Catalyst-transfer polymerization (CTP) is a living, chain-growth method for synthesizing conjugated polymers, which are attractive materials for organic electronics. What separates CTP from traditional cross-coupling polymerizations is a metal–polymer π-complex that enables the catalyst to stay associated to the growing polymer chain. This association yields polymers with targeted molecular weights, narrow dispersities, and tunable sequences. However, the utility of CTP is limited by a narrow monomer scope, wherein the most desirable polymers remain inaccessible via controlled methods. This thesis aims to advance CTP by designing catalysts capable of widening monomer pairings for block copolymers, exploring ligand electronics in designing an optimal CTP catalyst for previously inaccessible monomers, and optimizing a new user-friendly CTP method.
Chapter 1 briefly summarizes CTP with a focus on how understanding polymerization mechanisms can facilitate catalyst design. Specifically, how exploiting the metal-π complex has led to expanded, albeit limited monomer scope, and new copolymer sequences. The major conclusions of chapters 2–5 and our efforts to expand CTP catalyst scope are briefly outlined followed by the implications of this work on future CTP systems.
Chapter 2 reports the trials and tribulations of designing a single catalyst to perform two sequential, living polymerizations to access thiophene/olefin block copolymers in a one-pot synthesis. Lessons learned include the influence of catalyst reactive ligand and cocatalyst identity on successful thiophene polymerization as well as the inhibitory nature of olefins on thiophene polymerization, requiring olefin monomer removal to induce a switch-in-mechanisms. While a small amount of copolymer was synthesized, the major products were undesired homopolymer. We attributed these homopolymers to a high-barrier reductive elimination when the catalyst switches mechanisms and subsequent chain-transfer during thiophene polymerization. This work highlights the need to identify conditions that facilitate living behavior for both polymerizations as well as promotes efficient cross-propagation.
Chapter 3 describes efforts to design catalysts for CTP that expand monomer scope by tuning ligand electronics to stabilize the metal-π complex. A pyrrolidinyl-based bisphosphine precatalyst was explored in poly(thiophene) and poly(hexylesterthiophene) synthesis and yields polymers with targeted molecular weights as well as high end-group fidelity, suggesting this newly designed catalyst forms a stabilized metal-π complex. While poly(phenylene) synthesis was attempted, gel permeation chromatography revealed a multimodal polymer trace, suggesting multiple catalytic species in the polymerization and an uncontrolled reaction. This catalyst should be further explored in polymerizing previously inaccessible monomers, whose polymerizations are often marred by chain-transfer events.
Chapter 4 describes efforts towards developing a more user-friendly CTP. An NHC-ligated palladium precatalyst with a 3–fluoropyridine ligand polymerized electron-rich and electron-poor monomers of the form, Ar–ZnCl-Mg(OPiv)2, in-air via a controlled, chain-growth method. Ongoing work is focused on showing the utility of this method to a broader community in synthesizing relevant materials for organic electronics.
Chapter 5 summarizes each chapter and provides an outlook for how these results can be informative for the CTP community. The results in accessing conjugated/olefin block copolymers will inform the design of alternative precatalysts that promote Csp2–Csp3 reductive elimination in copolymerizations. The pyrrolidinyl-based bisphospine precatalyst for CTP will add to the toolbox of catalysts, particularly for electron-deficient polymerizations. Finally, our work in identifying a user-friendly CTP route will aid researchers from a variety of backgrounds in synthesizing conjugated polymers with control over molecular weight open-to-air.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/146125/1/kendrads_1.pd
