Superparamagnetic and biodegradable/biocompatible core-corona nanocomposite particles were prepared by ring-opening polymerization of E-caprolactone initiated from the surface of maghemite. As was done in a previous work, an aminosilane coupling agent was chosen as the coinitiator and immobilized at the surface of the maghemite particles to allow the growth of the poly(E-caprolactone) (PCL) chains from the solid surface. Two different catalytic systems based on aluminum and tin alkoxides were investigated. Whatever the catalyst used, diffuse reflectance Fourier transform spectroscopy brought evidence for polymer anchoring through a covalent bond, whereas thermogravimetric analysis attested to the presence of high amounts of PCL around the maghemite. Magnetization measurements proved that the nanocomposites kept their superparamagnetic properties after coating. The polymer contents obtained by this grafting-from route were compared with the results obtained by a more classical grafting-to process. (c) 2005 Wiley Periodicals, Inc.