The synthesis and characterization of a series of polystyrene (PS)-based copolymers, bearing 2-ureido-4[1H]-pyrimidinone (UPy) units pendant from the polymer backbone, is reported. The PS-based copolymers are efficiently synthesized by RAFT-mediated free radical polymerization performed on styrene and protected 4-vinylbenzylamine, and post-modification with suitable UPy-derived isocyanates to give the target polymers. Analysis of the samples in solution reveals that the polymer chains aggregate to give organic nanoparticles of discrete and uniform dimensions, whose direct visualization when deposited on surfaces can be achieved using AFM.