The optimal ratio of diisocyanate to polyol for the synthesis of polyurethane depends on the desired mechanical properties and the specific types of diisocyanate and polyol being used. In general, the ratio is determined by the equivalent weights of the diisocyanate and polyol, which are based on their respective reactive groups.A 1:1 ratio of isocyanate NCO groups to hydroxyl OH groups is typically used as a starting point for the synthesis of polyurethane. This stoichiometric ratio ensures that all the reactive groups are consumed during the reaction, resulting in a high molecular weight polymer with good mechanical properties.However, the optimal ratio can vary depending on the desired properties of the final material. For example:1. If a more flexible polyurethane is desired, a slight excess of polyol can be used, resulting in a higher concentration of hydroxyl groups. This can lead to the formation of a polymer with more urethane linkages and increased flexibility.2. If a more rigid polyurethane is desired, a slight excess of diisocyanate can be used, resulting in a higher concentration of isocyanate groups. This can lead to the formation of a polymer with more urea linkages and increased rigidity.The final characteristics of the polyurethane material, such as hardness, tensile strength, elongation, and abrasion resistance, can be fine-tuned by adjusting the ratio of diisocyanate to polyol, as well as by selecting specific types of diisocyanates and polyols with different molecular weights and structures.In summary, the optimal ratio of diisocyanate to polyol for the synthesis of polyurethane depends on the desired mechanical properties and the specific reactants being used. Adjusting this ratio can significantly affect the final characteristics of the material, allowing for the production of polyurethane materials with a wide range of properties for various applications.