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 (1H NMR, 13C 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.