## 3 Results and Discussion
The enhanced thermodynamic stability of the _exo_ adduct explains the shift of the peak toward higher temperatures after the first cycles [26]. Although the next cycles appear to be very similar, a slight decrease in the peak area of the DA peak is observed. This may be due to the fact that the system does not have enough time to fully recover all the DA bonds [6; 26] or that the _endo/exo_ equilibrium is still shifting toward the more stable _exo_ adduct [1]. The equality of the subsequent thermal cycles is a strong indication of thermoreversibility. When comparing the different composition ratios of the polyester resin, the integral of the peak around 140 to 160 \({}^{\circ}\)C corresponding to the rDA reaction decreases in the line of PE-fur/Bism-2:1 > 4:1 > 8:1. These results confirm that lowering the amount of cross-linker reduces the amount of coupled groups and subsequently the energy needed to decouple them. This has also been reported for other systems with a fixed furan content and lower  
Figure 3: DSC measurements of **(a)** Tg of PE-fur. **(b)** PE-fur/Bism-2:1. **(c)** PE-fur/Bism-4:1. **(d)** PE-fur/Bism-8:1.  
cross-linking concentrations [26]. Furthermore, the temperature at which the rDA reaction occurs only changes marginally for the other composition ratios of the polyester and bismaleimide, which is in agreement with reports on other furan/bismaleimide networks [26]. \({}^{1}\)H-NMR was employed to give more insight into the stereoselectivity of the DA reaction. It is well known that the DA reaction of furan and maleimide forms a stereoisomeric mixture. Initially, the kinetically-favored _endo_ adduct is formed though at higher temperatures, the thermodynamically favored _exo_ conformation is obtained [45; 46; 47]. Hence, after heating the material above the rDA temperature, the more stable _exo_ adduct will mainly be formed upon cooling down from a high-temperature starting point. The DSC results indicate that after multiple heating cycles, the conformation of the formed adducts gradually shifts from _endo_ to _exo_, resulting in the higher thermal stability of the adducts, as evident from the shifting of the DSC RDA peak to higher temperatures. The shift from _endo_ to _exo_ configurations at higher temperatures is confirmed by multiple sources [45; 46; 47; 48; 49], often accompanied by NMR studies on model compounds. Figure 4 shows the NMR spectra of PE-fur and bismaleimide after several stages. First, a 10%wt amount of PE-fur and bismaleimide was dissolved in DMSO-d\({}_{6}\) in a teflon-capped NMR tube. The first spectrum was recorded directly after mixing, (Figure 4 top (Pre, \(t\) = 0)). Subsequently, the NMR tube was kept at room temperature for 24 h; during this time, a gel was formed, indicating cross-link formation via the DA reaction. Then, the next spectrum was recorded (Figure 4 (Pre, \(t\) = 24 h)). The NMR tube containing the gelated sample was heated at 120 \({}^{\circ}\)C for 30 min in an NMR tube oven, after which the oven was switched off. The sample was kept in the oven for 24 h, in order to slowly cool down the sample to room temperature. (Figure 4 (Post, \(t\) = 24 h)). Finally, the last spectrum was recorded after keeping the sample at room temperature for another 24 hours, (Figure 4 (Post, \(t\) = 48 h)). Multiple peaks corresponding to protons in the Diels-Alder adduct can be observed in the \({}^{1}\)H-NMR spectra. The peaks belonging to the proton that is adjacent to the adduct bridge are most clearly differentiated from the others. It is known from the literature that the peaks around \(\delta\)5.2 and \(\delta\) 5.3 ppm belong to this adjacent bridge proton and correspond to the _exo_ and _endo_ conformations, respectively [6; 17; 50]. Figure 4: \({}^{1}\)H-NMR spectra of PE-fur/Bism recorded at various moments before and after heating. It was found that initially (at \(t=0\) h) no adduct protons are present, showing that the DA reaction has not yet taken place.