Ring opening polymerisation of novel sugar-derived cyclic monomers and strategies towards degradable materials

Abstract

The potential degradability of carbohydrate-derived polymers is but one of several features making them an attractive class of materials responsible for driving scientific research towards a more sustainable future. In addition to providing additional sustainability the characteristic structures of carbohydrates also have significant structural and stereo-chemical diversity that enables the production of highly functional polymers with a wide scope of properties. As such, the ring opening polymerisation of sugar-derived cyclic monomers presents a promising strategy towards degradable materials. In fact, through their incorporation into naturally un-degradable materials the methodology also presents a means to effectively enhance the degradability of pre-existing polymers.

Chapter 2 of this thesis explores the synthesis and radical ring opening polymerisation of novel cyclic ketene acetals derived from tri-O-acetyl-ᴅ-glucal and their potential application towards degradable materials. Quantitative 13C{1H} characterisation of the polymers revealed a hybrid of both polyester and polyacetal linkages and demonstrated the radical stabilisation capability of the ᴅ-glucal alkene functionality. Copolymerisation of the unsaturated cyclic ketene monomer with methyl methacrylate (MMA) as the principal monomer produced well-defined, degradable PMMA materials (via base initiated hydrolytic degradation) with similar thermal properties to pure PMMA.

Chapter 3 covers the copolymerisation of the previously reported ᴅ-mannose derived cyclic carbonate with L-lactide (LLA), which successfully produces degradable PLLA materials by allowing the tapered integration of carbonate linkages within the polylactic acid (PLA) backbone. The degradation was investigated in both alkaline and ‘natural’ environments, allowing for precise evaluation of the degradation potential for the PLLA materials. Thermal analysis of the tapered copolymers revealed improved thermal properties for the PLLA materials, specifically, increased thermal stability for the carbonate derived copolymers. In addition, the influence of exchanging oxygen atoms for sulfur on the polymerisability as well as the properties of the subsequent polymers was demonstrated.

Chapter 4 investigates the variations around the presence and position of sulfur in sugar-derived cyclic monomers, including their influence on the subsequent polymers. As such, a previously unexplored series of 6-membered cyclic carbonate and thiocarbonate monomers derived from tri-O-acetyl-ᴅ-glucal are synthesised, in which the number and position of sulfur atoms have been varied. The ring opening polymerisation is demonstrated and contrasted in terms of thermodynamics, polymer sequence, regioregularity and thermal properties. While the influence of the thiocarbonyl function varied across the series, the presence of a carbon-sulfur (C–S) bond in the polymerisable ring reduced the ring strain and subsequently the maximum monomer conversion at equilibrium. The onset of thermal degradation and the glass transition temperature decreased with the amount of sulfur in the polymer linkages, regardless of its position, and the degradation of the polymers under UV light was also explored.

Chapter 5 details the copolymerisation of cyclic (thio)carbonate monomers, derived from tri-O-acetyl-ᴅ-glucal, with LLA to successfully incorporate thiocarbonyl linkages into a polyester backbone. Copolymerisation produces tapered copolymers with similar thermal properties to PLA. The introduction of photosensitive sulfur-containing linkages results in unprecedented degradation of these PLLA materials under UV light, even at low monomer incorporations. Early investigations into real-life plastics objects have also demonstrated the potential to increase the environmental degradability of PLA without significantly effecting its material properties. Finally, the copolymerisation of the xanthate monomer was extended to the cyclic lactone monomers, ε-caprolactone and δ-valerolactone, which successfully expanded the substrate scope of the co-polymerisation methodology.

Finally, Chapter 6 outlines the preliminary investigations into the synthesis and the subsequent polymerisation of novel siloxane, and cyclic acetal monomers derived from tri-O-acetyl-ᴅ-glucal. The 8-membered siloxane monomer was amenable to both anionic and cationic polymerisation under mild reaction conditions, however, due to the uncontrolled reactivity of the monomer only low molar mass polymers were attained via these methods. In contrast, the unsaturated acetal monomer was successful copolymerised with glutaric anhydride, to produce amorphous co-polymers containing both acetal and ester linkages within the polymer backbone, as such, it is hoped this initial work will provide a basis for future research.

Date of Award	13 Sept 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Antoine Buchard (Supervisor) & Alex Cresswell (Supervisor)