Influence of the 2,5-Furandicarboxamide Moiety on Hydrogen Bonding in Aliphatic-Aromatic Poly(ester amide)s

We address the synthesis of novel poly(ester amide)s containing 2,5-furandicarboxylic acid (2,5-FDCA). A dimethyl 4,4'-((furan-2,5-dicarbonyl)bis(azanediyl))dibenzoate (2,5-FDCAn) model compound is synthesized to protect the thermally instable carboxylic acid groups of the 2,5-FDCA moiety. The properties of 2,5-FDCAn are compared with its terephthalic acid (TA), 2,5-thiophenedicarboxylic acid (2,5-TDCA), and isophthalic acid (IA) analogues. Furthermore, the feasibility of the melt polycondensation reaction of these model compounds with different aliphatic diols is investigated. The polymers thus synthesized are analyzed using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarization optical microscopy (POM), nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD). Solution NMR analysis indicates that the oxygen heteroatom in the furan ring functions as a hydrogen bond acceptor, resulting in the formation of intramolecular hydrogen bonds with the amide hydrogens in 2,5-FDCAn. It is anticipated that such intramolecular hydrogen bonds compete with the formation of intermolecular hydrogen bonds, which is generally considered to be the driving force for polyamide crystallization. As a consequence, 2,5-FDCAn exhibits a suppressed melting temperature, decreased crystallinity, and an increased solubility. The FTIR and WAXD data confirm the perturbed hydrogen bonding nature of 2,5-FDCAn and its limited ability to crystallize. The application of this low melting compound allows for the synthesis of novel poly(ester amide)s via a melt polycondensation reaction. In contrast, to obtain high molar mass polymers containing the TA, 2,5-TDCA, and IA analogue, solvent is required for polymerization. Similar to the properties of 2,5-FDCAn, the obtained polymers exhibit increased solubility, decreased melting temperatures, and low degrees of crystallinity compared to their TA, 2,5-TDCA, and IA based analogues