To obtain an optical polymer with a high refractive index ( n D ) and Abbe's number (n D ), a series of 1,4-dithiane-2,5-bis(thiomethyl) (DBT) group-bearing poly( Salkylcarbamate) and polyvinylsulfide were synthesized by the polyaddition of 2,5bis(mercaptomethyl)-1,4-dithiane (BMMD) with diisocyanates and by the addition polymerization of 2,5-bis(2-thia-3-butenyl)-1,4-dithiane (TBD) in bulk, respectively. Their n D : n D values, ranging from 1.598 : 38.2 to 1.678 : 34.8, are comparable to those of flint glass. Some of the polymers having a high transparency and glass transition temperature were suggested to be useful as optical polymers. The DBT group contribution to the increase of the n D value of the obtained polymer was explained in terms of the f value defined by the ratio of molar refraction to molar volume and was concluded to be of primary importance because the value for the DBT group (0.38) was greater than that for the residual group making up the polymer structure. This group's contribution in preventing the degradation of the n D of the polymers was also discussed based on the UV spectrum of BMMD, which only showed a suppressed absorption band. BMMD and TBD are useful monomers for preparing an optical polymer with high n D and n D values.