Why do plastics last so long?

Plastics last so long because they are made of very long chains of carbon atoms that few natural processes know how to break. Plastics are synthetic or semisynthetic materials composed primarily of polymers. The vast majority of those polymers are chains of carbon atoms, and each chain comprises several thousand repeating units formed from monomers, like a necklace of thousands of identical beads.

The weak point of a chain is a particular group of atoms in it. In general, how easily a polymer degrades correlates with the presence of functional groups such as esters and amides. The polyolefins, which lack such groups, degrade very slowly, especially polyethylene. PET, a polyester with ester groups, degrades relatively quickly by several pathways. In nature, the most important processes are hydrolysis and photo-oxidation, but they find little to grip on in a plain carbon chain. Microbes capable of degrading plastics are known, but they remain curiosities.

That same stability is why plastics are useful. They are not acutely toxic, in large part because their large molecular weight makes them insoluble or indigestible. But traits such as durability and resistance to biodegradation, desirable in many applications, have led to significant environmental issues.

The scale is what makes it matter. Between 1950 and 2017, an estimated 9.2 billion metric tons of plastic were made, more than half of it since 2004. Of all plastic discarded so far, some 14% has been incinerated and less than 10% has been recycled. In other words, the chemistry that makes plastic last is the same chemistry that makes its waste so hard to get rid of.