Polyethylene glycol

Polyethylene glycol (PEG) and polyethylene oxide (PEO) are polymers composed of repeating subunits of identical structure, called monomers, and are the most commercially important polyethers. Poly (ethylene glycol) or poly (ethylene oxide) refers to an oligomer or polymer of ethylene oxide. The two names are chemically synonymous, but historically PEG has tended to refer to shorter polymers, PEO to longer.^[1] PEG and PEO are liquids or low-melting solids, depending on their molecular weights. Both are prepared by polymerization of ethylene oxide. While PEG and PEO with different molecular weights find use in different applications and have different physical properties (e.g. viscosity) due to chain length effects, their chemical properties are nearly identical. Derivatives of PEG and PEO are in common use, the most common derivative being the methyl ether (methoxypoly (ethylene glycol)), abbreviated mPEG.

Their melting points vary depending on the Formula Weight of the polymer. PEG or PEO has the following structure:

HO-(CH₂-O-CH₂)_n-H

The numbers that are often included in the names of PEGs and PEOs indicate their average molecular weights, e.g. a PEG with n=80 would have an average molecular weight of approximately 3500 daltons and would be labeled PEG 3500. Most PEGs and PEOs include molecules with a distribution of molecular weights, i.e. they are polydisperse. The size distribution can be characterized statistically by its weight average molecular weight (Mw) and its number average molecular weight (Mn), the ratio of which is called the polydispersity index (Mw/Mn). Mw and Mn can be measured by mass spectroscopy.

PEGylation is the act of covalently coupling a PEG structure to another larger molecule, for example, a therapeutic protein (which is then referred to as PEGylated). PEGylated interferon alfa-2a or -2b is a commonly used injectable treatment for Hepatitis C infection.

PEG is soluble in water, methanol, benzene, dichloromethane and is insoluble in diethyl ether and hexane. It is coupled to hydrophobic molecules to produce non-ionic surfactants.

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Production

Poly (ethylene glycol) is produced by the interaction of ethylene oxide with water, ethylene glycol or ethylene glycol oligomers.[2] The reaction is catalyzed by acidic or basic catalysts. Ethylene glycol and its oligomers are preferable as a starting material instead of water, because it allows the creation of polymers with a low polydispersity (narrow molecular weight distribution). Polymer chain length depends on the ratio of reactants.

HOCH₂CH₂OH + n(CH₂CH₂O) → HO(CH₂CH₂O)_n+1H

Depending on the catalyst type, the mechanism of polymerization can be cationic or anionic. The anionic mechanism is preferable because it allows one to obtain PEG with a low polydispersity. Polymerization of ethylene oxide is an exothermic process. Overheating or contaminating ethylene oxide with catalysts such as alkalis or metal oxides can lead to runaway polymerization which can end with an explosion after few hours.

Polyethylene oxide or high-molecular polyethylene glycol is synthesized by suspension polymerization. It is necessary to hold the growing polymer chain in solution in the course of the polycondensation process. The reaction is catalyzed by magnesium-, aluminium- or calcium-organoelement compounds. To prevent coagulation of polymer chains from solution, chelating additives such as dimethylglyoxime are used.

Alkali catalysts such as sodium hydroxide NaOH, potassium hydroxide KOH or sodium carbonate Na₂CO₃ are used to prepare low-molecular polyethylene glycol.

