Synthesis, characterization and behavior in aqueous solution of star-shaped poly(acrylic acid)

We report the synthesis of star-shaped poly(acrylic acid) (PAA), with 5, 8, and 21 arms, by atom transfer radical polymerization of tert-butyl acrylate. We employ the core-first approach using glucose-, saccharose- and cyclodextrin-based initiators. Subsequent acidic treatment of poly(tert-butyl acrylate) (PtBA) leads to star-shaped poly(acrylic acid) (PAA). Alkaline cleavage of the arms enabled us to determine the initiation site efficiency. The PAA stars and arms were esterified to poly(methyl acrylate) (PMA). Molecular weight determination by means of GPC/viscosity, MALDI-TOF MS and NMR end-group determination showed that the initiation site efficiency is close to unity. Results from potentiometric titration of PAA arms and stars show that the apparent pK(a) values increase with increasing arm number, which is a direct result of increasing segment density. Osmometry measurements of aqueous solutions of the PAA stars result in osmotic coefficients between 0.05 and 0.38, indicating that most of the counterions are confined within the star. The confinement increases with arm number.