BACK-SKIP OF THE GROWING CHAIN AT MODEL COMPLEXES FOR THE METALLOCENEPOLYMERIZATION CATALYSIS

A molecular mechanics analysis on model metallocene complexes, as poss ible intermediates for the propene polymerization, in which the two co ordination positions available for the monomer and the growing chain a re diastereotopic, is presented. The energy difference between the cor responding 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 hyl(eta(5)-cyclopentadienyl))(et a(5)-9-fluorenyl)] ligand (2), and the eta(5)-9-fluorenyl)-1-phenyl-2- (eta(5)-1-indenyl)] ligand (3), while it appears to be small for those based on the )-cyclopentadienyl)-1-phenyl-2-(eta(5)-1-indenyl)] ligan d (4). It is suggested that these energy differences can be related to an increased probability of a back-skip of the growing chain toward t he 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 coordinat ion is able to rationalize the kinds of defects in prevailingly hemi-i sotastic chains obtained with systems based on 2, as well as the incre ased 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 accoun t for the influence of the ethylene bridge conformation on the stereos pecificity of the propene polymerization in ansa-zirconocene complexes .