Novel Methods for the Photocatalytic Oxidative Degradation of Polymers

Polymer degradation, specifically for polyethylene or polypropylene, some of the most important polymers currently produced at a rate of 80 and 60 Mt per annum, respectively, is still insufficiently understood. In order to deal effectively with all our waste plastic, one approach will be to design better materials that meet current performance standards, but are also able to degrade effectively in the environment. Polyethylene does degrade over time, but the degradation process is normally very slow (often intentionally slow due to the addition of anti-oxidants). When exposed to oxygen and light, initial oxidation of C-H bonds to C-OH and further to C=O bonds occurs, which are then prone to light-driven C-C bond cleavage reactions (Norrish type I and type II reactions), which will break down polymer chains into smaller fragments. Once the polymer chains are below 5000 g/mol molecular weight, they are generally considered digestible to microorganisms.[1] The mechanism of these degradation processes and their underlying fundamental chemistry is poorly understood. A better understanding and the ability to control the degradation processes will lead to improved performance materials that are also degradable within the natural environment.

In this project we will investigate the use of first row transition metal catalysts (Cat 1, for example Fe or Mn based complexes) that will enable the oxidation of long chain alkanes with oxygen, combined with light exposure to facilitate Norrish C-C bond cleavage reactions. The latter may also be catalyzed by an additional catalyst (Cat 2).[2] Initial catalyst screening for the oxidation of long chain alkanes and polyethylene with oxygen will be carried out in the GB research lab, as well as using high throughput facilities within ROAR. Mechanistic studies will be carried out using NMR and EPR spectroscopy, using fiber optic guided light sources to carry out spectroscopic measurements under light irradiation. The experimental setup for NMR studies using a LED source and fibre optics to guide light into NMR tubes has been developed by GB and ASR and has been effectively used to study oxidation reactions and this will be extended to EPR measurements together with MR. Analysis of the time-resolved NMR spectroscopic measurements is carried out using Dynamics Centre software (Bruker), which will enable kinetic analysis of the oxidation and C-C bond cleavage reactions. Time-resolved EPR spectroscopy will provide information on radical lifetimes, the extent of delocalization of the triplet exciton through hyperfine (pulse) measurements in the excited state, and identification of energy transfer partners through analysis of the electron spin polarization. Moreover, the wavelength of light primarily responsible for radical generation will be determined. Detection, quantification and characterisation of the radicals formed during the Norrish reaction and the ability to control and catalyse their formation will be essential for effective polymer degradation. Actual polymer degradation studies will be carried out at Polymateria using Q-SUN equipment and polymer analysis techniques such as GPC, TGA and DSC.

The student on this project will be working in a team consisting of 1 final year PhD student and 2 PDRA's who work on related projects on polymer degradation funded by Polymateria, UKRI and Innovate UK, respectively (GB lab). In addition, a PDRA (funded by the Leverhulme Trust) will provide expertise with EPR measurements and analysis (MR lab), and the student will also be supported by the SPIN-Lab manager. Furthermore, the project will fit very well within the Ocean Plastic Solutions Network at Imperial College http://www.imperial.ac.uk/ocean-plastic-solutions).

[1]: Ammala, A., Dean, K. et al., An Overview of Degradable and Biodegradable Polyolefins, Progress in Polymer Science, 36 (2011) 1015-1049. [2]: Hirashima, S., Nobuta, T., Norihiro, T., Itoh, A. Acceleration of Norris

Academic impact:
Recent advances in data science and digital technology have a disruptive effect on the way synthetic chemistry is practiced. Competence in computing and data analysis has become increasingly important in preparing chemistry students for careers in industry and academic research.

The CDT cohort will receive interdisciplinary training in an excellent research environment, supported by state-of-the-art bespoke facilities, in areas that are currently under-represented in UK Chemistry graduate programmes. The CDT assembles a team of 74 Academics across several disciplines (Chemistry, Chemical Engineering, Bioengineering, Maths and Computing, and pharmaceutical manufacturing sciences), further supported by 16 industrial stakeholders, to deliver the interdisciplinary training necessary to transform synthetic chemistry into a data-centric science, including: the latest developments in lab automation, the use of new reaction platforms, greater incorporation of in-situ analytics to build an understanding of the fundamental reaction pathways, as well as scaling-up for manufacturing.

All of the research data generated by the CDT will be captured (by the use of a common Electronic Lab Notebook) and made openly accessible after an embargo period. Over time, this will provide a valuable resource for the future development of synthetic chemistry.

Industrial and Economic Impact:
Synthetic chemistry is a critical scientific discipline that underpins the UK's manufacturing industry. The Chemicals and Pharmaceutical industries are projected to generate a demand for up to 77,000 graduate recruits between 2015-2025. As the manufacturing industry becomes more digitised (Industry 4.0), training needs to evolve to deliver a new generation of highly-skilled workers to protect the manufacturing sector in the UK. By expanding the traditional skill sets of a synthetic chemist, we will produce highly-qualified personnel who are more resilient to future challenges. This CDT will produce synthetic chemists with skills in automation and data-management skills that are highly prized by employers, which will maintain the UK's world-leading expertise and competitiveness and encourage inward investment.

This CDT will improve the job-readiness of our graduate students, by embedding industrial partners in our training programme, including the delivery of training material, lecture courses, case studies, and offers of industrial placements. Students will be able to exercise their broadened fundamental knowledge to a wide range of applied and industrial problems and enhance their job prospects.

Societal:
The World's population was estimated to be 7.4 billion in August 2016; the UN estimated that it will further increase to 11.2 billion in the year 2100. This population growth will inevitably place pressure on the world's finite natural resources. Novel molecules with improved effectiveness and safety will supersede current pharmaceuticals, agrochemicals, and fine chemicals used in the fabrication of new materials.

Recent news highlights the need for certain materials (such as plastics) to be manufactured and recycled in a sustainable manner, and yet their commercial viability of next-generation manufacturing processes will depend on their cost-effectiveness and the speed which they can be developed. The CDT graduates will act as ambassadors of the chemical science, engaging directly with the Learned Societies, local council, general public (including educational activities), as well as politicians and policymakers, to champion the importance of the chemical science in solving global challenges.