Conformational energies and random-coil configuration of poly(3,3-dimethyl oxetane) (PDO)

Abstract

A sample of poly(3,3‐dimethyl oxetane) was synthesized by cationic polymerization. A fraction having M_n=8500 and melting point of 45 °C was isolated and purified; its dipole moment, measured in benzene solutions at several temperatures in the interval 20–60 °C, yielded values of <μ²≳/nm²=0.249–0.297 and a temperature coefficient d(ln<μ²≳)/dT=4.4×10⁻³ K⁻¹. Semiempirical potentials were used to compute conformational energies as function of rotations over the backbone bonds; the results of these calculations indicate that skeletal C–O bonds are always placed in trans conformation, whereas gauche states over C–C bonds have an energy of about 0.4 kcal mol⁻¹ lower than their trans conformation, (E_σ?‐0.4); a second order interaction energy E_ω?1.3 kcal mol⁻¹ arises in conformations g^±g^∓ over two consecutive C–C skeletal bonds because of the interactions between two consecutive oxygen atoms. Agreement between theoretical (calculated with the RIS theory) and experimental results of dipole moment and its temperature coefficient is achieved using E_ω=1.3 and E_σ=−0.5 kcal mol⁻¹.