For kinetics experiments involving UV-Vis absorption spectroscopy, we prepared stock solutions of polymer and DNA in PBS (pH 7.4) with final concentrations of 0.5 µM for DNA, 200 µM for linear PEI, and 500 µM for branched PEI. We then added approximately 1 µL aliquots of polymer stock solutions with different volumes to DNA stocks to create solutions with N/P ratios (the molar ratios of polymer amines to DNA phosphate) of 1 and 5 for use in the kinetics experiments. We maintained the pH of all samples at 7.4. Since the polymer volumes were very small, the final concentration of DNA was maintained around 0.5 µM (see the Supplementary Information for details).