PCL(10k)-PEG(3k)-NHS is an active ester block copolymer for bioconjugation with primary amines at pH 7-9 to form a stable amide linkage. The self-assembling polymer can be used to prepare nanocarrier used in nanomedicine. The polycaprolactone (PCL) block is biodegradable via hydrolysis of the ester bonds. Reagent grade, for research purpose.