A heterobifunctional 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. Carboxy PEGs may be used for conjugation to amine substrates (forming amides) or other nucleophiles (forming esters) . However, activation is necessary prior to conjugation. Different PEG carboxy derivatives display a different degree of stability and reactivity of the corresponding active ester.