Linear Polyethylenimine (CAS 9002-98-6): Structure, Properties & Applications

Polyethylenimine (PEI) is a multifunctional cationic polymer widely used in biomedicine, materials science, and environmental engineering. Depending on the molecular architecture, PEI can be divided into branched and linear types. This article focuses on linear polyethylenimine (Linear PEI), systematically reviewing its structural characteristics, synthesis pathways, and key application directions.

1. Structure and Composition

Linear PEI is composed of repeating –CH₂CH₂NH– units, containing only secondary amine groups. The main chain exhibits a regular, continuous linear conformation. Compared with branched PEI, the linear variant lacks side-chain branching, resulting in a more uniform distribution and regular molecular chain.

Key implications of this linear structure include:

• Reduced steric hindrance, facilitating tighter molecular complexation;
• Lower charge density, improving biocompatibility while retaining function;
• Predictable amine group distribution, beneficial for chemical modifications and structural modeling. 

2. Synthesis and Molecular Weight Control

Linear PEI is typically synthesized via hydrolysis of poly(2-ethyl-2-oxazoline) (PEtOx) using acid or base catalysts. Compared to direct polymerization, this approach offers better control over molecular weight and product purity.

The general process includes:

1.Ring-opening polymerization to produce poly(2-ethyl-2-oxazoline);

2.Acidic or basic hydrolysis to obtain linear PEI;

3.Molecular weight (e.g., 2 kDa, 10 kDa, 25 kDa) controlled by polymerization time and monomer ratio.

Advantages of this route:

• Structurally uniform, narrow distribution of linear products;
• Mild reaction conditions suitable for further functionalization;

3. Physicochemical Properties

Compared to branched PEI, linear PEI has more balanced amine density, which helps reduce membrane disruption while maintaining binding capacity—making it more suitable for sensitive biological applications.

4. Major Application Areas

4.1 Gene Delivery and Transfection

Linear PEI can form polyplexes with negatively charged DNA, siRNA, or mRNA through electrostatic interactions and is widely used as a non-viral gene delivery vector.

Compared to the branched form, linear PEI offers:

• Lower cytotoxicity;
• More controllable particle size and surface charge;
• Better suited for sensitive cells or long-term expression studies.

4.2 Nanomaterial Surface Modification

Linear PEI is frequently used to functionalize the surface of nanoparticles (e.g., magnetic nanoparticles, quantum dots, gold nanoparticles), offering:

• Improved dispersion and hydrophilicity;
• Surface charge tuning;
• Reactive sites for grafting functional molecules.

Such PEI-modified nanocarriers are applied in targeted delivery, cell imaging, and photothermal therapy.

4.3 Environmental Adsorbent Materials

Due to its rich amine content, linear PEI exhibits strong chelation ability for metal ions such as Cu²⁺, Pb²⁺, and Cd²⁺. Applications include:

• Preparation of heavy metal adsorbents;
• Functionalization of membranes for wastewater treatment;
• Enhanced selectivity of biochar or resin-based materials.

• Controlled drug release;
• Hydrogel formation;
• Advanced polymer composites.

5. Summary

Linear polyethylenimine (Linear PEI, CAS: 9002-98-6) is a structurally well-defined, highly reactive cationic polymer. It finds broad applications in gene delivery, nanotechnology, environmental materials, and smart polymer systems. Compared to its branched counterpart, linear PEI features lower toxicity and better structural control, making it especially suitable for applications requiring high uniformity and functional precision.

With continuous advancement in polymer design, linear PEI is expected to play an increasingly central role in multifunctional polymer platforms of the future.