Poly(2-hydroxyethylacrylate)-poly(a-butyl acrylate) block copolymers were synthesized with the reversible addition-fragmentation chain transfer (RAFT) process. The block copolymers were synthesized successfully with either poly(2-hydroxyethyl acrylate) or poly(n-butyl acrylate) macro-RAFT agents. The resulting block copolymers had narrow molecular weight distributions (polydispersity index = 1.3-1.4). Copolymer self-aggregation in water yielded micelles, with the hydrodynamic diameter (D-h) values of the aggregates dependent on the length of both blocks according to D-h similar to (NBANHEA0.57)-N-1.17, where N-BA is the number of repeating units of n-butyl acrylate and N-HEA is the number of repeating units of 2-hydroxyethyl acrylate. The micelles were subsequently stabilized via chain extension of the block copolymer with a crosslinking agent. The successful chain extension in a micellar system was confirmed by an increase in the molecular weight, which was detected with membrane osmometry. The crosslinked particles showed noticeably different aggregation behavior in diverse solvent systems. The uncrosslinked micelles formed by the block copolymer (N-HEA = 260, N-BA = 75) displayed a definite critical micelle concentration at 5.4 x 10(-4) g L-1 in aqueous solutions. However, upon crosslinking, the critical micelle concentration transition became obscure. (c) 2006 Wiley Periodicals, Inc.