## 6 Future Perspectives  
Many scientists concentrated on a non-isocyanate process for PU production at the beginning of the 20 th century [220]. The most significant drawbacks that led to an increase in interest in non-isocyanate methods for synthesizing green PUs are toxicity, susceptibility to moisture, and diisocyanate synthesis based on phospenes [14]. To manufacture NIPU foams, [PERSON] et al. have been working on the isocyanate-free synthesis of PUs based on the reaction between diamines and cyclic carbonates [221; 222; 223]. The scientists reported that the most prospective alternative to Isocyanate PUs is polyhydroxyurethane (PHU). The cyclic carbonate aminolysis reaction can generate these materials. PHU is distinguished by its unique characteristics as higher tolerance to chemical degradation and higher adhesion and resistance to wear than standard PU materials because of the presence of hydroxyl groups in the principal macromolecular chain [150; 224]. The results demonstrated that the future outlook of the NIPUs in the plastics industry is positive. All the above examples demonstrate that bio-renewable products are among the most popular products developing in the chemical industry sector. The research on bio-based components for PU synthesis confirmed that they have good characteristics as an alternative to the petroleum-derived components used. Though NIPU research has been thoroughly developed over the recent years, NIPU synthesis still has an important issue, namely the cyclic carbonate amine reaction's low reactivity. Consequently, it is crucial to synthesize NIPU with a high response temperature and long reaction time, which means that the object NIPU curing takes a long time. The epoxy group can be used as a \"bridge\" to prepare solvent-based ambient curable NIPUs to improve the curing process. A modern hybrid NIPU epoxy resin, which has demonstrated well-balanced flexibility of traditional polyurethane resins and physical-chemical properties with conventional epoxy-amine [225]. The synthesis consists of two steps: creating an amine-finished NIPU and a small molecular epoxy resin chain extension. The response time for preparing NIPU has significantly decreased because epoxy-amine reactivity is much higher than that of cyclic carbonate-amine reactivity, and NIPU-epoxy hybrid resins are also very strong. However, these hybrid resins were still organic solvent-borne, and there are currently no waterborne NIPU-epoxy hybrid resins. Due to the urgency of developing high-efficiency NIPU resins, other bio-based resources, such as adipic acid and 1,6-hexanediol, for the development of polyols will soon be made accessible [226; 227; 228]. The above chemicals are substrates for the manufacture of aliphatic polyester polyols. Aromatic monomers for PU synthesis gained more attention in recent years. The most important examples of chemicals that demonstrated a significant application in the development of polyester polyols and PU are 2,5-furandicarboxylic acid and vanillic acid [229]. Polyesters, e.g., furandicarboxylic acid, developed using the bio-based aromatic monomer, were investigated as an alternative to bio-based terephthalates. Polymers such as poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), and poly(butylene terephthalate) (PBT), are all petroleum-based thermoplastic polymers. 2,5-furandicarboxylic acid and 1,4-butanediol are used to synthesize poly(butylene 2,5-furandicarboxylate) (PBF) [230; 231]. The thermal properties of poly(butylene terephthalate) and poly(ethylene terephthalate) of petrochemical origin compare with the achieved thermal properties of PBF. Other bio-based aromatic monomers, which are essential ingredients