Poly(ethylene glycol) (PEG), a highly biocompatible hydrophilic polyether, is tethered to poly(propylene fumarate) (PPF), a biodegradable polyester. To avoid change in molecular weight distribution of PPF, end hydroxyl groups of PPF are reacted with bis-carboxymethyl PEG after being treated with thionyl chloride. New end carboxyl groups of the PEG-tethered PPF are further reacted with N-hydroxysuccinimide (NHS) in the presence of dicyclohexylcarbodiimide (DCC) to couple bioactive molecules. Glutamine and glycine-arginine-glycine-aspartic acid (GRGD) are attached to the PEG-tethered PPF in 50 mM phosphate buffer of pH of 7.4. The method is valuable for the preparation of a triblock copolymer with PEG end blocks and the coupling of biologically active molecules.