The optimal ratio of diisocyanate and polyol required to synthesize polyurethane with desirable mechanical properties depends on the specific application and desired properties of the final product. In general, the ratio is determined by the stoichiometry of the reaction between the diisocyanate and the polyol.For a typical polyurethane synthesis, the ratio of diisocyanate to polyol is based on the equivalent weight of the reactive groups. The equivalent weight is the molecular weight of the compound divided by the number of reactive groups isocyanate or hydroxyl in the molecule.To achieve desirable mechanical properties, the ratio of isocyanate groups NCO to hydroxyl groups OH should be close to 1:1. This is known as the isocyanate index, and it is usually expressed as a percentage. An isocyanate index of 100% indicates a stoichiometric balance between the NCO and OH groups.However, in practice, a slight excess of isocyanate groups is often used to ensure complete reaction of the hydroxyl groups and to achieve the desired properties. This excess is typically in the range of 5-10%, resulting in an isocyanate index of 105-110%.It is important to note that the optimal ratio may vary depending on the specific diisocyanate and polyol used, as well as the desired properties of the final polyurethane product. Adjusting the ratio can influence the hardness, flexibility, and other mechanical properties of the polyurethane. Therefore, it is crucial to carefully determine the appropriate ratio for each specific application through experimentation and characterization of the resulting material.