AbstractSubstantial concern over the extensive environmental harm caused by persistent plastic refuse is driving research towards sustainable polymer alternatives. In addition to providing renewability, the characteristic structures of bio-derived polymers afford unique handles to enhance macromolecular properties through stereochemistry and post-polymerisation functionalisation.
Chapter 2 of this thesis develops a novel strategy for the direct coupling of CO2 with diols to form a range cyclic carbonates. The method exhibits the broadest scope of any reported diol-CO2 coupling, including the first examples of 7-,8- and trans-5-membered cyclic carbonates synthesised directly from CO2. DFT was used in conjunction with experimental evidence to propose a cyclisation mechanism. The strategy was also applied to synthesis of three novel terpene-derived cyclic carbonates with the preliminary attempts into their ROP disclosed.
Chapter 3 explores stereodivergent mechanisms for the ROP of a dual-functional olefin/cyclic carbonate monomer. Through judicious choice of conditions, fine control over the stereochemical outcomes of the polymerisation is possible to form geometrically opposed, polycarbonate isomers. While high cis-content polymers were shown to be crystalline and ‘hard’, the trans isomer is fully amorphous and ‘soft’. This was exploited to form the first-reported single-monomer ABA triblock, which may have potential as a sustainable thermoplastic elastomer. The polycarbonate backbones were also shown to be amenable to thermoset formation.
Chapter 4 details efforts to vary the chain chirality of a polysaccharide mimetic as a means to modulate polymer properties. This study necessitated the development of a novel strategy for the highly regioregular polymerisation of oxetane-functionalised xylofuranose derivatives, D-Ox and L-Ox. DFT-modelling revealed a strong kinetic origin for the high regioselectivity observed in the polymerisation. The synthesised polymers were found to exhibit remarkable thermal properties with modulation from crystalline to amorphous states, demonstrated through variation of chain tacticity. Crystallinity could also be enhanced through stereocomplexation. Exposure of the xylofuranose hydroxyls facilitated reversible cross-linking between polymer chains, suggestive of prospective applications in self-healing materials.
Chapter 5 covers investigations into the amenability of D-Ox towards ROCOP with various co-monomers. The strategy provides facile access to 7 novel polyesters which demonstrate excellent, and in one case unique, thermal properties. Further to this, the xylofuranose core imparts hitherto inaccessible reactivity profiles for this class of polymer. The ROCOP strategy may also be applied to the synthesis of a polythiocarbonate, with control exhibited regarding the type of polymer linkage formed during polymerisation. Foundational inquiries into block-copolymers synthesis have also been undertaken in the hope of creating a family of highly tuneable polymers by way of tandem, post-polymerisation functionalisation.
Chapter 6 details collaborative work with Professor Charlotte Williams (University of Oxford). The aim of the study was to ascertain a theoretical understanding of the basis of the high isoselectivity exhibited by a series of In phosphasalen catalysts in the ROP of rac-lactide. This was achieved through extensive DFT-modelling, which established a clear kinetic origin for the experimentally observed enantioselectivites. Modelling of the metal-complex surface in a key transition state posited a link between the buried volume of the catalysts and the observed activation barriers to ROP.
Finally, Chapter 7 outlines the preliminary investigations into the synthesis of a series of ambiphilic iminophosphoranes. Although the isolated structures were found to be unsuited to ROP catalysis, it is hoped this initial work will provide a basis for future research.
|Date of Award||20 Jan 2020|
|Supervisor||Antoine Buchard (Supervisor) & Matthew Jones (Supervisor)|