Covalent adaptable networks from functionalised unsaturated polyesters via Diels-Alder cycloaddition

Unsaturated polyesters (UPEs) containing fumarate groups were functionalised via the thiol-Michael addition of furfuryl mercaptan. This “click” reaction occurs rapidly in ambient conditions in the presence of an organic catalyst. The partial addition of the diene moiety allows the polyester chain to participate in Diels-Alder (DA) cycloaddition reactions with the remaining fumarate groups to form a crosslinked network. Such crosslinks dissociate at 90 °C through the retro-Diels-Alder (rDA) reaction and reform upon cooling to ambient temperature, highlighting the reprocessability of these polyester networks. The unsaturated groups in poly(propylene fumarate) were functionalised to varying degrees, producing a range of materials (12–100 % addition of furan) and it was shown via differential scanning calorimetry that 50 % functionalisation produced the highest enthalpy change for the rDA reaction. This composition also produced the highest bond strength in adhesive joints. Similar results were obtained with compositions comprising both 100 % furan functionalised polyester and unfunctionalised fumarate backbone.