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Recyclable and malleable thermosets enabled by activating dormant dynamic linkages

Abstract

Chemical recycling of polymers is critical for improving the circular economy of plastics and environmental sustainability. Traditional thermoset polymers have generally been considered permanently crosslinked materials that are difficult or impossible to recycle. Herein, we demonstrate that by activating ‘dormant’ covalent bonds, traditional polycyanurate thermosets can be recycled into the original monomers, which can be circularly reused for their original purpose. Through retrosynthetic analysis, we redirected the synthetic route from forming conventional C–N bonds via irreversible cyanate trimerization to forming the C–O bonds through reversible nucleophilic aromatic substitution of alkoxy-substituted triazine derivatives by alcohol nucleophiles. The new reversible synthetic route enabled the synthesis of previously inaccessible alkyl-polycyanurate thermosets, which exhibit excellent film properties with high chemical resistance, closed-loop recyclability and reprocessing capability. These results show that ‘apparently dormant’ dynamic linkages can be activated and utilized to construct fully recyclable thermoset polymers with a broader monomer scope and increased sustainability.

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Fig. 1: Synthetic strategies of polymers.
Fig. 2: SNAr in cyanurate exchange reactions.
Fig. 3: Preparation and characterization of PCNs.
Fig. 4: Chemical recycling of PCNs.

Data availability

Data supporting the findings of this study are included in the Article and its Supplementary Information. Data are also available upon request. Source data are provided with this paper.

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Acknowledgements

We thank D. Walba for the help with differential scanning calorimetry characterization. We acknowledge Colorado State University Analytical Resources Core (RRID SCR_021758) for gas chromatography–mass spectrometry and thermogravimetric analysis characterizations. K.Y. acknowledges the support from the National Science Foundation (grant CMMI-1901807). W.Z. acknowledges the support from University of Colorado Boulder.

Author information

Authors and Affiliations

Authors

Contributions

Z.L. and W.Z. conceived the concept and led the project. Z.L., H.C. and Y.R. performed the small-molecule model study, polymer synthesis, characterization and recycling. C.L. performed the dynamic mechanical analysis. Y.H. performed the differential scanning calorimetry tests. Z.L., Y.J. and W.Z. wrote the manuscript. All authors discussed and revised the manuscript.

Corresponding author

Correspondence to Wei Zhang.

Ethics declarations

Competing interests

Z.L. and W.Z. are coinventors on a provisional US patent covering the methods of polymerization and composition of matter presented in this work, filed through the University of Colorado Boulder (application no. 63/354,754). The other authors declare no competing interests.

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Nature Chemistry thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Small molecule study of cyanurate exchange.

a, No reaction between TETA and methanol was observed without TBD catalyst. b, The first step of exchange reaction between TETA and deuterated methanol can be considered as an irreversible pseudo first-order reaction when the deuterated methanol is used as solvent.

Extended Data Fig. 2 FTIR comparison.

a, FTIR spectra of PCN-A4 film and its corresponding monomers. b, FTIR spectra of PCN-A6 film and its corresponding monomers. c, FTIR spectra of PCN-A12 film and its corresponding monomers.

Extended Data Fig. 3 Chemical resistance test of PCN films.

a, Chemical resistance test for PCN-A4 film. b, Chemical resistance test for PCN-A6 film. c, Chemical resistance test for PCN-A12 film. The PCN films were cut into the rectangular shape and submerged in different solutions (1 M HCl, 1 M NaOH, 30% H2O2 and 1 M NaBH4); the top, middle and bottom photos were taken before submerging, after 48-hour submerging, and after drying, respectively. No change in appearance was observed for all the PCN films.

Extended Data Fig. 4 Chemical recycling of PCN-A12.

a, 1H-NMR spectra show that the film degradation in ethanol is clean (TMB, 1,3,5-trimethoxybenzene, used as internal standard) and the recycled DO-12 and TETA are in high purity. b, Nearly identical loss factors for the original and recycled PCN-A12 samples. c, Nearly identical FTIR spectra of the original and recycled PCN-A12 samples.

Extended Data Fig. 5 Chemical recycling of PCN-A4.

a, 1H-NMR spectra show that the film degradation in ethanol is clean (TMB used as internal standard). and the recycled TETA is in high purity. b, Nearly identical loss factors for the original and recycled PCN-A4 samples. c, Nearly identical FTIR spectra of the original and recycled PCN-A4 samples.

Extended Data Fig. 6 Chemical recycling of PCN-A6.

a, 1H-NMR spectra show that the film degradation in ethanol is clean (TMB used as internal standard) and the recycled TETA is in high purity. b, Neary identical loss factor for the original and recycled PCN-A6 samples. c, Nearly identical FTIR spectra of the original and recycled PCN-A6 samples.

Extended Data Fig. 7 Kinetic study of cyanurate exchange in polymer.

a, Bond exchange reaction can be triggered under heat in the presence of 23 mol% excess of diol monomers and 10 mol% of TBD. b, Stress relaxation test of PCN-A6-m at various temperatures. c, Arrhenius plot and its linear fitting. The activation energy was calculated to be 76.7 kJ/mol.

Supplementary information

Supplementary Information

Supplementary Figs. 1–26, Tables 1–5 and Discussion.

Source data

Source Data Fig. 2

Concentration data points.

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Lei, Z., Chen, H., Luo, C. et al. Recyclable and malleable thermosets enabled by activating dormant dynamic linkages. Nat. Chem. 14, 1399–1404 (2022). https://doi.org/10.1038/s41557-022-01046-4

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