The product S-1034 (PEG4/PFB Long Chain Crosslinker) does react with primary amine groups and does have a PEG4 linker arm. When looking at the structure on the product data sheet, you notice the penta-fluoro phenyl group. This is the leaving group, and the mechanism is very similar to the NHS ester mechanism. The nitrogen of the amine group attacks the carbonyl carbon adjacent to this potential leaving group, and with some rearrangement, along with participation of water, the phenyl group leaves.