From Plastics to Rings and Back Again: Catalytic Recycling of Waste Oxygenated Plastics

Despite the exponential increase in plastic usage in the past century, little attention has been paid to dealing with the huge amount of plastic left at the end of life. A solution to the growing plastic waste is desperately needed. The majority of plastics are derived from petrochemicals and whilst mechanical recycling is well developed, it is typically associated with high energy costs and material degradation. Chemical recycling has the potential to create a circular plastic economy as the original starting materials can be recovered under mild conditions, allowing for their reincorporation into new plastics and avoiding the need for new raw materials.

Plastics are made up from long, chain-like molecules called polymers. These in turn are built up from a series of monomers. The order in which the monomers are connected, how they are connected and the individual structure of each monomer determines the properties of the polymer and which types of plastic they are used for. Chemical recycling breaks down the polymers in the plastic to their starting monomers which can then be reused.

Chemical recycling faces many challenges. Many commercial plastics are incompatible with chemical recycling due to their chemical make-up and therefore alternative plastics which can be chemically recycled are needed to replace these. Polymers which contain oxygen linkers, such as polycarbonates, are particularly promising as they can be broken down much more readily. Polycarbonates can be produced by combining carbon dioxide with an epoxide. Epoxides are three-membered rings which contain an oxygen atom. Altering the epoxide used affects the properties of the resulting polycarbonate. Whilst previous research has shown that various polycarbonates can be broken down to recover the starting epoxide, the selectivity towards these starting epoxides is a major issue as a cyclic carbonate product is often formed instead. Catalysts are key tools in overcoming this challenge as they can help to favour the epoxide formation as well as speed up the decomposition reaction. Although the catalysed decomposition of several different polycarbonates to their starting epoxides has been successfully demonstrated, for polycarbonates to be adopted on an industrial scale their properties must be competitive with current commercial polymers. Poly(cyclohexene carbonate) (PCHC) and poly(propylene carbonate) (PPC) are two types of polycarbonate which have the greatest potential to replace current commercial polymers. Unfortunately the chemical recycling of these polycarbonates has proven more difficult due to selectivity issues. Recent work by the Williams group has developed a new and highly effective catalyst to chemically recycle PCHC to its epoxide1 but the chemical recycling of PPC remains a challenge.

This project aims to develop catalysts for the chemical recycling of PPC back to its starting epoxide. Using PCHC as a case study, this project will investigate the mechanism behind the catalysed decomposition of polycarbonates and how changing the shape and structure of the polycarbonate affect its rate of decomposition. This information will be used to inform future catalyst and polymer designs. In addition, the chemical recycling of other types of oxygenated polymers, specifically poly(esters-alt-ethers), will be explored.

The ultimate goal is to develop effective methods of chemically recycling polymers to enable a sustainable circular plastic economy. This project falls within the EPSCR 'Manufacturing the Future' research area.

The primary impact of the OxICFM CDT will be the highly-trained world-class scientists that it delivers. This impact will encompass both the short term (during their doctoral studies), the medium term (subsequent employment) and ultimately the longer timescale defined by their future careers and consequent impact on science, engineering and policy in the UK.

The impact of OxICFM students during their doctoral studies will be measured by the culture change in graduate training that the Centre brings about - in working at the interface between inorganic synthesis and manufacturing, and fostering cross-sector industry/academia working practices. By embedding not only from larger companies, but also SMEs, we have developed a training regime that has broader relevance across the sector, and the potential for building bridges by fostering new collaborations spanning enormous diversity in scientific focus and scale. Moreover, at a broader level, OxICFM offers to play a unique role as a major focus (and advocate) for manufacturing engagement with academic inorganic synthetic science in the UK.

From a scientific perspective, OxICFM will be uniquely able to offer a broad training programme incorporating innovative and challenging collaborative projects spanning all aspects of fundamental and applied inorganic synthesis, both molecular and materials based (40+ faculty). These will address key challenges in areas such as energy provision/storage, catalysis, and resource provision/renewal necessary to enhance the capability and durability of UK plc in the medium term. To give some idea of perspective, the output from previous CDTs in Oxford's MPLS Division include two start-up companies and in excess of 30 patents.

It is not only in the industrial and scientific realms that students will have impact during their timeframe of their doctorate. Part of the training programme will be in public engagement: team-based challenges in resource development/training and outreach exercises/implementation will form part of the annual summer school. These in turn will constitute a key part of the impact derived from the CDT by its engagement with the public - both face-to-face and through electronic/web-based media. As the centre matures, our aspiration is that our students - from diverse backgrounds - will act as ambassadors for the programme and promote even higher levels of inclusion from all parts of society.

For our partners, and businesses both large and small in the manufacturing sector, it will be our students who are considered the ultimate output of the OxICFM CDT. Our programme has been shaped by the need of such companies (frequently expressed in preliminary discussions) to recruit doctoral graduates who can apply themselves to a broad spectrum of multi-disciplinary challenges in manufacturing-related synthesis. OxICFM's cohort-based training programme integrates significant industry-led training components and has been designed to deliver a much broader skill set than standard PhD schemes. The current lack of CDT training at the interface of inorganic chemistry and manufacturing (and the relevance of inorganic molecules/materials to numerous industrial sectors) heightens the need for - and the potential impact of - the OxICFM CDT. Our students will represent a tangible and valuable asset to meet the long-term skills demand for scientists to develop new materials and nanotechnology identified in the UK Government's 2013 Foresight report.

In the longer term, the broad and relevant training delivered by OxICFM, and the uniquely wide perspective of the manufacturing sector it will deliver, will allow our graduates to obtain (and thrive in) positions of significant responsibility in industry and in research facilities/institutes. Ultimately we believe that many will go on to be future research leaders, driving innovation and changing research culture, and thereby making a lasting contribution to the UK economy.