Using Mechanistic Insight to Develop Living Polymerizations for Conjugated Homopolymers and Conjugated/Olefin Copolymers

Abstract

Catalyst-transfer polymerization (CTP) is a useful living, chain-growth polymerization method for synthesizing conjugated polymers with targetable molecular weights, narrow dispersities, and controllable copolymer sequences—all properties that significantly influence their performance in devices. Several phosphine- and N-heterocyclic carbene (NHC)-ligated Ni- and Pd-based precatalysts have been shown to be effective in CTP. One current limitation is that these traditional CTP catalysts lead to nonliving, non-chain-growth behavior when polymerizing complex monomers. Because these monomers are found in high-performing materials, there is a need to identify alternative CTP catalysts. Mechanistic insight has laid the foundation for designing new CTP catalysts. Building off this insight, we have designed and implemented model systems to identify catalysts by understanding their mechanistic behaviors and systematically modifying catalyst structures to improve their chain-growth behavior. In Chapter 1, we describe how each catalyst parameter influences CTP. Ancillary ligands can be used to promote the key intermediate (a metal–arene associative complex) and its reactivity. Reactive ligands can improve catalyst solubility and accelerate initiation. While most CTP catalysts contain nickel, palladium-based catalysts exhibit a higher functional group tolerance and broader substrate scope. Overall, we anticipate that applying the tools and lessons detailed in Chapter 1 to other monomers should facilitate a better “matchmaking” process that will lead to new CTPs. Few studies have elucidated the impact of these identities on the stability and reactivity of the key intermediate, especially under polymerization-relevant conditions. In Chapter 2, we developed a simple experiment to identify catalyst stability and ring-walking ability using in situ-generated polymers. The combined results show that the ancillary ligand, metal, and polymer identity all play a crucial role. While each catalyst studied walks efficiently over large distances in poly(thiophene), the trends observed for poly(phenylene) highlight the differing roles of transition metal and ancillary ligand identities. The insights gained herein should be useful for extending CTP to other monomer and copolymer scaffolds. Recently, diimine-ligated Ni complexes have been employed for CTP; however, in most cases nonliving pathways become dominant at high monomer conversions and/or low catalyst loading. In Chapter 3, we report 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. There are a limited number of living polymerization methods for generating copolymers from dissimilar monomers. In Chapter 4 we describe a model system to identify potential precatalysts for synthesizing thiophene/olefin block copolymers. We identified a potential living copolymerization systems involving a ligand-switch from a diimine ancillary ligand to an NHC. Currently, this method generates homopolymers rather than the desired copolymers. Future efforts are focused on elucidating potential termination pathways to circumvent them and enable future copolymerizations. In Chapter 5 we highlight palladium precatalysts with promise for polymerizing complex monomers. Palladium precatalysts demonstrate good functional group tolerance and can polymerize monomers with various transmetalating groups. Few ancillary ligands have been explored for palladium-catalyzed CTP. Here, we describe ancillary ligands used in small- molecule cross-couplings that should be evaluated for CTP. Precatalysts capable of cross- coupling motifs found in complex monomers are highlighted. We anticipate the mechanistic insight and precatalysts discussed herein should facilitate designing future CTPs.