A-B-A type block copolymers (GIG) composed of poly(~-benzyl L-glutamate) (PBLG) as the A component and polyisoprene (PI) as the B component were prepared. The method of synthesis consists of the polymerization of the ),-benzyl L-glutamate N-carboxy anhydride initiated by the amine groups capped at both ends of the PI block which has a moderate degree of polymerization. When dissolved in a helicogenic solvent, such as N,N'-dimethylformamide (DMF) or ethylene dichloride (EDC), the GIG is expected to take a conformation which may be pictured as two rigid blocks linked by a flexible joint, since the PI block portion is intolerant of intrachain hydrogen bonding. Hence, the GIG block copolymers would be considered as a macromolecular model of a once-broken rod with equal rod sections, if the molecular weight of the PBLG portion was relatively much higher than that of the PI portion. After molecular characterization on GIG by the circular dichroism measurement, the viscosity behavior in a helicogenic solvent was determined, and the result was compared with the corresponding data on PBLG, which assumes the form of a straight rod in a helicogenic solvent. It was found that the value of [t/] of GIG block copolymers was about 5/8 of those of PBLG of the equal molecular weight. This finding is in good agreement with the theoretical prediction.