title: Bis aromatic urea hydrogen bonding motifs for use in supramolecular materials
Creators: name: O'Donnell, Adam David
Publisher: name: University of Reading
Year: 2022

description: The design of hydrogen bonding motifs is an incredibly attractive proposition for a wide range 
of applications; these motifs can be integrated into polymeric materials to produce stronger, 
tougher, stiffer, or more elastic materials that exhibit thermo-reversible behaviour. This thesis 
describes the design of hydrogen bonding motifs and their successful incorporation in 
supramolecular materials through the formation of low molecular weight ‘super’ hydrogelators
and elastomeric materials. Hydrogelators were chosen as they offer a unique insight into 
supramolecular assembly, and they remove the influence of the polymeric backbone which is 
often responsible for the mechanical properties of polymer networks. The objective was to 
understand the structure to property relationships for low molecular weight substrates and how 
that translated to functional materials such as hydrogels. With an appreciation of the mode of 
assembly of these units and an understanding of how to tailor the assembly of the hydrogen 
bonding motif, these motifs could then be applied to polymeric systems to generate novel 
supramolecular elastomers.
Chapter 1. provides a summary of molecular recognition phenomena, the application of weak 
non-covalent interactions and self-assembly processes in polymeric systems to create 
functional supramolecular materials.
Chapter 2. details the synthesis of eight low molecular weight bis-aromatic ureas which contain 
carboxylic acid groups such that, upon pH switching form extended networks through 
carboxylate-carboxylate interactions. The supramolecular assembly was tailored by 
introducing methyl substituents on the aromatic rings, which attenuated the assembly by 
providing steric bulk and forcing the urea and phenyl substituents into a non-planar 
arrangement, thus promoting urea-urea stacking interactions. The expectation was that forcing 
the urea-phenyl into a non-planar arrangement would strengthen the urea-urea interaction and,
by extension improve the robustness of the hydrogelators. The strengthening of the urea-urea 
interaction was evident, as rather than form stable gels, the bis-ureas with the strongest urea-urea interactions were found to undergo gel to crystal transitions.
Chapter 3. outlines the synthesis and characterisation of thirteen telechelic poly(butadiene)-
based elastomers whose properties were modulated by changing the assembly motif at the 
end of the polymer chain. The effect of the end-group was stark upon the elastomers’
properties; in the absence of a nitro substituent to act as a hydrogen bonding acceptor, the 
polyurethanes would form exceptionally phase-separated materials that were unsuitable for 
extensive mechanical testing as they presented behaviour comparable to as crosslinked 
rubbers. Extensive mechanical property assessment revealed that in the presence of a nitro 
moiety, the resultant polyurethanes would behave as thermoreversible elastomers that 
demonstrated self-healing and reversible adherence to a range of different substrates.
Chapter 4. describes the shift in approach to supramolecular polyurethanes, in that, rather 
than end functionalise telechelic apolar polymer backbones, an apolar backbone, namely 
PTMG, was chain extended with a bis aromatic urea assembly motif to form chain extended 
polyurethanes. The synthesised polyurethanes exhibited a significant increase in their 
mechanical properties because of the addition of a supramolecular assembly within the repeat 
unit of the resultant polyurethanes. Additionally, the resultant supramolecular elastomers 
demonstrated self-healing capabilities.
The General and Specific Experimental Procedures are given at the end of each Chapter
— as are the References relevant to a specific Chapter. The thesis also features an Appendix 
section that contains the analytical data for Chapters 2–4.