Abstract
Polyurethanes (PUs) are among the most widely produced polymers globally and are extensively utilized across numerous industries due to their versatile properties. However, increased use of toxic isocyanates and petroleum-based diols in the synthesis of conventional PUs has raised serious concerns due to their environmental impact, health risks, and recycling challenges. Alternative approaches for non-isocyanate polyurethanes (NIPUs) have been developed in response to the high toxicity associated with isocyanates. Additionally, there has been growing emphasis on using renewable raw materials to produce more environmentally friendly PUs. Notably, lignin, the second most abundant biopolymer, has gained attention due to its rigidity and rich functionalities that can be modified, thus enabling the design of bio-based NIPUs with tailored chemical and mechanical characteristics. This review presents a comprehensive overview of the latest progress in the synthesis of lignin-based non-isocyanate polyurethanes (LNIPUs) with particular focus on the replacement of toxic isocyanate precursors and the use of greener processes. It begins with a brief overview and the current global status of PUs, followed by an in-depth exploration of various synthetic routes for NIPUs. Finally, recent research efforts devoted to LNIPUs are discussed.