The present application can provide a preparation method capable of preparing a desired polymer or conductive polymer film with excellent polymerization efficiency and conversion rates without consumption or modulation in the polymerization process, and a polymer and a conductive polymer film formed by the method. The present application can provide a method for preparing a polymer or a conductive polymer film having a desired level of transparency and conductivity, wherein desired physical properties such as solubility in a solvent or resistance to a solvent are effectively imparted thereto as necessary, and a polymer and a conductive polymer film formed by the method.