eli5: Why do some chemical compounds have “poly” in the name?

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I understand that poly means “many” or something along those lines. But whenever I hear something with poly in the name it feels like it gives literally no additional info other than it has two or more of something. Is it more than a specified number? I’m confused.

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Basically it often means that the chemical is repeated a great many times so it forms a chain like a polymer.

**P**oly **v**inyl **c**hloride = repetition of [vinyl (two carbons in a single bond) chloride (with a chlorine atom attached to one of the carbons] = **PVC**

“Poly” in a chemical name usually mean you’re making a chain of some kind.

For example: ethylene is a C2H4. Polyethylene is chains of that, often separated by carbon or nitrogen in a way that allows the chains to branch.

It means it’s arbitrarily many copies of a particular molecule bound together in a regular way. So ‘polyX’ means we have millions of X molecules strung together like a chain of paper people.

While the prefix ‘poly’ means ‘more than one’, in effect it usually means ‘more than any other common prefix’ as well; otherwise we’d just use that common prefix instead. It can also mean that there isn’t an exact amount of units, because then we’d use that exact number instead of the uncertainty of ‘poly’.

In chemistry, a polymer (mer meaning the smallest shape of atoms that is repeated, so a polymer is the generic name for molecules that you are talking about) has an arbitrary amount of repeating units that are all connected. It might be 2, 3, 20, or 20 billion. Many of the most useful chemicals have this format (such as all plastics, many nutrients, and many organic molecules), so we’d often see these chemicals named in studies that affect our daily lives.