Computational Investigations of Conformational Effects in Organometallic Polymerization Catalysts

Abstract

Computational chemistry is a powerful tool for increasing understanding of known chemistry, discovering new reaction mechanisms, and improving catalyst properties and design. It can provide physical insights that would be difficult or costly to achieve through experiments alone. The insight that can be gained from computational studies, however, can be limited by the accuracy of the models used and often requires an established working knowledge of the chemical system of interest. Below are two examples where computational chemistry is used to provide insight into chemical reactions catalyzed by flexible organometallic catalysts. Chapter 1 provides a brief overview of computational chemistry fundamentals that are needed to understand reaction landscapes. This introduction includes topics include such as the potential energy surface, molecular dynamics, and transition states. The reaction path finding and transition state finding methods used in this work are also described. Chapter 2 reveals the interactions between the counteranion and flexible polymer during propagation and termination with the Ti(IV) constrained geometry catalyst. The work studies the dynamic interplay between counteranion and growing polymer chain on ethylene polymerization through conformer sampling and reaction pathway simulations. The results show that the length of the polymer chain substantially influences the counteranion position, where longer polymer chains tend to limit the counteranion’s access to the monomer coordination site. Meanwhile, productive insertion and chain termination reaction pathways prefer ion pair orientations where the counteranion is on the opposite side of the Ti catalyst as the ethylene monomer. The effective activation barriers for insertion and termination reactions calculated from the reaction path ensemble explain the difference in catalytic activity and molecular weight between the two different counteranions observed in experiment. Chapter 3 elucidates how the differences in ion pair interactions between diastereomers during activation with the Hf(IV)-pyridylamido catalyst influence low active-site counts. Extensive sampling of the ion pair with molecular dynamics simulations reveals distinct preferred counteranion orientations, which are differentiated by the location of the bridgehead substituent. One diastereomer enables the counteranion to slide to favorable staggered orientations suitable for outer sphere coordination, whereas the other diastereomer prefers the counteranion with inner sphere coordination. Ion pair preferences not only affect which conformations are accessible, the ion pair also affects the free energy profile associated with ethylene monomer coordination. The ethylene monomer is shown to coordinate preferentially to one, but not the other, of the two potentially active catalysts. The free energy of these processes are linked to the conformational flexibility of the diastereomers, giving new physical insight into design principles for highly active polymerization catalysts. The concluding chapter reflects on each study and discusses possible future considerations. Overall, this work highlights accurate models of polymerization catalysts to gain a deeper understanding of organometallic chemistry.