X-ray standing waves at grazing incidence are used to trace the position of the reactive chain ends of a surface-initiated (via protons in the underlying PEDOT:PSS layer) polymerization in a bulk polymer film, a procedure recently used to realize insoluble functional films for mutlilayer organic light-emitting diodes. The reactive chain ends as well as their respective counterions are found within a few nanometers thin layer, which moves away from the surface as the reaction proceeds. This reactive front remains narrow for polymerized films that are significantly thicker than the front width. This result can be rationalized by assuming that all oxetane units in contact with the PEDOT:PSS are initiated before the reaction proceeds at elevated temperature.