The polystyrene-block-polyacrylic acid (PSt-b-PAA) copolymer is obtained through the reversible addition-fragmentation chain transfer polymerization (RAFT). Then the cobalt (Co) nano-particles are synthesized by loading the Co2+ salt into the polymer and reducing them. Polymermatrix composites with an average particle size of 10 nm have been formed by a simple technique. The technique exploits the dispersed phase of the polymer to form numerous identical nanoreactors. In this approach, nanoparticles are rigidly attached to the support surface to prevent their agglomeration. FT-IR and TEM analysis of these polymermatrix composites revealed that nano-structures are formed and the shape can be controlled. Vibrating sample magnetometer is also used to study the magnetic property of the materials at room temperature. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012