Introduction of Polyethylene Glycol (PEG)
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Introduction of Polyethylene Glycol (PEG)

Polyethylene glycol (PEG), a compound composed of repeated ethylene glycol units [-(CH2CH2O)n], is also known as Macrogols. PEG and PEG derivatives have the characteristics of non-irritation, good water solubility, good compatibility, etc. . At the same time, they possess excellent lubricity, moisture retention and dispersibility so that they can be used as adhesives, antistatic agents and softeners, and widely applied in cosmetics, pharmaceuticals, chemical fibers, rubber, plastics, paper, paint, electroplating, pesticides, metal processing and food processing industries.

Synthesis of PEG

PEG can be synthesized via anionic polymerization of ethylene oxide and any hydroxyl initiators. Hydroxyl group could be from water or ethylene glycol or any diols. They can also be derived from epoxyethane by ring-opening polymerization. Usually, the polymerization process produces a family of PEG molecules with wide Gaussian's distribution of molecular weights. PEG bears only two functional groups limiting the scope for further derivatization with other residues or targeting ligands. Therefore, commercial PEGs are available with different degrees of polymerization and activated functional groups.

Mechanism-of-anionic-polymerization-for-PEG-preparationFig. 1 Mechanism of anionic polymerization for PEG preparation.

PEG Properties

PEG earns its fame because of its high structure flexibility, biocompatibility, amphiphilicity, devoid of any steric hindrances, and high hydration capacity. PEG is soluble in water, ethanol and many other organic solvents, like DMF, dichloromethane, toluene, acetonitrile, acrylonitrile, etc. . It shows little toxicity and can be eliminated from the body intact by either the kidneys (for PEGs < 30 kDa) or in the faeces (for PEGs > 20 kDa). Other than that, PEG is also non-irritating and stable to heat, acid and alkali, thus it can be widely used in various pharmaceutical applications.

PEG series products have different properties depending on the different molecular weights (Mw). From the perspective of appearance, PEG with Mw of 200-700 are normally liquid at room temperature, and those with Mw > 700 will gradually change from semi-solid to soft solids (Mw between 1000~2000), and finally to hard crystalline solids (Mw > 2000). On the other hand, as the Mw increases, the water solubility, vapor pressure, water absorption, and solubility in organic solvents of PEG will correspondingly decrease, while the freezing point, relative density, flash point and viscosity will increase accordingly. The following table briefly lists several properties of PEG products with different Mw.

PEG and PEG Derivatives

PEG-NHS ester; PEG-aldehyde; PEG-maleimide; PEG-hydrazide; PEG-amine; PEG-alkyne; PEG-azide; PEG-vinyl sulfone; PEG-thiol

Chemical-structures-of-PEG-and-key-PEG-derivativesFig. 2 Chemical structures of PEG and key PEG derivatives. (Expert Opinion on Drug Delivery 2016, 13 (9), 1257-1275)

PEG, a polyether backbone polymer, is synthesized by ring opening polymerization of ethylene oxide using in common cases or usually methanol or water as an initiator. The reaction gives products with one or two end chain hydroxyl groups termed monomethoxy-PEG (mPEG-OH) or diol-PEG (HO-PEG-OH), respectively (Fig. 2).

The lone hydroxyl group in the case of the methoxy form, or the two in the case of PEG diol, can be modified to

be reactive towards different chemical groups by several activation strategies. Nowadays, a number of activated PEGs are commercially available. These derivatives are commonly monofunctional, either in their linear or branched structure. The chemical structures of several key PEG derivatives are shown in Figure 2. These derivatives are mainly known by their Mw.

