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 , followed by hydrolytic breakdown of the conjugate and release of the bound molecule . in some cases , drugs that demonstrate reduced activity when permanently coupled to peg maintain a therapeutically suitable activity when coupled to a degradable peg in accordance with the invention . the peg derivatives of the invention can be used to impart improved water solubility , increased size , a slower rate of kidney clearance , and reduced immunogenicity to a conjugate formed by attachment thereto . controlled hydrolytic release of the bound molecule into an aqueous environment can then enhance the drug &# 39 ; s delivery profile by providing a delivery system which employs such polymers and utilizes the teachings provided herein .