Crystallinity of methylene‐oxyethylene‐methylene triblock oligomers derived from pentadeca(ethylene glycol)

Block oligomers of the general formula CH 3 (CH 2 ) n O[CH 2 CH 2 O] 15 (CH 2 ) n CH 3 where n + 1 = n c = 1 to 18 and 21 have been prepared. Both the oxyethylene and the methylene blocks are monodisperse. Examination of their crystallinity (by differential scanning calorimetry (DSC); X‐ray scattering; IR, Raman, and 1 H NMR spectroscopy) shows two important structures I and II. I: 3 ⩽ n c ⩽ 6: oxyethylene blocks crystalline, methylene blocks non‐crystalline; II: 13 ⩽ n c ⩽ 18: oxyethylene and methylene blocks crystalline. Samples with n c = 1 or 2 crystallise essentially as poly(oxyethylene). Samples with 7 ⩽ n c ⩽ 12 are intermediate in structure between I and II. Structure II has 7/2 helical oxyethylene blocks in high density crystalline layers, with a structure closely related to that of monoclinic poly(oxyethylene), and planar zig‐zag methylene blocks in low density crystalline layers. The oxyethylene helices are normal to the lamella end‐planes, or almost so, and the methylene planar zig‐zags are tilted. The enthalpies of fusion of the type II block oligomers show an even‐odd effect. There is evidence of chain folding in the block oligomer with n c = 21.