Catalyst and Reaction Development in Conjugated Polymer Synthesis

Abstract

Conjugated polymers are lightweight, flexible, solution-processible materials which can be used in organic electronic devices, including photovoltaics and light-emitting diodes. Catalyst-transfer polymerization (CTP) is a chain-growth method for synthesize conjugated polymers with targeted molecular weight and sequence, as well as narrow dispersity. Several factors continue to limit the utility of CTP. Slow precatalyst initiation leads to broad dispersity and limited sequence control. Furthermore, the monomer scope of CTP is limited, especially for electron-deficient monomers. The highest performing polymers in photovoltaic devices are still synthesized by step-growth polymerizations. This thesis describes our efforts to address both limitations of CTP by designing new precatalysts which undergo fast initiation, using model reactions to screen catalysts to expand CTP scope, and finally, to develop new, non-CTP chain-growth syntheses of conjugated polymers through single-electron reactions. Chapter 1 provides a history of CTP for conjugated polymer synthesis. It describes progress that has been made in precatalyst design to improve dispersity and enhance initiation. We then discuss the use of small-molecule screens to identify catalysts for CTP as a method for expanding the monomer scope. Finally, we briefly introduce the precedent for single-electron reactions, both radical and electrochemical, for conjugated polymer synthesis and explain our proposal to develop chain-growth syntheses by modifying these methods. Chapter 2 reports our design of a new precatalyst which has faster initiation than propagation. It describes our development of a new method for measuring initiation rates during polymerization, as well as our discovery that adding triphenylphosphine to a polymerization and incorporating a trifluoroethoxy group into a precatalyst both affected initiation rates. Chapter 3 describes our efforts to identify catalysts and reaction conditions for CTP of phenylene-ethynylene by using a small-molecule reaction to probe for catalyst association by looking at mono-coupling versus di-coupling. It explains our discovery that small-molecule systems, which we and others have previously used, can have false positives when there are large reactivity differences between substrates, and proposes additional experiments to increase the accuracy of such models for predicting CTP. Chapter 4 describes our efforts to synthesize conjugated polymers through single-electron reactions rather than CTP. We present the precedent for conjugated polymer synthesis via SRN1 reactions, and our attempts to expand the monomer scope to several monomers used in organic photovoltaics. It further explains the history of step-growth electropolymerizations and our efforts to use indirect electrolysis to develop a chain-growth electropolymerization for conjugated polymers. We successfully synthesized one conjugated polymer using a perylene diimide mediator. Chapter 5 summarizes our progress in catalyst and reaction development for conjugated polymer synthesis and the future directions and impact of each chapter. It describes the impacts that our discoveries in Chapter 2 could have for precatalyst design, rate enhancement, and rate studies. It discusses the impact that our work in Chapter 3 will have on using model systems to identify CTP conditions, including a recent example that citing our work. Finally, it describes the promising results in Chapter 4 and the future directions of indirect electrocatalysis for chain-growth conjugated polymer synthesis. Chain-growth methods are needed to control molecular weight, dispersity, and sequence in conjugated polymer synthesis, to ensure reproducibly high-performing devices.