For a polymer composed of very simple repeating units, polyethylenimine (PEI) has an astounding number of practical applications, including detergents, adhesives, cosmetics, industrial agents, CO2 capture, and even cellular cultures. In general, PEI is synthesized by the ring-opening polymerization of ethylenimine, also known as aziridine. When produced this way, the result is a liquid polymer with a branching structure.