Thermal Pyrolysis of Polystyrene Aided by a Nitroxide End-Functionality Improved Process and Modeling of the Full Molecular Weight Distribution

A significantly improved thermal pyrolysis process for polystyrene (PS) is reported and mathematically modeled, including the description of the time evolution of the full molecular weight distribution of the polymer during its degradation by direct integration of the balance equations without simplifications. The process improves the styrene yield from 28-39%, reached in our previous report, to 58-75% by optimizing the heating ramp during the initial stage of the pyrolysis process. The process was tested at 390 and 420 degrees C on samples of conventional PS synthesized via free-radical polymerization (FRP) and PS with a nitroxide end-functionality synthesized via nitroxide mediated polymerization (NMP) with three levels of the nitroxide to initiator (N/I) molar ratio: 0.9, 1.1 and 1.3. The NMP-PS produced with N/I = 1.3 generates the highest styrene yield (75.2 +/- 6.7%) with respect to the best FRP-PS yield (64.9 +/- 1.2%), confirming the trends observed in our previous study. The mathematical model corrects some problems of a previous model that was based on assumptions that led to significant errors in the predictions; this is achieved by solving the full molecular weight distribution (MWD) without assumptions. The model provides further insight into the initial stages of the pyrolysis process which seem to be crucial to determine the chemical paths of the process and the styrene yield, as well as the influences of the initial heating ramp used and the presence of a nitroxide end-functionality in the polymer.

Automated summary

A significantly improved thermal pyrolysis process for polystyrene is reported, which achieves higher styrene yield by optimizing the heating ramp during the initial stage of the process. The process was tested on samples of polystyrene synthesized via free-radical polymerization and nitroxide mediated polymerization, and the latter showed the highest styrene yield. The mathematical model used in this study corrects the errors of a previous model and provides further insight into the initial stages of the pyrolysis process.