Chain folding in cyclic oligo(oxyethylene)s Crystallinity of large unsubstituted crown ethers by X-ray scattering and differential scanning calorimetry

Abstract

Uniform cyclic oligo(oxyethylene)s with chain lengths E₁₅ to E₁₈ (unsubstituted 45-crown-15 to 54-crown-18) have been prepared by reaction of corresponding linear glycols with tosyl chloride under alkaline conditions, purified by preparative gel permeation chromatography (GPC), and characterised by analytical GPC, NMR spectroscopy, and other methods. Wide-angle and small-angle X-ray scattering were used to investigate crystal structure and layer spacing. Comparison was made with scattering from linear oligomers crystallised in unfolded conformation. It was found that cyclo-E₁₈ crystallised into the poly(oxyethylene) structure (monoclinic sub-cell), while the lower oligomers crystallised into different structures. In confirmation of this result, the layer spacing of cyclo-E₁₈ was that expected for the twice-folded conformation. Melting temperatures and enthalpies of fusion of cyclo-E₁₈ and cyclo-E₂₇ (from previous work), measured by differential scanning calorimetry, were compared with those of linear oligomers to obtain values of the enthalpy and entropy of fold formation in the crystallisation of uniform oligo(oxyethylene).