Effects from various initial molar ratios of aniline (AN), ammonium peroxydisulfate (APS) and HCl ( [AN]: [APS]: [HCl]) on the polymerization of AN were investigated. First, a scheme derived from a molecular point of view was proposed to distinguish formation mechanisms based on their initial conditions. Thereafter, by choosing a relatively low ratio of Keywords: aniline; formation mechanism; phenazine-like oligomers; polyaniline INTRODUCTION Polyaniline (PANI) is one of the most important conducting and semi-conducting polymers widely used in organic optoelectronic devices and has attracted considerable attention recently because of its tunable electrical conductivity, easy preparation from common chemicals and excellent thermal and environmental stabilities. 1,2 The electrical properties of PANI are sensitively dependent on its oxidation and protonation states, of which the emeraldine state (see Figure 1) with a wide range of electrical conductivity can be achieved by simple doping/dedoping chemistry based on acidic/basic reactions. 1-3 The conventional chemical oxidative polymerization of aniline (AN) is carried out in a strong acidic solution and initiated by adding an oxidant (such as ammonium peroxydisulfate, APS). 3 From a technical point of view, the above-mentioned preparation for PANI is a simple process, but the formation mechanism involves an intricate interplay of consecutive chemical and physical reactions. [3][4][5][6][7][8][9][10][11][12] To account for the formation of PANI, the following basic mechanisms were reported. The redox reaction of neutral ANs with APS produces AN radicals. 4 Under a neutral or alkaline condition, the mixed ortho-para coupling of AN radicals together with oxidative intramolecular cyclization forms AN oligomers with phenazine-like structures at the initial stage (see Figures 2a and b for phenazine and phenazine-like trimer). 9-13 The oxidation of AN and AN oligomers with APS both release protons, and the pH of the solution falls. [12][13][14][15][16][17][18][19][20] In a strong acidic environment, the formation of a para-coupled structure