Both acPEI and zPEI have been previously demonstrated to improve transfection efficiency compared to unmodified bPEI. Here, zPEI and acPEI were synthesized under identical conditions, with differing molar ratios of acetic or succinic anhydride reacted with bPEI in sodium bicarbonate buffer (pH 9) for 4 h at 60 ºC. Primary and secondary bPEI amines react with anhydride to convert some amines to uncharged acetyl (acPEI) or negatively charged carboxylate (zPEI) groups. The reaction products were subsequently purified via dialysis, filtered through a 0.22 μm filter, and then lyophilized. Successful conjugation of the anhydrides was confirmed by FT-IR (Figures ). Two characteristic new peaks at 1635 cm -1 and 1563 cm -1 were observed in both acPEI and zPEI characteristic of the carbonyl stretch and N-H bend of the amide moiety formed after the reaction of bPEI amine with acetic or succinic anhydride. The degree of modification was determined by 1 H-NMR spectroscopy (Figures ), from the relative proton peak area ratio between the methylene of PEI (2.45-3.6 ppm) and the methylene of the succinyl protons (2.3-2.45 ppm) or the methyl protons of the acetyl group (1.85-2.1 ppm). Here, the degree of modification refers to the percentage of the total amines in bPEI that underwent acetylation or succinylation. Results for all polymers are tabulated in Table . For most polymers, nearly complete conjugation of the PEI amines with the anhydrides was observed, so polymers are named after the molar ratio of anhydride relative to bPEI amines in the monomer feed. The lowest reaction yield was observed for high ratios of succinic anhydride (zPEI40) with ~35% of the PEI amines being conjugated.