Silica nanoparticles (SNs) were grafted with epsilon-caprolactone using an environmentally friendly approach. By using tartaric acid as a catalyst and the silanol groups as initiators, grafted nanoparticles (GNs) with organic weight fractions (w(of)) within the range 0-46 wt% were synthesized. Thermogravimetric (TGA) and infrared analysis were used to measure the w(of) and to corroborate the covalent bond between the SN and the caprolactone monomer. Transmission electron micrographs of the polyurethane (PU) nanocomposites based on the SN and the GN revealed that the interfacial area of the GN-based PU increased by the reduction of agglomerate dimensions from 10 mu m to around 0.1 mu m. Dynamic mechanical analysis showed that the GN nanocomposites improved the storage shear modulus from 616 +/- 11 to 849 +/- 8 MPa for a GN with w(of) = 16.7% and 3 wt% filler concentration. In addition, the GN particles prevented a relevant decrease of the transition temperature (T-g). Differential scanning calorimetry corroborated that GN increased the enthalpic energy associated to the physical crosslinking of the hard segments (HS). Wide-angle X-ray diffraction proved that the GN formed a HS structure with improved crystallinity. The thermal stability of the GN-based PU a nanocomposite was improved by an increase of the thermal stability of the castor oil soft segments. (C) 2013 Society of Plastics Engineers