This work deals with the cationic ring-opening polymerization of the cyclic thiocarbonates 5-benzoyloxymethyl-5-methyl-1,3-dioxane-2-thione (1), 5,5-dimethyl-1,3-dioxane-2-thione (2), and 4-benzoyloxymethyl-1,3-dioxane-2-thione (3). The polymerization was carried out with 2 mol % trifluoromethanesulfonic acid, methyl trifluoromethanesulfonate, boron trifluoride etherate, or triethyloxonium tetrafluoroborate as the initiator to afford the polythiocarbonate with a narrow molecular weight distribution accompanying isomerization of the thiocarbonate group. The molecular weight of the obtained polymer could be controlled by the feed ratio of the monomer to the initiator and increased when the second monomer was added to the polymerization mixture after the quantitative consumption of the monomer in the first stage. The block copolymerization of 2 and 3 was also achieved, and this supported the idea that the cationic ring-opening polymerization of these monomers proceeded via a living process. The order of the polymerization rate was 3 Ͼ 2 Ͼ 1. The cationic ring-opening polymerization of 1 and 3 involved the neighboring group participation of ester groups according to the polymerization rate and molecular orbital calculations with the ab initio method.

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The cationic ring-opening polymerization of a seven-membered cyclic monothiocarbonate, 1,3-dioxepan-2-thione, produced a soluble polymer through the selective isomerization of thiocarbonyl to a carbonyl group {O[SC(CAO)O(CH 2 ) 4 ] n O}. 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. EXPERIMENTAL MaterialsDichloromethane and pyridine were dried over CaH 2 and distilled under a nitrogen atmosphere.