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 NOct(4)Br 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-Tudos approach to be r(GME) = 0.98 and r(EGE) = 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 H-1 and C-13 NMR spectroscopy in D2O and revealed only a partial dehydration during the collapse of the copolymer affecting both side chains and polymer backbone.