Reversible Interpolyelectrolyte Shell Cross-Linked Micelles from pH/Salt-Responsive Diblock Copolymers Synthesized via RAFT in Aqueous Solution

Abstract

Herein we report the synthesis and characterization of a series of pH reversible shell cross-linked micelles. A series of novel pH/salt-responsive block copolymers of poly(sodium 2-acrylamido-2-methyl-1-propanesulfonate-block-N-acryloyl-l-alanine) (P(AMPS-b-AAL)) were synthesized utilizing aqueous reversible addition−fragmentation chain transfer (RAFT) polymerization. Micellization of P(AMPS-b-AAL) is induced by rendering the PAAL block hydrophobic through protonation of the carboxylic acid (pH 1−3). The pH at which micelle formation occurs and the hydrodynamic diameters of the resultant micelles are dictated by block copolymer composition and electrolyte concentration. The anionic PAMPS micelle shells were subsequently cross-linked with a RAFT synthesized cationic homopolymer of either poly(N-[3-(dimethylamino)propyl]acrylamide) (PDMAPA, pKa = 8.5) or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA, pKa = 7.3). Upon deprotonation of the PAAL block, these cross-linked micelles swell but remain stable and intact. Significantly, the reversibility of the cross-linking was determined to be tunable by utilizing the different cationic homopolymers for cross-linking. This was demonstrated by increasing the pH above the pKa of the cationic homopolymer cross-linker, resulting in deprotonation of the cationic cross-linker and dissociation of the cross-linked micelles.