Molecular design of novel pH/temperature-sensitive hydrogels

A series of poly(β-amino ester)–poly(ɛ-caprolactone)–poly(ethylene glycol)–poly(ɛ-caprolactone)–poly(β-amino ester) pentablock copolymers (PAE–PCL–PEG–PCL–PAE) were designed and prepared to examine factors affecting sol–gel phase transition behavior. First, the composition of a series of PCL–PEG–PCL copolymers was controlled by changing the feed ratios of PCL/PEG and the molecular weight of PEG. Second, the composition of pentablock copolymers was varied using different PCL–PEG–PCL copolymers and several feed ratios of PAE monomers. The physicochemical properties of triblock and pentablock copolymers were characterized by 1H NMR and gel permeation spectroscopy. The PAE–PCL–PEG–PCL–PAE copolymers in aqueous solution (20–30 wt%) underwent sol–gel transitions with changes in both pH change and temperature. With increasing molecular weight of PAE, the sol–gel transition zone became narrower because the hydrophobic character of the copolymers decreased. Also, with increases in PCL/PEG ratio and PEG molecular weight, changes in the hydrophobic/hydrophilic balance within copolymers resulted in alterations in sol–gel phase transitions.