Researchers at the University of Osaka have discovered that adjusting the size of specific ring-shaped molecules added to biodegradable plastics can influence both their strength and how quickly they break down. Their findings, published in ACS Sustainable Chemistry & Engineering, focused on a type of biodegradable polyester called polycaprolactone (PCL), which has potential uses in eco-friendly materials. While methods like blending or combining PCL with other chemicals can improve its strength, they often complicate how the material degrades, making it difficult to achieve both durability and controlled breakdown. To solve this, the researchers used a novel approach involving "movable crosslinks"—flexible chemical bonds that reinforce the material but can also be adjusted to control its degradation. They used a byproduct of industrial chemical production called cyclic poly(phenylene sulfide) (c[n]PS), and isolated three different ring sizes: c[5]PS, c[7]PS, and c[9]PS. These were added to PCL using a solvent-free process that created special crosslinks known as pseudorotaxanes, which can slide along the polymer chains. This allows the material to absorb and distribute stress more effectively, improving its toughness. The results showed that even at a small concentration of 0.5 weight percent, the c[7]PS ring significantly increased PCL’s toughness without compromising its flexibility or ability to be reprocessed. The size of the rings also affected how quickly the material degraded when exposed to enzymes. Smaller rings (c[5]PS) slightly sped up the breakdown, medium-sized rings (c[7]PS) slowed it, and the largest (c[9]PS) caused the fastest degradation, leaving no trace after 48 hours. In contrast, traditional PCL without these crosslinks didn’t show such clear differences based on ring size. This discovery could lead to the development of biodegradable plastics that are strong enough for everyday use but can break down at a rate that matches their intended disposal method. It also repurposes a waste product from industrial processes into a new kind of material, supporting a more circular approach to polymer use. Professor Yoshinori Takashima, a senior researcher on the project, explained that making materials both durable and easily degradable might seem contradictory, but this research shows that both properties can be controlled, allowing plastics to be tailored for specific applications.