wherein Z is the protecting group, NH--R8 is the residue of the alpha-amino acid, R3 is a hydrogen atom or a hydrocarbyl group, and R4 is a are hydrocarbyl group; (b) removing the protecting group and reacting the amino acid ester with a dihalophosphazene polymer in a liquid reaction solvent and in the presence of a hydrogen halide acceptor to effect partial substitution of the halogen of the diha