PCL-PEG-COOH is an amphiphilic block copolymer consisting of a hydrophilic polyethylene glycol (PEG) and a hydrophobic polycaprolactone (PCL). The PEG polymer is capped a carboxylic acid COOH group. The hydrophobic block PCL terminates with a hydroxyl group. Biocompatible PEGylated copolymers with biodegradable polyester segments can be used to prepare core-shell micelles, nanoparticles and microspheres. The hydrolytic degradability of the polyester block depends on the molecular weight and the spacer between ester groups (lactide, glycotide or caprolactone). PEG-PCL with other MWs is also offered.