A temperature, pH, and ion sensitive block copolymer with an unusual phase transition: Monomethoxy polyethylene glycol-block-polyethyleneimine

A temperature, pH and ion sensitive, block copolymer, monomethoxypolyethylene glycol (MPEG)-block-polyethyleneimine (PEI) was synthesized by carbodiimide activation. MPEG-PEI block copolymer exhibited a thermally reversible phase transition from insoluble to soluble form with increasing temperature at neutral pH. MPEG-PEI chains are insoluble in the temperature range of 4-20 degrees C in an aqueous medium containing 3.5 mM phosphate ion. By increasing temperature, homogeneous and transparent solutions including dissolved copolymer are obtained at temperatures higher than 20 degrees C. The observed thermoresponsive behavior was opposite to that seen with poly(N-isopropylacrylamide). Depending on the phosphate ion concentration, the phase transition temperature for passing from insoluble to soluble form could be also precisely controlled between 4-40 degrees C. The reversible phase transition behavior of MPEG-PEI copolymer was confirmed by ten consecutive heating-cooling cycles between 4-50 degrees C. MPEG-PEI copolymer also exhibited another responsivity against pH. More appreciable increase in the average hydrodynamic size with the rising temperature was observed at pH 9.0 with respect to those obtained at pH 5 and 7. Temperature, pH and ion sensitive behavior of MPEG-PEI is documented for the first time. The reversible phase transition of MPEG-PEI is a promising tool which may open novel pathways, particularly for enzyme immobilization and drug release studies.