Conformational. characteristics of poly(methylene sulfide) (PMS) and its oligomeric model compounds have been investigated. Carbon-13 NMR measurements for a dimeric model compound, bis(methylthio)methane, in the gas phase as well as in solutions were carried out, and the first-order interaction energy E-sigma representing the gauche stabilization of the C-S bond was determined from observed vicinal C-H coupling constants. For example, the E, value for the gaseous dimer was evaluated as -1.43 +/- 0.01 kcal mol(-1), being in good agreement with the ab initio molecular orbital calculation (-1.38 kcal mol(-1)) at the B3LYP/6-311+G(2d,p)//BSLYP/6-31G(d) level. The conformational energy E, showed solvent dependence; polar solvents stabilize the trans conformation, in which dipole moments are parallel to each other and hence the molecule becomes polar. The characteristic ratio, dipole moment ratio, and configurational entropy of unperturbed PMS were estimated by the rotational isomeric state scheme and compared with those of poly(methylene oxide) (PMO). The PMS chain was indicated to be more flexible than PMO. The difference in melting point between PMS (245 degreesC) and PMO (180 degreesC) was suggested to come mainly from that in enthalpy (DeltaH(u)) of fusion: DeltaH(u)(PMS) > DeltaH(u)(PMO). The geometrical parameters, electron densities, and atomic charges of trimers of PMS and PMO, obtained from the MO calculations, showed that the gauche stability in the C-S bond of the PMS homologues comes partly from antiparallel dipole-dipole interaction and n(s) --> sigma*(C-S) hyperconjugation formed in the gauche state, partly from steric (SS)-S-... repulsion occurring in the trans form. It was also shown that sulfur electrons have such flexibility as to reduce the (SS)-S-... repulsion and unfavorable (parallel) dipole-dipole interaction.