Formic Acid as Feedstock in the Phosgene-Free Dehydrogenative Coupling of Formamides and Alcohols to Polyurethanes

This study introduces a novel polyurethane (PU) synthesis approach, aiming to eliminate the health risks associated with conventional methods using isocyanates and phosgene. Dehydrogenative PU synthesis is explored using iron and ruthenium-based catalysts to couple formamides and alcohols. The applied monomers can be easily prepared using formic acid in the presence of ZnCl₂. Various parameters influencing the polymerization process are investigated including catalyst choice, monomer structure, spacer length, and solvent effects. While oligomers with low molar masses are obtained for N-alkyl formamides, N-aryl formamides yield polymers with average molar masses M_n in the range of 7.60–14.4 kg/mol and polydispersities of 2.0–3.4. NMR, EDX, and elemental analysis reveal potential catalyst degradation and formation of inactive species due to ligand dissociation. Polar solvents mitigate solubility limitations, allowing for higher molar masses. The highest molar masses were achieved using RuCl₂(PPh₃)₃ as a catalyst with an N-aryl diformamide and alkyl diol in an anisole/DMSO with LiBr (1 wt %) mixture (6:1), analyzed by GPC, NMR, and ESI-MS. The thermal properties of the synthesized polymers were analyzed by TGA and DSC. This phosgene-free approach represents a promising avenue for sustainable PU synthesis, addressing both environmental and health concerns associated with traditional methods.