A zwitterionic polymer with a sticky catechol chain end was successfully synthesized by ARGET-ATRP (activators regenerated by electron transfer for atom transfer radical polymerization) and used to modify membranes for separation of oil/water mixtures and emulsions. The zwitterionic polymer was found to adhere to various substrates, including stainless steel meshes and polyethersulfone microfiltration membranes, via in-situ formation of polydopamine at the substrate surfaces by copolymerization of the catechol chain ends with a small amount of dopamine monomers added. The polyzwitterion coating was thin and uniform, which endowed the metal mesh and the polymeric microfiltration membrane with superhydrophilicity and underwater superoleophobicity without any marked change in morphology. The coated mesh and membrane, operating in the "water-removing" mode, exhibited very high fluxes and efficiency in separating oil/water mixtures and surfactant-stabilized emulsions, respectively. They also showed excellent anti-fouling ability against oil and protein as well as stability under various harsh environments, making them promising in commercial applications in various fields.