A theoretical study of the electronic structure of some aniline derivatives is performed, aimed at a microscopic explanation of the observed polymerizability of these compounds. Monomers with methyl and methoxy substituents located at various positions on the benzenoid ring are considered using the density functional theory program deMon-KS. The calculations indicate that the distribution of the net radical cation spin among the atoms (atomic spin populations) is very sensitive to the nature and the position of the substituents. In fact this distribution reflects specifics of the electronic structure of these monomers related to their polymerizability. It is shown that polymerization is favored when the net spin population on the nitrogen atom is similar to that on the ring carbon atom at the para position. On the contrary, in the species that do not polymerize, these two atoms have substantially different spin populations. A possible explanation of this result is discussed in terms of chemical reactivity based on the softness-hardness concept.