Relationships able to predict the behavior of polymers based on their chemical structure are necessary for the rational synthesis of materials with desired properties. The state-of-art of polymer chemistry allows one to synthesize, using techniques such as RAFT, polymers with tailored chemical structures to study that kind of relationships. In this study, RAFT was used to synthesize copolymers-poly(methyl methacrylate)-block-poly[(2-dimethylamino)ethyl methacrylate]-that form aggregates in water at neutral and acidic pH's. Cmc of each copolymer was determined by fluorescence using the probe N-phenyl-1-naphthylamine (NPN), and the values are consistent with the particular chemical structure of each one. These findings can be useful in future studies of dependence of cmc on the chemical structure and solution conditions and to tailor polymers with specific properties and applications, making the cmc determination faster and more accurate. To the best of our knowledge, this it the first time that NPN is used to determine the cmc of this kind of positively charged copolymers.