To determine optimal conditions (temperature and time)) synthesis was carried out at three different temperatures, with sampling at a time interval of 15 min. The first experiment was performed at the lowest possible temperature (120℃) due to the melting point of reactants. In the following experiments, the temperature increased by 20 degrees, up to the highest (160℃) in which the process was stable enough to end it just before gelling. Each reaction was ended when viscosity drastically increased, then the stirring was stopped, and the reaction mixture was poured into the PTFE tray. The soluble samples had a maximum reaction of functional groups of approximately 75%, which is consistent with polymerization theories. The conversion of COOH groups (pCOOH) determined from NMR spectra is shown in figure Figure .