Polyethylenimine 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.
Properties The linear PEIs are solids at room temperature
where branched PEIs are liquids at all molecular weights. Linear polyethyleneimines are soluble
in hot water, at low pH, in methanol, ethanol, or chloroform. They are insoluble in cold
water, benzene, ethyl ether, and acetone. They have a melting point of 73-75°C. They
can be stored at room temperature. Synthesis
Branched PEI can be synthesized by the ring opening polymerization of aziridine. Depending
on the reaction conditions different degree of branching can be achieved. Linear PEI is
available by post-modification of other polymers like poly(2-oxazolines) or N-substituted polyaziridines.
Linear PEI was synthesised by the hydrolysis of poly(2-ethyl-2-oxazoline) and sold as jetPEI.
The current generation in-vivo-jetPEI uses bespoke poly(2-ethyl-2-oxazoline) polymers
as precursors. Applications
Polyethyleneimine finds many applications in products like: detergents, adhesives, water
treatment agents and cosmetics. Thanks to ability to modify the surface of cellulose
fibres, PEI is employed as a wet-strength agent in the paper-making process. It is also
used as flocculating agent with silica sols and as a chelating agent with the ability
to complex metal ions such as zinc and zirconium. There are also other highly specialized PEI
applications: Attachment promoter
Polyethyleneimines are used in the cell culture of weakly anchoring cells to increase attachment.
PEI is a cationic polymer; the negatively charged outer surfaces of cells are attracted
to dishes coated in PEI, facilitating stronger attachments between the cells and the plate.
However, polyethylenimine has very strong toxicity.
Transfection reagent Poly(ethylenimine) was the second polymeric
transfection agent discovered, after poly-l-lysine. PEI condenses DNA into positively charged
particles, which bind to anionic cell surface residues and are brought into the cell via
endocytosis. Once inside the cell protonation of the amines results in an influx of counter-ions
and a lowering of the osmotic potential. Osmotic swelling results and bursts the vesicle releasing
the polymer-DNA complex into the cytoplasm. If the polyplex unpacks then the DNA is free
to diffuse to the nucleus. PEI is extremely cytotoxic, by two different mechanisms, the
disruption of the cell membrane leading to necrotic cell death and disruption of the
mitochondrial membrane after internalisation leading to apoptosis.
CO2 capture Both linear and branched polyethylenimine
have been used for CO2 capture, frequently impregnated over porous materials. First use
of PEI polymer in CO2 capture was devoted to improve the CO2 removal in space aircraft
applications, impregnated over a polymeric matrix. After that, the support was changed
to MCM-41, an hexagonal mesostructured silica, and large amounts of PEI were retained in
the so-called "molecular basket". MCM-41-PEI adsorbent materials led to higher CO2 adsorption
capacities than bulk PEI or MCM-41 material individually considered. The authors claim
that, in this case, a synergic effect takes place due to the high PEI dispersion inside
the pore structure of the material. As a result of this improvement, further works were developed
to study more in depth the behaviour of these materials. Exhaustive works have been focused
on the CO2 adsorption capacity as well as the CO2/O2 and CO2/N2 adsorption selectivity
of several MCM-41-PEI materials with PEI polymers. Also, PEI impregnation has been tested over
different supports such as a glass fiber matrix and monoliths. However, for an appropriate
performance under real conditions in post-combustion capture it is necessary to use thermally and
hydrothermally stable silica materials, such as SBA-15, which also presents an hexagonal
mesostructure. Moisture and real world conditions have also been tested when using PEI-impregnated
materials to adsorb CO2 from the air. A detailed comparison among PEI and other amino-containing
molecules showed an excellence performance of PEI-containing samples with cycles. Also,
only a slight decrease was registered in their CO2 uptake when increasing the temperature
from 25 to 100°C, demonstrating a high contribution of chemisorption to the adsorption capacity
of these solids. For the same reason, the adsorption capacity under diluted CO2 was
up to 90% of the value under pure CO2 and also, a high unwanted selectivity towards
SO2 was observed. Lately, many efforts have been made in order to improve PEI diffusion
within the porous structure of the support used. A better dispersion of PEI and a higher
CO2 efficiency were achieved by impregnating a template-occluded PE-MCM-41 material rather
than perfect cylindrical pores of a calcined material, following a previously described
route. The combined use of organosilanes such as aminopropyl-trimethoxysilane, AP, and PEI
has also been studied. The first approach used a combination of them to impregnate porous
supports, achieving faster CO2-adsorption kinetics and higher stability during reutilization
cycles, but no higher efficiencies. A novel method is the so-called "double-functionalization".
It is based on the impregnation of materials previously functionalized by grafting. Amino
groups incorporated by both paths have shown synergic effects, achieving high CO2 uptakes
up to 235 mg CO2/g. CO2 adsorption kinetics were also studied for these materials, showing
similar adsorption rates than impregnated solids. This is an interesting finding, taking
into account the smaller pore volume available in double-functionalized materials. Thus,
it can be also concluded that their higher CO2 uptake and efficiency compared to impregnated
solids can be ascribed to a synergic effect of the amino groups incorporated by two methods
rather than to a faster adsorption kinetics. References