Theoretical study on the factors controlling the accessibility of cationicmetal centers in zirconocene polymerization catalysts

The influence of the ligand structure of zirconocene polymerization catalys ts on the accessibility of the active reaction center was studied by an ab initio Hartree-Fock method. The variations in the accessibility were elucid ated by comparing molecular structures and relative stabilities of 54 bridg ed zirconocene catalysts with 19 different bridging units, 18 ancillary Cp' ligands, and 18 ligand substituents. Ligand variations gave rise to variou s steric and electronic effects affecting both the nature and the concentra tion of active metal centers in the system, such as steric blocking or shie lding of the metal center, and stabilization or destabilization of the acti ve cationic species. Comparisons to experimental work demonstrated clear co rrelations between accessibility of the active reaction center and observed polymerization activity. Furthermore, interactions between the Lewis acidi c aluminum centers and Lewis basic functionalities were observed. The conse quences of such interactions are discussed.