Atomistic Simulation of Ion Pair Aggregates of a (Pyridylamido)Hf(IV) Catalyst: Energetic Destabilization and Structural Diversity Induced by Monomer, Chain Transfer Agent, and Growing Polymer Chain

Ion pair aggregates (IPAs) in olefin polymerization catalysts have attracted growing attention due to their potential influence on catalytic activity and selectivity. In this study, employing (pyridylamido)Hf(IV) as a model system, we investigated the destabilization and structural diversity of IPAs by using the molecular dynamics method. The potential of mean force (PMF) analysis indicates that the growing polymer chains, ethylene, and ZnEt₂ destabilize the IPAs. In addition, the energy decomposition analysis demonstrated that not only the dominant electrostatic interaction but also relatively small yet qualitatively different contributions from dispersion and coordination interactions play an essential role in the structures of IPAs. These findings can be regarded as a piece of evidence that chemical species other than the catalyst itself in the reaction medium, through relatively small interactions, should modulate catalytic behaviors. Furthermore, we also found that the growing polymer chains, ethylene, and ZnEt₂ induce structural diversity of IPAs, and classified IPAs' structures into three distinct classes: (1) inner-sphere ion pair aggregate (ISIPA), (2) bridged ion pair aggregate (BIPA), and (3) outer-sphere ion pair aggregate (OSIPA). Our findings have laid the groundwork for a broader understanding of IPAs' behaviors in olefin polymerization catalysts across diverse catalytic environments.