Synthesis and characterization of a thermosensitive hydrogel based on biodegradable amphiphilic PCL-Pluronic (L35)-PCL block copolymers

A series of PCL–Pluronic–PCL biodegradable block copolymers were successfully synthesized by ring opening polymerization of ɛ-caprolactone (ɛ-CL) initiated by Pluronic (PEG–PPG–PEG) macromonomer, which were characterized by 1H NMR and FTIR. Their aqueous solution displayed special gel–sol transition with temperature increased from 4 °C to 70 °C, when the concentration was above corresponding critical gel concentration (CGC). The gel–sol phase diagram was recorded using test tube-inverting method, which depended not only on the hydrophilic/hydrophobic balance in macromolecular structure, but also on heating history of copolymer's aqueous solution. As a result, the gel–sol transition temperature range could be altered according to its practical application, which would be very useful as injectable drug delivery system. To illuminate the mechanism of gel–sol transition, we introduced a typical double layers shell/core structure of PCL–Pluronic–PCL micelles here. The hydrophobic interactions and partly crystallization might be the key gelation machanism for such special gel–sol transition behavior of PCL–Pluronic–PCL aqueous solution.