Influence of the ligand structure of hafnocene polymerization catalysts:: A theoretical study on ethene insertion and chain propagation

The influence of the ligand structures of hafnocene polymerization catalysts on ethene insertion and chain propagation was systematically studied by quantum chemical methods. Altogether 54 hafnocenes were studied as a function of the ligand structures. Two consecutive ethene insertions and chain propagations were performed for the catalysts, giving rise to 15 intermediate structures along the reaction pathway. The behavior of the catalysts was analyzed as a function of ancillary ligands, ligand substituents, and bridging units. The differences along the reaction pathway are dominated by the changes in relative stabilities of the catalytic intermediate products. Large aromatic ancillary ligands and electron-donating ligand substituents strongly stabilize the catalyst cations. Steric effects introduced by the ligand framework mostly affect the feasibility of ethene pi-coordination and the activation energy for chain propagation. The dominant effect of the relative stabilities of the catalyst intermediates sheds light on the catalytic performance of metallocenes, which may turn out to be useful in further catalyst development.