A series of near-monodisperse sulfopropylbetaine homopolymers have been prepared by reaction of 1,3-propanesultone with poly(2-(dimethylamino)ethyl methacrylate)s produced by group-transfer polymerization. Block copolymers of 2-(dimethylamino)ethyl methacrylate and n-alkyl methacrylates gave sulfobetaine block copolymers when treated with 1,3-propanesultone. Betaine formation dramatically reduces the surface activity of both the homopolymers and the block copolymers. Chromatography and light scattering show that the betaine copolymers form polydisperse micelles on direct dissolution in water, and preliminary dissolution in a nonselective solvent (2,2,2-trifluoroethanol) is needed to give near-monodisperse micelles. Micelle aggregation numbers are controlled by the length of the hydrophobic block and are essentially independent of the hydrophilic block. The betaine homopolymers show chain expansion in aqueous NaCl solutions up to 0.1 M, due to the anti-polyelectrolyte effect. A similar effect is also observed for the block copolymer micelles, which expand significantly with increasing salt concentration up to 0.1 M.