## 3 Results and Discussion
After 24 h at room temperature, the adduct formation is indicated by the appearance of multiple peaks (Figure 4, Pre, \(t=24\) h, peaks 1-5) corresponding to the DA adduct [6; 14; 17]. The intensity of the peak at \(\delta\) 5.33 ppm (3) corresponding to the _endo_ conformation, is much larger than that of the peak at \(\delta\) 5.20 ppm (3) corresponding to the _exo_ conformation. The ratios of the _endo_/_exo_ conformations after 24 h at room temperature are 77/23%, respectively. After heating to 120 \({}^{\circ}\)C and cooling down for, respectively, 24 and 48 h, \({}^{1}\)H-NMR peaks are again observed that correspond to the DA adduct. However, when comparing the _endo_/_exo_ ratio to the gel obtained at room temperature, after heating, the integral of the _exo_ peak is larger; the ratios of _endo_/_exo_ after 24 and 48 h post-heating are 32/68%, respectively. The fact that the spectra recorded after 24 and 48 h post-heating are identical, indicates that the equilibrium conditions in the DA adduct formation are reached within the first 24 h or that the adduct is stable at room temperature. It is worth noting that multiple reactions can occur in a small temperature window close to the rDA temperature: adduct formation via the DA reaction, adduct breakage via the rDA reaction, and isomerization of the _endo_ to the _exo_ adduct [50]. The results obtained via the NMR experiments would indicate that the shift in the ratio of _endo_/_exo_ conformations after a heating cycle confirms that the _exo_ conformation is indeed favored at higher temperatures. This further justifies the conclusions drawn from the DSC spectra. Differences in thermal reversibility due to the influence of either _endo_ or _exo_ adducts can have an important impact on the material properties of thermoremendable polymers and networks in particular [51]. The influence of stereoselectivity on the mechanical properties of the prepared polyester resin was examined and will be discussed later on _(vide infra)_. The first (strong) indication of thermoreversible cross-linking is the ability to produce homogeneous test specimens using hot compression molding from material that had already been cross-linked in solution. This would not have been possible using a material containing non-reversible cross-links, as the final material would be a sintered compressed powder [26]. First, it is observed that during the first cycle, E' and E\" drop at elevated temperatures (most clearly seen for ratio 2:1, Figure 5). This is due to the buckling of the bars as a result of the sideways forces exerted by the DMTA machine sample holder. At elevated temperatures (starting from 132 \({}^{\circ}\)C for ratio 2:1 and 114 \({}^{\circ}\)C for ratio 4:1), a large decrease in moduli is observed; it is also in this region that tan \(\delta\) increases rapidly (Figure 5a-c). This change is due to the DA-rDA equilibrium that shifts to favor the rDA reaction. This results in de-cross-linking in the polymer resin, which yields a softening point at elevated temperatures after which the polymer exhibits viscous behavior, rather than elastic [52]. During the multiple DMTA cycles, the mechanical properties remain the same in the glassy region, whereas the temperature of the softening point shifts to higher values after each cycle. The shift of the softening point has previously been attributed to an increase in cross-link density [21]. However, recent publications attribute this to the change in conformation of the DA adduct from _endo_ to _exo_[53]. Since the _exo_ adduct is more thermally stable, a shift of the softening point to higher temperatures seems logical. The change in softening temperature implies that the rDA reaction becomes more difficult (due to the fact that the adduct becomes more stable) after multiple heating cycles [53]. When comparing the mechanical properties for the different ratios of polyester/cross-linker, a slight decrease in the storage modulus (E') is observed for the ratio of 4:1 (polyester/bismaleimide) with respect to 2:1.