Molecular Energetics and Non-Isothermal Kinetics of Polystyrene Degradation: An Integrated Oligomeric DFT–TGA Study

Polystyrene (PS) thermal degradation involves localized molecular bond-cleavage events that are not directly equivalent to the apparent kinetic parameters obtained from bulk thermal analysis. In this study, a finite hydrogen-terminated PS oligomeric model was examined using density functional theory at the M06-2X/LANL2DZ level, whereas the non-isothermal degradation behavior of a PS sample was independently evaluated by thermogravimetric analysis under nitrogen. The computational analysis considered frontier molecular orbital distributions and site-specific thermodynamic descriptors associated with homolytic C–C cleavage and radical-mediated β-scission reactions. The calculated HOMO–LUMO gap of 742.62 kJ mol−1 indicated a comparatively large orbital-energy separation within the selected oligomeric model, while the localization of the frontier orbitals over aromatic and benzylic regions revealed a spatially heterogeneous electronic distribution. Homolytic C–C cleavage exhibited bond dissociation energies ranging from 414.09 to 481.24 kJ mol−1, demonstrating that the thermodynamic requirement for radical generation depends on the local molecular environment of the evaluated structure. The Gibbs free-energy changes calculated for the selected radical β-scission reactions ranged from 55.44 to 189.41 kJ mol−1. These quantities represent model-dependent reaction thermodynamics and should not be interpreted as activation barriers because transition states were not calculated. Thermogravimetric analysis showed systematic increases in Tonset and Tmax with increasing heating rate, consistent with kinetic delay and thermal-lag effects under non-isothermal conditions. The Kissinger method yielded a global apparent activation energy of 186.61 kJ mol−1, whereas the residual-mass-corrected Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods produced average apparent activation energies of 180.81 and 178.49 kJ mol−1, respectively, over α = 0.05–0.95. Across the same conversion interval, the FWO apparent activation energy increased from 143.10 to 221.71 kJ mol−1, while the KAS values increased from 140.10 to 220.23 kJ mol−1, indicating an evolving macroscopic degradation response with greater uncertainty toward high conversion. The computational and experimental datasets were therefore interpreted as complementary but non-equivalent scale-dependent descriptions: DFT compares the relative thermodynamics of selected molecular reactions within a finite isolated oligomer, whereas TGA characterizes the global apparent kinetic behavior of the condensed polymer sample. No direct numerical correspondence was established between the molecular reaction energies and the TGA-derived apparent activation energies, and no individual cleavage reaction was assigned to a specific conversion interval. Extrapolation of these results to high-molecular-weight, polydisperse, additive-containing, cross-linked, or environmentally aged PS microplastics should therefore be made with caution.