We found by surprise that omega -metal carboxy-terminated linear polystyrene chains could self-assemble into stable surfactant-free colloidal nanoparticles if the polymer solution in THF or DMF is added dropwise into an excess of water. Our results showed that the self-assembly was controlled by competition between the intrachain contraction and interchain association. There is a delicate balance between thermodynamic and kinetics. Inside the self-assembled micelle-like nanostructure, the polymer chains were in st metastable state and kinetically frozen. The self-assembly resembles the initial stage of surfactant-free emulsion polymerization. The size of the nanostructures was essentially governed by the surface area occupied per ionic end group. A variation of the initial conditions, such as the polymer concentration in THF and DMF, solvent, and counterions, can make either the intrachain contraction or interchain association dominant so that the self-assembly can be tailored.