A series of modified succinylated PEI (zPEI, 2-40%) and acetylated PEI (acPEI, 10-50%) were synthesized from unmodified 25-kDa branched PEI (bPEI) as follows. Half a gram of bPEI was dissolved in 3 mL of 0.5 M sodium bicarbonate buffer (pH 9) and reacted with the desired mole ratio of either succinic or acetic anhydride (Table ). Succinic anhydride was added directly as a dry powder, while acetic anhydride was added dropwise to the PEI solution. These reactions then proceeded for 4 h at 60 ºC, and the products were filtered using a 0.22 μm PVDF syringe filter. The modified PEI products were subsequently purified by dialysis (membrane molecular weight cutoff = 3.5 kDa) against double distilled water for 48 h with the water replaced every 4 h. After dialysis, the products were then lyophilized for 48 hours. The dried polymers were stored in a -80 ºC freezer before use.