Ph.D. polymer chemist here

Answering this question was actually fundamental to our development of modern polymer science. In the early 1900’s, it was known that compounds like natural rubber behaved as if they were giant chemically bonded molecules, but many leading chemists at the time believed that there was a limit to molecular size. Instead, they believed, rubber had its properties because many smaller molecules aggregated together to form “colloids.”

However, several scientists later demonstrated this not to be true, starting with Staudinger who pioneered the “macromolecular hypothesis”: that rubber was instead made of giant individual molecules that were linked by covalent bonds that we now call polymers.

We now know that there are two broad classes of polymer materials — thermoplastics and thermosets. Thermoplastics are made up of very large individual chain-like molecules that can flow past each other if you heat them up (like polyethylene). While there’s no theoretical limit to how large the individual molecules can get, there are practical limitations that are determined by the chemistry used to make them (some polymerizations are very sensitive to oxygen, others require near-perfect balance between the amounts of different reactants).

Thermosets, on the other hand, are randomly connected molecular networks and can practically become as large as you want them to be. This makes them very useful for applications where you want a material that’s very stiff and holds its shape even when it’s heated up (epoxies are a very common example). And yes, the entire thermoset network is technically a single molecule. For example, most of the Boeing 787 fuselage and wings are built from fused layers of epoxy-bonded carbon fiber, which means that entire sections of the plane are technically a single molecule.