Unraveling the Role of Lignin Oligomers Functionality for the Synthesis of Nonisocyanate Polyurethanes

Lignins are the main source of aromatic compounds in nature. Their depolymerization most often leads to oligomeric products, which lack proper valorization. In this study, two lignins (Kraft and organosolv) were subjected to two depolymerization methodologies, leading to a total of six lignin samples, which were characterized and functionalized to cyclocarbonate (CC) building blocks through a green and optimized one-pot procedure. The lignin oligomers obtained by depolymerization were compared to the raw lignins for the synthesis of safe and sustainable polyhydroxyurethane (PHU) thermosets by aminolysis with lignin contents between 28 and 34 wt%. The synthesized PHU materials displayed a wide range of mechanical properties and behaviors, with a tensile modulus ranging from 50 to 1400 kPa, and elongation at break ranging from 46% to 217%. While lignin depolymerization had a negligible effect on the polymerization kinetics, it led to softer and more flexible materials at equivalent lignin contents. Interestingly, it was found that the functionality of the lignin-CC building blocks is the main factor affecting the crosslinking density, and consequently the mechanical properties of the materials. Other factors such as the lignin type (e.g., Kraft vs. organosolv) and the depolymerization method (e.g., hydrothermal vs. reductive) did not have a strong influence on the final material properties. These different points establish a strong foundation for the understanding of "structure-properties" relationships in lignin-based materials, a key point toward the valorization of lignin into renewable, safe, and high-performance materials.