# Bio-Based Aromatic Polyesters Reversibly Crosslinked via the Diels-Alder Reaction
## 3 Results and Discussion
First, a sample containing uncrosslinked polymers and bismaleimide in a 1:1 ratio of maleimide to furan groups in DMSO-d6 was prepared. Immediately after mixing, an NMR spectrum was recorded (spectrum I in Figure 2). Next the sample was heated to 50 \({}^{\circ}\)C for 3 h; this should ensure complete formation of the crosslinks. Afterwards, another NMR spectrum was recorded, and the signals attributed to the DA adduct of furan and maleimide could clearly be detected (spectrum II in Figure 2). There have been various reports describing [12; 22] the assignments of these signals based on model compounds. Interestingly, there are two clearly distinct peaks for the endo and exo adducts visible. To the best of our knowledge, this is the first direct confirmation for the formation of both species since similar works on different systems describe an NMR spectrum of a crosslinked polymer network [22], but only see a single peak which is attributed to both isomers. When the sample was subsequently heated to 150 \({}^{\circ}\)C for 5 min and a new spectrum was recorded. the adduct signals disappeared indicating that the RDA reaction had completely occurred (spectrum III in Figure 2). This is further proof, in addition to the gelation process and its reverse (vide supra), of the reversibility of this crosslinking system in solution. Being able to observe the two DA adducts in the NMR, an attempt was made to quantify their relative ratio as a function of temperature. This is quite interesting as such a shift in the ratio of these two adducts has been frequently invoked in the literature in order to clarify the thermal behavior of thermal reversible networks [12; 33], yet no clear proof has ever been reported for a crosslinked polymeric system. For the present system, in another experiment, the same solution containing uncrosslinked polymer and bismaleimide in a 1:1 maleimide:furan group ratio was created. A spectrum was recorded immediately after mixing (spectrum I in Figure 3). Next, the sample was heated to 65 \({}^{\circ}\)C for 2 h, and a new spectrum was recorded (spectrum II in Figure 3). Once again, the signals belonging to the DA adduct were found, and the ratio of endo:exo adducts was 58:42. It is well known that the DA reaction is an equilibrium reaction, and that the exo isomer is thermodynamically favored and the endo adduct is kinetically favored. When the sample \begin{table}
\begin{tabular}{c c c c c} \hline Ratio maleimides to furanes (mol:mol) & 1:1 & 2:1 & 3:1 & 4:1 \\ \hline \multicolumn{6}{c}{Gelation time (min)} \\ \hline \(1\),\(1^{\prime}\)-(methylenedi-1,\(4\)-phenylene)bismaleimide & 45 & 32 & 25 & 15 \\ \(1\),\(1^{\prime}\)-(methylenedi-1,\(4\)-phenylene)bismaleimide & & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Gelation times for various ratios of two different bismaleimides. was kept as 65 \({}^{\circ}\)C for 48 h and another spectrum was recorded (spectrum III in Figure 3), the ratio of endoexo isomers shifted significantly to 11:88. This constitutes, to the best of our knowledge, the first direct confirmation of the formation of two adducts in as well as their nature (thermodynamically and kinetically favored) for furan maleimide crosslinking reactions on a polymeric system. Indeed, other studies have shown a similar behavior, but only model compounds or singular adducts have been used [22]. The formation of the two DA adducts and their relative ratio might be of pivotal importance for thermal reversibility of the system, as suggested for the related ones [34]. Thermal reversibility was tested by cyclical DSC measurements, according to which the material was heated to 180 \({}^{\circ}\)C and subsequently cooled to room temperature eight consecutive times (Figure 4 for DPA-fur/tereph). The DA coupling is exothermic, while the RDA decoupling is an endothermic process. The integral of the corresponding peaks is an indication of the extent of the reaction.