Polyethylene glycol (PEG) is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular weight, and under the tradename Carbowax.
PEG, PEO, or POE refers to an oligomer or polymer of ethylene oxide. The three names are chemically synonymous, but historically PEG has tended to refer to oligomers and polymers with a molecular mass below 20,000 g/mol, PEO to polymers with a molecular mass above 20,000 g/mol, and POE to a polymer of any molecular mass. PEG and PEO are liquids or low-melting solids, depending on their molecular weights. PEGs are prepared by polymerization of ethylene oxide and are commercially available over a wide range of molecular weights from 300 g/mol to 10,000,000 g/mol. 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. Different forms of PEG are also available dependent on the initiator used for the polymerization process, the most common of which is a monofunctional methyl ether PEG (methoxypoly(ethylene glycol)), abbreviated mPEG. Lower-molecular-weight PEGs are also available as purer oligomers, referred to as monodisperse, uniform or discrete. Very high purity PEG has recently been shown to be crystalline, allowing determination of an xray crystal structure. Since purification and separation of pure oligomers is difficult, the price for this type of quality is often 10-1000 fold that of polydisperse PEG. PEGs are also available with different geometries. Branched PEGs have three to ten PEG chains emanating from a central core group. Star PEGs have 10–100 PEG chains emanating from a central core group. Comb PEGs have multiple PEG chains normally grafted to a polymer backbone.
Their melting points vary depending on the Formula Weight of the polymer. PEG or PEO has the following structure:
The numbers that are often included in the names of PEGs indicate their average molecular weights, e.g., a PEG with n=9 would have an average molecular weight of approximately 400 daltons and would be labeled PEG 400. Most PEGs 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 spectrometry.
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