A mercapto group generated by addition of 5-phenoxymethyl-1,3-oxathiolane-2-thione and benzylamine could react with cyclic acid anhydrides to give the corresponding carboxylic acids. This method was applied to a polymer modification. Poly(thiourethane)s containing mercapto groups were prepared from difunctional five-membered cyclic dithiocarbonates and diamines could react with succinic or glutaric anhydride to give the corresponding poly(thiourethane)s bearing carboxyl group in the side chains in high yield. Thermal properties of the obtained polymers were examined by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The obtained polymers showed 5 % weight loss temperatures (T (d5)) at 170 and 216 A degrees C. These polymers exhibited glass transition temperatures (T (g)) at 52 and 57 A degrees C, respectively. Furthermore, a cross-linked poly(thiourethane) was readily obtained using difunctional cyclic acid anhydride, 1,2,3,4-butanetetracarboxylic dianhydride as a cross linker.