Chemical oxidative polymerization is a traditional and commonly used method for the polymerization of aniline in large quantities. When aniline is mixed with an oxidant in an acidic solution, polymerization will occur. The most important parameter to be controlled in this method for the synthesis of polyaniline nanofibers is the domination of homogeneous nucleation over heterogeneous nucleation. Homogeneous nucleation describes when the nuclei are formed spontaneously in solution while heterogeneous nucleation describes when the nuclei are grown on other species. In the early stages of this polymerization, only nanofibers are formed since there are no heteronuclei available for heterogeneous nucleation. However, if the reaction is left uncontrolled, heterogeneous nucleation will begin to dominate as the polyaniline will preferentially grow on existing particles, leading to irreversible agglomeration. The reaction can be made to favor homogeneous nucleation throughout by increasing reaction speed, temperature of the reaction, and allowing the reaction to proceed without stirring.
The diameter of the polyaniline nanofibers can be controlled with this method through choice of acid. Hydrochloric acid produces nanofibers with a diameter of about 30 nm, while camphorsulfonic acid and perchloric acid produce a diameter of 50 nm and 120 nm respectively. Under normal synthetic methods polyaniline derivatives, such as ones that are alkyl and fluoro substituted, do not exhibit a well-defined fibrous shape, however, in the presence of an aniline oligomer nanofibers of certain derivatives can be synthesized. While the most common oxidant is ammonium peroxydisulfate (APS), various others can be used. One study shows the use of potassium biiodate (KH(IO)) as an oxidant, claiming it to lead to polyaniline nanofibers that are longer, have higher crystallinity, and have higher electrical conductivity.