Exploring the depolymerization of an intrinsically recyclable hyperbranched polyester using density functional theory calculations Alexander Shaw 1 , Changxia Shi 2 , Matthew W. Coile 1 , Sai Phani Kumar 1 , Eugene Y.-X. Chen 2 , and Linda J. Broadbelt 1 1 Northwestern University, USA, 2 Colorado State University, USA The flow of waste polymeric materials into aquatic and terrestrial environments continues to intensify in line with increasing polymers use. This problem is further exacerbated by limitations in current approaches to polymer reuse and recycling. Improvements to end-of-life operations are imperative if the environmental impacts of polymer use are to be minimized. The development and application of circular polymers offers a promising pathway to the sustainable use of polymers going forward. The primary design feature of circular polymers is their capacity to be deconstructed to their constituent monomers which, through selection of appropriate catalysts, can be achieved under moderate reaction conditions. In this work, we have explored the catalytic depolymerization of a hyperbranched polyester to yield its bicyclic lactone monomer. The vicinal hydroxyl groups of the end-chain depolymerizing unit provide potential for the formation of both a 5-membered lactone containing monomer (M5) and a 6-membered lactone monomer (M6), however laboratory depolymerization experiments found a complete absence of the M6 form. Using density functional theory calculations, we observed that the epimerization about the carboxyl adjacent ring-carbon that is necessary to form the M6 monomer is energetically unfavorable under the depolymerization reaction conditions and that formation of the M5 monomer is significantly more facile. Additionally, we modelled ten unique depolymerization pathways that encompass all possible ester linkages within the polymer network and calculated reaction free energies and activation barriers for all steps. Our results showed that inclusion of explicit DMSO solvent molecules is essential to accurately capture the effect of hydrogen-bond interactions between the reactants and the solvent environment and to obtain polymerization free-energies that more closely conform experimental measurements of polymer ceiling temperature and monomer conversion.
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© The Author(s), 2023
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