Intramolecular Interactions of Polyethers and Polysulfides, Investigated by NMR, Ab Initio Molecular Orbital Calculations, and Rotational Isomeric State Scheme: An Advanced Analysis of NMR Data

Abstract

Intramolecular interactions of representative polyethers and polysulfides such as poly(methylene oxide) (PMO), poly(ethylene oxide) (PEO), poly (propylene oxide) (PPO), poly(methylene sulfide) (PMS), poly(ethylene sulfide) (PES), and poly(propylene sulfide) (PPS) have been investigated by NMR, ab initio molecular orbital (MO) calculations, and statistical mechanics of chain molecules, viz., the so-called rotational isomeric state (RIS) scheme. A conformational analysis of isotactic PPO was carried out from vicinal 1H–1H coupling constants observed from its monomeric model compound, 1,2-dimethoxypropane, and the gauche preference of the OCC∗O bond was found to due to intramolecular (CH)⋯O hydrogen bonds formed in the g±g∓ conformations for the CO/CC∗ bond pair. Carbon-13 NMR chemical shifts of six PPO dimers having different regio- and stereosequences were assigned and simulated by RIS calculations based on γ- and δ-effects of substituents. As a consequence, the hydrogen bonds were proved to exist in the PPO dimers as well. Conformational analyses of PMS, PES, and PPS were carried out from vicinal 1H–1H and 13C–1H coupling constants to determine their conformational energies defined on the RIS approximation. In parallel, conformational characteristics of PMO and PEO were investigated, and the origin and nature of intramolecular interactions of the polyethers and polysulfides were successfully elucidated as follows: (1) gauche stabilizations of CX bonds (X=O and S) of PMO and PMS, due to lone pair (nX)→antibonding orbital (σCX∗) hyperconjugations and intramolecular dipole–dipole interactions; (2) a competitive equilibrium between the intramolecular (CH)⋯O hydrogen bond and the so-called attractive gauche effect of PEO; (3) gauche preferences of CS bonds of PES and PPS, mainly due to nS→σCC∗ and nS→σCH∗ delocalizations; (4) trans preferences of CC bonds of PES and PPS, due to steric S⋯S repulsions in the gauche conformations; (5) induced dipole–dipole interactions in g±tg∓ states of PES. The NMR experiments combined with ab initio MO calculations and the RIS scheme have been demonstrated to be a powerful methodology for conformational analysis of polymers.