If a polyol has four active sites per molecule and an isocyanate also has four active sites per molecule, why isn't a 1:1 mixture by weight a likely ideal concentration mixture? What should be the correct ratio?

Question

What will be an ideal response?

Answer 1

Although the number of active sites is in the ratio of 1:1, the differences in molecular weight of the two components must be taken into account. For instance, assume that the polyol is a long-chain polymer with a molecular weight of 10,000 and that the diisocyanate is a small molecule with a molecular weight of only 100 . To achieve complete reaction of all the reactive groups, there would need to be the same number of groups, not the same weight. Therefore, the weight of polyol should be 100 times the weight of diisocyanate to give the same number of polymers and the same number of reactive sites, a weight ratio of 100:1.