Novel five-membered cyclic monothiocarbonate derivatives, 4-benzoyloxymethyl-1,3-dioxolane-2-thione (TC1) and 4-phenoxymethyl-1,3-dioxolane-2-thione (TC2), were synthesized by reaction of the corresponding diols and thiophosgene in the presence of 2,3-dimethyl-1-phenyl-3-pyrazolin-5-one (antipyrine) in chloroform. The cationic ring-opening polymerization of TC1 and TC2 using several cationic initiators afforded polythiocarbonate with good solubility in common organic solvents accompanying isomerization of thiocarbonate group. The polymerization of TC1 proceeded involving the neighboring group participation of the ester group. The main chains of the polymers obtained from TC1 and TC2 were different. The main chain structure depends on the substituent on the thiocarbonate ring, i.e., whether neighboring ester group participation was involved or not.