PEG crosslinkers with Fmoc (fluorenylmethoxycarbonyl) -protected amino groups on one chain end. The Fmoc protecting group is stable in a wide range of conditions (including highly acidic environments) . It may be easily removed by primary and secondary amine bases (such as piperidine) in polar solvents to release the primary amine which allows for selective and covalent attachment to other molecules with complementary functional groups, such as carboxylic acids or activated esters, aldehydes and epoxides. Applicable for reversible or permanent crosslinking with PEG. The butyric acid endgroup is directly connected to the PEG chain via a very stable ether linkage. After activation it is used to conjugate the PEG derivative to nucleophilic substrates, mainly amines (to form amides) or hydroxyls/thiols (forming esters/thioesters) .