Limiting Side-Reaction Impact on Recyclability of Diels-Alder Networks Containing Furan-Maleimide Resins

Among recyclable polymers, those functionalized with furan-maleimide chemistry using a Diels-Alder (DA) mechanism are a popular choice. Due to the retro-DA (rDA) reaction at high-temperature (80-140°C), a range of re-processable thermoplastic and thermosetting polymers are possible. However, potential side reactions are undesirable in the development of multiple self-healing, processing, and recyclable polymers. Hydroquinone (HQ), a well-known free-radical inhibitor, is often added to prevent the maleimide free-radical homopolymerization mechanism that occurs above 110°C, the predominantly cited side reaction. We demonstrate HQ’s inhibiting ability through spectral, thermal, and rheological analyses of thermosetting furan-maleimide polyethers (4+2 or 4+3 functionalities). FTIR and NMR confirm the epoxy-amine reaction is near completion, which is involved with furfuryl functionalization, ruling out amine-maleimide reaction. Further, by using non-stoichiometric ratios of the reactive groups and longer chain lengths, which changes the network connectivity and properties, the rate of maleimide homopolymerization is also affected.