Polyglycolide

Polyglycolide has a glass transition temperature between 35 and 40 °C and a melting point 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. The high molecular weight form is insoluble in common organic solvents (acetone, dichloromethane, etc.), whereas low molecular weight oligomers are more soluble. Polyglycolide is soluble in highly fluorinated solvents like hexafluoroisopropanol (HFIP) and hexafluoroacetone sesquihydrate, that can be used to prepare solutions of the high MW polymer for melt spinning and film preparation. Fibers of PGA exhibit high strength and modulus (7 GPa) and are particularly stiff. ==Synthesis==

Polyglycolide can be obtained through several processes starting with different materials: • polycondensation of glycolic acid • ring-opening polymerization of glycolide • solid-state polycondensation of halogenoacetates Polycondensation of glycolic acid is the simplest process available to prepare PGA, but it is not the most efficient because it yields a low molecular weight product. The most common synthesis of high molecular weight form of the polymer is ring-opening polymerization of glycolide, the bislactone cyclic dimer of glycolic acid. Glycolide can be prepared by thermal cracking, collecting the diester by means of distillation. Ring-opening polymerization of glycolide can be catalyzed using diverse 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). Another procedure consists in the thermally induced solid-state polycondensation of halogenoacetates with general formula X-—CH2COO−M+ (where M is a monovalent metal like sodium and X is a halogen like chlorine), resulting in the production of polyglycolide and small crystals of a salt. Polycondensation is carried out by heating an halogenoacetate, like sodium chloroacetate, at a temperature between 160 and 180 °C, continuously passing nitrogen through the reaction vessel. During the reaction polyglycolide is formed along with sodium chloride which precipitates within the polymeric matrix; the salt can be conveniently removed by washing the product of the reaction with water. PGA can also be obtained by carbonylation (reaction with carbon monoxide) of formaldehyde or the related compounds like paraformaldehyde or trioxane. ==Degradation==

The hydrolytic degradation 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. ==Uses==

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. ==References==