PEG succinamic acid derivatives have a C4 acid group attached to PEG via a very stable amide linkage. The active ester derivatives of succinamic acids are very reactive and can be used for pegylation of amine or other nucleophilic substrates. But at the same time they are prone to hydrolysis with hydrolysis half life of under 5 minutes at pH 8. 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.