# Reactive Molecular Dynamics Simulations of Polystyrene Pyrolysis
## 2 Results and Discussions
### Effects of Heating Rate on the Pyrolysis Products  
Firstly, the temperature of the PS precursor was raised to 3000 K with different heating rates (5 K/ps, 10 K/ps, 50 K/ps, 100 K/ps, and 200 K/ps), and we examined the effect of different heating rates on pyrolysis intermediates. We found that PS pyrolysis occurs after 2000 K, so Figure 1a shows the trends in the number and types of pyrolysis species during the heating process above 2000 K. According to the changes in pyrolysis intermediates, the heating process can be divided into low-rate (5 K/ps, 10 K/ps) and high-rate heating (50 K/ps, 100 K/ps, 200 K/ps). Overall, the number and type of pyrolysis intermediates at the low-rate heating are more significant than those at the high-rate heating. A peak occurs in the number and type during the low-rate heating, which involves the further cross-linking transformation of intermediate products obtained from the early pyrolysis of PS into larger carbon-hydrogen macromolecules. However, the number and type of pyrolysis intermediates show a negative correlation with the heating rate during the high-rate heating, which indicates that a higher heating rate suppresses the thermal decomposition of the precursor.  
Then, 6 ns isothermal simulations at 3000 K were performed for both low- and high-rate heating systems. As Figure 1b shows, in this stage, the total number of pyrolysis intermediates increases while types decrease for every heating rate, which suggests that diverse molecules during the heating process undergo secondary cross-linking reactions that eventually converge to produce the same final pyrolysis equilibrium products. Therefore, the heating rate will not affect the equilibrium carbon products at 3000 K after the isothermal process.  
A more detailed pyrolysis products and tissue analysis further proves the above discussion. As shown in Figure 2, hydrocarbon intermediates increase for high-rate heating, while the number and type of hydrocarbon intermediates peak and gradually decrease for low-rate heating. As a result, the heating rate affects the type and quantity of pyrolysis intermediates and their appearance time. In experiments, [PERSON] et al. [28] reported the pyrolysis of PS in an intermittent autoclave, where PS was degraded at around 350 \({}^{\circ}\)C,and the graphitic carbon increased slightly until 425 \({}^{\circ}\)C. The graphitic carbon increased significantly at 450-500 \({}^{\circ}\)C [20; 22]. In the absence of a catalyst, PS undergoes pyrolysis at temperatures above 500 degrees Celsius, resulting in the production of methane, acetylene, ethylene, styrene, and styrene-like compounds, as well as the formation of graphitic carbon [20; 22]. [PERSON] et al. [29] studied the differences in PS' thermal decomposition and gasification characteristics by employing three gradual heating rates of 8-12 \({}^{\circ}\)C/min. It was demonstrated that a slower heating rate during the pyrolysis process generates a greater variety of substances. [PERSON] et al. [30] and [PERSON] et al. [31] discovered, through their experiments on the pyrolysis of polystyrene, that styrene comprises the highest proportion among the pyrolysis products. Thus, the experiments are consistent with our simulations.  
Figure 1: (**a**) Changes in the type and number of pyrolysis products during the heating process; (**b**) changes in the type and number of pyrolysis products during the isothermal process.  
Figure 2: Tissue and species changes during heating and isothermal simulations. To facilitate the comparison of the evolution of the carbon organization during pyrolysis, hydrogen atoms in the polymer are not shown in the figure, since they are released as hydrogen gas in the final pyrolysis products. Detailed hydrogen evolution can be found in the second part of the SI. (Please refer to Figures S3-S5 for more details.).