Molecular Tug-of-War to Address Plastic Pollution

Korea University has developed a novel polymeric material that is decomposed by mechanical force but that also has excellent thermal stability. The results of the study were published in Angewandte Chemie (IF = 15.336), a globally acclaimed chemistry journal published by the German Chemical Society, on September 9 (German local time; September 10 Korean local time).
- Author : 정상민 (고려대학교, 제1저자), 윤효재 (고려대학교, 교신저자) 총 2명
- Title of Paper : Mechanical Force for the Transformation of Aziridine into Imine
- Title of Journal: Angewandte Chemie
  (2021. 9. 9. online published; https://onlinelibrary.wiley.com/doi/10.1002/anie.202109358)

With the development of petrochemistry, materials manufactured from synthetic polymers have enabled humankind to enjoy a more convenient life. Representative examples of synthetic polymers include the synthetic fibers used in clothes and the plastics used for disposable containers. As the technology has advanced, polymeric materials that are light, have better durability, and have various functions have been developed, and have been employed in various products.

The downsides of convenient synthetic polymers are the social and environmental problems they cause. According to one study, a PET plastic bottle buried in the ground requires 450 years to fully decompose. The images of the sea creatures killed by undecomposed plastic straws have reminded people of the problems underlying the convenience of using synthetic polymers. Waste synthetic polymers are mostly ground into microplastics, which are not completely decomposed. These microplastics penetrate deep into the global ecosystem, even harming humans through the food chain. A recent report showed that an individual person consumes the weight of a credit card of microplastics per week.

To address these problems, scientists are making efforts to develop novel polymeric materials that can decompose within a relatively short time or that are recoverable after use. They are attempting this through the development of biodegradable polymers or enzymes that can decompose polymers.

Professor Yoon Hyo-jae’s group developed a novel polymeric material that can be decomposed through the use of mechanical force. The strong carbon-carbon covalent bonds of most polymers cannot be easily broken by mechanical force. Completely breaking them in order to decompose the polymers fully requires high-energy chemical reactions, which may cause additional environmental pollution. Professor Yoon Hyo-jae’s group decided to design a polymer structure that includes a ring strain of extra energy in order to weaken the carbon-carbon bonds. As shown in Figure 1, aziridine, the smallest heterocyclic compound that includes a nitrogen atom, was introduced to the polymeric material. The team was the first to show that when a phthalimido functional group is introduced as a substituent of the nitrogen atom, the polymer that includes aziridine undergoes a ring opening reaction in response to mechanical force, as it is converted into an imine structure. This mechanism is like a molecular tug-of-war in which the ‘main chain,’ serving as the backbone of the polymer, is pulled in both directions in order to change the molecular structure. Imine is a well-known functional group, which seems to be strong due to the double bonds between the nitrogen atom and the carbon atom, but easily undergoes hydrolysis in the presence of water. The experimental results showed that when the polymer that includes aziridine had mechanical force applied to it and was then exposed to moisture, it decomposed at room temperature and under normal pressure, as the bonds were easily broken.

Another stunning finding from this study was that the polymeric material that responded to mechanical force did not respond to heat. Most existing materials that are responsive to mechanical force are also responsive to heat, and such an undesired response, which is commonly generated in a multitude of circumstances, often makes the application of force problematic.

Mechanical force to polybutadiene, which is widely used in synthetic rubber. As shown in Figure 2, phthalimidoaziridine was introduced to the double bond of the polymer, and the resulting polymer was subject to mechanical force and exposed to a small amount of water. Like the previous experimental results, the polymer decomposed. Considering that various double bond-based polymeric materials are used routinely, this method may be employed as an environment-friendly polymer decomposition technology. Furthermore, the functional groups generated at the terminals of the polymers through their decomposition may be used to develop new materials.

The present study was supported by the Basic Research Programs and the Academic Research Support Program of the National Research Foundation of Korea (NRF).