Thermal thresholds for styrene recovery: overcoming kinetic limits in polystyrene pyrolysis

Abstract The escalating accumulation of polystyrene (PS) waste has driven the development of chemical recycling technologies. However, optimizing the yield of valuable styrene monomers remains challenging due to complex and poorly understood product distributions. Therefore, this study aimed to investigate the competitive reaction mechanisms of PS thermal degradation, providing a theoretical foundation for experimentally observed temperature-dependent selectivities. The pyrolysis products were analyzed using Gas Chromatography-Mass Spectrometry (GC–MS), while reaction pathways, energetics, electronic reactivity, and theoretical temperature-dependent kinetics (623–723 K) were analyzed using Density Functional Theory (DFT) (ωB97X-D3/6-31G(d, p), ORCA 6.1.1), and the Eyring–Polanyi equation. Results revealed that low-temperature pyrolysis (350 °C) predominantly produced heavy oligomers and dimeric compounds, exhibiting a GC-MS peak area of 12.12% for 1,1’-(1,3-propanediyl)bisbenzene, while raising the operating temperature to 400 °C could maximize the styrene relative peak area up to 28.56%. Computational chemistry revealed that intramolecular hydrogen transfer (backbiting) was the kinetically and thermodynamically dominant initiating step (Δ G ‡ = 14.68 kcal/mol; Δ G = -5.23 kcal/mol), which mechanistically predicts a high theoretical accumulation of mid-chain radicals. At lower temperatures, a kinetic bottleneck occurred due to the high activation barrier of mid-chain β-scission (Δ G ‡ = 27.85 kcal/mol). Reaching a temperature of 400 °C provided sufficient thermal excitation to overcome this barrier and simultaneously accelerate direct end-chain unzipping (Δ G ‡ = 24.04 kcal/mol). This study highlights its key novelty by directly bridging quantum-mechanical theory with macroscopic reactor operations, demonstrating that the theoretically calculated unraveling of the kinetic bottleneck precisely matches the experimentally observed surge in styrene selectivity.