The sequential polypeptide [Ala-Gly-Ala-Gly-Ser-Gly]n has been prepared by polymerization of a hexapeptide p-nitrophenyl ester. This polymer contains the repeating sequence of amino acid residues present in the crystalline regions of Bombyx mori silk fibroin. The o-nitrophenylsulphenyl amino-protecting group was used extensively in the synthesis of the hexapeptide active monomer from which the polypeptide was obtained.