MECHANISTIC STUDIES ON PROPENE POLYMERIZA TION WITH THE CATALYTIC-SYSTEM EN-1-YLIDENE(METHYLETHYLIDENE)-1H-INDEN-1-YLIDENE] ZIRCONIUM DICHLORIDE METHYL ALUMINOXANE

Reducing the symmetry of the C-2-symmetric Me(2)Si[Ind]2ZrCl2 to asymm etric Pr-i[CpInd]ZrCl2 should change the microstructures of resulting polypropenes from isotactic to hemi-isotactic. As the polypropenes obt ained show microstructures between ideal hemi-isotactic and ideal atac tic we developed a new model to explain these experimental results. Ou r model is able to describe the polymer structures obtained by four co nformations in the ground state of cationic metallocene species with d ifferent propene coordinations ((S)R(re), (S)S-re, (S)R(si) and (S)S-s i). Furthermore we calculated 15 probable transitions of polymer chain isomerization with and without propene insertion. Molecular modelling calculations yield the energies of four conformations in the ground s tate. The energy differences (maximum 1.4 kcal) are decisive for polyp ropene microstructures and explain the frequent appearence of pentad e rrors (mrmr, rrmr and mmrm) as well as the microstructure change frome hemi-isotactic to isotactic with increasing polymerization temperatur e. By means of comparison between isomerization with and without prope ne insertion for the first time one succeeds in gaining insight into t he dynamic behaviour of a metallocene catalyst.