ISOCYANATE DERIVATIVE AND ITS USE

PROBLEM TO BE SOLVED: To obtain a new isocyanate derivative containing sulfur atom, useful for plastic lens having an extremely high refractive index and an excellent heat resistance and surface hardness. SOLUTION: This diisocyanate derivative is represented by formula I [R is ethylene or propylene (carbon in the middle of propylene may be substituted by S); R' is methylthio or methylthiomethylthio; (n) is 1-3], e.g. 2,4- dithiapentane-1,3-diisocyanate of formula II. The compound of formula I, e.g. 2,4-dithiapentane-1,3-diisocyanate of formula II is obtained by subjecting a dihydrazide derivative obtained by reacting 2-methylthio-3-thiaglutaric acid dihydrazide with nitrous acid in an aqueous solution of hydrochloric acid to Curtius rearrangement. The new diisocyanate derivative of formula I and at least one kind of polythiol compound are included to provide a composition for optical resins having high refractive index. The composition is polymerized to produce a lens for various kinds of optical uses.