The protected hydrazide may be liberated by treating the Boc group with acid. The hydrazine group is used to conjugate carbonyl compounds like aldehydes or ketones with the formation of hydrolysable and pH sensitive hydrazone links. This PEG derivative has a sulfhydryl group (-SH) on one chain end. The free thiol groups react readily with maleimido or iodacetamido functionalities or transition metal surfaces like gold, silver etc. Reaction with other free thiols leads to S-S disulfide bonds which can be cleaved by reducing agents. PEG thiols may also be conjugated by thiol-ene chemistry.