Described is a polymerizable organic composition comprising at least one capped polyisocyanate having nonfugitive capping groups, e.g., a 1H-azole, a polythiol having two or more thiol groups, and optionally a reactive hydrogen compound selected from polyols, compounds having both hydroxy and thiol groups, and mixtures thereof. A polymerizate, having a thickness of at least 0.5 millimeters, e.g., an optical lens, obtained from the composition of the present invention has an initial Barcol hardness of at least 1, a refractive index of at least 1.57 and an Abbe number of at least 33. Also described are photochromic articles prepared from such polymerizates.