Date of Award
Doctor of Philosophy
Brian K. Long
Mark Dadmun, Ampofo Darko, Joseph Bozell
Over the past few decades, interest in the design and synthesis of tailorable polymeric materials has grown due to the well documented correlation between structure, property, and function. However, in order to obtain polymers with desired microstructures, well controlled synthetic methods are needed. Therefore, the continued investigation of homogeneous, single-site polymerization catalysts is important to gain a deeper understanding of how systematic modifications of polymerization conditions, ligand scaffold, metal center identity, cocatalyst or activator identity, etc. affect the catalytic activity, selectivity, and/or polymer topology obtained when using these catalysts for the polymerization of a variety of monomers. This dissertation will describe a portion of the advances and insights contributed by the Long Research Group towards furthering the fundamental understanding of catalyst structure for the polymerization of α-olefins and cyclic esters.
First our work within the field of redox-switchable catalysis (RSC) will be examined. RSC enables catalytic activity and/or selectivity to be oscillated based upon the oxidation-state of the ligand or active metal center. Unfortunately, though myriad redox-switchable catalysts have been developed for the ring-opening polymerization of cyclic esters, further fundamental structure-catalytic performance studies are needed to better ascertain how systematic changes in ligand structure impact catalytic activity and redox-switchability. Herein, we describe our studies designed to determine how the number of ligand-based redox-active moieties impact catalytic performance. More specifically, we compare symmetric catalysts bearing tetradentate [ONNO] ligands with two redox-active moieties to related asymmetric catalysts bearing tridentate [ONN] ligands featuring only a single redox-active moiety for the polymerization of L-lactide (L-LA). The results of these studies reveal that the number of redox-active moieties may not play a crucial role in the catalysts’ switchability; however, the choice of metal center may dramatically influence catalyst activity, stability, and redox-switchability. Furthermore, we examine how ligand connectivity and conjugation affects catalytic activity, selectivity, and redox-switchability by comparing catalysts bearing salen-type ligands with two redox-active moieties to salan-type ligands with two redox-active moieties for the polymerization of L-LA and ε-caprolactone (CL). The results of these studies reveal that ligand connectivity and conjugation may both play a role in catalyst activity, selectivity, and redox-switchability.
Additionally, the ability to control the branching density of polyethylene (PE) has been of great interest within the field of olefin polymerization catalysis as a means by which polymer properties may be modulated. One method by which PE branching density may be control is through modulation of the catalysts via the inclusion of electron-withdrawing (EW) or electron-donating (ED) substituents to the ligand scaffold. However, while myriad of Pd- and Ni- centered α-diimine catalysts have been studied, a few key fundamental studies are still absent from the literature including a comprehensive examination of Ni-based α-diimine catalysts, as well as an examination of how placement of the substituent on the backbone versus placement on the N-aryl moieties affect polymer topology. Additionally, no method currently exists by which the polymer topology may be predicted based on an intrinsic characteristic of the catalyst-ligand combination without needing to perform exhaustive polymerization studies. Herein, we use a joint experimental and computational approach to demonstrate how placement of EW or ED substituents on the acenaphthene backbone, N-aryl moieties, or both the acenaphthene backbone and N-aryl moieties of Ni-based α-diimine catalyst affects PE branching density as compared the analogous unsubstituted catalyst. More specifically, we will show that inclusion of EW substituents results in decreased PE branching density, whereas inclusion of ED substituents results in little to no change in PE branching density. We will also show that as the placement and identity of the ligand substituents are varied, so too is the catalysts’ reduction potential and this relationship can be used to generate a calibration curve from which PE branching density may be predicted for other substituted Ni-based α-diimine catalysts without the need for extensive polymerization studies.
Doerr, Alicia, "Development of Catalysts for the Ring-Opening Polymerization of Cyclic Esters and the Coordination-Insertion Polymerization of Olefins. " PhD diss., University of Tennessee, 2021.