(465d) Multiscale Computational Calculations for Ceiling Temperature of Ring Opening Polymerization

Redesigning next-generation circular polymers that possess intrinsic chemical recyclability and/or biodegradability shows promising potential in tackling the current plastics crisis.^1,2 The primary obstacle of this chemical recycling technique is that not all polymers can be efficiently depolymerized. However, a unique class of polymers, which are synthesized through the opening of cyclic monomers, presents a promising opportunity for depolymerization. Thus, the development and optimization of ring-opening polymerization (ROP) are timely in the realm of sustainability.³ The Gibbs free energy of polymerization is expressed as ΔG_P ≡ ΔH_P – TΔS_P, where T represents temperature, and ΔH_P and ΔS_P denote the enthalpy and entropy of polymerization, respectively.⁴ The ceiling temperature (T_c) is the temperature at which the rate of polymerization of monomers is equal to the rate of depolymerization of polymers, making it a crucial property in the design of new monomers for the development of circular polymers.²

Although the experimental measurement of ΔH_P and ΔS_P is feasible, computing these values in silico for high-throughput screening is attractive yet remains a challenge. This limitation has hindered our ability to achieve our ultimate objective of designing depolymerizable polymers with desirable T_c values and other attractive properties. Recently, Tran et al. reported a workflow to calculate ΔH_P for ring-opening reaction that employs size-dependent loop structures to represent finite-size polymer models without any artificial effects resulting from different end groups.⁵ However, the workflow does not address the calculation of ΔS_P, which depends on a well-represented solvent environment, which cannot be captured by gas-phase calculations.

In this study, we developed a robust, quantitative, and highly accurate method for computing both ΔH_P and ΔS_P. Our approach takes into account the local solvation environment differences that arise from monomer polymerization in multiple solvents and varying monomer/solvent concentration ratios. Our workflow involves the design of realistic extrapolative models for monomers and polymers, followed by extensive sampling of their configuration spaces using ab initio molecular dynamics (AIMD) simulations in conjunction with density functional theory (DFT) calculations. Our computational results aim to assist, guide, and effectively narrow down the extensive design space of circular polymers.

References

(1) Borrelle, S. B.; Ringma, J.; Law, K. L.; Monnahan, C. C.; Lebreton, L.; McGivern, A.; Murphy, E.; Jambeck, J.; Leonard, G. H.; Hilleary, M. A.; Eriksen, M.; Possingham, H. P.; De Frond, H.; Gerber, L. R.; Polidoro, B.; Tahir, A.; Bernard, M.; Mallos, N.; Barnes, M.; Rochman, C. M. Predicted Growth in Plastic Waste Exceeds Efforts to Mitigate Plastic Pollution. Science 2020, 369 (6510), 1515–1518. https://doi.org/10.1126/science.aba3656.

(2) Shi, C.; Reilly, L. T.; Phani Kumar, V. S.; Coile, M. W.; Nicholson, S. R.; Broadbelt, L. J.; Beckham, G. T.; Chen, E. Y.-X. Design Principles for Intrinsically Circular Polymers with Tunable Properties. Chem 2021, 7 (11), 2896–2912. https://doi.org/10.1016/j.chempr.2021.10.004.

(3) Tardy, A.; Nicolas, J.; Gigmes, D.; Lefay, C.; Guillaneuf, Y. Radical Ring-Opening Polymerization: Scope, Limitations, and Application to (Bio)Degradable Materials. Chem. Rev. 2017, 117 (3), 1319–1406. https://doi.org/10.1021/acs.chemrev.6b00319.

(4) Greer, S. C. Physical Chemistry of Equilibrium Polymerization. J. Phys. Chem. B 1998, 102 (28), 5413–5422. https://doi.org/10.1021/jp981592z.

(5) Tran, H.; Toland, A.; Stellmach, K.; Paul, M. K.; Gutekunst, W.; Ramprasad, R. Toward Recyclable Polymers: Ring-Opening Polymerization Enthalpy from First-Principles. J. Phys. Chem. Lett. 2022, 13 (21), 4778–4785. https://doi.org/10.1021/acs.jpclett.2c00995.