Well-defined diblock copolymers of isoprene and 2-vinylpyridine (2VP) were prepared by sequential anionic polymerization at temperatures of 45 and 6-8 degreesC, respectively, in benzene solution using sec-BuLi as initiator in the presence of lithium chloride ([LiCl]/[PLi] = 5-11). Rapid quenching with acidic methanol 4-7 min after 2VP addition was required to obtain controlled polymerization. The polyisoprene-bloch-poly(2-vinylpyridine) diblock copolymers with high 1,4-polyisoprene microstructure exhibited narrow molecular weight distributions [M-w/M-n(SEC) = 1.01-1.06] and molecular weights (light scattering, universal calibration) that were in good agreement with calculated values. Molecular weights (M-n) determined by SEC with polyisoprene standards were much lower than calculated values.