Interfacial polymerization of uniform template-free nanostructures is very challenging since many factors play determinant roles in the final structure of the resulting nanoassemblies. Here, we present a single oxidative coupling method for the synthesis of different nanoshapes by addition or substitution of a methyl group on aniline monomers to freely alter the mechanism of monomer-to-polymer conversion. Well-defined nanotubes, nanohollows, and solid nanospheres are obtained from aniline, N-methylaniline, and 2-methylaniline polymerizations, respectively. We found that the extent of hydrophobicity and protonation under mild acidic conditions determines the monomers' arrangement in micelle or droplet form, reactivity, and nucleation mechanism. These can subsequently affect the final morphology through a fusion process to form tubular structures, external flux of monomers to form nanohollows, and intradroplet oxidation to form solid nanospheres. Altered biological responses, such as cytocompatibility, redox response, hemocompatibility, and cell proliferation, are also found to be dependent on the position of the methyl group in the nanostructures.