A biocompatible, bioerodable polyanhydride polymer having a Young's modulus between about 1.5 and 3 and a selected rate of surface degradation, and methods of forming and using the polymer, are disclosed. The polymer is formed of a polyester prepolymer having a preferred molecular weight of greater than 5 and less than 7.5 Kdaltons, and a selected number of anhydride linkages between 5 and about 30.