A one-pot synthesis of oleic acid end-capped temperature- and pH-sensitive amphiphilic polymers

A one-pot synthesis of an amphiphilic oleic acid-end capped random poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (2) is reported. In aqueous media, the solubility of 2 was temperature- and pH-sensitive. Both the lower critical solution temperature and critical micelle concentration (CMC) of 2 were pH-dependent. The LCST of 2 was around 35.2 degrees C in acidic buffer solutions (pH=2.00-5.00), and it was increased significantly to around 38.4 degrees C in neutral and alkaline buffer solutions (pH 7.00-11.00). Polymer 2 exhibited a phase transition pH around 6.7, below which the polymer became significantly less water-soluble. The CMC of 2 was 40 mg/l in pH 2.0 buffer solution, and it was increased markedly to 60-67 mg/l in pH 7.0 and pH 11.0 buffer solutions. Micelle solutions of 2 in different pH conditions were prepared by a membrane-dialysis method. In aqueous solution, dynamic light scattering studies revealed that the size of micelles was 50-90 nm with the particle size being larger in acidic solutions. In solid state, transmission electron microscope studies showed that micelles were roughly spherical, their sizes were 25-90 nm and it decreased with the increase of solution pH. The pH-sensitivity of 2 was triggered by the -COOH in the hydrophobic segment. The temperature- and pH-sensitivity of the novel polymeric micelles would make an interesting drug delivery system.