Biomass Lignin- and CO₂-Based Non-Isocyanate Polyhydroxyurethanes

Polyhydroxyurethanes (PHUs) synthesized from cyclic carbonates have gained attention as sustainable alternatives to conventional, hazardous isocyanate-based polyurethanes. However, most PHUs are currently synthesized from petroleum-based or toxic phosgene-based precursors under high CO2 pressure. Lignin, an abundant and low-cost biomass polymer, offers a promising alternative to traditional PHU synthesis methods. New technology can be developed to overcome lignin’s intrinsic challenges, including low chemical reactivity, heterogeneity, and structural complexity. This study presents the first report of biomass-based non-isocyanate PHUs prepared from biomass lignin and CO2 in the presence of amine monomers. Lignin-based cyclic carbonate, synthesized via CO2 gas valorization, is reacted with various bi- and triamines to produce seven distinct and chemically well-defined PHUs. These PHUs are structurally characterized using 1H, 13C, and 2D-HSQC NMR along with FT-IR spectroscopy, confirming the precise synthesis of the designed polymer chemical structures. The molecular weights are assessed using GPC, revealing values ranging from 182.4 to 248.7 kDa. The study also explores the structure–property relationships of the PHUs, focusing on their thermal and mechanical properties. The synthesized PHUs exhibit sufficient thermal stability (Td5%: 248–259 °C), as determined by TGA, and glass-transition temperatures (Tg: 45–73 °C), as measured by DSC. The mechanical stability is noteworthy, with stress at 10% strain reaching up to 69.17 kPa and modulus values up to 691 kPa. Moreover, these polymers demonstrated high solubility in organic solvents, facilitating easier processing, in contrast to the poor solubility observed in previously reported lignin-based polyurethanes. Therefore, the current findings indicate that adjusting the chemical structure of biomass-derived polymers can significantly enhance their molecular, thermal, and mechanical properties, providing greater controllability and making them viable for a wide range of applications.