Aqueous RAFT synthesis of stimuli-responsive, amphiphilic block copolymers and self-assembly behavior in solution and incorporation into LBL films
Of all the living radical polymerization techniques, reversible addition-fragmentation chain transfer (RAFT) polymerization is arguably the most versatile in terms of the reaction conditions (e.g. temperature and solvent selection), monomer selection (e.g. neutral, anionic, cationic, and zwitterionic), and purification. Since the introduction of RAFT in 1998, the McCormick research group and others including the Lowe, Sumerlin, and Davis research groups have synthesized a wide range of (co)polymers with predetermined molecular weights, low polydispersities, and advanced architectures utilizing aqueous RAFT (ARAFT) polymerization. These research groups have also studied how various block copolymers exhibit stimuli-responsive behavior due to a change in temperature, solution pH, or electrolyte concentration. However the stimuli-responsive behavior of unprotected, chiral, amino acid-based polymers had yet to be reported. The incorporation of these homopolymers into stimuli-responsive block copolymers will create novel polymer systems that can be reversibly "locked" under facile conditions and have potential applications in sequestration and targeted delivery. The overall goal of this research is to utilize the RAFT process for the synthesis of such block copolymers directly in water, investigate the relationship between block copolymer composition and solution properties on the self-assembly behavior of the copolymers, and incorporate these micelles within films via the layer-by-layer technique to produce stimuli-responsive films for applications such as drug release from surfaces. The first section concerns utilizing ARAFT polymerization for the successful synthesis of a series of novel pH-responsive block copolymers containing an unprotected amino acid-based block. Block copolymers containing a permanently anionically charged hydrophilic block of AMPS and a pH-responsive AAL block were subsequently synthesized and the aqueous self-assembly behavior was investigated. The aggregation behavior for a series of P(AMPS- b -AAL) was determined at varying pH values and salt concentrations. The effect of the permanently hydrophilic and responsive block lengths on the stimuli driven assembly behavior was examined. The second section details the cross-linking of these micelles using interpolyelectrolyte complexation (IPEC) with cationic polymers. This is the first report of a pH reversible IPEC cross-linked micellar system. The third section details work done in collaboration with Christopher Harris and concerns the incorporation of micelles possessing anionically charged coronas within layer-by-layer films. The effect of salt concentration on film thickness and morphology was studied. Also because the films are made using pH-responsive block copolymers, the responsive behavior of the polyelectrolyte multilayer films was also investigated.