Preparation of an amphiphilic triblock copolymer with pH- and thermo-responsiveness and self-assembled micelles applied to drug release

A novel amphiphilic ABC triblock copolymer, poly(N-isopropylacrylamide)-b-poly(styrene-alternate-maleic anhydride)-b-polystyrene (PNIPAAm-b-PSMA-b-PSt), was designed and synthesized by reversible addition fragmentation chain transfer polymerization. The copolymer can disperse in aqueous media at room temperature to self-assemble into pH- and thermo-responsive core-shell-corona micelles, with the hydrophobic PSt block as core, the pH-sensitive PSMA block as shell, and the thermo-sensitive PNIPAAm block as corona. The aqueous copolymer solutions exhibited lower critical solution temperature (LCST) values around 33.5-35 degrees C under pH 2.1-9.2 conditions via optical transmittance measurements. The critical micelle concentration Values of the self-assembled micelles were about 18.6 mg/L at pH 2.1 and 21.1 mg/L at pH 6.9, respectively, relying on fluorescent probe techniques. An interesting pH- and thermo-dependent size of the micelles was observed by dynamic light scattering techniques, which showed that the micelle diameters were about 100-120 nm at 40 degrees C and 120-160 nm at 25 degrees C, corresponding to pH 2.1-9.2, respectively. Transmission electron microscopy observations showed that the resulting micelles as uniform nanoparticles existed in regularly spherical shapes. The folic acid-loaded micelles showed a remarkable pH- and thermo-responsive drug release behavior. These results indicate that the copolymer micelles may serve as a promising "intelligent" drug delivery system. (C) 2009 Elsevier Inc. All rights reserved.