Expanding the Utility of Catalyst-Transfer Polymerization

Abstract

Conjugated polymers are electronically tunable semiconductors that can be solution-processed onto flexible substrates, making them valuable materials for electronic devices including bulk-heterojunction (BHJ) solar cells. Historical syntheses of π-conjugated polymers have been step-growth; however, the development of catalyst-transfer polymerization (CTP) has led to precise control over molecular weight, dispersity, and copolymer sequence. To date, CTP has not been widely adopted to prepare materials for devices. This thesis describes our efforts to improve the utility of CTP through developing new catalysts for the controlled synthesis of pi-conjugated polymers, investigating the mechanism of non-living behavior in CTP of challenging substrates, and understanding the impact of dispersity on thin-film morphology. Chapter 1 provides an overview of BHJ solar cells and a brief history of catalyst-transfer polymerization. Investigations of the mechanism of CTP are described, with a focus on the purported key intermediate, a catalyst-polymer π-complex formed following reductive elimination. We focus on the monomer scope, illustrating the current limitations, and connect the challenge posed by electron-deficient monomers to the proposed mechanism. The catalyst scope, and efforts to expand CTP catalysis to alternate ancillary ligand scaffolds, is also described. Chapter 2 describes the use of a palladium-N-heterocyclic carbene catalyst for CTP. We observe the controlled polymerization of both phenylene and thiophene monomers, while the polymerization of fluorene is nonliving. Excitingly, block copolymers of thiophene and phenylene can be prepared regardless of addition order, indicating more complicated copolymer sequences could be achieved. We suggest further investigation of this catalyst scaffold as an alternate path for new CTP conditions. Chapter 3 describes mechanistic studies into the CTP of thiazole, an electron-deficient analogue of thiophene. Using reaction-discovery calculations, we identify a facile pathway for chain-transfer to monomer. The chain-transfer pathway is enabled by preferential association of the catalyst following reductive elimination, inhibiting catalyst transfer to the chain-end. We selectively inhibit this chain-transfer pathway and promote chain propagation via ancillary ligand modification. End-group analysis confirms the greatly enhanced living character of the polymerization. We also report the autopolymerization of certain thiazole Grignard monomers. Chapter 4 reports initial investigations into the role of dispersity on thin-film morphology. The existing literature on dispersity’s impact is in poor agreement, and we believe that the control CTP provides over molecular weight is necessary to properly investigate this question. We utilize two methods to vary dispersity, preparing three series of polymer samples with similar Mn or Mw. Using UV-vis spectroscopy and optical microscopy, we find that fullerene aggregation increases with dispersity, and tentatively attribute this to the presence of more low-molecular-weight polymer. Preparation and characterization of solar cells is underway. We expect the large morphological differences we observe to significantly impact device performance and lifetime. The control over molecular weight distribution that CTP provides will be invaluable for future BHJ research. Chapter 5 summarizes our efforts expanding the scope of CTP and applying it towards the synthesis of polymers for bulk-heterojunction photovoltaic solar cells. Future directions are outlined for each chapter, highlighting areas of research needed to address limitations of CTP. Additionally, relevant external papers that have been influenced by our work are also briefly discussed. Widespread adoption of CTP will require continued expansion of the monomer scope to include polymers used in high-efficiency devices, and this thesis describes some fruitful strategies for targeting useful monomers.