Different diblock copolymers constituted by one segment of a monomer supporting a reactive functional group, like allyl methacrylate (AMA), were synthesized by atom transfer radical polymerization (ATRP). Bromo-terminated polymers, like polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(butyl acrylate) (PBA) were employed as macroinitiators to form the other blocks. Copolymerizations were carried out using copper chloride with N,N,N',N ',N '-pentamethyldiethylenetriamine (PMDETA) as the catalyst system in benzonitrile solution at 70 degrees C. At the early stage, the ATRP copolymerizations yielded well-defined linear block copolymers. However, with the polymerization progress a change in the macromolecular architecture takes place due to the secondary reactions caused by the allylic groups, passing to a branched and/or star-shaped structure until finally yielding gel at monomer conversion around 40% or higher. The block copolymers were characterized by means of size exclusion chromatography (SEC), H-1 NMR spectroscopy, and differential scanning calorimetry (DSC). In addition, one of these copolymers, specifically P(BA-b-AMA), was satisfactorily modified through osmylation reaction to obtain the subsequent amphiphilic diblock copolymer of P(BA-b-DHPMA), where DHPMA is 2,3-dihydroxypropyl methacrylate; demonstrating the feasibility of side-chain modification of the functional obtained copolymers. (C) 2007 Wiley Periodicals, Inc.