The self-assembling behavior of poly(methacrylic acid-block-ethyl acrylate) (P(MAA-b-EA)) copolymer synthesized via atom transfer radical polymerization (ATRP) was examined and compared with that of random and cross-linked MAA-EA copolymers. The polymer exists as spherical micelle containing 60 polymer chains. The micelle consists of a hydrophobic EA core surrounded by a hydrophilic MAA shell, which expands when the MAA segments are ionized. During the course of neutralization, the particle size increases from similar to22 nm at alpha=0 to 30 nm at (alpha=0.15, thereafter it decreases to a minimum of similar to12 nm at alpha=0.29, suggesting that the polymer undergoes a structural change (expansion-dissociation) upon ionization. When alpha exceeds 0.6, the micelles completely disintegrate into single polymer chains.