## 3 Route for Non-Isocyanate PUs (NIPUs)  
PUs can be manufactured by various methods [143]. A reaction between the polyol (an alcohol with two or more hydroxyl groups in one molecule) and the diisocyanate is the most important and common process [76; 144]. For PU preparation, isocyanates are highly required components. They can be classified as difunctional or heterofunctional, and aromatic or aliphatic by nature. Methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI), and aliphatic diisocyanates are among the most frequently used options available. Environmental concerns led researchers to look at better ways in which environmental problems and other toxicity issues related to diisocyanates can be minimized or possibly eliminated. For example, sustainable PU from carbonated soy oil, 3-aminopropyltriethoxysilicane, and lignin was studied. A reaction between cyclic carbonate and amines [145] was followed on the non-isocyanate path, and polyol was prepared from lignin using the oxypropylation method [146]. An additional analysis used lignin-based PU with oligomeric polybutadiene diisocyanate [147]. A related study responded to the preparation of PUs by lignin-aminated polyol and diphenyl diisocyanates. The properties of formulated non-isocyanate-based PUs are mainly dependent on lignin content in most reporting. The quality of the lignin defines the relationship between the materials and the modulus. Efficient alternative methods for PU synthesis must be established with due regard to environmental protection and health safety. The replacement of fossil-fuel feedstock with renewable bio-based resources associated with reducing petrochemical procedures and using fossil fuels is one of the potential green alternative routes for PU synthesis [16; 148]. DuPont (Wilmington, DE, USA), BASF (Ludwigshafen, Rhineland-Palatinate, Germany), BioAmber (Princeton, NJ, USA), Myriant Technologies LLC (Quincy, MA, USA), General Mills Co. (Minneapolis, MN, USA), Covestro (Leverkusen, Rhine-Westphalia, Germany), Henkel Company (Dusseldorf, Rhine-Westphalia, Germany) and Allessa GmbH (Frankfurt, Hesse, Germany) are the most renowned companies supplying organic raw materials to synthesize PU. Furthermore, a phosgene-free and isocyanate-free synthetic strategy was developed [16]. The use of toxic and moisture-sensitive diisocyanates provided by toxic phosgene eliminated from the non-isocyanate synthesis routes to PUs. Four different methods for NIPUs synthesis, i.e., step-growth polyaddition, polycondensation, ring-opening polymerization (ROP), and rearrangement reactions, were reported in the literature [14; 149; 150; 151; 152; 153; 154; 155], as shown schematically in Figure 3.  
Figure 2: Thermosetting polyurethane synthetic pathway (TSPUs) using pMDI, polyol, and glycerol [142]. Published research papers showed the feasibility of using Hoffman rearrangement, Curtius rearrangement, and Lossen rearrangement. Unfortunately, substrates' toxicity such as carboxamide, acyl-acid, and hydroxamic acids limits their use in non-isocyanate PU synthesis. This issue also concerns the reagents used in ring-opening polymerization. Aliphatic cyclic carbamates are typically derived from highly dangerous phaggenes and aziridines [150]. [PERSON] and [PERSON] carried out the first polycondensation synthesis of NIPUs [156]. The polycondensation pathway synthesis of NIPUs is linked to the reaction between di-or polycarbamates and diols [80], di-or polycarbonates and amino alcohols [145] or diamines [157], polychloroformate and polyamines, as well as polyacrylamide chloride and polyol [150]. This approach to industrial application is constrained by the development of side-products (e.g., H\({}_{2}\)O, HCl, alcohols). Furthermore, the synthesis is typically carried out in solvents, using catalysts and under particular reaction conditions, such as long reaction times or high temperatures (150-220 \({}^{\circ}\)C), which are economically unfavourable from an industrial point of view [14].