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) but may be easily removed by primary and secondary amine bases (such as piperidine) in polar solvents to release the primary amine. Applicable for reversible or permanent crosslinking with PEG.