A series of expanded oligo- and poly((L)-leucine)s, with an alternating arrangement of p-phenylene moieties and (L)-leucine residues, were synthesized by stepwise elongation or polycondensation and characterized by H-1 NMR, SEC, ESI-MS, UV, and CD spectra. The degree of polymerization was found to be about 19 by SEC. Each monomer unit, i.e., expanded Lleucine, behaved like a chiral unit with a rigid bend body. Elongation of the peptide chain significantly increased the CD intensity per phenylene chromophore; however, the CD intensity decreased at high temperatures. This cooperative behavior strongly suggested the formation of a secondary structure. H-1 NMR analysis revealed a right-handed helical structure in dimethyl sulfoxide. The theoretical calculations were consistent with these results.