Novel Methods for the Photocatalytic Oxidative Degradation of Polymers

聚合物的降解，特别是对于聚乙烯或聚丙烯，目前分别以每年80公吨和60公吨的速度生产的一些最重要的聚合物仍然没有得到充分的了解。为了有效地处理我们所有的废塑料，一种方法将是设计出更好的材料，既能满足当前的性能标准，又能在环境中有效降解。聚乙烯确实会随着时间的推移而降解，但降解过程通常非常缓慢(通常是由于添加了抗氧化剂而故意减缓)。当暴露在氧气和光线下时，C-H键最初被氧化为C-OH，进而发生C=O键，然后容易发生光驱动的C-C键断裂反应(Norrish类型I和类型II反应)，这将把聚合物链断裂成更小的片段。一旦聚合物链低于5000克/摩尔分子质量，它们通常被认为是可被微生物消化的。[1]这些降解过程的机制及其潜在的基础化学知识知之甚少。更好地了解和控制降解过程将导致性能提高的材料，这些材料也可以在自然环境中降解。在这个项目中，我们将研究第一排过渡金属催化剂(Cat 1，例如铁基或锰基络合物)的使用，它将使长链烷烃与氧气氧化，结合光暴露促进Norrish C-C键裂解反应。后者也可以由额外的催化剂(Cat 2)催化。[2]将在GB研究实验室内进行长链烷烃和聚乙烯氧气氧化的初步催化剂筛选，并使用Roar内的高通量设施。将使用核磁共振和电子顺磁共振光谱进行机理研究，使用光纤引导光源在光照射下进行光谱测量。GB和ASR开发了使用LED光源和光纤将光引导到核磁共振管中进行核磁共振研究的实验装置，该装置已有效地用于研究氧化反应，并将与MR一起扩展到EPR测量。使用动力学中心软件(Bruker)分析时间分辨的核磁共振波谱测量，将能够对氧化和C-C键断裂反应进行动力学分析。时间分辨EPR谱将提供关于自由基寿命的信息，通过超精细(脉冲)测量在激发态中三重态激子的离域程度，以及通过分析电子自旋极化来识别能量转移伙伴。此外，主要负责产生自由基的光的波长将被确定。检测、量化和表征诺里什反应过程中形成的自由基，以及控制和催化它们的形成的能力，将是有效降解聚合物的关键。实际的聚合物降解研究将使用Q-SUN设备和聚合物分析技术，如GPC、TGA和DSC进行。这个项目的学生将在一个团队中工作，团队由1名最后一年的博士生和2名PDRA组成，他们分别参与由PolyMateria、UKRI和Innovate UK(GB实验室)资助的聚合物降解相关项目。此外，PDRA(由Leverhulme Trust资助)将提供EPR测量和分析(MR实验室)方面的专业知识，学生还将得到Spin-Lab经理的支持。此外，该项目将非常适合帝国理工学院http://www.imperial.ac.uk/ocean-plastic-solutions).的海洋塑料解决方案网络[1]：Ammara，A.，Dean，K.等，可降解和可生物降解聚烯烃概述，聚合物科学进展，36(2011)1015-1049。[2]：Hirashima，S.，Nobuta，T.，Norihiro，T.，Itoh，A.

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