The first ferrocene-containing epoxide monomer, ferrocenyl glycidyl ether (fcGE), is introduced. The monomer has been copolymerized with ethylene oxide (EO). This leads to electroactive, water-soluble, and thermoresponsive poly(ethylene glycol) (PEG) derived copolyethers. Anionic homo- and copolymerization of fcGE with EO was possible. Molecular weights could be varied from 2000 to 10 000 g mol(-1), resulting in polymers with narrow molecular weight distribution (M-w/M-n = 1.07-1.20). The ferrocene (fc) content was varied from 3 to 30 mol %, obtaining water-soluble materials up to 10 mol % incorporation of the apolar ferrocenyl comonomer. Despite the steric bulk of fcGE, random copolymers were obtained, as confirmed via detailed H-1 NMR kinetic measurements as well as C-13 NMR studies of the polymer microstructure, including detailed triad characterization. In addition, the poly(fcGE) homopolymer has been prepared. All water-soluble copolyethers with fc side chains exhibited a lower critical solution temperature (LCST) in the range 7.2-82.2 degrees C in aqueous solution, depending on the amount of fcGE incorporated. The LCST is further tunable by oxidation/reduction of ferrocene, as demonstrated by cyclic voltammetry. Investigation of the electrochemical properties by cyclovoltammetry revealed that the iron centers can be oxidized reversibly. Further, to evaluate the potential for biomedical application, cell viability tests of the fc-containing PEG copolymers were performed on a human cervical cancer cell line (He La), revealing good biocompatibility only in the case of low amounts of fcGE incorporated (below 5%). Significant cytotoxic behavior was observed with fcGE content exceeding 5%. The ferrocene-substituted copolyethers are promising for novel redox sensors and create new options for the field of organometallic (co)polymers in general.