# Bio-Based Aromatic Polyesters Reversibly Crosslinked via the Diels-Alder Reaction
## 3 Results and Discussion
The results suggest that the system shows thermoreversible coupling and decoupling: the heat capacity of the material did not change significantly over the course of all cycles, which is an indication of full reversibility [12; 35]. The peak at 150 \({}^{\circ}\)C is related to the RDA decoupling reaction, while the DA coupling is not immediately apparent in this graph as it occurred gradually while cooling the sample. Over the course of all cycles, the maximum of the RDA peak at 150 \({}^{\circ}\)C shifted to higher values. A similar shift has been reported for related systems [12; 22], for which this was firstly and hypothetically attributed to the RDA reaction being too slow with respect to the DSC timescale. However, this was later disproven, and the difference likely lies in a change in the ratio of endo and exo DA adducts. It has previously been established [34] that the exo isomer is thermodynamically favored, which makes it likely that the kinetically favored endo adduct is initially formed in excess. Nevertheless, during the course of heating and cooling this equilibrium shifts towards  
Figure 2: NMR spectra of uncrosslinked polymers and bismaleimide; (**I**): no reaction, (**II**): 3 h at 50 \({}^{\circ}\)C, (**III**): 5 min at 150 \({}^{\circ}\)C. Inset: both regioisomers and assignments. the more favored exo product, which in turn should have a higher temperature of RDA decoupling. This is in striking agreement with the NMR experiments (vide supra) and thus provides direct conformation of the formation and importance of the adducts on the thermal behavior. In order to assess the mechanical properties of the material, DMTA measurements were performed. The formation of solid, homogeneous bars is an indication of the reversibility of the compound, as without reversible crosslinking taking place, only a compressed  
Figure 4: DSC measurements of reversible network formation for DPA-fur/terephthal. Figure 3: 1H-NMR spectra of unreacted polymer and crosslinker (**I**) after 2 h (**II**) and 48 h (**III**) at 65 \({}^{\circ}\)C (only the endo/exo protons are shown). powder would be obtained. In order to elucidate the effect of the crosslinking degree on the softening temperature (tan (\(\delta\)) peak), various samples containing different maleimide:furan ratios were prepared. Figure 5 shows G\({}^{\prime}\) and G\({}^{\prime\prime}\) as well as the tan(\(\delta\)) for samples with 1:1 1:2 and 1:10 mol:mol ratios of maleimide:furan groups. The softening temperatures were 137 \({}^{\circ}\)C, 135 \({}^{\circ}\)C, and 120 \({}^{\circ}\)C for the 1:1, 1:2, and 1:10 ratios of maleimide:furan, respectively. The effect of the MAL:FUR ratio on the softening point was less expressed than that reported for the related systems (Figure 6). A highly substituted (60%) polyketone-based system (PK50) displays a drop in softening temperature from 102 \({}^{\circ}\)C to 86 \({}^{\circ}\)C upon changing the MAL:FUR ratio from 1:1 to 1:2 (halving the amount of bismaleimide). An additional halving of the amount of bismaleimide led to an even larger decrease of the softening point (22 \({}^{\circ}\)C lower). A similar trend was observed when looking at the system obtained by the copolymerization of bis(hydroxymethyl)furane and succin acid (PPS) [36]: when halving the amount of bismaleimide, the softening point dropped from 38 \({}^{\circ}\)C to 9 \({}^{\circ}\)C. Since this value is below the T\({}_{\text{g}}\) of the uncrosslinked material (15 \({}^{\circ}\)C) [37], the authors attribute this to a disturbance in the chain packing due to the presence of a crosslinker. For DPA-fur, the effect of halving the amount of bismaleimide on the softening temperature was practically insignificant; only a drop of 2 \({}^{\circ}\)C was observed. The large difference in the softening point for the other systems probably stems from the low T\({}_{\text{g}}\) of the uncrosslinked polymer. When the amount of crosslinker was reduced, the material behaved increasingly similar to a branched polymer structure than a three-dimensional network.