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°C in acidic buffer solutions (pH=2.00–5.00), and it was increased significantly to around 38.4°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.