PEG butyric acid NHS derivatives have a C4 aliphatic chain directly attached to PEG and the NHS active ester is a useful reagent to pegylate proteines, peptides or any other surfaces with a free primary amino group to form stable amide bonds. PEG crosslinkers with Boc (tert-butyloxycarbonyl)-protected amino groups on one chain end. The Boc protecting group is stable in a wide range of conditions (including highly basic environments) but may be easily removed by (strong) acids such as TFA or at low pH in water to release the primary amine.