pH-responsive micellization of amphiphilic diblock copolymers synthesized via reversible addition-fragmentation chain transfer polymerization

Well-defined poly(sodium 2-(acrylamido)-2-methylpropanesulfonate-block-sodium 6-acrylamidohexanoate) (pNaAMPS-AaH) was synthesized by reversible addition-fragmentation chain transfer (RAFT) radical polymerization of sodium 6-acrylamidohexanoate (AaH) using the sodium 2-(acrylamido)2-methylpropanesulfonate-based macrochain transfer agent. The living polymerization of AaH was evidenced by the fact that the number-average molecular weight increased linearly with monomer conversion while the molecular weight distribution remained narrow independent of the conversion. pH-induced association and dissociation behavior of the diblock copolymers was investigated by quasi-elastic light scattering (QELS), static light scattering (SLS), H-1 NMR spin-spin relaxation time, and fluorescence probe techniques. At pH < 4, the diblock copolymers exhibited large values of the hydrodynamic radius and small values of the 1H NMR spin-spin relaxation time. These observations indicated that micellization occurred to form polymer micelles comprising hydrophobic protonated AaH cores and hydrophilic NaAMPS coronas at pH < 4. On the other hand, these diblock copolymers dissolved in aqueous solutions as a state of unimer under high-pH conditions. 8-Anilino-1-naphthalenesulfonic acid, ammonium salt hydrate (ANS), as a fluorescence probe could be incorporated into the protonated AaH core of diblock copolymer micelles at low pH and released upon dissociation of the micelles at high pH, which was completely reversible.