Abstract: A first prepolymer contains a structural unit represented by the formula ##STR1## where m denotes a positive number of from 2 to 20 with a polymerization degree of 2 to 1000. A second prepolymer contains a structural unit represented by the formula ##STR2## where X denotes a hydrogen atom or a halogen atom, m.sup.1 and m.sup.2 denote positive numbers of from 2 to 20 and n.sup.1 +n.sup.2 denote positive numbers of from 2 to 1000, with n.sup.1 and n.sup.2 being not equal to 0, the prepolymer having a polymerization degree of 2 to 1000. An optical material with a high refractive index is obtained by polymerizing and curing a polymerizable composition containing the first or second prepolymer.

Abstract: A light curable dental liquid or paste comprises an ethylenic unsaturated monomer, a light polymerization initiator and an organic filler. The light polymerization initiator is a combination of an organic peroxide and a pyrylium salt compound of: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 each represents a hydrogen atom, an alkyl group and the like, X stands for an oxygen or sulfur atom and Y for an anionic functional group.