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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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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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.
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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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.
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.
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.
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.
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.
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.
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.
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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