Renewable Furanic Cyclobutanes as Potential Building Blocks for Circular Polymers via Diels–Alder Reaction

The circular economy represents an essential shift from the current linear “take–make–use–dispose” model toward a more sustainable economic paradigm. In polymer science, this shift requires prioritizing renewable raw materials, developing synthetic processes that generate minimal waste and incorporate most starting materials into the final product, and designing materials that can be easily repaired, reprocessed, or recycled. Hence, the development of dynamic network polymers, which are emerging as more sustainable alternatives to conventional materials such as thermosets, has become particularly relevant. Within this framework, the present study aimed to explore the potential of using 3,4-di(furan-2-yl)cyclobutane-1,2-dicarboxylic acid (CBDA) as a renewable chemical platform for synthesizing thermoreversible cross-linked polymers via the Diels–Alder reaction. CBDA was synthesized via UV-promoted dimerization of trans-3-(2-furyl)acrylic acid. This synthesis, performed using commercial UV LEDs, achieved over 90% conversion in 12 h. The Diels–Alder reactivity of CBDA toward mono-, bi-, and trifunctional maleimides was confirmed by 1H NMR, which revealed slow adduct formation at 65 °C and rapid retro-Diels–Alder dissociation at 110 °C. Network polymers in the form of gels were produced from CBDA and a trifunctional maleimide (TMI), although thermoreversibility in the bulk material was not observed. A sustainability assessment highlighted CBDA’s green credentials: an E factor of 1.6, 100% atom economy, and 87% reaction mass efficiency. In contrast, TMI synthesis presented a significant sustainability bottleneck, with a high E factor of 270 and a low reaction mass efficiency of 26%. However, the final Diels–Alder polymerization step showed excellent theoretical efficiency, with an atom economy of 100% and an estimated reaction mass efficiency of approximately 72%. Thus, this work is the first to link the green, accessible CBDA molecule with Diels–Alder chemistry, providing a foundation for its use as a building block in biobased, dynamic polymer networks.