A general concept for the preparation of junction-point-reactive, amphiphilic Y-shaped block copolymer brushes for ultrathin polymer films is demonstrated, combining anionic polymerization techniques. The polymer films (1-3 nm) exhibit reversible, stimuli-responsive wetting behavior upon application of different external stimuli (temperature and solvent). Contact angle measurements confirm the reversible hydrophobization with shifts up to 23 degrees.