Ionic liquid-based copolyesters containing an imidazolium cation backbone and PF6- or Tf2N- counter-anion are synthesized from N,N'-bis(omega-hydroxyalkyl)imidazolium salts (PF6- or Tf2N-) (0.5 equiv.), poly(ethylene glycol) (PEG; number average molecular weight: 1000 or 2000) (0.5 equiv.), and sebacoyl chloride (1.0 equiv.). The imidazolium monomers are room-temperature ionic liquids. In the ionic copolymers, the 1:1 ratio between imidazolium and PEG, and number average molecular weights were confirmed using H-1 NMR. Crystallization of the PEG segments was affected by the alkyl spacer length of the ionic units and by the anion type. Softer Tf2N- can prevent copolyester crystallization during cooling when the anions are close to the PEG segments; moreover, some Tf2N- copolymers show crystallization during a heating scan. The crystalline phase of the PEG-2000 segments gives distinct X-ray diffraction peaks at 2 theta approximate to 19.0 and 23.1 degrees, and the lateral chain packing of the C-11-(imidazolium PF6)-C-11 structures is shown at 2 theta approximate to 21.5 degrees. These crystalline domains are shown as T-c peaks during a cooling scan. The room-temperature ionic conductivities of the Tf2N- copolyesters are above 10(-5) S cm(-1); however, the ionic conductivity drops significantly below the melting temperatures of the PEG copolyesters. In contrast, the predominantly amorphous polymers that contain PF6- and Tf2N- counter-anions do not show a sudden conductivity drop over the entire temperature range. The imidazolium-PEG copolyesters, which have a low ion density, show a higher ionic conductivity than a reported poly(imidazolium-sebacate) without PEG segments because of the lower activation energy of the isolated conducting anions. (C) 2018 Elsevier Ltd. All rights reserved.