Polyamide-based pH and temperature-responsive hydrogels: Synthesis and physicochemical characterization

The pH- and temperature-responsive pentablock copolymers that could form in situ hydrogels, composed of pH-responsive polyamide and temperature-responsive poly(ε-caprolactone)-b-poly(ethylene glycol)-b-poly(ε-caprolactone) (PCL-b-PEG-b-PCL), have been synthesized. The pH-responsive polyamide blocks containing diamide linkages were synthesized using condensation polymerization of adipic acid dihydrazide and phthalic anhydride; whereas, temperature-responsive PCL-b-PEG-b-PCL was synthesized by ring-opening polymerization of ε-caprolactone in the presence of bifunctional PEG macroinitiator. Pentablock copolymer was then synthesized by the chemical conjugation of carboxylic acid-terminated pH-responsive copolymer to the chain end of temperature-responsive triblock copolymer through ester bond formation. The pK_a values of pH-sensitive polyamide, measured using titration method, varied depending on the average molecular weight of polymers. In aqueous solutions, the pentablock and triblock copolymers formed spherical micelles and their particle sizes were found to be 55 to 100 nm. The pentablock copolymer exhibited pH- and temperature-induced sol-to-gel phase transition. At high pH and low temperature, the copolymers were freely soluble even at 20 wt% concentration and form stable hydrogel at the physiological condition (pH 7.4, 37 °C). Furthermore, the sol-to-gel window of the pentablock copolymers was controlled by varying the composition of polyamide to PCL/PEG and covers the physiological region. Overall, these results suggest that the sharp physicochemical change of the pentablock copolymers may be exploited in controlled delivery applications.