Polyethylenimine (PEI) or polyaziridine is a polymer with repeating unit composed of the amine group and two carbon aliphatic CH₂CH₂ spacer. Linear polyethyleneimines contain all secondary amines, in contrast to branched PEIs which contain primary, secondary and tertiary amino groups. Totally branched, dendrimeric forms were also reported. PEI is produced on industrial scale and finds many applications usually derived from its polycationic character.