Synthesis and characterization of polymers with densely-grafted oligo(ethylene glycol)s (OEGs) are described. Polymerization of OEG-containing diazoacetates with (η³-C₃H₅)PdCl-based initiating systems afforded C–C main chain polymers bearing OEGs on each main chain carbon atom. The polymers with a short OEG side chain showed a lower critical solution temperature-type phase separation in an aqueous medium; the transition temperatures were higher than those of the corresponding vinyl polymers with the same oxyethylene chain length. In addition, the Li-ion conductivity was examined to reveal that the polymers with densely-grafted OEGs showed higher values than the corresponding vinyl polymers, despite the fact that no significant difference in the glass transition temperature was observed between the two types of polymers.