The cationic ring-opening polymerization of a seven-membered cyclic mono-thiocarbonate, 1,3-dioxepan-2-thione, produced a soluble polymer through the selective isomerization of thiocarbonyl to a carbonyl group {-[SC(C=O)O(CH2)(4)](n)-}. The molecular weights of the polymer could be controlled by the feed ratio of the monomer to the initiators or the conversion of the monomer during the polymerization, although some termination reactions occurred after the complete consumption of the monomer. (C) 2005 Wiley Periodicals, Inc.