Synthesis and Characterization of Novel Non-Isocyanate Polyurethanes Derived from Adipic Acid: A Comprehensive Study

The increasing quest for greener and more sustainable polymeric materials has gained interest in the past few decades. Non-isocyanate polyurethanes (NIPUs) have attracted attention considering that they are produced through less toxic methods compared to the conventional polyurethanes (PUs) obtained from petroleum resources and toxic isocyanates. In this context, adipic acid, glycerol carbonate, 1,2-ethylenediamine, and 1,6-hexamethylenediamine, were used to synthesize NIPU_ethyl and NIPU_hexa, respectively. The obtained NIPUs were characterized using nuclear magnetic resonance spectroscopy (H-NMR spectra) and Fourier-transform infrared spectroscopy (FTIR) analysis, which verified the structures of the intermediate and final products. Calorimetric and dielectric studies provided direct and indirect support for the facilitated thermal stability of NIPU_ethyl and NIPU_hexa. Compared to the intermediate product, the NIPUs exhibit elevated glass transition temperatures, suggesting the formation of more rigid structures. The NIPUs were also tested in terms of swelling properties, and the results indicated that NIPU_hexa absorbs and withholds increased amounts of water for longer time periods compared to NIPU_ethyl, and their hydrolysis and enzymatic hydrolysis confirmed that NIPU_hexa is more stable in aqueous environments than NIPU_ethyl. Therefore, the successful production of adipic-acid-based NIPUs through a novel perspective of the polyaddition path is reported and complemented by the characterization of the obtained materials with several techniques.