Comprehensive characterisation of polypropylene pyrolysis mechanisms and chemical species formation pathways using ReaxFF MD simulations

• Reactive molecular dynamics (ReaxFF-MD) simulations integrated with experimental TGA data were applied to systematically elucidate the pyrolysis mechanisms of polypropylene. • Temperature-dependent evolution pathways of key intermediates (C₂H₄, C₃H₆, CH₄, C₂H₂) were fully mapped and quantified. • Distinct pyrolysis regimes from low-temperature β-scission to high-temperature radical-driven pathways were characterized at atomic scale. • Activation energies from simulation (140.84 kJ/mol) and experiment (128.64 kJ/mol) agree with a deviation of 9.49 %. • The comprehensive multi-scale kinetic framework offers insights for polymer recycling, thermal conversion, and reactor optimization. Polypropylene (PP) is among the most versatile polymers, widely used in packaging, containers, and films, owing to its lightweight, chemical stability, and durable properties. Given the global market size of over 120 billion USD and its continued growth, insights into thermal degradation are crucial for the recycling and waste management of PP. Nevertheless, its underlying mechanisms have not been fully explored, especially the detailed reaction pathways and gas volatiles produced at different temperatures. In this study, the pyrolysis mechanisms of PP were investigated through in-depth tracking of the molecular breakdown process using reactive molecular dynamics (ReaxFF-MD) simulations at elevated temperatures to accelerate sampling. Kinetics are benchmarked against thermogravimetric analysis (TGA) under various temperature conditions to validate the degradation rates. Using chemical tracing techniques, it was discovered that the primary driving breakage forces of PP pyrolysis were the cleavage of C-C and C-H bonds and bond-scission reactions..Propylene (C 3 H 6 ) is abundant at lower temperatures, whereas ethylene (C 2 H 4 ) and methane (CH 4 ) accumulate at higher temperatures; hydrogen (H 2 ) increases with temperature, and acetylene (C 2 H 2 ) remains minor and is only detectable at the highest temperature.Moreover, the pyrolysis kinetics were elucidated based on thetemperature-dependent product formation rates. The activation energy of the ReaxFF simulations of 140.84 kJ/mol was found to be reasonably consistent with the experimental results of 128.64 kJ/mol, supporting the ability of the ReaxFF model to capture key features of PP pyrolysis. Further analysis of the reaction pathways revealed the evolution patterns of intermediate and final products at different temperatures. Additionally, this study provides fundamental insights for future optimisation of pyrolysis processes to mitigate plastic waste, enhancing recycling and energy conversion operations.