Random ionomers based on methacryl poly(ethylene oxide) with nine ethylene oxide units on the side chain and sodium sulfonated styrene (NaSS) are synthesized by reversible addition fragmentation chain transfer polymerization. The glass transition temperature increases gradually as ions are incorporated at low ion content then more strongly as the ion content reaches 51 mol % NaSS comonomer (1:9 ion to ether oxygen (EO) ratio), exceeding the polarizability volume overlap for contact pairs at 35 mol % NaSS comonomer. Ionomers with 51 and 70 mol % NaSS comonomer show microphase separation, as determined by X-ray scattering, where ion-rich microphases have glass transition temperature T-g = 165 degrees C excluding many of the PEO side chains, which primarily reside in a dilute ion-pair microphase. Fourier transform infrared spectroscopy was used to characterize the fraction of sulfonates in three different association states that can be related to the static dielectric constant from dielectric relaxation spectroscopy through the Onsager equation. The ionomer with 10 mol % NaSS comonomer exhibits the highest room-temperature conductivity (similar to 10(-7) S/cm) that results from the best combination of number density of simultaneously conducting ions and their mobility, with no ion-rich microphase below 100 degrees C.