Mixtures of amino acids with hydroxy acids allow for the formation of peptide bonds in a plausible prebiotic scenario via ester bond formation followed by ester amide exchange. Here, we investigate the ability of the ester-mediated reaction pathway to form even longer polymers with peptide backbones based on the specific details of the reaction protocol. Fresh monomers were fed to the polymer/monomer mixture periodically by an automated "day-night machine" that was designed to simulate wet-dry cycles that would have been common on the prebiotic Earth. Quantitative analysis of peptide bond formation in the complex oligomer mixture was enabled by a simple hydrolysis treatment. In the ester mediated peptide elongation process, new monomers add to one end of the chain step-by-step without termination. The feed composition (hydroxy acids and/or amino acids) was found to determine the final oligomer distribution. Production of longer oligomers enriched in peptide bonds was more efficient when only amino acids were fed because of a smaller number of active oligomer chains. These results reveal a process for synthesizing longer depsipeptides and/or peptides that could form secondary structures, and possibly functional polymers.