PLGA-PEG-Maleimide (MAL) is a thiol-reactive amphiphilic AB diblock copolymer. It consists of a hydrophilic block polyethylene glycol (PEG) and a hydrophobic block poly(lactide-co-glycolide) (PLGA). The PEG polymer has a terminal maleimide group to react with thiol molecules. The hydrophobic block terminates with a hydroxyl group. PEGylated copolymers with biodegradable and biocompatible polyester segments can be useful to prepare core-shell micelles, nanoparticles and microspheres. The hydrolytic degradability of polyester segments depend on the molecular weight. The spacer between ester groups (lactide, glycolide or caprolactone) also determines the ester degradability. PEG-PLGA with other MWs and various lactide to glycolide ratios is also available.