Well-defined amphiphilic block copolymers were prepared by ring opening metathesis polymerization and their stimuli responsive behavior of formed micelles in aqueous solution was investigated. The hydrophobic core of the micelles consists of either a poly[5,6-bis(ethoxymethyl)bicyclo[2.2.1]hept-2-ene]-block with a glass transition T-g at room temperature or a poly[endoexo[2.2.1]bicyclohept-5-ene-2,3-diylbis (phenylmethanone)] with a Tg of 143 degrees C. For the polyelectrolyte shell, the precursor block poly[endo,exo[2.2.1]bicyclohept-5-ene-2,3-dicarboxyclic tert-butylester] was transformed into the free acidic block by cleavage of the tert-butyl groups using trifluoroacetic acid. Micellar solutions were prepared by dialysis, dissolving the copolymers in dimethyl sulfoxide which was subsequently replaced by water. All polymers form micelles with radii between 10 and 20 run at a pH-value below 5, where the carboxylic acid groups are in the protonated state. The block copolymer micelles show a strong increase of the hydrodynamic radius with increasing pH-value, due to the repulsion among the formed carboxylate anions resulting in a stretching of the polymer chains. In this state, the micelles exhibit responsive behavior to ionic strength where a contraction of the micelles is observed as the carboxylate charges are balanced by sodium ions, whereas no changes of the hydrodynamic radius on addition of salt are observed at low pH. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47:1178-1191, 2009