The maleimide functional group at each end of the PEG linker molecule allows selective and covalent attachment to other molecules with complementary functional groups, such as thiol groups on proteins or peptides. This can be achieved through thiol-maleimide coupling reactions, which result in the formation of a stable and covalent bond between the two molecules. PEG linker molecules with maleimide endgroups are particularly useful for the PEGylation of proteins and peptides, as the maleimide group reacts selectively with the thiol group on cysteine residues, which are abundant in many proteins and peptides. The resulting PEGylated product has improved solubility, stability, and biocompatibility compared to the original molecule.