Полигликолевая кислота: синтез, свойства и применение в биомедицине.

Polyglycolide or poly(glycolic acid) (PGA), also spelled as polyglycolic acid, is a biodegradable, thermoplastic polymer and the simplest linear, aliphatic polyester. It can be prepared starting from glycolic acid by means of polycondensation or ring opening polymerization. PGA has been known since 1954 as a tough fiber forming polymer. Owing to its hydrolytic instability, however, its use has initially been limited. Currently polyglycolide and its copolymers (poly(lactic co glycolic acid) with lactic acid, poly(glycolide co caprolactone) with ε caprolactone and poly (glycolide co trimethylene carbonate) with trimethylene carbonate) are widely used as a material for the synthesis of absorbable sutures and are being evaluated in the biomedical field.

Polyglycolide has a glass transition temperature between 35 and 40 °C and its melting point is reported to be in the range of 225 to 230 °C. PGA also exhibits an elevated degree of crystallinity, around 45–55%, thus resulting in insolubility in water. Fibers of PGA exhibit high strength and modulus (7 GPa) and are particularly stiff. The most common synthesis used to produce a high molecular weight form of the polymer is ring opening polymerization of "glycolide", the cyclic diester of glycolic acid. Glycolide can be prepared by heating under reduced pressure low MW PGA, collecting the diester by means of distillation. Ring opening polymerization of glycolide can be catalyzed using different catalysts, including antimony compounds, such as antimony trioxide or antimony trihalides, zinc compounds (zinc lactate) and tin compounds like stannous octoate (tin(II) 2 ethylhexanoate) or tin alkoxides. Stannous octoate is the most commonly used initiator, since it is approved by the FDA as a food stabilizer. Usage of other catalysts has been disclosed as well, among these are aluminium isopropoxide, calcium acetylacetonate, and several lanthanide alkoxides (e. g. yttrium isopropoxide). The procedure followed for ring opening polymerization is briefly outlined: a catalytic amount of initiator is added to glycolide under a nitrogen atmosphere at a temperature of 195 °C. The reaction is allowed to proceed for about two hours, then temperature is raised to 230 °C for about half an hour. After solidification the resulting high MW polymer is collected. PGA can also be obtained by reacting carbon monoxide, formaldehyde or one of its related compounds like paraformaldehyde or trioxane, in presence of an acidic catalyst. In a carbon monoxide atmosphere an autoclave is loaded with the catalyst (chlorosulfonic acid), dichloromethane and trioxane, then it is charged with carbon monoxide until a specific pressure is reached; the reaction is stirred and allowed to proceed at a temperature of about 180 °C for two hours. Upon completion the unreacted carbon monoxide is discharged and a mixture of low and high MW polyglycolide is collected.

Polyglycolide is characterized by hydrolytic instability owing to the presence of the ester linkage in its backbone. The degradation process is erosive and appears to take place in two steps during which the polymer is converted back to its monomer glycolic acid: first water diffuses into the amorphous (non crystalline) regions of the polymer matrix, cleaving the ester bonds; the second step starts after the amorphous regions have been eroded, leaving the crystalline portion of the polymer susceptible to hydrolytic attack. Upon collapse of the crystalline regions the polymer chain dissolves. When exposed to physiological conditions, polyglycolide is degraded by random hydrolysis, and apparently it is also broken down by certain enzymes, especially those with esterase activity. The degradation product, glycolic acid, is nontoxic, but like ethylene glycol, it is metabolized to oxalic acid, which could make it dangerous. A part of the glycolic acid is also excreted by urine. Studies undergone using polyglycolide made sutures have shown that the material loses half of its strength after two weeks and 100% after four weeks. The polymer is completely resorbed by the organism in a time frame of four to six months.

While known since 1954, PGA had found little use because of its sensitivity to hydrolysis when compared with other synthetic polymers. However, in 1962 this polymer was used to develop the first synthetic absorbable suture which was marketed under the tradename of Dexon Production is at Belle, West Virginia, with an intended capacity of 4000 annual metric tons. Its attributes as a barrier material result from its high degree of crystallization, the basis for a tortuous path mechanism for low permeability. It is anticipated that the high molecular weight version will have use as an interlayer between layers of polyethylene terephthalate to provide improved barrier protection for perishable foods, including carbonated beverages and foods that lose freshness on prolonged exposure to air. Thinner plastic bottles which still retain desirable barrier properties may also be enabled by this polyglycolide interlayer technology. A low molecular weight version (approximately 600 amu) is available from The Chemours Company (formerly part of DuPont) and is purported to be useful in oil and gas applications.