Interpreting “Acidity” as a Global Property Controlling Comonomer Reactivity in Olefin Polymerization

Abstract

A possible rationale for the different catalytic behaviors of systems based on <i>rac</i>-(ethylenebis­(1-indenyl))zirconium dichloride (<i>rac</i>-EBIZrCl₂), <i>rac</i>-(ethylenebis­(1-indenyl))­hafnium dichloride (<i>rac</i>-EBIHfCl₂), and <i>rac</i>-(isopropylidenebis­(1-indenyl))zirconium dichloride (<i>rac</i>-iPrBIZrCl₂) toward ethene–styrene copolymerization has been sought by studying related active systems. For this purpose, the metallocene ion pairs (IPs) <i>rac</i>-EBIZrMeMeB­(C₆F₅)₃, <i>rac</i>-EBIHfMeMeB­(C₆F₅)₃, and <i>rac</i>-iPrBIZrMeMeB­(C₆F₅)₃ have been synthesized and their structures in solution explored with ROESY and pulsed gradient NMR spectroscopy. The energetics of dynamical processes relevant for catalysis that can be used as indicators of the cation acidity have been studied with variable-temperature NMR experiments and density functional theory (DFT). NMR experiments successfully provided IP structural details in solution and also indicated the presence of an intricate dynamic behavior for all the IPs. DFT results, instead, indicated quantitatively how changing the metal and/or the ancillary ligand bridge influences the energetics of the active species and modifies the reaction energy profile. The theoretical results also drew attention to the fact that finding a rationale for the ligand influence on the catalytic behavior of <i>rac</i>-EBIZrCl₂/MAO and <i>rac</i>-iPrBIZrCl₂/MAO in ethene–styrene copolymerization requires not only considering the steric effects but also determining how subtle changes in the ligand sphere affect the capability of the metal center to accept electrons from the counteranion or the olefins