The amine functional group at one end of the PEG linker molecule allows for selective and covalent attachment to other molecules with complementary functional groups, such as carboxylic acids or activated esters, aldehydes and epoxides. The newly formed bonds are amides, imines or secondary amines. Applicable for reversible or permanent crosslinking with PEG. The hydroxy group on one side of this PEG crosslinker is able to undergo esterification reactions with acids for reversible pegylation. It also enables further derivatization or replacement with other reactive functional groups.