Metal-free and room-temperature photodegradation of styrene-incorporated polymers: A visible light-induced HAT approach

Abstract

This thesis focuses on the development of photochemical strategies for the degradation of non hydrolyzable vinyl polymers, with the main emphasis on polystyrene (PS) degradation under visible light. Commodity vinyl polymers such as PS and poly(methyl methacrylate) (PMMA) are widely used because of their durability, low cost, and useful material properties. However, their stable carbon based backbones make their degradation and recycling difficult under mild conditions. Therefore, new approaches are needed to enable more practical and sustainable degradation pathways. In the first part of the study, the degradation of post consumer expanded polystyrene waste was investigated using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) under visible light irradiation. DDQ was used as an organic photoactive compound capable of promoting hydrogen atom transfer from the benzylic C-H bonds of PS. The reaction was examined under different conditions, including solvent, atmosphere, DDQ concentration, polymer concentration, irradiation wavelength, and reaction time. The system was able to operate under ambient air and in ethyl acetate, which supports the practical character of the method. Under selected conditions, the molecular weight of PS decreased significantly, indicating efficient chain scission. Spectroscopic, chromatographic, gravimetric, and thermal analyses showed that degradation was accompanied by the formation of oxidized oligomers and low molecular weight aromatic products, including acetophenone, benzaldehyde, and benzoic acid derivatives. These results indicate that DDQ can act as an effective metal free organic mediator for the visible light degradation and upcycling of PS waste under mild conditions. As an additional approach, a preliminary copolymerization strategy was examined to introduce possible degradation sites into PMMA. Since PMMA does not contain readily abstractable benzylic hydrogens, styrene was incorporated into the polymer structure as a commercially available comonomer. Both block and random copolymers of methyl methacrylate and styrene were synthesized and characterized. These copolymers were then exposed to UVA irradiation in the presence of phenacyl bromide (PAB), which can generate bromine radicals capable of initiating hydrogen atom transfer from the styrene units. The degradation behavior was evaluated mainly by comparing the effects of solvent, atmosphere, and post-reaction treatment. The results showed that the presence of styrene units can provide reactive sites for chain scission, while the reaction medium and workup procedure influence the observed molecular weight changes. This part of the study was intended as a preliminary evaluation rather than a full optimization or mechanistic investigation. Overall, this thesis demonstrates that hydrogen atom transfer chemistry can be used as a useful tool for designing degradation pathways for persistent vinyl polymers. The DDQ system provides a metal free and mild approach for the degradation of PS waste, while styrene incorporation offers an initial route for exploring degradable PMMA copolymers. These findings contribute to the development of more sustainable polymer degradation strategies and provide insight into the design of future degradable or recyclable vinyl polymer systems.