Design, synthesis and characterisation of new photo-reversible thyminyl-based polymers
thesisposted on 06.02.2017, 05:42 by Johnston, Priscilla
Traditionally, polymers have been designed to possess irreversible covalent bonds between the repeating units in polymer molecules. However, significant interest has more recently been directed toward the synthesis polymers that are held together by reversible covalent or non-covalent bonds. Linear polymers that incorporate reversible bonds are particularly interesting materials as they are able to polymerise and depolymerise in response to an external stimulus by the reversible connection and disconnection of monomers. Reversible linear polymers could therefore be a platform for the development of novel recyclable polymer systems. Several reversible polymerisations have been reported in the literature, however many of these are based on thermally-reversible reactions, like the Diels-Alder reaction, which require high energies for depolymerisation. Photo-chemical reactions, on the other hand, are considered to be green synthetic pathways because they can be conducted at ambient temperature and often in the solid-state. Thymine, one of the nucleic acid bases of DNA, has the propensity to reversibly photo-dimerise in the solid-state by a [2π+2π]-cycloaddition reaction. The focus of this Thesis is to exploit the reversible photo-dimerisation of thymine in order to develop a novel, reversible linear polymer system using di-thymine monomers. In this research, several new bis-thyminyl monomers were designed and synthesised, and their solid-state photo-reactivity was examined using films or crystals of the monomers. Where possible, the crystal structures of the bis-thyminyl monomers were determined by single crystal X-ray diffraction (SC XRD). The photo-reactions of the monomers were studied using a number of spectroscopic techniques such as UV-visible (UV-Vis), nuclear magnetic resonance (NMR) and infrared (IR). Where possible, the molecular weights of the photo-products were determined using gel permeation chromatography (GPC) and/or matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS). The thermal properties of some of the new materials were also evaluated using thermoanalytical techniques including thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Overall, these studies demonstrated the importance of monomer structure, alignment and conformation for the photo-polymerisation of bis-thyminyl compounds. Photo-reactivity investigations of thin monomer films revealed that the bis-thyminyl monomers only underwent partial multimerisation/oligomerisation. Nevertheless, the polymerisation yield was modestly improved in the films, when the alignment of the monomers was controlled using a polyvinylpyrrolidone (PVP) template. Quantum chemical studies confirmed that the reaction barriers and reaction energies of [2π+2π]-cycloaddition reactions were not significantly effected by the nature of the thymine substituents, but were instead governed by the alignment of the reactive thyminyl moieties. Moreover, based on crystallographic studies using simple N1-modified thymine derivatives, it was proposed that the ideal alignment of thyminyl moieties could be achieved if other disruptive intermolecular interactions were eliminated. Using this design principle, a number of bis-thyminyl derivatives were synthesised and successfully photo-polymerised by irradiating crystalline samples of the compounds with 302 nm UV light. The polymerisations were also found to be photo-reversible. In the solid state partial depolymerisation occurred to produce oligomers, but when the depolymerisation reactions were conducted using polymer samples suspended in MeCN, complete photo-depolymerisation was achieved in some cases. The polymers also demonstrated varied susceptibility to thermally-induced depolymerisation reactions. Ongoing work aimed at the synthesis of photo-reactive coordination compounds using bis-pyridyl thyminyl monomers is also presented.