A biodegradable block copolymer made from the transesterfication of a poly(propylene fumarate) prepolymer and a poly(ethylene oxide) prepolymer. The block copolymer poly(propylene fumarate -co- ethylene oxide) is capable of crosslinking at body temperature. Crosslinking requires an appropriate crosslinking monomer and an initiator. The biodegradable block copolymer has utility as a vascular implant became it can be injected as a fluid into the vascular system and crosslinked in situ.