Preparation and Rheology of Polymeric Frustrated Lewis Pairs

  • Utku Yolsal

Student thesis: Phd

Abstract

Presence of sterically demanding substituents around Lewis acids (LAs) and bases can preclude the required close contact to form classical Lewis pair adducts. These instead form frustrated Lewis pairs (FLPs) which can be used for the activation of various small molecules, including H2, CO2, diethyl azodicarboxylate (DEAD) and cyclic ethers. Applying this concept into polymeric frameworks results in the formation of a novel class of stimuli-responsive polymers, which demonstrated significant potential as small molecule triggerable polymer gels. This thesis provides the first insights into the rheology of poly(FLP) networks. It provides a detailed rheological characterisation of the DEAD-triggered poly(FLP) gels, which were shown to behave similarly to the supramolecular assemblies linked by non-covalent bonds. It was shown that the FLP-DEAD crosslinks are highly dynamic, and the self-healing occurred immediately after the damage cycles, with up to 91% recovery of the original moduli values within 15 mins. It was found that an increase in crosslink density (4 to 7 mol%) increased the stiffness of the networks 3-fold, whereas switching the polymer backbone from styrene to methyl methacrylate resulted in easier chain rearrangements, thus higher number of effective crosslinks. Inspired by these promising results, an attempt was made to ease access to the preparation of poly(FLP)s by synthesising a range of accessible polymeric LAs with high Lewis acidity moieties which usually require challenging synthetic steps. This part reports the syntheses of poly(styrene-co-bis(pentafluorophenyl)pentafluorophenylborinate) and poly(styrene-co-cyclohexylbis(pentafluorophenyl)borane), 1, as well as the preparation of a tris(pentafluorophenyl)borane-mimicking functional monomer and the subsequent attempts to polymerise it. Finally, 1 was matched with poly(styrene-co-triphenylphosphine) to explore its potential as poly(FLP)s. Although earlier poly(FLP) networks relied entirely on the activation of DEAD, this substrate scope was expanded to cyclic ethers, which also triggered gelations. It was shown that the steric and electronic properties of the ethers directly affect the extent of crosslinking and can be used to tune the mechanical properties of the gels. Unlike the DEAD-triggered poly(FLP)s, these systems behaved similarly to the covalently crosslinked polymer networks, suggesting the strong effect of FLP interactions on the overall rheology of the polymer networks as well as the versatility of the poly(FLP)s as platforms for polymer networks with tuneable mechanical properties.
Date of Award31 Dec 2021
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMichael Shaver (Supervisor) & Lee Fielding (Supervisor)

Keywords

  • Polymer Gels
  • Polymeric FLPs
  • Stimuli Responsive Polymers
  • Frustrated Lewis Pairs

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