Clinical uses

Polyethylene glycol has a low toxicity^[2] and is used in a variety of products. It is the basis of a number of laxatives (e.g. macrogol-containing products such as Movicol and polyethylene glycol 3350, or MiraLax or GlycoLax). It is the basis of many skin creams, as cetomacrogol, and sexual lubricants, frequently combined with glycerin. Whole bowel irrigation (polyethylene glycol with added electrolytes) is used for bowel preparation before surgery or colonoscopy and drug overdoses. It is sold under the brand names GoLYTELY, GlycoLax, Fortrans, TriLyte, and Colyte. When attached to various protein medications, polyethylene glycol allows a slowed clearance of the carried protein from the blood. This makes for a longer acting medicinal effect and reduces toxicity, and it allows longer dosing intervals. Examples include PEG-interferon alpha which is used to treat hepatitis C and PEG-filgrastim (Neulasta®) which is used to treat neutropenia. It has been shown that polyethylene glycol can improve healing of spinal injuries in dogs.^[3] One of the earlier findings that polyethylene glycol can aid in nerve repair came from the University of Texas (Krause and Bittner).^[4] Polyethylene glycol is commonly used to fuse B-cells with myeloma cells in monoclonal antibody production. PEG has recently been proved to give better results in constipation patients than tegaserod.^[5]

Research for New Clinical Uses

• High-molecular weight PEG, e.g., PEG 8000, is a strikingly potent dietary preventive agent against colorectal cancer in animal models.^[6]

The Chemoprevention Database shows it is the most effective agent to suppress chemical carcinogenesis in rats. Cancer prevention in humans has not yet been tested in clinical trials.

• The injection of PEG 2000 into the bloodstream of guinea pigs after spinal cord injury leads to rapid recovery through molecular repair of nerve membranes.^[7] The effect of this treatment to prevent paraplegia in humans after an accident is not known yet.
• Research is being done in the use of PEG to mask antigens on red blood cells. Various research institutes have reported that using PEG can mask antigens without damaging the functions and shape of the cell.

PEG is being used in the repair of motor neurons damaged in crush or laceration incidence in vivo and in vitro. When coupled with melatonin, 75% of damaged sciatic nerves were rendered viable.^[8]

Other uses

PEG is used in a number of toothpastes as a dispersant; it binds water and helps keep gum uniform throughout the toothpaste. It is also under investigation for use in body armor^[9] and tattoos to monitor diabetes.^[10] Functional groups of PEG give polyurethane elastomers their "rubberiness", for applications such as foams (foam rubber) and fibers (spandex). Its backbone structure is analogous to that of silicone, another elastomer.

Since PEG is a flexible, water-soluble polymer, it can be used to create very high osmotic pressures (tens of atmospheres). It also is unlikely to have specific interactions with biological chemicals. These properties make PEG one of the most useful molecules for applying osmotic pressure in biochemistry experiments, particularly when using the osmotic stress technique.^{[citation needed]}

PEO (poly (ethylene oxide)) can serve as the separator and electrolyte solvent in lithium polymer cells. Its low diffusivity often requires high temperatures of operation, but its high viscosity even near its melting point allows very thin electrolyte layers. While crystallization of the polymer can degrade performance, many of the salts used to carry charge can also serve as a kinetic barrier to the formation of crystals. Such batteries carry greater energy for their weight than other lithium ion battery technologies.

When working with phenol in a laboratory situation, PEG 300 can be used on phenol skin burns to deactivate any residual phenol.

Poly (ethylene glycol) is also commonly used as a polar stationary phase for gas chromatography, as well as a heat transfer fluid in electronic testers.

PEG is included in many or all formulations of the soft drink Dr Pepper, purportedly as an anti-foaming agent.

PEG has also been used to preserve objects which have been salvaged from underwater, as was the case with the warship Vasa in Stockholm.^[11] It replaces water in wooden objects, which makes the wood dimensionally stable and prevents warping or shrinking of the wood.

PEG is often seen (as a side effect) in mass spectrometry experiments with characteristic fragmentation patterns.

In the field of microbiology, PEG precipitation is used to concentrate viruses.

PEG is also used in lubricant eye drops. PEG derivatives such as narrow range ethoxylates are used as surfactants.

Dimethyl ethers of PEG are the key ingredient of Selexol, a solvent used by coal-burning, integrated gasification combined cycle (IGCC) power plants to remove carbon dioxide and hydrogen sulfide from the gas waste stream.

References

See also

• PEGylation