Configurational chracteristics of the polysulfides, 3. Dipole moments of poly(trimethylene sulfide) and comparisons between some polysulfides and the corresponding polyoxides

Abstract

AbstractConformational energy calculations indicate that poly(trimethylene sulfide) (P3MS) approximates the idealized freely‐rotating chain in that gauche‐trans energy differences in this molecule are very small. This occurs because the relatively long SC bond tends to relieve congestion within the P3MS chain. The bonds in poly(trimethylene oxide) (P3MO) are much more constrained to a particular rotational state because the OC bond is shorter than the SC one, and in this sense P3MO is considerably less flexible. In this study the dipole moments 〈μ2〉0 of P3MS are measured as a function of temperature, and the results analyzed in terms of the rotational isomeric‐state theory. Using conformational energies derived from semiempirical potential energy functions, a three‐rotational‐state model gave values of the dipole moment in good agreement with experiment. Theoretical and experimental values of the temperature coefficient of 〈μ2〉0 were in disagreement, however, and a five‐rotational‐state model based on that adopted by Flory in a similar study of polymethylene failed to remove this discrepancy. Some statistical properties are tabulated for the n‐alkylene polysulfides studied to date and for structurally related polyoxides. In these types of molecules, strong preferences for pairs of gauche states in CH2SCH2SCH2 and CH2 OCH2OCH2 sequences give rise to unusually small values of the dipole moment.