The graft polymerization of e-caprolactone (F.-CL) onto magnetite was carried out under microwave irradiation in the presence of tin(II) 2-ethylhexanoate. The molar ratio of a-CL to tin(II) 2-ethylhexanoate was 300, whereas the molar ratio of F.-CL to magnetite was 5. The chemical structures of the obtained poly(epsilon-caprolactone) coated magnetic nanoparticles were characterized by FTIR and XPS spectroscopy. These magnetic-polymer hybrid nanostructures were further investigated by X-ray diffraction and magnetization measurements. The morphology of the magnetic core-shell nanostructures were determined by TEM. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5397-5404, 2009