1H-NMR spectra of poly(ethylene glycols) in the presence of shift reagent

To determine the content of trans-gauche-trans (TGT) conformation in poly(ethylene glycol) (PEG) ¹H-NMR spectra were measured with tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-1,4,6-octanedinato)europium(III) [Eu(fod)₃] as a shift reagent. As a result the peaks assigned to the ethylene protons in the polymeric segments separated from those assigned to four ethylene protons as the “near-end” groups, and the singlet peak in the former at the lowest field was assigned to the TGT conformation of the COCCOC sequence. It was confirmed that the conformation of the polymeric segments in PEG complexed with Eu(fod)₃ remained unchanged with benzene-d₆ content as the solvent. The concentration of TGT conformation increased with the number-average molecular weight (M _n): it increased rapidly until about 70% at M _n = 1500.