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Polyethyleneimine, often abbreviated as PEI, is a synthetic polymer that consists of repeating ethyleneimine monomer units. It is a water-soluble and highly cationic polymer, meaning it carries a significant positive charge. This cationic nature is key to many of its applications. PEI is produced through the ring-opening polymerization of aziridine, a cyclic amine.

The structure of PEI features a branched or linear chain with numerous primary, secondary, and tertiary amine groups. The degree of branching can vary depending on the synthesis method, leading to different properties. Linear PEI is typically synthesized using a different process than branched PEI.

Due to its positive charge, PEI readily interacts with negatively charged molecules and surfaces. This property makes it highly effective as a gene delivery vector in biotechnology, where it can complex with DNA or RNA and facilitate their entry into cells. It is also used as a flocculant in water treatment, helping to aggregate and remove suspended particles.

Other applications of PEI include its use as an adhesive, a coating material, and as a chelating agent for metal ions. Its reactivity allows for further chemical modification, expanding its utility in various scientific and industrial fields. The safety and biodegradability of PEI are subjects of ongoing research, particularly concerning its use in biomedical applications.