Chemical recycling to monomer for an ideal, circular polymer economy

Abstract

The majority of post-consumer plastic waste is not recycled. Impediments to the recycling of commodity polymers include separation, impurities and degradation of the macromolecular structures, all of which can negatively affect the properties of recycled materials. An attractive alternative is to transform polymers back into monomers and purify them for repolymerization — a form of chemical recycling we term chemical recycling to monomer (CRM). Material recycled in this way exhibits no loss in properties, creating an ideal, circular polymer economy. This Review presents our vision for realizing a circular polymer economy based on CRM. We examine the energetics of polymerization and other challenges in developing practical and scalable CRM processes. We briefly review attempts to achieve CRM with commodity polymers, including through polyolefin thermolysis and nylon 6 ring-closing depolymerization, and closely examine the recent flourishing of CRM with new-to-the-world polymers. The benefits of heterocycle ring-opening polymerization are discussed in terms of synthetic control and kinetically accessible polymer-backbone functionality. Common chemical and structural characteristics of CRM-compatible ring-opening-polymerization monomers are identified, and the properties, benefits and liabilities of these recyclable polymers are discussed. We conclude with our perspective on the ideals and opportunities for the field.

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Fig. 1: Present and ideal polymer economies.
Fig. 2: Conceptual model of polymerization energetics.
Fig. 3: Comparison of highly exergonic and ergoneutral polymerization and depolymerization reactions.
Fig. 4: Polycarbonate synthesis and depolymerization modes with alicyclic epoxides and CO2 or alicyclic carbonates.
Fig. 5: Polycarbonate synthesis and selective CRM.
Fig. 6: Related polycarbonate synthesis and depolymerization reactions.
Fig. 7: ROP and CRM of lactones.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (award no. DE-FG02-05ER15687) and Cornell University.

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Coates, G.W., Getzler, Y.D.Y.L. Chemical recycling to monomer for an ideal, circular polymer economy. Nat Rev Mater (2020). https://doi.org/10.1038/s41578-020-0190-4

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