In this manuscript, a biscyclic monomer with an epoxide and a thiolactone ring connected by a urethane bond is used for the synthesis of amino acid-functional polyelectrolytes. In a first step, lithium salts of amino acids react selectively with the thiolactone ring by ring-opening, formation of an amide bond, and a thiol group. In a second step and in the presence of a base a polymeric building block is formed by polyaddition of the thiolate to the epoxide ring. The reaction occurs at room temperature in water as solvent. The resulting polymeric building block has a poly(thioether urethane) backbone, with hydroxyl- and amino acid side groups; the connection of the amino acid to the backbone occurs by an amide bond. As proof of concept, a selected series of amino acids and derivatives of such is used: glycine, alanine, tyrosine, glutamic acid, ε-amino caproic acid (as a lysine surrogate), BOC-lysine-O-methyl ester, BOC-lysine, and the dipeptide carnosine. The resulting polymer building blocks with molecular weights of Mn = 1830-9590 g/mol are entirely based on both bio-based and biodegradable components. Exemplarily, using the lithium salts of glycine and lysine methyl ester, anionic and cationic polyelectrolyte building blocks are obtained. A mixture of the two polyelectrolyte solutions results in the formation of polyelectrolyte complexes (PECs). With decreasing concentration of the polyelectrolyte solutions, the radii of PECs decrease.