# Characteristics of Polycarbonate Soft Segment-Based Thermoplastic Polyurethane
Because solvent-free methods involve bulk polymerization, the synthesized product has a higher molecular weight and stronger mechanical strength than PU produced using WPU or traditional synthesis methods. In addition to exhibiting various advantages, solvent-free methods require little time for synthesis given their reaction speed. One-shot solvent-free PU synthesis is commonly used in industrial production. However, relatively few studies have compared solvent-free PU with solvent-based and waterborne PU. The present study investigated one-shot solvent-free PU synthesis and conducted an in-depth discussion of its properties. Aliphatic, cyclic-aliphatic, and aromatic isocyanates are isocyanate monomers commonly applied in one-shot solvent-free PU synthesis. Because the reaction properties of aromatic isocyanates are more favorable than those of aliphatic and cyclic-aliphatic isocyanates, aromatic isocyanates are commonly used in industrial fields [9]. Among aromatic isocyanates, 4,4\"-methylene diphenyl diisocyanate (MDI) has replaced the commonly-used toluene diisocyanate (TDI), which has been listed as a restricted substance because of the extremely harmful effects of VOCs on human health. MDI is not volatile and has favorable reaction properties and outstanding symmetry; moreover, high tacitity in the hard segment enhances the tensile strength and hardness of PU [10; 11]. Polyols commonly used for PU synthesis include polyester, polyether, and polycarbonate polyols. In particular, polycarbonate-polyol-synthesized PU exhibits the excellent mechanical properties of polyester and the hydrolytic stability of polyether. The various advantages of carbonate groups in polycarbonate polyols have been mentioned in the literature [12]. Functional groups in polycarbonate polyols are rigid functional groups, which engender a high glass transition temperature (T\({}_{\text{g}}\)) and increased plasticity in the synthesized PU. Consequently, polycarbonate PU has higher mechanical strength and greater Young's modulus than do polyester or polyether PU made of a similar recipe [13]. Because carbonate groups have one more O atom than ester groups do, polycarbonate PU is more resistant to hydrolysis and oxidation than polyester or polyether PU [14]. Furthermore, the high polarity of carbonate groups results in polycarbonate PU having excellent wearability [15; 16], enabling its application in the coating, binder, elastomer, and foaming material fields [17; 18; 19]. PU is a class of segmented copolymers composed of soft and hard segments. The soft segment is usually polyols, and the hard segment includes the diisocyanate and the chain extender. Due to the immiscibility of hard and soft segments, micro-phase separation occurs. The properties of the micro-phase separation copolymers are directly related to their two-phase microstructure, with the hard segments possessing more hydrogen bonds acting as physical crosslinks, for the flexible soft segments, and provide the material with high modulus and elastomeric properties [20; 21]. The present study aimed to synthesize a solvent-free thermoplastic PU (TPU) with mechanical properties suitable for widespread application. Bifunctional monomers were selected to ensure the polymerization of a thermoplastic linear polymer. MDI was employed as the isocyanate, and polycarbonate polyol with a molecular weight of 1000 was adopted as the polyol. Two common bifunctional monomers, 1,3-propane diol (1,3-PDO) and 1,4-butane diol (1,4-BDO), were used as chain extenders. One-shot solvent-free PU synthesis was performed to polymerize polycarbonate-based TPU. The present study discusses the TPU's properties.