To optimally combine different types of diisocyanates and polyols to synthesize polyurethane with desirable mechanical, thermal, and chemical properties, several factors need to be considered:1. Selection of diisocyanates: Diisocyanates are the primary building blocks of polyurethane. Commonly used diisocyanates include toluene diisocyanate TDI , methylene diphenyl diisocyanate MDI , and hexamethylene diisocyanate HDI . The choice of diisocyanate will influence the final properties of the polyurethane. For example, TDI-based polyurethanes generally have higher mechanical strength, while MDI-based polyurethanes have better chemical resistance and lower temperature flexibility.2. Selection of polyols: Polyols are the other main component of polyurethane and can be classified into two main categories: polyether polyols and polyester polyols. Polyether polyols generally provide better hydrolytic stability, low-temperature flexibility, and resilience, while polyester polyols offer better mechanical properties, chemical resistance, and heat resistance. The choice of polyol will depend on the desired properties of the final polyurethane product.3. Molecular weight of polyols: The molecular weight of the polyol will influence the mechanical properties of the polyurethane. Higher molecular weight polyols will generally result in polyurethanes with better mechanical properties, such as tensile strength and elongation at break. However, higher molecular weight polyols may also result in increased viscosity, which can affect processing and curing.4. Ratio of diisocyanate to polyol: The ratio of diisocyanate to polyol, also known as the isocyanate index, is a critical parameter in determining the final properties of the polyurethane. A higher isocyanate index will result in a more cross-linked and rigid polyurethane, while a lower isocyanate index will produce a more flexible and less cross-linked material. The optimal ratio will depend on the desired balance of mechanical, thermal, and chemical properties.5. Additives and catalysts: Various additives and catalysts can be used to modify the properties of the polyurethane. For example, chain extenders can be added to increase the molecular weight of the polyurethane, improving its mechanical properties. Catalysts can be used to control the reaction rate and curing process, affecting the final properties of the polyurethane.6. Processing conditions: The processing conditions, such as temperature, pressure, and mixing, can also influence the properties of the polyurethane. Proper control of these parameters is essential to ensure the desired properties are achieved.In summary, to optimally combine different types of diisocyanates and polyols to synthesize polyurethane with desirable mechanical, thermal, and chemical properties, it is crucial to carefully select the appropriate diisocyanate, polyol, molecular weight, ratio, additives, catalysts, and processing conditions based on the specific requirements of the final product.