Abstract: The geometries of the three structures of poly-(3-methylpyrrole-4-carboxylic acid) (PMPC), reduced PMPC(a), oxidized PMPC(c) and half-oxidized PMPC(b), have been optimized by means of MNDOprogram.The electronic and energy band structures of these systems have been studied by means of EHMO-COmethod.The self-doping conducting mechanism was explored in terms of the calculated results.After β-Hatom of PPy is substituted by methyl and carboxy group, E_g of system become smaller and conductivity increases because π-orbital of pyrrole and p-orbital of carboxy group form LUCOand NLUCOwhich corresponds to two impurity bands between valence band and conductive band.Reduced PMPCcan spontaneously dope to form stable oxidized PMPCby the lass of proton of carboxy group, which makes HOBWincreasing and E_g decreasing furtherly.The groups with the atoms of bigger electroaffinity play an important role in the self-doping conducting process.

Key words: Polypyrrole, Substituted polypyrrole, Self-doping