Ring-opening polymerization of cyclic 1,3-oxathiolane-2-thione promoted by neighboring sulfide group and ring contraction

Abstract

Methyl triflate-mediated polymerization of cyclic oxathiolane-2-thione was investigated with a focus on the influence of several neighboring substituents. The unusual migration of a neighboring sulfide was observed in a poly(dithiocarbonate) prepared from PhS-substituted monomer, in contrast to polymers prepared from PhO and PhN analogs. The migration suppressed the competitive isomerization of a monomer to cyclic dithiocarbonate and led to high polymer conversion. 4-Phenylthio-1,2-dithiolane was isolated as a main fragment product from the prepared poly(dithiocarbonate) under basic cleavage condition, the formation of which indicated the PhS migration. A plausible mechanism for the migration was proposed, wherein ring opening is promoted by the formation of an intermediate thiiranium salt involving a neighboring PhS, followed by regioselective attack of the monomer for propagation. The spiro monomer with thia-heterocycle as a neighboring group provided an interesting ring contraction during the polymerization. Six-, seven- and eight-membered heterocycles with one or two sulfures were apparently reduced to smaller heterocycles in the corresponding polymers. The S-participation of heterocycles resulted in ring contraction, which probably progressed via an intermediate bicyclic thiiranium salt. The polymerization exhibited the characteristics of living polymerization such as a narrow distribution of average molecular weights, which were controllable according to the amount of initiator. Efficient S-migration was ensured by the formation of a homopolymer.