Synthesis and characterization of pH-sensitive and thermosensitive double hydrophilic graft copolymers

Abstract

A series of poly(N-isopropylacrylamide)-co-poly(Ne-benzyloxycarbonyl-L-lysine) graft copolymers (PNIPAm-co-PZLLys) with different side chains (degree of polymerization, DP = 5∼40) and unit ratios (from 30 to 70 mol %) were prepared via free radical polymerization, followed by cleaving benzyloxycarbonyl groups (Z groups) to obtain the double hydrophilic graft copolymer, poly(N-isopropylacrylamide)-co-poly(L-lysine) (PNIPAm-co-PLLys). The pH- and temperature-response properties of the graft copolymers in aqueous solution were studied. The experimental results indicate L15-N30 and L15N-70, that is, the PNIPAm-co-PLLys having the poly(L-lysine) of DP = 15 as side chains as well as 30 and 70 mol %, respectively, of PNIPAm as backbone, have coil-to-helix transitions from pH 6 to pH 12 at room temperature and form uniform nanoscale micelle-like dispersions in aqueous solution at pH 12. The graft copolymers also could form uniform and nanoscale micelle-like structures at 50 °C in pH 6 buffer solution due to slightly polymer aggregation. With temperature and pH increased, both the deprotonated PLLys side chains and PNIPAm backbone become hydrophobic, leading to polymer precipitation. These results illustrate that a double tunable hydrophilic graft copolymer had been successfully synthesized via a simple radical polymerization, and could form micelles without serious polymer aggregation at a lower pH and a higher temperature. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011