Name = Polyglycolide
ImageFile = PGA.png
ImageName = Polyglycolide
IUPACName = 1,4-Dioxane-2,5-dione
Section1 = Chembox Identifiers
CASNo = 26124-68-5
SMILES = O=C(O)CO
Section2 = Chembox Properties
Formula = (C2H2O2)n
MolarMass = N/A
Density = 1.530 g/cm³ at 25 °C
MeltingPt = 225-230 °C
BoilingPt = Decomposes
Polyglycolide or Polyglycolic acid (PGA) is a
biodegradable, thermoplastic polymerand the simplest linear, aliphatic polyester. It can be prepared starting from glycolic acidby means of polycondensation or ring-opening polymerization. PGA has been known since 1954as a tough fiber-forming polymer. Owing to its hydrolytic instability, however, its use has initially been limited.cite journal | last = Gilding | first = D. K. | coauthors = A. M. Reed | title = Biodegradable polymers for use in surgery - polyglycolic/poly (lactic acid) homo- and copolymers: 1 | journal = Polymer | volume = 20 | pages = 1459–1464 | date = December 1979 | doi = 10.1016/0032-3861(79)90009-0 ] 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.cite journal | last = Middleton | first = J. | coauthors = A. Tipton | title = Synthetic biodegradable polymers as medical devices | journal = Medical Plastics and Biomaterials Magazine | date = March 1998 | url = http://www.devicelink.com/mpb/archive/98/03/002.html | format = HTML| accessdate = 2006-07-04 ]
Polyglycolide has a
glass transition temperaturebetween 35-40 °C and its melting pointis reported to be in the range of 225-230 °C. PGA also exhibits an elevated degree of crystallinity, around 45-55%, thus resulting insoluble in water. The solubilityof this polyester is somewhat unique, in that its high molecular weightform is insoluble in almost all common organic solvents ( acetone, dichloromethane, chloroform, ethyl acetate, tetrahydrofuran), while low molecular weight oligomers sufficiently differ in their physical properties to be more soluble. However 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 spinningand film preparation.Schmitt, E.: "Polyglycolic acid in solutions", U.S. Pat 3 737 440, 1973] Fibers of PGA exhibit high strength and modulus (7 GPa) and are particularly stiff.
Polyglycolide can be obtained through several different processes starting with different materials:
# polycondensation of glycolic acid;
# ring-opening polymerization of glycolide;
# solid-state polycondensation of
# acid catalyzed reaction of
carbon monoxideand formaldehyde
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. Briefly, the procedure is as follows: glycolic acid is heated at atmospheric pressure and a temperature of about 175-185°C is maintained until water ceases to distill. Subsequently, pressure is reduced to 150 mm Hg, still keeping the temperature unaltered for about two hours and the low MW polyglycolide is obtained.Lowe, C. E.: "Preparation of high molecular weight polyhydroxyacetic ester", U.S. Pat 2 668 162, 1954]
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 antimonycompounds, such as antimony trioxideor antimony trihalides, zinccompounds (zinc lactate) and tincompounds 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 aluminum isopropoxide, calcium acetylacetonate, and several lanthanidealkoxides (e.g. yttrium isopropoxide). cite journal | last = Bero | first = Maciej | coauthors = Piotr Dobrzynski, Janusz Kasperczyk | title = Application of Calcium Acetylacetonate tothe Polymerization of Glycolide and Copolymerization of Glycolide with ε-Caprolactone and L-Lactide | journal = Macromolecules | volume = 32 | issue = 14 | pages = 4735–4737 | publisher = ACS | date = 18 June 1999 | doi = 10.1021/ma981969z ] cite journal | last = Stridsberg | first = Kajsa M. | coauthors = Maria Ryner, Ann-Christine Albertsson | title = Controlled Ring-Opening Polymerization: Polymers with designed Macromolecular Architecture | journal = Advances in Polymer Science | volume = 157 | pages = 41–65 | publisher = Springer| date = 2002 | id = ISSN 0065-3195 | doi = 10.1007/3-540-45734-8_2 ] The procedure followed for ring-opening polymerization is briefly outlined: a catalytic amount of initiator is added to glycolide under a nitrogenatmosphere 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.
Another procedure consists in the thermally induced solid-state polycondensation of halogenoacetates with general formulaX-—CH2COO-M+ (where M is a monovalent metal like
sodiumand X is a halogenlike 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 160-180°C, continuously passing nitrogen through the reaction vessel. During the reaction polyglycolide is formed along with sodium chloridewhich precipitates within the polymeric matrix; the salt can be conveniently removed by washing the product of the reaction with water.cite journal | last = Epple | first = Matthias | title = A detailed characterization of polyglycolide prepared by solid-state polycondensation reaction | journal = Macromolecular Chemistry and Physics | volume = 200 | issue = 10 | pages = 2221–2229 | publisher = Wiley | date = 1999 | doi = 10.1002/(SICI)1521-3935(19991001)200:10<2221::AID-MACP2221>3.0.CO;2-Q ]
PGA can also be obtained by reacting carbon monoxide, formaldehyde or one of its related compounds like
paraformaldehydeor trioxane, in presence of an acidic catalyst. In a carbon monoxide atmosphere an autoclaveis loaded with the catalyst ( chlorosulfonic acid), dichloromethaneand trioxane, then it is charged with carbon monoxide until aspecific 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.Masuda et al.: "Biodegradable plastic composition", U.S. Pat 5 227 415, 1993]
Polyglycolide is characterized by hydrolytic instability owing to the presence of the
esterlinkage 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 esteraseactivity. The degradation product, glycolic acid, is non toxic and it can enter the tricarboxylic acid cycleafter which it is excreted as water and carbon dioxide. A part of the glycolic acid is also excreted by urine.cite journal | last = Gunatillake | first = Pathiraja A. | coauthors = Raju Adhikari | title = Biodegradable Synthetic Polymers for tissue engineering | journal = European Cells and Materials | volume = 5 | pages = 1–16 | date = 2003 | url = http://www.ecmjournal.org/journal/papers/vol005/pdf/v005a01.pdf | format =
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 ease of degradation when compared with other synthetic polymers. However in
1962this polymer was used to develop the first synthetic absorbable suture which was marketed under the tradenameof Dexon by the Davis & Gecksubsidiary of the American Cyanamid Corporation. Because polyglycolide give strongs fibers and degrades into water soluble monomers, sutures made with this polymer have found use in certain surgical applications, since there is no need for further medical attention to remove them. Implantable medical devices have been produced with PGA as well, including anastomosisrings, pins, rods, plates and screws.
The traditional role of PGA as a biodegradable suture material has led to its evaluation in other biomedical fields, such as
tissue engineeringor controlled drug delivery. Tissue engineering scaffolds made with polyglycolide have been produced following different approaches, but generally most of these are obtained through textiletechnologies in the form of non-woven meshes.
Surgicryl becomes a reference for PGA sutures.
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