US Patent 7396900 Thiourethane-based optical material

A composition for making a high-refractive-index resin contains a polythiol compound and a compound containing at least two iso(thio)cyanato groups, the polythiol compound having a dithioacetal, dithioketal, orthotrithioformic ester, or orthotetrathiocarbonic ester skeleton. The molar ratio of the mercapto group to the iso(thio)cyanato group in the composition is greater than 1.0 but not more than 3.0. A method for making a resin by curing the composition, a resin prepared by curing the composition, an optical element made from the resin, and a lens made from the optical element are also provided.