Solar-Driven Upcycling of Polystyrene Enabled by Elemental Sulfur

Polystyrene (PS) is one of the most widely used plastics, and its notably low recycling rate has prompted the consideration of chemical upcycling of waste PS. Oxidative methodologies have been extensively investigated, employing molecular oxygen to upcycle PS into oxygenated products. Sulfur, a group congener of oxygen, has rarely been exploited in PS upcycling. Furthermore, elemental sulfur currently confronts an issue of an annual production surplus. Here, we report the coupcycling of PS and elemental sulfur into 2,4-diphenylthiophene and 1,3,5-triphenylbenzene by leveraging solar energy as the driving force. The practicality of this strategy is demonstrated by the rapid and efficient conversion of various postconsumer waste PS plastics under solvent-free, ambient air conditions. The photothermal effect of elemental sulfur and the generation of sulfur radicals at elevated temperatures are critical to enable this transformation. Experimental and computational studies of dimeric and tetrameric PS model compounds show that sulfur-radical-mediated hydrogen atom abstraction is the key to sustaining the reaction. These findings provide valuable insights into the photothermal behavior and reaction characteristics of elemental sulfur and hold significant implications for polymer conversion.