Electrogeneration of reducers at a microelectrode allows us to locally debrominate a surface bound polymerization initiator. Once submitted to atom transfer radical polymerization, patterns of nanometer polymer structures are formed on the surfaces. The surface reduction rate, controlled by the microelectrode scan rate and the reducer strength, affects the dimensions of the patterns. The reducer strength allows to tune the surface density of the initiator layer and then the elongation of the polymer brushes grown in the locally etched regions.