Formic Acid\nas Feedstock in the Phosgene-Free Dehydrogenative\nCoupling of Formamides and Alcohols to Polyurethanes

This study introduces a novel polyurethane (PU) synthesis\napproach,\naiming to eliminate the health risks associated with conventional\nmethods using isocyanates and phosgene. Dehydrogenative PU synthesis\nis explored using iron and ruthenium-based catalysts to couple formamides\nand alcohols. The applied monomers can be easily prepared using formic\nacid in the presence of ZnCl<sub>2</sub>. Various parameters influencing\nthe polymerization process are investigated including catalyst choice,\nmonomer structure, spacer length, and solvent effects. While oligomers\nwith low molar masses are obtained for <i>N</i>-alkyl formamides, <i>N</i>-aryl formamides yield polymers with average molar masses <i>M</i><sub>n</sub> in the range of 7.60–14.4 kg/mol and\npolydispersities of 2.0–3.4. NMR, EDX, and elemental analysis\nreveal potential catalyst degradation and formation of inactive species\ndue to ligand dissociation. Polar solvents mitigate solubility limitations,\nallowing for higher molar masses. The highest molar masses were achieved\nusing <b>RuCl</b><sub><b>2</b></sub><b>(PPh</b><sub><b>3</b></sub><b>)</b><sub><b>3</b></sub> as a\ncatalyst with an <i>N</i>-aryl diformamide and alkyl diol\nin an anisole/DMSO with LiBr (1 wt %) mixture (6:1), analyzed by GPC,\nNMR, and ESI-MS. The thermal properties of the synthesized polymers\nwere analyzed by TGA and DSC. This phosgene-free approach represents\na promising avenue for sustainable PU synthesis, addressing both environmental\nand health concerns associated with traditional methods.