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

Abstract

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 °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.

Reversed Controlled Polymerization (RCP): Depolymerization from Well-Defined Polymers to Monomers

“…These results are intriguing and suggest that ball-mill grinding could be a route to efficient depolymerization of more “challenging” polymers such as PS in bulk. Pyrolysis of PS prepared by nitroxide mediated polymerization has recently been reported to be enhanced (increased monomer yield) compared to PS from conventional radical polymerization, presumably due to thermally promoted homolytic cleavage of the chain-end nitroxide species enabling more efficient radical formation than relying on chain scission. , …”

“…Pyrolysis of PS prepared by nitroxide mediated polymerization has recently been reported to be enhanced (increased monomer yield) compared to PS from conventional radical polymerization, presumably due to thermally promoted homolytic cleavage of the chain-end nitroxide species enabling more efficient radical formation than relying on chain scission. 99,100 Depolymerization of polyolefins, such as polyethylene (PE) and polypropylene (PP), in a low energy and cost-effective manner would be a highly valuable development from an environmental standpoint, as polyolefins make up the majority of commodity polymers; yet their highly stable nature makes depolymerization very challenging. Recent pioneering work by Buchwald has shown the generation of propylene from commercial PE by employing a dual catalytic system whereby PE was first partially dehydrogenated to give unsaturated segments which then underwent catalytic isomerization by metathesis with ethylene.…”

Catalytic routes towards polystyrene recycling

“…25 In the case of PS, the T c has been reported to be 310 °C (277 °C in gaseous conditions, 397 °C when melted). 26 While in principle, to achieve complete chemical recycling to monomer (CRM) a temperature above the T c is necessary (Fig. 3), the reaction equilibrium can be driven by specific setup configurations, addition of solvents or product (monomer) removal.…”

Simulation time analysis of kinetic Monte Carlo algorithmic steps for basic radical (de)polymerization kinetics of linear polymers

“…Specifically for the depolymerization case study, it should be noted that the availability of reliable kinetic parameters for elementary reactions is far less advanced compared to polymerization. 72,80,[93][94][95] The current work, which aims at reducing simulations times of elementary reaction based kMC modelling tools however contributes to resolving this shortcoming, as it will facilitate parameter tuning/regression using these advanced elementary reaction based models in the future.…”