ethylene oxide polymer
Pure polyethylene glycol HO(CH2CH2O)nH (PEG) have been characterized as clear viscous liquid (molecular weight less than 200), wax-like substance (molecular weight 200 ÷ 2000) and as opaque white crystalline solid (higher molecular weight). Polyethylene glycols are soluble in most organic solvents, such as benzene, carbon tetrachloride, chloroform, dimethylformamide (DMF), acetonitrile. PEG is perfectly soluble in water, but solubility decreases with molecular weight of polymer increasing.
The two names polyethylene glycol and polyethylene oxide are chemically synonymous, but historically PEG has tended to refer to shorter polymers with molecular weight Mr<20000, polyethylene oxide to high-molecular adducts. PEG undergoes thermo-oxidative and oxidative destruction at the temperature above 310°C, also it is destructed by action of high-speed stirring. Polyethylene glycol forms complexes with earth-alkaline and alkaline metals, mercury chloride HgCl2, thiourea and polyacrylic acid.
Polyethylene glycols are non-toxic compounds and can be used in pharmaceuticals compositions and as a food additives;
Polyethylene glycol is produced by interaction of calculated amount of ethylene oxide with water, ethylene glycol or ethylene glycol oligomers. The reaction is catalyzed by acidic or basic catalysts. Ethylene glycol and its oligomers are preferable as a starting material than water, because of it allows to obtain polymer with narrow molecular weight distribution (low polydispersity). 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. Anionic mechanism is more preferable and it allows to obtain PEG with low polydispersity. Polymerization (polycondensation) of ethylene oxide is an exothermic process. Contamination of ethylene oxide with catalysts such as alkalis or metal oxides or overheating 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 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.
Low-molecular PEG is used:
High-molecular PEG is used:
- as a textile auxiliaries;
- as the basis of a number of laxatives;
- as the basis of many skin creams and sexual lubricants, frequently combined with glycerin;
- as a dispersant in toothpastes;
- to create very high osmotic pressures (tens of atmospheres) in water systems in biochemistry experiments;
- as a thickening agent e.g in hydraulic fluids;
- as a basis and binding agent of a number of tablets, cosmetic creams, suppositories;
- as a binding agent in making molds for ceramics, casting and powder metallurgy;
- as an oligomer in polyurethanes manufacturing;
- to produce non-ionic detergents, pharmaceuticals and other compounds by coupling a PEG structure to another larger molecule, such as castor oil, interferon alfa-2a and so on. This procedure is known as PEGylation;
- as a flocculating agent and coagulant in ore dressing processes, for sedimentation of dredges, paper pulp, breeze;
- as an agent used to decrease hydrodynamic resistance in industrial processes and in medicine for injections;
- as a packing material for foods (water-soluble film);
- as a binding agent and thickener for latex and paints;
- as the separator and electrolyte solvent in lithium polymer cells;
- as a polar stationary phase for gas chromatography;