Thermal Degradation of Polystyrene via Radical Initiator Pathways

• Mateo Johnson, Chemical Engineering, San Jose State University

• Raul Lobo, Chemical & Biomolecular Engineering, University of Delaware

Polystyrene (PS) accounted for approximately 5% of global plastic waste in 2019, totaling ~15 million metric tons. Commercial forms such as Expanded Polystyrene (EPS), which consists of ~98% air, are widely employed in single-use packaging due to their low density and insulating properties. Cumulatively, PS waste is estimated to occupy up to 30% of landfill volume. Pyrolysis is a common method for plastic chemical recycling, but requires high temperatures to cleave polymer backbones and is thus energy-intensive. Radical initiators can be used to promote plastic depolymerization and it is underexplored. The presence of initiator-generated radicals could promote chain scission at lower temperatures, potentially increasing the rate of depolymerization. Herein, the effect of Dicumyl Peroxide (DCP) on the low-temperature degradation of PS is investigated. PS with varied amounts of DCP was loaded into a closed, nitrogen-purged Parr reactor and isothermally held for 30 minutes. Soluble components were analyzed via Gas Chromatography Mass Spectrometry (GC-MS) and Quantified via Gas Chromatography Flame Ionization Detection (GC-FID). A positive correlation was observed between initiator quantity (0-10%) and yield. Notably, product composition becomes more complex with increases in initiator concentration and temperature. Gel Permeation Chromatography (GPC) showed a decrease in molecular weight with increasing degradation temperature, consistent with progressive backbone cleavage. At a fixed temperature, samples containing DCP exhibited greater reductions in molecular weight compared to control groups, indicating that the presence of excess radicals promoted further chain scission. Differential Scanning Calorimetry (DSC) revealed a correlation between higher degradation and lower, broader glass transition temperatures, indicating a reduced, heterogeneous mix of chains. Nuclear Magnetic Resonance (NMR) spectroscopy confirmed the presence of terminal vinylidene groups formed during chain scissions, which may serve as reactive sites for further upcycling purposes into value-added terpolymers.