PEGylation Technique

PEGylation is defined as the covalent attachment of poly(ethylene glycol) (PEG) chains to bioactive substances, such as nanoparticles, proteins, peptides or non-peptide molecules. This technology emerged gradually with progress in field of biologicals and polymer chemistry and was exploited to improve pharmaceutical applications of a wide range of therapeutics. Nowadays, PEGylation has become the leading approach for overcoming most of the limits of biologics and numbers of FDA approved PEGylated products like enzymes (bovine adenosine deaminase and urate oxidase), cytokines (interferon-α2a, interferon-α2b), granulocyte colony stimulating factors, hormones (epoetin-β), antibodies and their fragments and other organic molecules (pegvisomant, pegatinib) are on the market.

Main-advantages-of-PEGylated-proteinsFig. 3 Main advantages of PEGylated proteins. (Drug discovery today 2005, 10 (21), 1451-1458)

Application Fields

  • Bio-medical Industry

As an amphiphilic polymer, PEG can be soluble in water and most organic solvents, and has the characteristics of good biocompatibility, non-toxicity, and low immunogenicity. It can be excreted through the kidneys, so there will be no accumulation phenomenon in the body. In addition, PEG is easy to bind with proteins, nanocarriers and small molecule drugs,i.e., PEGylation, after the terminal hydroxyl group is activated, thus being widely applied into various biomedical areas, such as drug delivery, imaging diagnosis, pharmaceutical preparation and tissue engineering.

In recent years PEG aqueous solutions have been widely applied in many different kinds of reaction systems. Their low-toxicity, low volatility, and biodegradability represent important environmentally benign characteristics, which are particularly attractive when combined with their relatively low cost as a bulk commodity chemical. In addition, aqueous PEG solutions may often substitute for other expensive and toxic phase-transfer catalysts.

Polyethylene glycols (PEGs) and their anionic or nonionic derivatives are widely used in cosmetics as surfactants, cleansing agents, emulsifiers, skin conditioners, and humectants, because of their good solubility, viscosity and low toxicity properties. The PEG, their ethers, and their fatty acid esters show little or no

ocular or dermal irritation and have extremely low acute and chronic toxicities.

  • Textile Industry

In the textile industry, PEG can be used as spinning aid, dyeing solvent and dispersant. The solid powder PEG has good properties of film-forming, lubrication and plasticization. It is used to treat fiber additives, which can make fabrics soft, plasticized and refined, and can inhibit the accumulation of static electricity. Above all, PEG can be used as detergent, softener, lubricant, anti-static agent, dispersant, dye carrier, air conditioner and finishing agent in the textile industry.

  • Plastic Industry

PEG is commonly used as an auxiliary agent in the plastic industry, for example, as a lubricant during extrusion molding, and as a release agent during mold molding. In addition, because of good water solubility, adding PEG into the resins can greatly improve their performance. Substituting PEG for glycerin can improve the toughness of the resin. Adding PEG to the polyurethane resin can improve its strength and elasticity.

  • Paint Industry

Using PEG as a matrix can replace water-quality emulsified coatings. Low molecular weight PEG has better dispersibility, while high molecular weight PEG can produce better film forming properties. In any cases, coatings with PEG as the matrix show more water-resistant than those with water as the matrix, and can improve the gloss of the film and reduce the odor.

  • Other

Apart from the application areas mentioned above, PEG also has a wide range of applications in other fields. For example, PEG has a strong binding force with rubber without degrading the rubber. It is often used as a release agent and loosening agent for rubber and polyurethane products. PEG has a dehydrating and fixing effect on tissues. There are also reports on the use of PEG in the preparation and preservation of animal and plant specimens (called plasticized specimens).

References

  1. D'souza, A. A.; Shegokar, R., Polyethylene glycol (PEG): a versatile polymer for pharmaceutical applications. Expert Opinion on Drug Delivery 2016, 13 (9), 1257-1275.
  2. Veronese, F. M.; Pasut, G., PEGylation, successful approach to drug delivery. Drug discovery today 2005, 10 (21), 1451-1458.
  3. Fruijtier-Pölloth, C., Safety assessment on polyethylene glycols (PEGs) and their derivatives as used in cosmetic products. Toxicology 2005, 214 (1-2), 1-38.

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