PLA-PEG-NHS is a useful reagent to prepare core-shell nanoparticles displaying amine-reactive succinimidyl NHS ester. The NHS ester is for conjugation with peptide or proteins bearing free amine. PEG-PLA is an amphiphilic AB diblock copolymer consisting of a hydrophilic block polyethylene glycol (PEG) or polyethylene oxide (PEO) and a hydrophobic block polylactic acid or polylactide (PLA). The hydrophobic block terminates with a hydroxyl group. PEGylated copolymers with biodegradable and biocompatible polyester segments is useful to prepare core-shell micelles, nanoparticles and microspheres.