Random-Coil configurations of the polymormals [CH2?O?(CH2)y?O?]. II. Dipole moments of poly(1,3–dioxepane) [CH2?O?(CH2)4?O?;]

Dielectric constant measurements were carried out on a sample of poly(1,3–dioxepane) CH₂? O? (CH₂)₄? O? ], in benzene over the range 25—60°C. This chain molecule was found to be very similar to poly(1,3–dioxolane) CH₂? O? (CH₂)₂? O? ] in having a relatively small dipole moment, which increases markedly with increasing temperature. Rotational isomeric-state calculations show that these two characteristics are due to very strong preferences for gauche states in the two central bonds in the sequence CH₂? O? CH₂? O? CH₂. Such pairs of gauche states, necessarily of the same sign, place neighboring-group dipoles in essentially antiparallel orientations. The resulting attenuation of dipole vectors is the origin of the small dipole moment, and the disruption of these preferred conformations with increasing temperature gives rise to the large positive temperature coefficient of the dipole moment.