Unexpected tremendous effect of poly(acrylic acid) (PAA) as polydonor on the thermoresponsive behaviour of the comb-like poly(poly(ethylene glycol) methacrylate) (PPEGMA(500)) with short poly(ethylene glycol) (PEG) side chains as H-bond acceptor, both synthesized by atom transfer radical polymerization (ATRP), has been found indicating the formation of a unique class of interpolymer complexes between these well-defined polymers. Strikingly, the aqueous solutions of the PPEGMA(500)-PAA inter polymer complexes possess LCST-type thermoresponsive behaviour with hysteresis at significantly lower temperatures (similar to 40-50 degrees C) than that of PPEGMA(500), which is attributed to the decreased hydrophilicity of the complexes. It is revealed that thermal phase transition temperatures (T-PT) of the PPEGMA(500)-PAA interpolymer complexes have a minimum in the near of 1.3 ethylene glycol/acrylic acid (EG/AA) monomer unit molar ratio, in accordance with the dynamic light scattering (DLS) results, and the T-PT can be fine tuned by varying the EG/AA ratio, the molecular weight of the PPEGMA(500) and pH of the solution. (C) 2017 Elsevier Ltd. All rights reserved.