Plastic optical materials are lightweight, hardly fragile and tintable compared to inorganic optical materials. Various plastic materials, such as resins, are currently used in optical materials and are gradually required to have better physical properties.
Polythiourethane optical resins produced using polythiol compounds and isocyanate compounds are widely used as optical lens materials due to their excellent optical properties, including high transparency, Abbe number, transmittance and tensile strength. However, optical materials produced by curing resin compositions including polythiol compounds and general-purpose isocyanate compounds suffer from frequent nonuniformity polymerization or whitening, which worsens the optical properties of the optical resins. General-purpose isocyanate compounds and polythiol compounds as main components of resin compositions for thiourethane lenses are prone to nonuniformity polymerization, whitening and clouding depending on their miscibility. In contrast, optical resins produced by heat curing of highly miscible isocyanate compounds and polythiol compounds do not substantially suffer from the problems of nonuniformity polymerization and whitening even when the polythiol compounds are not specially treated. Such isocyanate compounds include, for example, 3,8-bis(isocyanatomethyl)tricyclo[5.2.1.02.6]decane, 3,9-bis(isocyanatomethyl)tricyclo[5.2.1.02.6]decane, 4,8-bis(isocyanatomethyl)tricyclo[5.2.1.02.6]decane, 4,9-bis(isocyanatomethyl)tricyclo[5.2.1.02.6]decane, 2,5-bis(isocyanatomethyl)bicyclo[2.2.1]heptane, and 2,6-bis(isocyanatomethyl)bicyclo[2.2.1]heptanes. However, the isocyanate compounds are difficult to prepare, which incurs considerable preparation costs. As a consequence, the use of the isocyanate compounds inevitably increases the production cost of thiourethane lenses. Meanwhile, optical resins obtained by curing inexpensive general-purpose isocyanates and polythiol compounds suffer from frequent nonuniformity polymerization or whitening. Such isocyanate compounds include, for example, isophorone diisocyanate, dicyclohexylmethane-4,4-diisocyanate (H12MDI), and 1,6-hexamethylenediisocyanate. Particularly, tape whitening and bubbling occur in some cases. These phenomena adversely affect the performance of optical materials and are causes of high defective proportion and low lens quality.