A class of cationic bottle-brush polymers that show ionic strength-dependent stimuli responsiveness is prepared. Brush polymers with norbornene as backbone and quaternary ammonium (QA)-containing polycaprolactone copolymers as side chains are synthesized by a combination of ring-opening metathesis polymerization, ring-opening polymerization, and click reaction. In water with low ionic strength, brush polymers are soluble due to the strong electrostatic repulsion between cationic QA groups. As the addition of salt to increase ionic strength, single brush polymers undergo a transition from extended conformation to collapsed state and finally become insoluble in solution due to the screening effect of salts that yield the once-dominant electrostatic interactions among QA species to hydrophobichydrophobic interactions.