Diblock copolymer poly (2-methylpropene)-b-poly (alpha-amino acid) was obtained by polymerization of the corresponding N-caboxy anhydride initiated by a poly (2-methylpropene) bearing a terminal amine in dioxane/CH2Cl2 mixture. Copolymers were analyzed by FT IR, H-1 and C-13 NMR. MBAR n of the alpha-amino acid segment determined by H-1 NMR fits well with those obtained by a theoretical calculation.