It has been previously reported that chain extension of a water-soluble poly(glycerol monomethacrylate) (PGMA) macro-CTA with 2-hydroxpropyl methacrylate (HPMA) via reversible addition-fragmentation transfer (RAFT) aqueous dispersion polymerization produces thermo-responsive diblock copolymer nano-objects. For a relatively narrow range of target diblock compositions and copolymer concentrations, a well-defined worm phase can be obtained. Such worms form soft, transparent, free-standing gels at 20oC. However, cooling to 5oC induces a worm-to-sphere transition due to the thermo-sensitive nature of the PHPMA block (see Blanazs et al. J. Am. Chem. Soc, 2012  , 134, 9741). Herein we demonstrate that preparation of such diblock copolymers using a carboxylic acid functionalized RAFT agent, 4-cyano-4-(2-phenylethanesulfanylthiocarbonyl)sulfanyl pentanoic acid (PETTC), results in worms that are pH-responsive as well as thermo-responsive. Transmission electron microscopy, dynamic light scattering and aqueous electrophoresis studies indicate a worm-to-sphere at around pH 4-5 due to ionization of the terminal carboxylic acid on the PGMA stabilizer block. This change in morphology is reversible on addition of acid. In contrast, preparation of the equivalent diblock copolymer using the methyl ester derivative of PETTC produces worms that do not exhibit pH-responsive behavior. This represents a new paradigm for pH-responsive block copolymers, since minimal acid/base is required to drive the morphological transition.

pH-responsive diblock copolymer nano-objects based on non-ionic monomers: Ionization of carboxylic acid end-groups induces an order-order morphological transition

248th National Meeting (2014)

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