The polymerizations of the diepisulfides 1,2:5,6-diepithio-3,4-di-O-methyl-D-mannitol (1a), 1,2:5,6-diepithio-3,4-di-O-methyl-L-iditol (1b), and 1,2:5,6-diepithio-3,4-di-0-methyl-allitol (1c) were carried out with ZnEt2/H2O, ZnEt2/alcohol, and ZnEt2/(S or R)-1,1'-bi-2-naphthol (BN) as the initiator systems. All the polymerizations proceeded without any gel formation and gave white, powdery products. The number-average molecular weights of the obtained polymers were changed in the range of 5300-33,600. The polymerization of la using the ZnEt2/H2O (1/1) catalyst in tetrahydrofuran proceeded through a regio- and stereoselective cyclopolymerization mechanism to produce thiosugar polymers mainly consisting of 2,5-anhydro-1,5dithio-D-glucitol as the five-membered ring units. The polymers obtained from 1b and 1c with ZnEt2/H2O exhibited lower stereoregularities than that from la. For the polymers obtained from la with the ZnEt2/alcohol systems, the molar fraction of the five-membered ring units depended on the alcohol used as a ligand. On the other hand, the polymerization of le using ZnEt2/(R or S)-BN asymmetrically proceeded, and optically active polymers consisting of desulfurized acyclic units were obtained. When ZnEt2/(R)-BN (1/1) was used in toluene, the polymer with [alpha](D)(23) = +56.9 degrees was obtained in an 88.6% yield. The resulting polymer had an isotactic-rich structure consisting of about 90% (R)-configurational units and about 10% (S)-units. (c) 2005 Wiley Periodicals, Inc.