Both homo- and copolymerization of the hitherto nonpolymerizable epoxide monomer epicyanohydrin (EPICH) with ethylene oxide (EO) have been studied, employing the monomer activation technique. Tetraoctylammonium bromide or tetrabutylammonium iodide was used as initiator combined with i-Bu3Al to activate the EPICH monomer. The EPICH content was varied from 4 to 16 mol %, yielding well-defined PEG-co-PEPICH copolymers with molecular weights M-n (SEC) ranging from 3700 to 8800 g mol(-1). The nitrile groups of the resulting polyethers were further reduced or hydrolyzed to introduce amino, amide, or carboxyl groups at the polyether backbone, circumventing protecting group chemistry. Successful transformation of the functional groups was proven by SEC measurements, H-1 NMR, C-13 NMR, and FT-IR spectroscopy. These carboxyl-functional PEG copolymers are anionic polyelectrolytes consisting only of purely aliphatic polyether structures with carboxyl groups. The hydrolyzed PEPICH homopolymers represent the first polyether-based analogues of poly(acrylamide) and poly(acrylic acid). In pronounced contrast to poly(acrylic acid), carboxyl-functional PEPICH shows excellent aqueous solubility even at low pH.