The synthesis of polyacrylonitrile-block-polystyrene (PAN-b-PS) copolymers by atom transfer radical polymerization (ATRP) is reported. Chain extension of bromine terminated PAN macroinitiators with styrene was performed using a CuBr/N,N,N',N",N"-pentamethyldiethyl-enetriamine catalyst system and 2-cyanopyridine as a solvent. The first-order kinetic plots of styrene consumption showed a significant curvature, indicating a progressive decrease in the concentration of active species during copolymerization. The loss of the bromide end group was mainly ascribed to the elimination of HBr, as shown by H-1 NMR spectroscopy. By varying the molar ratio of either the catalyst or the monomer to the initiator, a series of PAN-b-PS copolymers were prepared, with polydispersities as low as 1.3, and molar compositions ranging from 8.6/91.4 to 35.5/64.5.