A convenient approach for the synthesis of narrowly dispersed polypeptide copolymers of defined compositions is presented. The controlled denaturation of the proteins serum albumin and lysozyme followed by an in situ stabilization with polyethylene(oxide) chains yields polypeptide side chain copolymers of precisely defined backbone lengths as well as the presence of secondary structure elements. Supramolecular architectures are formed in solution because of the presence of hydrophobic and hydrophilic amino acids along the polypeptide main chain. Polypeptide copolymers reported herein reveal excellent solubility and stability in aqueous media and no significant cytotoxicity at relevant concentrations, and they can be degraded via proteolysis, which is very attractive for biomedical applications. This "semi-synthetic chemistry" approach is based on a novel and convenient concept for producing synthetic polypeptides from native protein resources, which complements traditional polypeptide synthesis and expression approaches and offers great opportunities for the preparation of diverse polypeptides with unique architectures.