Polyethylene glycol ester or polyglycolic acid (PGA) is a biodegradable, thermoplastic polymer and the simplest linear aliphatic polyester. It is prepared from glycolic acid through condensation or ring opening polymerization. PGA has been known as a tough polyester fiber since 1954. Due to its hydrolytic instability, its use is still limited to low-level categories. At present, polyethylene glycol and its copolymers polyethylene propylene glycol and polyethylene caprolactone, as well as poly (ethylene glycol co trimethylene carbonate), are widely used in the synthesis of absorbable suture materials and have extremely high value in the biomedical field.
The biomedical applications of PGA are mainly manifested in medical sutures, drug controlled release carriers, fracture fixation materials, tissue engineering scaffolds, and suture reinforcement materials. The application of PGA in ecology is as a fully biodegradable plastic that is beneficial to the environment, replacing the widely used bio stable universal plastics in the plastic industry. PGA is mainly used as a slow-release system to control the release rate of herbicides. The most obvious advantage of using PGA agricultural film is that it will not cause environmental pollution like the widely used PE and PVC. This film can automatically degrade after a few years of use and will not pollute the land and water sources.
physical property
The glass transition temperature of polyethylene glycol is 35-40 ℃ and its melting point is in the range of 225-230 ℃. PGA also exhibits high crystallinity, approximately 45-55%, resulting in insolubility in water. The solubility of this polyester is somewhat unique, as its high molecular weight form is almost insoluble in all common organic solvents (acetone dichloromethane, chloroform, ethyl acetate, tetrahydrofuran), while low molecular weight oligomers have significant differences in their physical properties and are more easily dissolved. However, polyethylene glycol is dissolved in high fluorinated solvents such as hexafluoroisopropanol and fluoroacetone multibar, which can be used to prepare solutions for high molecular weight polymer melt spinning and thin film preparation.
PGA fibers have high strength and modulus (7GPa) and are particularly hard.
synthesis
1. Condensation reaction of glycolic acid;
2. Ring opening polymerization of ethyl ester;
3. Solid phase condensation of halogenated acetic esters
The condensation of glycolic acid is the simplest process for preparing PGA, but it is not the most effective because its yield is a low molecular weight product. The simple steps are as follows: Ethanol acid is heated at atmospheric pressure and approximately 175-185 ℃ until no more water evaporates, then the pressure is reduced to 150mmHg, and the temperature remains unchanged for about two hours. Low molecular weight PGA is obtained.
The most common synthesis process for producing high molecular weight polymers is ring opening polymerization of ethyl ester, which can be obtained by heating low molecular weight PGA under reduced pressure and collecting the ester by distillation. The ring opening polymerization of ethylene glycol can be catalyzed by different catalysts, including antimony compounds such as antimony trioxide or antimony trihalide, zinc compounds (zinc lactate), and tin compounds such as stannous octoate or stannous alcohol.
Since obtaining approval from the Food and Drug Administration in the United States, stannous octoate has become the most commonly used initiator for this reaction. With the continuous deepening of research on this reaction, a series of catalysts that can be used for this reaction have gradually been discovered by researchers, including aluminum isopropoxide, calcium acetylacetonate, and several other rare earth alkoxides, such as yttrium isopropoxide
The open-loop polymerization process is described as follows: under nitrogen atmosphere and 195 ℃, a certain amount of catalyst initiator is added to the monomer, and the reaction lasts for two hours. Then, the temperature is raised to 230 ℃ for about half an hour. Collect high molecular weight polymers after solidification.
Another thermal process for synthesizing polyethylene glycol is the solid-state polymerization of halogenated acetate salts. Halogenated acetates refer to a series of acetates with the general formula X-CH2COO-M+, where M is a partially monovalent metal (such as sodium) and X is a halogen. The solid-phase condensation products of halogenated acetic acid salts are polyhydroxyacetic acid and a series of smaller volume crystalline salts. The reaction process is as follows: Heat the halogenated acetate salt under nitrogen conditions to 160-180 ° C and maintain the reaction temperature until the reaction is terminated. During the reaction process, metal halide impurities will be generated within the polymer, and the removal of impurities can be achieved by washing the product with water.
PGA can also be obtained by reactingcarbon 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 (chlorosulfonicacid), dichloromethane and trioxane, then it is chargedwith carbon monoxide until a specific pressure is reached; the reaction isstirred 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 oflow and high MW polyglycolide is collected.
degradation
There are two steps in the process of converting polymers into their monomer glycolic acid: first, water diffuses to the amorphous region of the polymer matrix, causing ester bonds to break; The second step begins after the erosion of the amorphous region, and the crystalline region of the polymer is prone to hydrolysis and cracking. The polymer chains in the crystalline region disintegrate and collapse. When exposed to physical conditions, polymers are degraded by free water, which is clearly broken by certain enzymes, especially those with ester activity. The degradation product glycolic acid is non-toxic and can enter the tricarboxylic acid cycle, where it is converted into water and carbon dioxide and excreted. Some of the glycolic acid is also excreted through urine.
