Exploring Polymeric Diene–Dienophile Pairs for Thermoreversible Diels–Alder Reactions

The thermoreversible Diels–Alder (DA) reaction provides access to reversible thermosets and thus a pathway to their circular recycling. However, the known thermoreversible diene–dienophile DA pairs are very limited and primarily involve the furan–maleimide pair; hence, there is a need to investigate novel pairs that can provide thermal reversibility in chemical binding at higher and lower temperatures. Hence, a set of 24 diene–dienophile pairs are screened for their tendency to undergo a Diels–Alder (DA) reaction at temperatures up to 140 °C. Of the 21 viable DA pairs, 16 DA pairs then successfully underwent gelation in an analogous polymer cross-linking system. The viability of the thermoreversible retro-Diels–Alder (rDA) reaction at elevated temperatures was then studied via a dissolution study, dynamic scanning calorimetry, and dynamic mechanical analysis. Two novel pairs were shown to undergo rDA degelation for the first time in a polymeric system. [Anthracene-9-methanol + citraconimide] and [anthracene-9-methanol + monomethyl fumarate amide] underwent degelation at 277 and 247 °C, respectively. Several additional novel gels showed dissolution at temperatures up to 250 °C, suggesting that their rDA processes may be accessible, albeit at higher temperatures. The partial self-healing of these two thermoreversible gels at temperatures of 100 and 150 °C, significantly below their degelation temperatures, is also demonstrated.