Synthesis of poly(cystine bisamide)‐PEG block copolymers grafted with 1‐(3‐aminopropyl)imidazole and their phase transition behaviors

New biodegradable and pH‐sensitive block copolymers were prepared by grafting 1‐(3‐aminopropyl) imidazole onto a backbone polymer formed via condensation polymerization between l ‐cystine and EDTA‐dianhydride. The copolymer with a graft ratio of 79% exhibited a good buffering capacity and pH sensitivity. These are attributed to protonation–deprotonation of the imidazole ring at around pH 7. The copolymers with less imidazole content did not show any apparent responses to changes in pH. The particle size of the copolymer aggregate formed under basic conditions was around 200 nm and increased with decreasing pH. The critical aggregation values at pH 6.0 and 8.0, derived from the changes of intensity ratios (I₁/I₃) in the emission spectrums of pyrene, were approximately 0.17 and 0.05 mg/ml, respectively. The surface charge of the aggregates increased with the decreasing pH as a result of the increase in protonation of imidazole and the tertiary amine in the polymer chain. The microviscosity of hydrophobic domains was estimated using 1,6‐diphenyl‐1,3,5‐hexatriene. The decrease of the anisotropy value under acidic conditions reflects a disruption of hydrophobic interaction. Copyright © 2008 John Wiley & Sons, Ltd.