Controlling shapes of polymer nanoparticles via single-step process is a challenge due to their amorphous chemical nature. Precise regulation of interfacial interactions, electrical charging and reaction dynamics during ongoing polymerization process provides an environment where uniform nucleation, growth and in situ assembling can be realized, and hence nanoparticles of complex shapes can be obtained. In this work, it is investigated how in situ assembling of the growing nanoparticles succeeds and specifically in different manners by using cationic, anionic, polyionic, and nonionic surface-active agents in a time-dependent blended form. Micelle of molecular surfactants leads the spheres, but long chained polyelectrolytes support in situ assembling of growing spheres to form the nonspherical polymer nanoparticles in order to minimize the surface energy of a system. Similarly, a nonionic polymer promotes the movement of growing species in solution and allows tunable aggregation-based growth which produces more complexed nanoparticles. Furthermore, the application of acid, base and salt solution also contribute specific effect where unexpected size and shape of nanoparticles can be obtained. Overall, the roles of limited polarizability, solvation power, mobility, ionic strength, pH, and microfluidics for the synthesis of various shape-controlled polymer nanoparticles are presented here.