AbstractThe series of poly(oxyethylene)s (poly‐EO) with number‐average degrees of polymerization ranging from ≈6 to ≈80, terminated at both ends with phosphonium ion groups, was prepared by conversion of hydroxyl end groups of α‐hydro‐ω‐hydroxypoly(oxyethylene)s into bromide end groups, followed by the reaction with triphenylphosphine. The functionality of the products (diionic poly‐EO) was determined by elemental analysis,1H and31P NMR as well as mass spectroscopy (MS). The observation of the signals corresponding to individual oligomers in31P NMR spectra of short chain (DPn<10) diionic poly‐EO was explained by intramolecular aggregation of ionic end groups leading to cyclic structures. The measurements of spin lattice relaxation times and the chemical shifts of phosphorus at different conditions (temperature, solvent, concentration) confirmed, that the aggregation of ionic end groups is clearly manifested in the31P NMR spectra