Methods and compositions for the generation of polypeptides having varied material properties are disclosed herein. Methods include means for initiating the polymerization of aminoacid-N-carboxyanhydride (NCA) monomer by combining the monomer with an amido-containing metallacycle, for making self assembling amphiphilic block copolypeptides and related protocols for adding oligo(ethyleneglycol) functionalized aminoacid-N-carboxyanhydrides (NCAs) to polyaminoacid chains. Additional methods include means of adding an end group to the carboxy terminus of a polyaminoacid chain by reacting an alloc-protected amino acid amide with a transition metal-donor ligand complex to forming an amido-amidate metallacycle for use in further polymerization reactions. Novel compositions for use in peptide synthesis and design including five and six membered amido-containing metallacycles and block copolypeptides are also disclosed.