Switchable Polymerisation Catalysis of Mixed-Monomer Feedstocks

Our dependence on synthetic polymers is such that it is becoming increasingly unsustainable to develop these materials using conventional polymerisation methods. Additionally, there remains the challenge to achieve efficient and selective polymerisation of multiple monomers to form well-controlled copolymers. Many synthetic polymers currently at our disposal rely on petrochemical feedstocks for production and, with resources dwindling, alternative methods to produce similar materials must be sought. Moreover, as we strive for a greener future, biodegradable materials with renewable feedstocks are an increasingly important aspect in polymer chemistry. Oxygenated polymers, such as polyesters, are an important class of biodegradable materials with a wide range of properties and applications. Whilst initially realised by step-growth polymerisation methods, modern approaches to polyester synthesis like ring-opening polymerisation (ROP) and ring-opening copolymerisation (ROCOP), have elicited more control to ultimately produce a better class of polyester materials. Generally, step-growth polymerisations give slow rates, poor dispersity and molecular weights with limited control over sequencing. Recent advancements in the field have seen the development of catalysts able to switch between polymerisations, with a trigger. The Williams group have developed a number of catalysts which exhibit this switchable property and in their case the switch is brought about by a change in the chemical bond between catalyst and growing polymer chain. Initially, a dizinc catalyst was found to be efficient for the switch polymerisation of mixtures of epoxide, CO2, lactone, and anhydride. Faster zinc-magnesium heterodinuclear catalysts have since been developed which show promise for individual ROP and ROCOP polymerisations. As yet this catalyst class has not been tested for "switch" polymerisations. If successful, this will be another key step towards the selective polymerisation of mixed monomer feedstocks to give block copolymers with well-controlled sequencing. Furthermore, the incorporation of CO2 into the polymer backbone, through ROCOP with epoxides or anhydrides, brings the prospect of greener materials and adds value to waste CO2. This also has the potential for monomer scope to be explored and to utilise bio-derived monomers and renewable feedstocks in switchable catalysis polymerisation. This in turn will aid in reducing the impact of unsustainable methods in polymer synthesis on the environment. The initial aim for this project is to investigate whether the heterodinuclear zinc-magnesium catalysts are viable for ROP/ROCOP "switch" polymerisations of monomer feedstocks involving mixtures of epoxides, carbon dioxide, and anhydrides or lactones. Following on from this, monomer loadings, temperature, solvent, and other reaction conditions will be altered to monitor the effect on the polymerisation mechanism, and resulting block composition. The scope of monomers in the feedstock will also be explored with the intention of later conducting research into post-polymerisation modification as well as the structure-property relationship of the materials developed. Additionally, with the use of the faster zinc-magnesium catalyst, more complex multiblock copolymers will potentially be accessible. This project falls within the EPSRC Physical Sciences research area.