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). This PEG linker with a maleimide endgroup is applied in sulfhydryl-selective coupling of PEG to proteins and other thiol substrates and for selective scavenging of thiol containing peptides.