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's delivery profile by providing a delivery system which employs such polymers and utilizes the teachings provided herein.