Model polyisoprene-polylactide (PI-PLA) diblock copolymers were synthesized by a combination of living anionic polymerization and controlled coordination-insertion polymerization. Living anionic polymerization of isoprene followed by end-capping with ethylene oxide yielded hydroxyl-terminated polyisoprenes (PI-OH) with narrow molecular weight distributions. In a second step, an aluminum alkoxide macroinitiator was formed from the equimolar reaction of triethylaluminum with the PI-OH prepolymer and subsequently utilized for the ring-opening polymerization of lactide to produce the desired PI-PLA diblock copolymer. The final molecular weight of each block was controlled through manipulation of the monomer-to-initiator ratio in both polymerizations. Well-defined blocks were obtained as evidenced by the narrow molecular weight distributions and the absence of homopolymer as characterized by GPC analysis. Molecular characterization of the block copolymers by spectroscopy ( 1 H NMR, 13 C NMR, and IR) and elemental analysis confirmed the relative compositions of the component blocks. We characterized the morphology of a representative PI-PLA diblock copolymer using DSC and SAXS. Both analyses indicated a microphase-separated structure characteristic of an ordered diblock copolymer. These model diblock copolymers are ideal materials for fundamental phase behavior and mechanical property studies.