Poly(amino acids) such as polypeptides and proteins are attractive biomass-based polymers that potentially contribute to circular economy for plastic. In the current study, we synthesized polypeptides containing periodic nylon units to incorporate thermal plasticity into polypeptides. We successfully synthesized polypeptides containing nylon units by chemoenzymatic polymerization. In addition to nylon 4, which is of biomass origin, nylon 3, nylon 5, and nylon 6 were chosen as the second monomer units with alanine, a natural amino acid. The synthesized polypeptides showed partial melting behavior prior to their thermal degradation. It was found that the nylon unit length significantly changed the thermal properties of the resulting polypeptides, as evidenced by WAXS and DSC analysis. Furthermore, the biodegradability and environmental toxicity of the synthesized polypeptides were evaluated using biological systems. The copolymer containing nylon 4 is considered as a highly potential biopolymer in terms of the polymerization results and environmental toxicity. These results open a new door for utilizing nylon-containing peptides as biomaterials, especially as thermally processable and eco-friendly structural materials.