Insights into the evolution mechanism of polyethylene pyrolysis: a combined experimental ReaxFF-MD and DFT study

• The pyrolysis mechanism of PE was studied by molecular dynamics and DFT. • The VRC-VTST theory was used to calculate the chemical reaction rate of the PE pyrolysis homolysis reaction. • SS-QRRK theory was used to study the influence of pressure on PE pyrolysis. Plastic pyrolysis is a promising technology for converting plastic waste into valuable fuels and chemicals; however, insufficient understanding of reaction mechanisms and kinetics limits reactor design and industrial application. In this study, the pyrolysis of polyethylene (PE) was systematically investigated through a combination of experiments with ReaxFF molecular dynamics (ReaxFF-MD) simulations and density functional theory (DFT) calculations. Experimental results for C 6 -C 29 products show that PE pyrolysis at 500 °C mainly yields wax (79.76 %) and oil (<C23, 20.26 %). ReaxFF-MD simulations reveal that PE conversion increases from 20 % at 2073 K to nearly complete decomposition at 2473 K within 200 ps, while ethylene (C 2 H 4 ) is the most abundant gaseous product. DFT calculations indicate that C–C bond cleavage (365.17–372.25 kJ/mol) is energetically more favorable than C–H bond cleavage (397.32–410.72 kJ/mol) during the initial pyrolysis stage. Subsequent β-scission reactions of macromolecular radicals exhibit energy barriers ranging from 127.98 to 163.69 kJ/mol, with the ethylene-forming pathway showing the lowest barrier (1127.98 kJ/mol). In addition, radical-assisted hydrogen abstraction reactions present much lower energy barriers (30.37–67.63 kJ/mol), significantly accelerating PE decomposition. The corresponding reaction rate constants and Arrhenius parameters for major reaction pathways were determined over a temperature range of 500–1000 K. These results provide quantitative mechanistic and kinetic insights into PE pyrolysis, offering theoretical guidance for optimizing product distribution and reactor design.