Entirely amino acid-based polymers were prepared by side-chain attachment to polysuccinimide derived from the thermal polycondensation of aspartic acid. Following deprotonation of various amino acid ester hydrochlorides by a secondary amine, the restored primary amino groups initiated the ring-pening of succinimide to form amide bonds. H-1 and C-13 NMR measurements revealed that the mole fraction of the introduced amino acid side chains could be controlled by the reaction time, while no hydrolysis of methyl ester groups was observed. The synthesized polymers contain exclusively amino acids, which makes them promising candidates as base materials of controlled drug delivery systems.