The NHS (N-hydroxysuccinimide) ester functional group at each end of the PEG linker molecule allows selective and covalent attachment to other molecules with complementary functional groups, such as primary amines on proteins, peptides, or other molecules. This particular PEG derivative has an amide linkage between PEG and the succinic acid (C4) active ester endgroup. It is highly reactive towards amines but at the same time prone to hydrolysis and self deactivation at elevated temperatures. The hydrolysis half-life is in the range of 10 min at slightly basic conditions in aqueous media.