Polymeric Frustrated Lewis Pairs as Catalytic Systems and Stimuli-Responsive Materials

Abstract

Frustrated Lewis Pairs (FLPs) are a rapidly expanding family of efficient catalysts with the potential to replace traditional transition-metal complexes. Although their activity is high, the separation of these catalysts from the resultant product mixture is often complicated. Polymer-supported catalysts have frequently been employed to facilitate product separation and catalyst reuse. Recently, a small number of polymeric FLP (poly(FLP)) systems have been reported to give similar benefits. In addition, poly(FLP) activation of small-molecule substrates forms crosslinked polymer networks. This thesis explores both uses of poly(FLP)s: introducing cyclic ether substrates to achieve catalysis across multiple cycles and form tuneable polymeric networks. CO2 insertion catalysis to these substrates (both terminal epoxides and oxetane) was successfully achieved using aryl borane/phosphine copolymer combinations (Chapter 2). An epoxide deoxygenation side reaction was discovered: formation of an alkene by-product promotes phosphine oxidation and loss of catalytic performance. Deoxygenation was mitigated using a sterically bulky phosphine (P2) to give a highly active FLP catalytic system when combined with a mildly Lewis acidic boron copolymer (B1). Stronger Lewis acid copolymers were also trialled, resulting in reduced catalytic activity. Catalyst reuse was achieved by precipitation and recovery, gradually reducing reactivity in subsequent cycles. Ring-expansion carbonylation catalysis was unsuccessful using these copolymer combinations, however a novel polymer-supported PPNCo(CO)4 derivative was synthesised to enable CO insertion to terminal epoxide substrates (Chapter 3). The formation of ketone-by-products was demonstrated, consistent with previously reported bimetallic systems. Build-up of these ketone by-products, and loss of -Co(CO)4 anion during precipitation, led to a significant reduction in catalytic activity in subsequent reaction cycles. Finally, the influence of Lewis acidity, basicity and cyclic ether substrate on polymer network formation was explored using rheology (Chapter 4). Strong Lewis acid (LA) and base (LB), which were further probed using amine copolymers, lead to covalently-linked networks, while weaker interactions give dynamic crosslinking. Dynamic networks were demonstrated to self-heal, with rates dependent on both steric bulk of the LB site and ring-strain of the crosslinker. Epoxide deoxygenation was also demonstrated within polyphosphine networks, leading to a change in crosslinking/healing dynamics over time and with increased temperature.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Shaver (Supervisor) & Stephen Edmondson (Supervisor)