The reaction is catalyzed by acidic or basic catalysts. Ethylene glycol and its oligomers are preferable as a starting material instead of water, because they allow 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 in an explosion after a few hours. Polyethylene oxide, or high-molecular weight 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-weight polyethylene glycol.
Chemical uses
Polyethylene glycol has a low toxicity and is used in a variety of products. The polymer is used as a lubricating coating for various surfaces in aqueous and non-aqueous environments.Since PEG is a flexible, water-soluble polymer, it can be used to create very high osmotic pressures (on the order of 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, in particular when using the osmotic stress technique.[citation needed] Polyethylene glycol is also commonly used as a polar stationary phase for gas chromatography, as well as a heat transfer fluid in electronic testers. PEO (polyethylene 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 to be created. 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. PEG has also been used to preserve objects that have been salvaged from underwater, as was the case with the warship Vasa in Stockholm, the Mary Rose in England and the Ma'agan Michael Ship in Israel. It replaces water in wooden objects, making the wood dimensionally stable and preventing warping or shrinking of the wood when it dries. In addition, PEG is used when working with green wood as a stabilizer, and to prevent shrinkage. PEG is often used (as an internal calibration compound) in mass spectrometry experiments, with its characteristic fragmentation pattern allowing accurate and reproducible tuning. PEG derivatives, such as narrow range ethoxylates, are used as surfactants. PEG has been used as the hydrophilic block of amphiphilic block copolymers used to create some polymersomes.
Biological uses
PEG is commonly used as a precipitant for plasmid DNA isolation and protein crystallization. X-ray diffraction of protein crystals can reveal the atomic structure of the proteins.
Polymer segments derived from PEG polyols impart flexibility to polyurethanes for applications such as elastomeric fibers (spandex) and foam cushions. In microbiology, PEG precipitation is used to concentrate viruses. PEG is also used to induce complete fusion (mixing of both inner and outer leaflets) in liposomes reconstituted in vitro.
Gene therapy vectors (such as viruses) can be PEG-coated to shield them from inactivation by the immune system and to de-target them from organs where they may build up and have a toxic effect. The size of the PEG polymer has been shown to be important, with larger polymers achieving the best immune protection.
PEG is a component of stable nucleic acid lipid particles (SNALPs) used to package siRNA for use in vivo.
In blood banking, PEG is used as a potentiator to enhance detection of antigens and antibodies.
When working with phenol in a laboratory situation, PEG 300 can be used on phenol skin burns to deactivate any residual phenol.
Commercial uses
PEG is the basis of many skin creams (as cetomacrogol) and sexual lubricants (frequently combined with glycerin).
PEG is used in a number of toothpastes as a dispersant. In this application, it binds water and helps keep xanthan gum uniformly distributed throughout the toothpaste.
PEG is also under investigation for use in body armor, and in tattoos to monitor diabetes.
In low-molecular-weight formulations (i.e PEG 400), it is used in Hewlett-Packard designjet printers as an ink solvent and lubricant for the print heads.
PEG is also one of the main ingredients in paintball fills, due to its thickness and flexibility. However, as early as 2006, some Paintball manufacturers began substituting cheaper alternatives for PEG.[citation needed]
PEG is a major ingredient in e-liquid, used in electronic cigarettes. It is generally used as a 30%-50% proportion of the liquid that is vaporized. Its use is designed to give a smoother effect to the vaporizing action.[citation needed] PEG is also used as an anti-foaming agent in food[28] - its INS number is 1521[29] or E1521 in the EU.
Industrial uses
Nitrate ester-plasticized polyethylene glycol is used in Trident II ballistic missile solid rocket fuel.
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.
PEG has been used as the gate insulator in an electric double-layer transistor to induce superconductivity in an insulator.
PEG is also used as a polymer host for solid polymer electrolytes. Although not yet in commercial production, many groups around the globe are engaged in research on solid polymer electrolytes involving PEG, with the aim of improving their properties, and in permitting their use in batteries, electro-chromic display systems, and other products in the future.
Source : http://en.wikipedia.org/wiki/Polyethylene_glycol
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