Insight into the competitive reaction mechanism of polyethylene terephthalate (PET) pyrolysis by ReaxFF-based reactive molecular dynamics simulation

Competitive chain scission reactions between homolytic cleavage and concerted pathways are pivotal in the pyrolysis mechanisms of polyethylene terephthalate (PET) but remain insufficiently understood. In this study, Reactive Force Field Molecular Dynamics (ReaxFF MD) simulations, coupled with pyrolysis-gas chromatography- mass spectrometry (Py-GC-MS) measurement, were employed to unravel the PET pyrolysis mechanisms. The applicability and reliability of the simulation were verified against the measured product distributions, revealing the formation pathways of typical products. Furthermore, the time- and temperature dependent competition between homolytic cleavage and concerted reactions was quantified for the first time through the development of a product distribution-based mathematic model. Results indicated that the PET pyrolysis is dominated by random chain scissions, evident from the calculated apparent chain scission and cross-linking ratios. Homolytic cleavage reactions dominate the early stage, constituting 67.2% of the total reactions. As pyrolysis progresses, concerted reactions, mainly beta-cis cis elimination, emerge and account for approximately 22.8% in the middle stage and 30% in the later stage. During these stages, homolytic cleavage contributes around 29.9% and 21.0%, while cross-linking reactions make up about 47.3% and 49.0%, respectively. The competition between these two chain scission reactions, along with cross-linking reactions, results in a fluctuating trend in the ratio of free radicals-increasing, decreasing, and stabilizing. The paper provides detailed insights into the temporal evolution of the three reaction pathways, offering crucial information for a profound understanding of PET pyrolysis mechanisms and the development of a detailed kinetic model.