The Reactivity of Ni Complexes in Conjugated Polymer Synthesis

Abstract

Catalyst-transfer polymerization (CTP) is a living, chain-growth polymerization method used to synthesize conjugated polymers with control over molar mass, sequence, and end-group identity. CTP is one of the few chain-growth polymerizations that can be used for accessing conjugated polymers, but its utility is limited by its monomer scope, which primarily includes small electron-rich arenes. This limit to the monomer scope arises in part from an inability to rationally identify catalysts for new monomers, which in turn arises from our inability to observe or study key intermediates of CTP. Discovering ways to observe intermediates during CTP would enable establishing structure–reactivity relationships and could enable us to new identify catalysts for CTP. My work in this dissertation is therefore focused on identifying intermediates in CTP and understanding the structure/property relationships that govern the reactivity of these intermediates. In Chapter 1, I describe previous work aimed at understanding the mechanism of CTP and identifying structure–reactivity relationships. I highlight the knowledge gaps with respect to the reactions of π-complexes and explain why these π-complexes cannot be observed easily via experimental studies, precluding our understanding of their reactivity. In Chapter 2, I discuss how Ni bidentate phosphines, a major class of complexes used in CTP, can be identified using the JPP value derived from 31P NMR spectroscopy. Specifically, I show that JPP depends on the oxidation state of Ni, the linker length of the phosphine, and the donating or withdrawing character of the ligands bound to Ni. Given these correlations between structure and JPP, I propose that JPP analysis can be used to identify complexes during reactions in which isolation and purification of intermediates is difficult or impractical, such as in CTP. In Chapter 3, I use JPP values along with computational and synthetic investigations to elucidate the structure of the catalyst trap in Ni-catalyzed CTP of thienothiophene. The catalyst trap we identify is a NiII species which arises from off-cycle C–S insertion of Ni into the thienothiophene ring. We propose that this C–S insertion trap may occur in CTP of other sulfur-containing arenes and that this reactivity mode may explain some of the limitations on large monomers in the scope of CTP. In Chapter 4, I describe our efforts at understanding the structure–reactivity relationships inherent to Ni and Pd π-complexes in CTP. Using density functional theory-based simulations, we find that the Dewar-Chatt-Duncanson model adequately describes binding in π-complexes. Additionally, we find that the barrier to ring-walking does not correlate strongly with the π-binding energy, indicating that ring-walking barrier and π-binding energy can be modulated independently based on the catalyst choice. Lastly, in Chapter 5, I describe the current limitations on CTP and open questions for future consideration. I anticipate that the work described herein will aide future investigators in uncovering relationships that are important for synthesizing conjugated polymers.