Thermoresponsive and highly biocompatible poly(glycidyl ether) copolymers of glycidyl methyl ether (GME) and ethyl glycidyl ether (EGE) with adjustable molecular weight and defined end groups are synthesized by a monomer-activated anionic ring-opening polymerization with NOct4Br as initiator and i-Bu3Al as activator. In contrast to a conventional oxyanionic (nonactivated) copolymerization, higher molecular weights and a truly random incorporation of the monomers are accomplished. The monomer reactivity ratios were determined by the Kelen–Tüdõs approach to be rGME = 0.98 and rEGE = 0.95. The thermoresponsive properties of these copolymers with varying molecular weight were characterized by UV–vis transmittance and dynamic light scattering. Conformational changes of the copolymer during the phase transition on the molecular level were studied by 1H and 13C NMR spectroscopy in D2O and revealed only a partial dehydration during the collapse of the copolymer affecting both side chains and polymer backbone.