## 3 Results and Discussion  
The addition of a chain stopper appears to have little effect on the polymerization reaction kinetics. The reaction still proceeds readily as is evident from the comparable yields under identical reaction conditions. There is, however, a small negative impact on the total yield: the precipitation step does not yield all the polymer as the molecular weight of the shortest fractions is limited and as such, they are still soluble in methanol. Furthermore, there is a very clear effect on the chain length as shown in the GPC elugraphs of five different batches prepared under the same experimental conditions. A significantly shorter polymer is obtained when comparing the obtained PE-fur with the long-chain PE-fur described before [1] (Figure 2). First, the presence of multiple peaks instead of a single smooth curve indicates that the sample consists of relatively small polymers containing relatively large monomeric repeating units. This is also reported for the synthesis of cyclic polycarbonates from various bisphenol analogs [38; 39; 40]. The difference in molar mass between the various peaks is more or less the same, ranging from 2300 g/mol to 2700 g/mol. Both the height and narrow appearance of the peaks, as well as the more or less constant difference in molar mass between them, are strong indications of the presence of short-chain polyesters, with each peak varying one monomeric repeating unit from its neighbor. The successful application of phenol as a chain stopper is evident from the elugrams of both polymers, comparing the long-chain polyester to the short-chain one (Figure 2), the latter obviously has a lower average Mw. The large peak at 200,000 g/mol in the long-chain PE-fur results from the limitations of the column used and it also contains all fractions with a higher Mw. Differential Scanning Calorimetry was used to determine the processability of the polyester as well as the thermal reversibility of the polyester/bismaleimide networks. The glass transition temperature of PE-fur lies at around 115 \({}^{\circ}\)C (Figure 3a). The long-chain polymer made without the use of phenol possesses a T\({}_{\text{g}}\) of approx5. 125 \({}^{\circ}\)C. The effect of shortening the polymer chains by employing phenol as a chain stopper has led to a decrease of approximately 10 \({}^{\circ}\)C in T\({}_{\text{g}}\). The decrease in T\({}_{\text{g}}\) with decreasing chain lengths is also seen for other linear bisphenolic polymers [41; 42] and can primarily be ascribed to the loss of entanglements due to the shortening of chains (Figure 3). Figure 2: GPC spectrum of multiple PE-fur batches (for reaction conditions see Experimental Section). When the temperature increases above 130 \({}^{\circ}\)C, a transition takes place, which corresponds to the rDA reaction [7; 9; 22], as evident from the peak in the DSC spectrum. This is observed in all three different resins and all of the heating steps. Subsequently, during cooling down, an exothermic peak is shown in the same region, indicating the occurrence of the DA reaction [22]. The position of the endothermic peak of the first cycle appears to differ from the subsequent cycles (most clearly observed in Figure 3b,d). This phenomenon has previously been reported for a furan/maleimide polyketone network [26] and a thermoresversible epoxy resin [23; 43]. The shift of the endothermic peak to higher temperatures could be explained by the transition of DA adducts from the _endo_ to the _exo_ conformation [1; 44] (the chemical structures of these adducts are provided in the Supplementary Materials). The step-in heat capacity observed for this transition instead of a proper melting/cooling peak is merely an artifact as the broad transition is cut in the DSC thermogram. This is necessary as running the DSC to a higher temperature entails the appearance of side reactions. The general shape is in agreement with the previous literature on this subject [23; 26; 43].