Advancing Conjugated Polymer Synthesis Through Catalyst Design

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

Trials and tribulations of designing multitasking catalysts for olefin/thiophene block copolymerizations

Block copolymers containing both insulating and conducting segments have been shown to exhibit improved charge transport properties and air stability. Nevertheless, their syntheses are challenging, relying on multiple post‐polymerization functionalization reactions and purifications. A simpler approach would be to synthesize the block copolymer in one pot using the same catalyst to enchain both monomers via distinct mechanisms. Such multitasking polymerization catalysts are rare, however, due to the challenges of finding a single catalyst that can mediate living, chain‐growth polymerizations for each monomer under similar conditions. Herein, a diimine‐ligated Ni catalyst is evaluated and optimized to produce block copolymer containing both 1‐pentene and 3‐hexylthiophene. The reaction mixture also contains both homopolymers, suggesting catalyst dissociation during and/or after the switch in mechanisms. Experimental and theoretical studies reveal a high energy switching step coupled with infrequent catalyst dissociation as the culprits for the low yield of copolymer. Combined, these studies highlight the challenges of identifying multitasking catalysts, and suggest that further tuning the reaction conditions (e.g., ancillary ligand structure and/or metal) is warranted for this specific copolymerization. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 132–137Block copolymers containing insulating segments (derived from 1‐pentene) and conducting segments (derived from 3‐hexylthiophene) are synthesized in one pot using a single multitasking catalyst. Notably, this process requires different enchainment mechanisms (coordination/insertion vs. cross‐coupling) mediated by the same precatalyst. Nevertheless, the block copolymer is the minor product due to a slow switching step between the mechanisms coupled with catalyst dissociation from the polymer chain.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139919/1/pola28885_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139919/2/pola28885.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139919/3/pola28885-sup-0001-suppinfo.pd

Air‐tolerant poly(3‐hexylthiophene) synthesis via catalyst‐transfer polymerization

The discovery of catalyst‐transfer polymerization and its further developments have led to unprecedented control over the length and sequence of conjugated polymers. However, the methods themselves are technically challenging to perform due to the air‐ and moisture‐sensitivities of the monomers and catalysts. Herein, we report a catalyst‐transfer polymerization method that affords poly(3‐hexylthiophene) in high yields without using an inert atmosphere. The synthesis capitalizes on a rapid Negishi cross‐coupling using a moisture‐tolerant organozinc monomer mediated by an air‐stable Pd precatalyst. This simple method should make conjugated polymer synthesis more accessible to a broader range of researchers and may be generalizable to other monomer scaffolds.Transition metal‐mediated cross‐coupling polymerizations typically require inert atmospheres to attenuate premature monomer quenching and/or catalyst deactivation. Herein, poly(3‐hexylthiophene) is synthesized without the need for an inert atmosphere by using bench‐stable Pd precatalysts and organozinc pivalates. The reaction yields polymers with specified molar masses and end‐group identities, as well as with narrow dispersities, consistent with a living, chain‐growth polymerization.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/166165/1/pol20200788-sup-0001-supinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/166165/2/pola29912.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/166165/3/pola29912_am.pd

Air‐tolerant

The discovery of catalyst‐transfer polymerization and its further developments have led to unprecedented control over the length and sequence of conjugated polymers. However, the methods themselves are technically challenging to perform due to the air‐ and moisture‐sensitivities of the monomers and catalysts. Herein, we report a catalyst‐transfer polymerization method that affords poly(3‐hexylthiophene) in high yields without using an inert atmosphere. The synthesis capitalizes on a rapid Negishi cross‐coupling using a moisture‐tolerant organozinc monomer mediated by an air‐stable Pd precatalyst. This simple method should make conjugated polymer synthesis more accessible to a broader range of researchers and may be generalizable to other monomer scaffolds.Transition metal‐mediated cross‐coupling polymerizations typically require inert atmospheres to attenuate premature monomer quenching and/or catalyst deactivation. Herein, poly(3‐hexylthiophene) is synthesized without the need for an inert atmosphere by using bench‐stable Pd precatalysts and organozinc pivalates. The reaction yields polymers with specified molar masses and end‐group identities, as well as with narrow dispersities, consistent with a living, chain‐growth polymerization.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/166165/1/pol20200788-sup-0001-supinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/166165/2/pola29912.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/166165/3/pola29912_am.pd

Mechanistic Insight into Thiophene Catalyst-Transfer Polymerization Mediated by Nickel Diimine Catalysts

Catalyst-transfer polymerization (CTP) is a living, chain-growth method for accessing conjugated polymers with control over their length and sequence. Typical catalysts utilized in CTP are either Pd or Ni complexes with bisphosphine or N-heterocyclic carbene ancillary ligands. More recently, diimine-ligated Ni complexes have been employed; however, in most cases nonliving pathways become dominant at high monomer conversions and/or low catalyst loading. We report herein an alternative Ni diimine catalyst that polymerizes 3-hexylthiophene in a chain-growth manner at low catalyst loading and high monomer conversion. In addition, we elucidate the chain-growth mechanism as well as one chain-transfer pathway. Overall, these studies provide insight into the mechanism of conjugated polymer synthesis mediated by Ni diimine catalysts

Mechanistic Insight into Thiophene Catalyst-Transfer Polymerization Mediated by Nickel Diimine Catalysts

Catalyst-transfer polymerization (CTP) is a living, chain-growth method for accessing conjugated polymers with control over their length and sequence. Typical catalysts utilized in CTP are either Pd or Ni complexes with bisphosphine or N-heterocyclic carbene ancillary ligands. More recently, diimine-ligated Ni complexes have been employed; however, in most cases nonliving pathways become dominant at high monomer conversions and/or low catalyst loading. We report herein an alternative Ni diimine catalyst that polymerizes 3-hexylthiophene in a chain-growth manner at low catalyst loading and high monomer conversion. In addition, we elucidate the chain-growth mechanism as well as one chain-transfer pathway. Overall, these studies provide insight into the mechanism of conjugated polymer synthesis mediated by Ni diimine catalysts