(662d) Computational Insights into the Thermodynamic Properties of Circular Polymers

To address end-of-life issues with current synthetic polymers and the lack of proper disposal and recycling strategies, increased attention has been paid to the development of circular polymers that exhibit monomer-polymer-monomer closed loop potential for a circular economy.^[1-3] Circular polymers that are produced from renewable resources can be recycled back to their monomer/building block chemicals by overcoming the kinetic barriers of depolymerization.^[1-2] This can be achieved by thermal depolymerization at moderate to elevated temperatures (thermolysis) or via the use of catalysts to depolymerize polymers to their monomers in the presence of suitable solvents and moderate temperatures (chemolysis).^[2-4] However, the chemical recycling of circular polymers through chemolysis and thermolysis depends on the thermodynamics and kinetics of the de/polymerization. Thermodynamic parameters including the change in enthalpy (ΔH), entropy (ΔS) and free energy (ΔG) govern the thermodynamic equilibrium of de/polymerization. In addition, a concise measure of the de/polymerization, ceiling temperature (T_c), describes the relative rates of the de/polymerization reactions at different working conditions, including monomer concentration.^[1-3] The present work focuses on the design of circular polymers through density functional theory (DFT) calculations as a screening method. DFT calculations have been performed to predict the thermodynamic properties (ΔH, ΔS and ΔG) and associated T_c values for oligomers of α-methylene-γ-butyrolacone (MBL), γ-methyl-α-methylene-γ-butyrolacone (MMBL) and α-ethylidene-γ-butyrolacone (EBL). In addition, oligomers of methyl methacrylate (MMA) and MEA were simulated to compare the ΔH, ΔS, ΔG and T_c values with the lactone oligomers.

Gaussian 16 was used to perform geometry optimization and vibrational frequency analyses of the oligomers of MBL, MMBL, EBL, MMA and MEA. The dispersion corrected functional wB97XD along with the 6-31g(d,p) basis set were chosen for the geometry optimization followed by vibrational frequency calculations. Contributions from vibrational frequencies were quantified from -123 °C to 627 °C to evaluate the thermodynamic parameters as a function of temperature and associated T_c. The T_c values for MBL, MMBL, MMA and MEA were found to be 297 °C, 205 °C, 211.5 °C and 112.9 °C, respectively, at 1 M concentration of the monomer. The effects of chain length and the presence of substituents on ΔH, ΔS and T_c values were investigated. Further, a linear correlation has been developed to compare the simulated results with the experimental T_c values, providing an approach to screen various combinations of monomers. This study provides guidelines to experimentalists to synthesize new lactone-based circular polymers that exhibit low to moderate T_c values and an approach that can be extended to other families of polymers to achieve a closed loop circular economy.

References:

[1] C. Shi, L. T. Reilly, S. P.K. Vangala, M. W. Coile, S. R. Nicholson, L. J. Broadbelt, G. T. Beckham, E. Y.-X. Chen, Chem, 2021, 7, 1.

[2] R. A. Gilsdorf, M. A. Nicki, E. Y.-X. Chen, Polym. Chem. 2020, 11, 4942.

[3] J-B. Zhu, E. M. Watson, J. Tang, E. Y.-X. Chen, Science, 2018, 360, 398.

[4] B. A. Abel, R. L. Snyder, G. W. Coates, Science, 2021, 373, 783.