peg and related polymer derivatives having weak , hydrolytically unstable linkages near the reactive end of the polymer are provided for conjugation to drugs , including proteins , enzymes , small molecules , and others . these derivatives provide a sufficient circulation period for a drug - peg conjugate and then for hydrolytic breakdown of the conjugate and release of the bound molecule . in some cases , drugs that previously had reduced activity when permanently coupled to peg can have therapeutically suitable activity when coupled to a degradable peg in accordance with the invention . the peg of the invention can be used to impart water solubility , size , slow rate of kidney clearance , and reduced immunogenicity to the conjugate . controlled hydrolytic release of the bound molecule in the aqueous environment can then enhance the drug delivery system .