Polyethyleneimine (PEI for short), with a CAS number of 9002-98-6, is a water-soluble cationic polymer. Its core feature is that its molecular chain is rich in a large number of amino groups (primary amines, secondary amines, and tertiary amines). This structure determines its strong alkalinity, high reactivity, and unique physicochemical properties. Polyethyleneimine has irreplaceable application value in multiple fields.
I. Basic Information: Structure and Classification
1. Chemical structure
The repeating unit of PEI is -CH₂CH₂NH-, and the amino groups on its molecular chain give it strong cationic properties (pKa≈10) – even under neutral or weakly acidic conditions, the amino groups can be protonated (-NH₂→-NH₃⁺), which is the core source of its water solubility, complexation ability and surface adsorption.
According to different structures, they are mainly divided into two categories:
Linear polyethyleneimine (L-PEI) : The molecular chain is linear, with secondary amines as the main amino group (about 90%), and the proportion of primary and tertiary amines is low. The relative molecular mass distribution is narrow, and the purity is higher.
Branched polyethyleneimine (B-PEI) : The molecular chain contains a large number of side chains, with a balanced proportion of primary amines (≈25%), secondary amines (≈50%), and tertiary amines (≈25%), and has higher reactivity. It is the most commonly used type in industry.
2. Key physical properties
| Nature | Typical value (branching PEI) |
| Appearance | Pale yellow to brown viscous liquid (low molecular weight) or solid (high molecular weight) |
| Water solubility | PEI is readily soluble in polar solvents such as water, ethanol and methanol, but insoluble in non-polar solvents |
| Molecular weight range | Several hundred to several hundred thousand (commonly 1,000 to 25,000 Da) |
| Density (25℃) | 1.05 to 1.10 g/cm³ |
| Refractive index (25℃) | 1.50 ~ 1.52 |
| Toxicity | Low-molecular-weight PEI has relatively low toxicity, while high-molecular-weight/branched PEI has certain cytotoxicity to cells |
Ii. Core Features: Why is PEI Widely Used?
Strong cationic property and complexation ability: The amino group after protonation carries a positive charge and can form stable complexes with negatively charged substances (such as DNA, RNA, anionic dyes, clays, and metal ions), which is the core mechanism of its action in gene delivery, water treatment, and metal adsorption.
High reactivity: Amino groups (especially primary amines) can participate in various reactions (such as Michael addition, epoxy ring-opening, acylation, crosslinking reactions), and can be used as crosslinking agents and modifiers to functionalize the material surface.
Water solubility and film-forming property: Good water solubility makes it easy to process. After drying, it can form a dense film, and the film has certain adhesion and barrier properties.
Strong alkalinity: The large number of amino groups in the molecule makes its aqueous solution strongly alkaline (pH≈10 to 12), and it can undergo neutralization reactions with acidic substances.
Iii. What is polyethylenimine used for (Classified by Demand Scenarios
1. Biomedical field (Core: Gene delivery vectors
PEI is one of the most commonly used non-viral gene vectors at present:
Principle Cationic PEI and anionic DNA/RNA form a “PEI-nucleic acid complex” through electrostatic interaction, which not only protects nucleic acids from being degraded by nucleases but also enables them to enter cells through endocytosis. Moreover, the “proton sponge effect” of PEI (absorbing protons within the cell after protonation) Triggering vesicle rupture can promote the release of nucleic acids into the cytoplasm;
Application: Gene therapy (such as gene delivery for tumors and genetic diseases), nucleic acid vaccine delivery, cell transfection experiments (commonly used PEI transfection reagent in laboratories);
Note: High-molecular-weight PEI has relatively high cytotoxicity. Currently, low-toxicity modified PEI (such as peG-modified PEI and cross-linked PEI nanoparticles) have been developed.
2. Water treatment field (Core: Flocculants, Adsorbents)
Flocculant: The cationic property of Polyethylenimine PEI can neutralize negatively charged suspended particles in water (such as sand, organic matter, and bacteria), causing the particles to aggregate and settle. It is used for drinking water purification and industrial wastewater treatment (such as printing and dyeing, papermaking wastewater), especially suitable for treating wastewater with high turbidity and high organic matter content.
Adsorbent The amino group of Polyethylenimine PEI can form coordination bonds with heavy metal ions (such as Cu²⁺, Ni²⁺, Cr⁶⁺, Pb²⁺), and can be used for the removal of heavy metals in industrial wastewater. It can also be made into polyethylenimine-modified adsorption materials (such as PEI-activated carbon, PEI-nanofibers) to enhance the adsorption capacity.
3. Field of material surface modification (Core: Functional Modification)
Paper/Fiber modification: Polyethylenimine PEI treatment of paper can enhance its wet strength, water resistance and printability (through cross-linking with hydroxyl groups on the fiber surface), and is used in the production of specialty paper (such as packaging paper, filter paper).
Metal surface treatment: Polyethylenimine can form a protective film on the metal surface, enhancing the metal’s corrosion resistance, and can also serve as an adhesion promoter for metal coatings.
Polymer material modification: PEI is used as a crosslinking agent or compatibilizer to improve the compatibility and adhesion of polymers (such as crosslinking modification of polyurethane and epoxy resins), or to enhance the hydrophilicity of materials (such as PEI-modified polyolefin films).
4. Other industrial applications
Adhesives and sealants: The amino group of PEI can undergo cross-linking reactions with aldehydes, isocyanates, etc., and can be used as a curing agent for water-based adhesives. Polyethylenimine is used for bonding wood, paper, and metal, with high bonding strength and good water resistance.
Dye and pigment industry: Polyethylenimine PEI can be used as a fixative for cationic dyes (especially for dyeing natural fibers such as cotton and silk), enhancing the adhesion and washability of dyes; Polyethylenimine can also be used as a pigment dispersant to prevent pigment agglomeration.
Petroleum extraction: PEI is used in oilfield water treatment (such as water shutoff and profile control agents), or as a drilling fluid additive to improve the stability of drilling fluids.
By adjusting the molecular weight (low molecular weight, low toxicity; high molecular weight, high activity), structure (linear vs. branched), and modification methods (peG-ylation, cross-linking, and compounding) of polyethylenimine, its application in high-end fields (such as targeted drug delivery and special functional materials) can be further expanded.
Post time: Nov-21-2025