Tandem Diels-Alder reaction overrules entropy: the gate to thermally stable, yet thermally recyclable furan-based polymers

The study investigates the kinetics and thermodynamics of the reversible tandem Diels-Alder (tDA) reaction between difuranic compounds and maleimides, leading to the quantitative formation of tDA adducts at rates comparable to their counterparts from the "classical" Diels-Alder (DA) reaction. The tDA adducts exhibited unprecedented thermal stability up to 250 degrees C, which is 100 degrees C higher than that of the DA adducts, owing to the higher activation energy (Ea) required for the initial intramolecular step of the reverse process. The stability of the tDA adducts was exploited in the AA + BB type polymerization of tetrafuranic monomers with bis(maleimides), yielding thermally stable (up to 200 degrees C) yet depolymerasable linear polymers with molecular weights of 10-20 kDa. Only furanic groups were identified as the end-groups of the resulting polymers, suggesting the possibility for post-polymerization and end-group modifications. NMR and GPC kinetic data offered insights into the intermediate formation of classical adducts during polymerization, as well as the stereochemistry of tDA adducts in the polymer chains. Combined thermal analysis (DSC, TGA, and TMA) provided a comprehensive understanding of the reverse DA reaction in the resulting materials. This relatively clean, catalyst- and byproduct-free, well-controlled process, which uses derivatives of biorenewables as monomers, heralds the formation of a new class of thermally recyclable polymers.