Tools for the Design of Diels–Alder-Based Reversible Polymer Systems: Reactivity and Reversibility of Furan/Maleimide Adducts

Abstract

The equilibrium conversion of 16 model furan derivatives in Diels–Alder (DA) reactions with an aromatic bis-maleimide was investigated across the temperature range of 30–110 °C. The impact of furan substituents on the reactivity and reversibility of DA adducts was systematically investigated for establishing a useful toolbox of structure–thermal property relationships and clear guidelines for designing reversible furan-maleimide DA-based polymers. Two strategies were considered for furan integration in a polymer network. Monosubstituted furans mimicking grafted-onto moieties were first categorized into two groups: electron-donating substituents (e.g., methylene oxide/sulfide) achieved ≈90% conversion at 30–60 °C and electron-withdrawing groups (e.g., carboxyl) showed lower conversions (30–40%) at 30 °C but full reversibility at 100 °C. The heteroatom nature (N, O, or S) poorly affected equilibrium conversion. On the other hand, bisubstituted furans considered for furan integration in the polymer backbone through copolymerization exhibited distinct behaviors: furan-2,5-diyldimethanol (2,5-BHMF) mirrored monofunctional trends, while dimethyl 3,4-furandicarboxylate (3,4-DMFC) interestingly peaked at 76% conversion at 70 °C. For each studied pair, the endo/exo adduct ratios continuously evolved toward the more stable exo adduct, potentially modifying network reversibility over time. This library of equilibrium conversions with temperature covers the main commercially available furans and can be used as a selection guide when designing a reversible cross-linked system.