Closed-loop recycling of plastics enabled by dynamic covalent diketoenamine bonds

Abstract

Recycled plastics are low-value commodities due to residual impurities and the degradation of polymer properties with each cycle of re-use. Plastics that undergo reversible polymerization allow high-value monomers to be recovered and re-manufactured into pristine materials, which should incentivize recycling in closed-loop life cycles. However, monomer recovery is often costly, incompatible with complex mixtures and energy-intensive. Here, we show that next-generation plastics—polymerized using dynamic covalent diketoenamine bonds—allow the recovery of monomers from common additives, even in mixed waste streams. Poly(diketoenamine)s ‘click’ together from a wide variety of triketones and aromatic or aliphatic amines, yielding only water as a by-product. Recovered monomers can be re-manufactured into the same polymer formulation, without loss of performance, as well as other polymer formulations with differentiated properties. The ease with which poly(diketoenamine)s can be manufactured, used, recycled and re-used—without losing value—points to new directions in designing sustainable polymers with minimal environmental impact.

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Fig. 1: Reversible, dynamic covalent diketoenamine bonds.
Fig. 2: Closed-loop recycling from dynamic covalent PDKs.
Fig. 3: Synthesis of network poly(ketoenamine)s by ball-milling.
Fig. 4: Mixed polymer decolouration, additive removal and closed-loop recycling of fibre-reinforced composites.
Fig. 5: Re-formulation of PDK networks.
Fig. 6: Dynamic covalent behaviour of PDK networks.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information, and also from the authors upon request. Crystallographic data for compounds 3, 5 and TK-6 are available free of charge from the Cambridge Crystallographic Date Centre (www.ccdc.cam.ac.uk) under reference nos. 1891131, 1891132 and 189113, respectively.

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Acknowledgements

The technical scope of this work was supported by the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under US Department of Energy contract no. DE-AC02–05CH11231. K.E.L. was supported by the US Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internship (SULI) programme. Portions of this work, including organic and polymer synthesis and characterization, were carried out as a User Project at the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231.

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Authors

Contributions

B.A.H. and P.R.C. designed and planned the project. P.R.C. synthesized and characterized all PDK materials and their recyclability. A.M.S. synthesized small molecules and carried out experiments to measure the activation energies for amine exchange. K.E.L. carried out experiments to characterize the extent of network formation by ball-milling. B.A.H. and P.R.C. wrote the manuscript, with contributions from all co-authors.

Corresponding author

Correspondence to Brett A. Helms.

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Competing interests

B.A.H. and P.R.C. are inventors on US provisional patent application 62/587,148 submitted by Lawrence Berkeley National Laboratory that covers poly(diketoenamine)s, as well as aspects of their use and recovery.

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Supplementary information

Supplementary information

Supplementary synthetic procedures, materials characterization, closed-loop polymer recycling, bond exchange kinetics, and Supplementary Figures 1–17.

Crystallographic data

CIF for compound 3; CCDC reference: 1891131.

Crystallographic data

CIF for compound 5; CCDC reference: 1891132.

Crystallographic data

CIF for compound TK-6; CCDC reference: 1891133.

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Christensen, P.R., Scheuermann, A.M., Loeffler, K.E. et al. Closed-loop recycling of plastics enabled by dynamic covalent diketoenamine bonds. Nat. Chem. 11, 442–448 (2019). https://doi.org/10.1038/s41557-019-0249-2

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