A process for producing a polythiol oligomer having disulfide linkages by reacting polythiol (with the functionality of two or above) with sulfur in a molar ratio of m to (m-1) (where m is an integer of 2 to 21) in the presence of a basic catalyst, whereby the oligomers produced have a degree of oligomerization of m (wherein m is defined as above). Specifically disclosed is a process for reacting 2,5-dimercaptomethyl-1,4-dithiane (DMMD) with sulfur in a molar ratio 2:1 or 3:2 in the presence of a basic catalyst, thereby producing an oligomer represented by the below formula: ##STR1## (wherein n is an integer of up to 21). Also disclosed are optical polymers made with the oligomer and optical materials and devices made with the polymer.