## 3 Results and Discussion
### Synthesis of Poly(Esteramide)s Via Transesterification Approach  
For the transesterification approach, a standard oligoester itaconate (OE) and an oligo(ester amide) (OEA) were synthesized separately. Then, the desired poly(ester amide) was obtained by a transesterification reaction of the two oligomers. This synthetic protocol should prevent the formation of free amines during the final step due to the high stability of the amide bond, resulting in the suppression of the undesired side-reaction. The complete synthetic strategy is shown in Scheme 2. It is noteworthy to mention that a high conversion of the transesterification is necessary to ensure that all polymer chains have a sufficient amount of unsaturated bonds. This is important to obtain materials that are reactive enough towards UV-induced radical crosslinking.  
The synthesis of the itaconic acid-based OE was conducted as described in previous studies [20; 25]. A ratio of 1.25:1 (HDO:IA) was chosen to obtain an OE with a lower molecular weight distribution usually used in UV-curing resin applications.  
The synthesis of the OEA was not straightforward. In the first set of experiments, dimethyl succinate was reacted with hexamethylene diamine (0.2 eq.) and an excess of ethylene glycol (2.8 eq). However, the use of the succinate leads to the formation of a substantial amount of succinimide instead of the desired oligo amide (see SI for details and spectra). Therefore, the synthesis was then conducted starting from dimethyl adipate, as here, the formation of an amide is not possible. An excess of glycol was used to obtain an OEA with low molecular weight, and therefore, low viscosity suitable for resin applications. In addition, the excess ensured that the oligomers were OH-terminated to avoid any free amine groups in the final products, as these would again lead to undesired side-reactions during the transesterification step. After the reaction, the OEA was purified by extraction with dichloromethane to remove unreacted ethylene glycol and small dimeric ester fractions. Removal by distillation was not desired, as this would lead to further polymerization of the OEA. The NMR spectra of the OEA obtained was free of unreacted ethylene glycol. The amount of amide in the oligomer was found to be 14%, which was determined by integration of the CH\({}_{2}\)-signals next to the ester or amide bond, respectively (6 and 6'' for the ethylene glycol esters and three for the amide, see Figure 1b). The OEA was obtained as a white waxy solid.  
In the last step, the OE was reacted with the OEA in different ratios (95/05 to 60/40, see Table 1). In this transesterification step, the two oligomers were heated to 160 \({}^{\circ}\)C in the presence of Ti(OBu)\({}_{4}\) as a catalyst. The reaction was conducted under ambient pressure to avoid removal of ethylene glycol from the system, as this would lead to an increase in molecular weight. This was not desired, as the  
Figure 1: \({}^{1}\)H NMR spectra of a) Oligoester (OE), b) Oligoester amide (OEA), c) and PEA-005.  
co-polymers should exhibit low enough viscosities that are compatible with coating applications. The percentage of amide in the final co-polymer was calculated from the integrals of the NMR signals corresponding to the ester and amide groups.