Maleimide-PEG-Hydroxyl (MAL-PEG-OH) is a linear heterobifunctional PEG reagent with a maleimide and a hydroxyl group. It is a useful crosslinking reagent with a PEG spacer. Maleimide reacts with thiol, SH, sulfhydryl or mercapto, and hydroxyl can react with carboxylic acid. MAL-PEG-OH can also be used to synthesize PEGylated block copolymer such as PLA-PEG (poly-L-lactic acid-co-polyethylene glycol) and PCL-PEG (polycaprolactone-co-polyethylene glycol) through hydroxyl initiated polymerization with lactide or caprolactone. Maleimide contains a reactive C=C double bond and is light or oxygen sensitive.