Back-skip of the growing chain at model complexes for the metallocene polymerization catalysis

A molecular mechanics analysis on model metallocene complexes, as possible intermediates for the propene polymerization, in which the two coordination positions available for the monomer and the growing chain are diastereotopic, is presented. The energy difference between the corresponding diastereoisomeric preinsertion intermediates appears to be relevant for the model complexes based on the meso-ethylenebis(4,5,6,7-tetrahydro-1-indenyl) ligand (1), the isopropylidene[(3-methyl(η⁵-cyclopen>r tadienyl))(η₅-9-fluorenyl) ligand (2), and the rac-ethylene[1-(η⁵-9-fluorenyl)-1-phenyl-2-(η ⁵-1-indenyl)] ligand (3), while it appears to be small for those based on the rac-ethylene[1-(η⁵-cyclopentadienyl)-1-phenyl-2-(η ⁵-1-indenyl)] ligand (4). It is suggested that these energy differences can be related to an increased probability of a back-skip of the growing chain toward the outward coordination position after the monomer insertion and prior to the coordination of a new olefin molecule. The kinetic competition between the back-skip of the chain outwards and the monomer coordination is able to rationalize the kinds of defects in prevailingly hemi-isotactic chains obtained with systems based on 2, as well as the increased stereoregularity at decreasing monomer concentration for systems based on 3 (not shown by systems based on 4). The molecular mechanics analysis shows that the nonbonded interactions are also able to account for the influence of the ethylene bridge conformation on the stereospecificity of the propene polymerization in ansa-zirconocene complexes.