New biodegradable-biocompatible amphiphilic block copolymers were prepared in good yields by SnOct(2) catalyzed ring opening polymerization of epsilon-caprolactone initiated by monomethoxy-terminated poly(ethylene glycol) (MPEG). Coupling of the AB copolymers with hexamethylene diisocyanate afforded ABA (formally ABBA) block copolymers. Both AB and ABA copolymers were thoroughly characterized by IR and NMR spectroscopy, size exclusion chromatography, TGA and DSC thermal analysis. In particular, DSC measurements evidenced that the copolymer hydrophilic-lipophilic balance appreciably affected the state of adsorbed water. Polarized optical microscopy of bulk materials and pyrene fluorescence emission of polymer water solutions highlighted the copolymer tendency to phase separation and self-organization, respectively. Most of the prepared materials formed micelles in water and the copolymer structure appreciably affected their-critical micellar concentration. In vitro cytocompatibility tests confirmed the low toxicity of the prepared polymeric materials which enhances their potential for biomedical applications. (c) 2005 Elsevier Ltd. All rights reserved.