Poly(E-caprolactone) (PCL)-polydimethylsiloxane diblock and triblock copolymers and poly(epsilon-caprolactone-co-4-ethylcaprolactone) random copolymers were prepared through the homogeneously catalyzed coordination anionic polymerization of E-caprolactone (CL) and the copolymerization of CL with 4-ethyl-epsilon-caprolactone (EtCL) in the presence of hydroxy-terminated polysiloxanes or allyl alcohol as chain-transfer agents, respectively. Polysiloxane precursors with hydroxypropyl or hydroxyethyl propyl ether end groups were obtained by the hydrosilation of the appropriate unsaturated alcohol with monofunctional or difunctional hydro-terminated polysiloxanes of different molecular weights. As proven by differential scanning calorimetry analysis, the presence of siloxane blocks and EtCL units determined the diminished copolymer crystallinity, which was shown by the reduced melting temperatures and enthalpy of fusion with respect to those of pure PCL. Both types of copolymers were found to form, in the presence of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) emulsifier, monodisperse and stable nanoparticles able to encapsulate different types of bioactive compounds (Vitamin E and indomethacin). (C) 2003 Wiley Periodicals, Inc.