The synthesis of diblock copolymers using atom transfer radical polymerization, ATRP, of n-butyl methacrylate, BMA, and methyl methacrylate, MMA, is reported. These copolymers were prepared from 2-bromoisobutyryl-terminated macroinitiators of poly(MMA) and poly(BMA), using copper chloride, CuCl,/N,N,N',N",N"-pentamethyldiethylenetretramine, PMDETA, as the catalyst system, at 100degreesC in bulk and in benzonitrile solution. The block copolymers were characterized by means of size-exclusion chromatography, SEC, and H-1-NMR spectroscopy. The SEC analysis of the synthesized diblock copolymers confirmed important differences in the molecular weight control depending on the reaction medium (solvent effect) and the chemical structure of the macroinitiator used. In addition, differential scanning calorimetry, (DSC) measurements were performed, showing for all the copolymers a phase separation,