Abstract
A molecular mechanics analysis of model catalytic complexes for the homogeneous Ziegler-Natta polymerization with oscillating stereocontrol, leading to atactic-isotactic stereoblock polymers, is presented. The analysis on the unbridged bis(2-phenylindenyl)zirconium dichloride complex, precursor of the catalyst, has allowed us to locate three minimum-energy rotational isomers, two being close to those observed in the crystalline structure. The enantioselectivity and the stereospecificity evaluated for models suitable for the propene monomer insertion, including the different torsional isomers, confirm the hypothesis that the behaviour of the catalytic system can be rationalized in terms of isomerization of the catalyst (through rotations of the unbridged pi ligands) between achiral and chiral coordination geometries, during the polymerization reaction. The analysis suggests a common enantioselectivity mechanism for complexes including bridged and unbridged pi-ligands.