The Biodegradable Polyesters

Most of the plastics on this Atlas were praised for their durability, for shrugging off water, weather, and time. The biodegradable polyesters were bred for the opposite virtue: they are built to fall apart, on purpose, when their job is done. The trick is beautifully simple. The ester bond that defines the whole polyester dynasty has one Achilles’ heel: it can be split by water (hydrolysis) and by the enzymes that microbes carry. In everyday polyesters that’s a liability to be managed. In this branch of the family, it’s the entire point.

The family lines up like a menu of controlled disappearances. Polylactic acid, brewed from fermented corn or cane sugar, composts under the right industrial conditions and shows up as cups, cutlery, and 3D-printer filament. The polyhydroxyalkanoates (PHAs, including PHB) are even more remarkable: they’re made by bacteria, as the microbes’ own energy stores, and they biodegrade in soil and seawater alike. Polycaprolactone, polyglycolic acid, and the succinate and adipate copolymers round out the set, several of them tuned so precisely that surgeons use them as dissolving sutures and drug-release capsules that simply vanish after delivering their dose. Across the family, chemists dial the degradation rate up or down by choosing which units to string between the esters, designing not just the material, but its lifespan.

These are the polymers wrestling most directly with the great unfinished problem of the plastics age: what happens after we’re done using them. They answer it by building the ending into the beginning.