You’ve probably heard of the monomer of glycolic acid. It’s the simplest alpha-hydroxy acid, consisting of only two carbons, and two functional groups: an alcohol and a carboxylic acid. The glycolic acid market is growing fast, thanks in large part to the use of glycolic acid in cosmetics and household products. In the pharmaceutical industry, glycolic acid is used as a topical skin exfoliant. The same properties lend themselves towards the use of glycolic acid in household cleaning agents. Glycolic acid, which is found naturally in some fruits, is even used in the food industry as a preservative and flavorant.
But did you know glycolic acid can be polymerized to produce a biodegradable polymer? It’s called poly(glycolic acid). The hope is that poly(glycolic acid) and other biodegradable polymers will eventually be able to eliminate the use of traditional single-use plastics.
Poly (glycolic acid) as a biodegradable plastic
Traditional petroleum-based plastics pose a serious threat to the environment. Of all the single-use plastics produced and discarded over time by our society, only a small portion are actually recycled. Nearly 80% accumulate in the environment, and especially in the oceans where they are irretrievable. They then begin to break down, forming microplastics that pollute the ocean, harming marine life, and bioaccumulating to the extent that they harm humans as well.
Unlike regular plastics, poly (glycolic acid) can degrade quickly, even without the need for extreme conditions. It has many useful mechanical properties. The polymer of lactic acid is structurally quite similar to poly (glycolic acid), and has already been studied for its use as a biopolymer. However, despite the structural similarities, these two polymers exhibit very different properties. For example, while it has excellent strength and durability, poly (lactic acid) does not degrade as readily as poly (glycolic acid).
Scientists would like to take advantage of these complementary properties by creating a co-polymer that incorporates both lactic acid and glycolic acid. The higher the ratio of glycolic acid to lactic acid, the shorter the degradation time. There is a trade-off between biodegradability and the (potentially) desirable mechanical properties of lactic acid polymer. However, glycolic acid itself offers a number of desirable mechanical properties that can overall improve the performance of poly (lactic acid).
In fact, the limiting factor to the inclusion of glycolic acid in industrial applications is not its mechanical or biodegradable properties, but its high production cost. In particular, the monomer glycolic acid is much more expensive compared to lactic acid monomer. For this reason, research on poly (glycolic acid) has usually been limited to bio-medical applications, where it is less replaceable. For example, it has been used to create absorbable sutures (stitches).
Environmentally-sustainable production of glycolic acid polymers and monomers
Poly (glycolic acid) can be produced from glycolic acid using a method called poly-condensation. While straight-forward, this method is not the most efficient and offers relatively low yields. Another option is ring-opening polymerization of glycolide. (Glycolide is the cyclic dimer of glycolic acid.)
How, then, is glycolic acid produced? Glycolic acid can be produced synthetically from formaldehyde and carbon monoxide with an acid catalyst. This is the most common approach because it is efficient and cost-effective. Another method can be used to make bio-based glycolic acid. Remember, although we differentiate it by calling it “bio-based,” this glycolic acid is chemically identical to synthetically produced glycolic acid. It is important to differentiate not because the compound is different, but because the footprint is different.
Following up on our discussion of environmental impact, it makes sense that there is interest in developing sustainable production pathways for widely used chemicals, including glycolic acid. Researchers are investigating production of glycolic acid from renewable resources. These processes must be cheap and efficient if they are to compete with the synthetic production of glycolic acid for commercial use. Recent research has demonstrated that glycolic acid can be produced from biological sources, like sugar cane and pineapple, with the help of genetically-engineered yeast strains.
The polymerization of glycolic acid produces a chemical that is just as useful as its monomer. As the simplest alpha-hydroxy acid, glycolic acid has widespread use in the food, cosmetics, and household products. Meanwhile, its polymer can be used to create bio-plastics that will eventually replace traditional single-use plastics. While traditional plastics accumulate in our oceans and slowly break down into harmful microplastics, poly (glycolic acid) is able to degrade quickly in the natural environment. Since it is more expensive than other bio-plastics like poly (lactic acid), it can also be used as a co-polymer in order to increase biodegradability. Furthermore, although glycolic acid is usually produced synthetically, new methods demonstrate that it can be made from renewable resources using genetically-modified yeast.
Jem, K. Jim, and Bowen Tan. “The development and challenges of poly (lactic acid) and poly (glycolic acid).” Advanced Industrial and Engineering Polymer Research 3.2 (2020): 60-70.
Salusjärvi, Laura, et al. “Biotechnological production of glycolic acid and ethylene glycol: current state and perspectives.” Applied microbiology and biotechnology 103 (2019): 2525-2535.< Back