# Bio-Based Aromatic Polyesters Reversibly Crosslinked via the Diels-Alder Reaction
## 3 Results and Discussion  
The overall strategy for the synthesis of the desired polymers relies on the possibility of modifying DPA and functionalizing it (Scheme 1, top) with a Diels-Alder reactive group (in this case, furan). The employed reaction conditions, based on similar ones for amidation reactions [30], entail the use of an intermediate (the imidazole peptide), subsequently reacted with fufurylamine to yield the desired product. This synthetic strategy allows for very controlled reaction conditions and a high purity and acceptable yield of the end product. Indeed, after crystallization, the DPA-fur was obtained in a 51% yield as an off-white powder characterized by a \({}^{1}\)H NMR spectrum with no relevant traces of impurities (Figure 1). DPA-fur was then subsequently polymerized with terephthaloxyl chloride in a two-phase system (Scheme 1, bottom) as described in the literature [31]. Clean polymer formation was evident from the complete disappearance of the signals at \(\delta\) 9.4 ppm in \({}^{1}\)H NMR (corresponding to the OH group on the DPA-fur monomer). Furthermore, aromatic protons from both terephthaloxyl chloride and the DPA-fur shifted significantly. Two distinct groups of peaks can be identified, one at \(\delta\) 7.42-7.12 ppm belonging to the aromatic protons next to an ester bond containing eight protons (4 \(o\)-from the DPA-fur, and 4 from the terephthaloxyl monomer), and another group at \(\delta\) 8.42-7.95 ppm containing four protons (the \(o\)-protons from DPA-fur away from the ester bond). Crosslinking of the prepared polymer via the Diels-Alder reaction was achieved by mixing it with an equimolar amount of bismaleimide in DMSO. The system was expected to crosslink by a reaction of the pendant furan moieties with the maleimide groups (Scheme 2). The reaction was initially carried out in DMSO in order to visually observe gel formation (indicative of the formation of a network structure), as well as to preliminarily investigate its reversibility as a function of temperature. Two types of bismaleimide were added in various ratios, and the gelation time (defined as the time it took the stirrer to stop spinning), a rough indication for the gel formation kinetics, was determined (Table 1). In the first instance, it is worth noting how both bismaleimides were able to induce gel formation, thus indicating network formation. It was also evident that the aromatic bismaleimide displayed faster kinetics than the aliphatic one. The significant difference in gelation times might be related to different reaction kinetics. On the other hand, this might also be due to the possibility of the aliphatic crosslinker (because of its flexible nature) to backbite coordination where both maleimide groups attach to the same polymer chain. The aromatic bismaleimide was expected to be far too rigid to be able to coordinate in this fashion [32]. Furthermore, increasing the maleimide to furan ratio led to a decrease in gelation time. This has been observed for other polymeric systems crosslinked in the same way [12] and is most probably related to the second-order kinetics (first-order in the bismaleimide) of the crosslinking reaction. Figure 1: NMR spectrum of DPA-fur. Generally speaking, the chemical reversibility of the furan-bismalimide systems has been already investigated and reported in previous studies (among others in our group [12; 22]). In all these studies, this reversibility (as proved by FT-IR, but also by using model compounds) is also coupled with the thermal and mechanical one, as also shown in this paper (see below). However, in these published works, no light could be shed on the chemistry of crosslinking as the prepared samples, in all cases insoluble networks, did not allow any accurate spectroscopic analysis. In the present case, the relatively low molecular weight allowed the elucidation of the mechanism of crosslinking at the molecular level by solution NMR.