Olefin cross-metathesis is an effective tool for the functionalization of biodegradable aliphatic polyesters through both pre- and post-polymerization of beta-heptenolactone (beta HL). Ring-opening polymerization of beta HL accesses both homopolymers and novel copolymers, producing strong gradient copolymers with lactide. A total of 15 different alkene cross-partners ranging from type I to type III olefins are readily incorporated to produce polyesters with a range of functionalities, altering the thermal and chemical properties of the resulting polymers. A novel methodology was also developed to introduce two unique functionalities into the polymer backbone based on manipulation of cross-partner reactivity, showcasing the scope and versatility of olefin CM as a strategy for post-polymerization modification of polymers.