monomer insertion, inc luding the different torsional isomers, confirm the hypothesis that th e behaviour of the catalytic system can be rationalized in terms of is omerization of the catalyst (through rotations of the unbridged pi lig ands) between achiral and chiral coordination geometries, during the p olymerization reaction. The analysis suggests a common enantioselectiv ity mechanism for complexes including bridged and unbridged pi-ligands .