Thermoreversible Natural Rubber Covalent Adaptable Networks via Diels–Alder Chemistry

Recycling sulfur-cross-linked natural rubber (NR) to enhance its sustainability and reduce waste remains challenging due to its permanent network and existing devulcanization approaches that are energy-intensive and inconsistent and risk backbone degradation. Here, we introduce a recyclable thermoreversible covalent adaptable network (CAN) based on furan-maleimide Diels–Alder (DA) chemistry, synthesized directly through batch mechanochemical processing. NR was functionalized with furfurylamine (FFA) and cross-linked in situ with bismaleimide (BM) to form dynamic DA bonds, and the resulting networks were characterized via VT-FTIR, XPS, DMA, DSC, and tensile testing to assess structural, thermal, and mechanical properties. The optimized formulation (20BM-5FFA-60) demonstrated large improvements in max stress relative to un-cross-linked furan-grafted NR, including a 121% increase in tensile strength, 51% and 26% increases in elastic modulus (at 50% and 100% strain, respectively), and a 138% rise in toughness. Variable-temperature FTIR and DMA analysis (temperature sweep, iso-strain stress relaxation) showed that retro-DA cleavage begins above 40 °C, with maximum dissociation between 75 and 115 °C. Solvent-assisted reprocessing of 20BM-5FFA-60 allowed the retention of 70% of its pristine mechanical integrity with a ∼7% reduction in storage modulus after 3 recycle cycles. These results demonstrate a scalable, sustainable route to reprocessable NR elastomers, where tuning BM and furan contents enables control over thermal response, cross-link density, and recyclability.