The bulk cyclopolymerization of diepisulfide, 1,2:5,6-diepithio-3,4-di-O-methyl-1,2:5,6-tetradeoxy-D-mannitol (1), was studied using R4N+Br (R = -CH3, C2H5, C3H7, C4H9, and C7H15) and (C4H9)(4)N+X- (X = Cl, I, NO3, and ClO4) as the initiators. All the bulk polymerizations of 1 using quaternary tetraalkylammonium salts at 90 degreesC proceeded without gelation even at high conversion to produce gel-free polymers consisting of 2,5-anhydro-1,5-dithio-D-glucitol (I) as the major cyclic repeating unit along with 1,5-anhydro-2,5-dithio-D-mannitol (II) and the desulfurized acyclic unit (III) as the minor units. The polymerization rate and molar fraction of the I unit increased with the increasing alkyl chain length of the tetraalkylammonium cation and the increasing nucleophilicity of the counteranion. Tetrabutylammonium chloride exhibited the highest catalytic activity and the highest stereoselectivity, that is, the thiosugar polymer with I:II:III = 81:15:4 and a number-average molecular weight of 31.9 x 10(3) was obtained in 85% yield for a polymerization time of 0.5 h.