Synthesis of Reprocessable Lignin-based Non-isocyanate Poly(imine-hydroxyurethane)s Networks

In this study, an environmentally friendly and non-toxic route to synthesize lignin-based non-isocyanate poly(imine-hydroxyurethane)s networks was explored. Specifically, the NH₂-terminated polyhydroxyurethanes (NPHUs) prepolymer was first synthesized from bis(6-membered cyclic carbonate) (BCC) and diamine via the ring-opening reaction. Subsequently, the corresponding lignin-based non-isocyanate polyurethanes (NIPUs) with tunable properties were synthesized from NPHUs and levulinate lignin derivatives containing ketone groups via the Schiff base reaction. The structural, mechanical, and thermal properties of NIPUs with different stoichiometric feed ratios of BCC and levulinate lignin were characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). The results indicated that the tensile strength, Young''s modulus, toughness, storage modulus, glass transition temperature, and thermal stability of lignin-based NIPUs gradually increased with increasing lignin content, and the highest Young''s modulus of 41.1 MPa was obtained when lignin content reached 45.53%. With good reprocessing properties, this synthetic framework of lignin-based NIPUs also provides sustainable non-isocyanate-based substitutions to traditional polyurethane networks.