ED Sciences Chimiques
Photocatalytic conversion of polystyrene in homogeneous solution and in aqueous dispersion
by Anne-Lise DE ABREU (Laboratoire de Chimie des Polymères Organiques)
The defense will take place at 14h00 - Salle de Conférence 3ème EST Institut des Sciences Moléculaires Bâtiment A12 — 351 Cours de la Libération 33405 TALENCE cedex
in front of the jury composed of
- Daniel TATON - Professeur des universités - Université de Bordeaux - Directeur de these
- Louis FENSTERBANK - Professeur des universités - Collège de France - Rapporteur
- Didier GIGMES - Directeur de recherche - Institut de Chimie Radicalaire - Rapporteur
- Dario BASSANI - Directeur de recherche - Institut des sciences moléculaires - CoDirecteur de these
- Alexandra TER HALLE - Chargée de recherche - Softmat - Examinateur
Chemical recycling of plastic waste remains critically underdeveloped, with less than 1% of discarded plastics undergoing chemical valorization. Polystyrene (PS), a widely produced plastic (~20 million tons annually), is particularly resistant to chemical recycling methods. Photocatalytic strategies have recently emerged as promising routes for PS upcycling into value-added chemicals. Herein, we investigate the photocatalyst 9-mesityl-10-methylacridinium perchlorate (MA), known for its strong photooxidative properties. We demonstrate that MA enables the efficient photo-upcycling of diverse PS grades—including post-consumer waste—under visible or solar irradiation, affording up to 40% yield of benzoic acid, 40% yield of formic acid, and a limited quantity of acetophenone. Spectroscopic analyses (IR, MALDI-TOF) confirm the formation of carbonyl-terminated oligomers. These observations are consistent with a mechanism involving hydrogen atom transfer (HAT) from benzylic positions by reactive oxygen species (ROS) generated via the photoinduced reaction between MA and O₂. This step is followed by the formation of peroxyl radicals through O₂ addition to the benzylic radical and subsequent β-scission of the polymer chain, leading to the observed oxidation products. To develop a more sustainable process, we further examined PS photodegradation in aqueous dispersions using anthraquinone as a photocatalyst. Latexes obtained by ab initio polymerization or post-dispersion were used as colloidal PS models. Comparisons with homogeneous solution-phase degradation revealed significative mechanistic divergences between the two media. Molar mass-dependent studies, combined with electron paramagnetic resonance (EPR) measurements, identified distinct radical pathways operating in aqueous dispersions versus homogeneous solutions. These findings refine the mechanistic understanding of PS photochemical transformation and highlight the potential of photocatalysis as a viable strategy for sustainable plastic upcycling.