Novel Methods for the Photocatalytic Oxidative Degradation of Polymers

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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