Research has shown that sutures made from polyethylene glycol lose half of their material strength after two weeks and 100% after four weeks. The polymer is completely absorbed by the body tissues within a range of four to six months. The degradation rate is faster in vivo than in vitro, which is attributed to the activity of cellular enzymes.
purpose
As is well known, since 1954, PGA has not been widely used compared to other synthetic polymers due to its sensitivity to hydrolysis. However, in 1962, this polymer was first used by Davis and Jocke of the American Cyanide company to study the synthesis of 'Dixon' absorbable sutures, which are now sold as Surgicrel
PGA suture is defined as a synthetic, absorbable, braided multi fiber suture. Apply N-glyceryl triacetate and L-lysine to make the suture extremely smooth, soft, and safe to weave. It can also be coated with magnesium stearate and disinfected with ethylene oxide. It is usually degraded in the body through hydrolysis and absorbed as a water-soluble monomer, taking approximately 60 to 90 days to complete. In the early stages, patients with anemia and malnutrition may absorb sutures faster. Its color is not dyed except for blue purple, It has the advantages of high initial tensile strength, smooth passage throughtissue, easy handling, excellent knotting ability, and secure knot tying. It is commonly used for subcutaneous suturing, intradermal closure, and surgical procedures on the abdomen and chest. The traditional role of PGA as a biodegradable suture material has enabled its value to be realized in other fields of biomaterials. PGA related implantable medical devices have been produced, including articulated loop rings, needles, rods, discs, and screws. It is also used to explore tissue engineering or drug controlled release. The tissue engineering scaffolds made from polyethylene glycol have been obtained through various methods, but most of them are generally made using non-woven weaving technology. Japan's Wu Yu Company has announced the industrialization of high molecular weight polyethylene glycol esters for food packaging under the kuredux brand The production is in Bell, West Virginia, with an expected capacity of 4000 tons per year according to a chemical technology report. Its properties as a moisture-proof material are due to its high crystallinity, and its properties as a barrier material are based on the low permeability tortuous path mechanism due to its high crystallinity Itsattributes as a barrier material result from its high degree ofcrystallization, the basis for a tortuous path mechanism for low permeability. This is expected, as the high molecular weight version will use an intermediate layer between polyethylene terephthalate layers to provide an improved protective barrier for perishable foods, including carbonated beverages and food, that are exposed to prolonged exposure without fresh air Itis anticipated that the high molecular weight version will have use as aninterlayer between layers of polyethylene terephthalate toprovide improved barrier protection for perishable foods, including carbonatedbeverages and foods that lose freshness on prolonged exposure to air. Thin plastic bottles still maintain ideal barrier properties, which can also be achieved through this polyethylene glycol interlayer technology. A low molecular weight (approximately 600 amu) version can be claimed by DuPont to be useful in oil and gas applications. Thinnerplastic bottles which still retain desirable barrier properties may also beenabled by this polyglycolide interlayer technology. Alow molecular weight version (approximately600 amu) is available from the DuPontCo. andis purported to be useful in oil and gas applications.
The large-scale promotion and application of biodegradable plastics is a key breakthrough in solving the problem of plastic pollution. As a type of polyester material that combines excellent biodegradability and biocompatibility, polyglycolic acid (PGA) has shown broad application prospects in packaging materials, agricultural production, medical devices, oil and gas extraction, and other fields. According to estimates, the demand for PGA in China's market will reach a scale of millions of tons in the future. However, the mainstream preparation process of PGA faces significant challenges: the traditional synthesis route of its monomer raw material glycolic acid relies on highly toxic precursors (such as chloroacetic acid or hydrocyanic acid), which poses safety risks and is difficult to scale up production. Meanwhile, the decomposition of products and high energy consumption during the synthesis of glycolic acid further increase production costs, making it a key bottleneck restricting the development of the PGA industry. As of 2024, China's annual production capacity of glycolic acid is less than 50000 tons, which creates a huge gap with the demand of the million ton market. It is urgent to develop green and economical synthesis routes.
In response to this major demand, the research team of Chen Yong from the Institute of Physical and Chemical Technology of the Chinese Academy of Sciences developed a new electrosynthesis strategy, using waste PET plastics as starting materials, and successfully achieved the preparation of glycolic acid in grams (Andrew Chem. Int. Ed.2023, 62, e202300094; Green Chem., 2023, 25, 5872; Adv. Energy Mate.2024, 14, 2304065; Andrew Chem. Int. Ed.2025, e202422183). In order to promote the industrialization of this technology and achieve the full process conversion from waste plastic PET to biodegradable plastic PGA, the team systematically analyzed the two core challenges in the electrocatalytic reforming of PET to prepare PGA: (I) low spatiotemporal yield of ethylene glycol to prepare glycolic acid; (II) The separation and purification cost of glycolic acid crystals is high.
Based on these two key technical issues, the team has recently developed a palladium catalyst loaded with strong Lewis acid (CoCr2O4), which promotes the rapid migration of OH - in the reaction system through strong Lewis acid, greatly improving the spatiotemporal yield of ethylene glycol to prepare ethanol acid; A new distillation and alcohol precipitation process has been developed, which not only achieves efficient separation of high-purity glycolic acid crystals, but also completes the recycling of alkali solution/ethylene glycol and the recovery of by-products. Based on these technological innovations, the team independently designed a 500 W level fuel cell stack system and successfully completed the full process amplification experiment of 20 kg waste PET to PGA. The economic and technical analysis results show that the cost of PGA prepared based on electrocatalytic reforming route is about 1240.12 US dollars/ton, which is close to the cost range of general polyolefin plastics, laying a solid foundation for the industrialization of this technology.
This achievement is based on Scale-up upcycling of waste polyethylene terephthalate plastics to biodegradable polyglycolic acid plastics The title was published in the journal Nature Communications, and the corresponding authors of the paper are Researcher Chen Yong and Associate Researcher Shi Rui from the Institute of Physical and Chemical Technology. The co first authors are PhD students Wang Yuxiang and Associate Researcher Liu Fulai. The research work has been supported by the National Key Research and Development Program, the National Natural Science Foundation of China, the Chinese Academy of Sciences Hong Kong University New Materials Joint Laboratory Fund and other supporting projects.
废弃塑料PET转化为生物可降解塑料PGA的过程示意图
