Conformational Analysis of Poly(propylene sulfide)

Abstract

Conformational characteristics of poly(propylene sulfide) (PPS, [CH2C*H(CH3)S]x) have been investigated. Proton and carbon-13 NMR vicinal coupling constants observed from its monomeric model compound, 1,2-bis(methylthio)propane (BMTP, CH3S−CH2−C*H(CH3)−SCH3), were analyzed to yield bond conformations of the S−C, C−C*, and C*−S bonds. Ab initio molecular orbital (MO) calculations at the MP2/6-311+G(3df,2p)//HF/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels were carried out for BMTP to evaluate free energies and dipole moments of all the possible conformers. Conformational energies and bond dipole moments of BMTP were estimated therefrom. Conformational energies of BMTP and PPS were also determined by simulations based on the rotational isomeric state scheme for experimental observations of bond conformations of BMTP, characteristic ratio of atactic PPS, and dipole moment ratios of isotactic and atactic PPS. The first-order interaction energies for the S−C (Eσ) and the C−C* (Eα and Eβ) bonds were obtained as follows: Eσ = −1.0 to −0.60 kcal mol-1, Eα = 0.5−0.6 kcal mol-1, and Eβ = 1.1−1.2 kcal mol-1. The second-order ω1 and ω2 interactions, representing intramolecular C−H···S interactions, are repulsive: Eω1 = 0.6−0.9 kcal mol-1 and Eω2 = 1.0−1.2 kcal mol-1. The S−C, C−C*, and C*−S bonds were found to prefer the gauche, trans, and trans states, respectively. The conformational characteristics of unperturbed PPS are similar to those of poly(ethylene sulfide) but significantly different from those of its corresponding polyether, poly(propylene oxide) (PPO), although isotactic PPS and PPO are isomorphous. The conformational characteristics of PPS are discussed in terms of solvent effect, crystal structure, and thermal properties.