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We developed thermally self-healing polymeric materials on the basis of furan-functionalized, alternating thermosetting polyketones (PK-furan) and bis-maleimide by using the Diels-Alder (DA) and Retro-Diels-Alder (RDA) reaction sequence. PK-furan can be easily obtained under mild conditions by the Paal-Knorr reaction of the polyketones with furfurylamine. The highly cross-linked polymers can be thermally remended to complete recovery in fracture loading, whereas the remending process can be repeated multiple times without any loss in mechanical properties. It is found that the achieved self-healing ability of this easily accessible system provides full recyclability and reworkability, which often is perceived to be difficult or impossible for thermosetting polymers. The simplicity of the synthesis, the broad range of available polyketone precursors, and the striking healing ability (kinetics and efficiency of mechanical properties recovery) of this system could expand the scientific understanding of self-healing materials and introduce the cradle-to-cradle concept for thermoset-based plastics and composites.

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A proof of principle for the use of Diels–Alder chemistry as a thermoreversible cross-linking tool for rubber products is demonstrated. A commercial ethylene-propylene rubber grafted with maleic anhydride has been thermoreversibly cross-linked in two steps. The pending anhydride rings were first modified with furfurylamine to graft furan groups onto the rubber backbone. These pending furans were cross-linked with a bismaleimide via a Diels–Alder coupling reaction. The newly formed Diels–Alder cross-links break at elevated temperatures (>150 °C) and can be re-formed by thermal annealing (50–70 °C). Reversibility of the rubber network was proven with infrared spectroscopy and on the basis of the mechanical properties. Furthermore, reversibility was also shown in a practical way, i.e., by cutting the used material into pieces and pressing them into new samples displaying comparable mechanical properties (impossible for conventionally cross-linked rubbers). The physical properties of the resulting products are comparable to those of conventionally cross-linked EPDM rubber and superior compared to those of their non-cross-linked precursors.

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