The use of a polymer depends on its properties, and the properties depend on the atoms in its repeating unit. Engineers choose the polymer whose properties match the job.
- Poly(ethene) is light, flexible, cheap and waterproof, and has a low density. It is used for carrier bags, food wrap and squeezy bottles.
- Poly(propene) is tough and stiff, and bends many times without breaking. It is used for crates, ropes and food tubs.
- PVC, poly(chloroethene), is strong, rigid and waterproof, and it is an electrical insulator, so it does not conduct electricity. It is used for window frames and drain pipes. With a plasticiser added it becomes flexible, and it covers electric cables.
- PTFE, poly(tetrafluoroethene), is very unreactive and has a very low friction, slippery surface. It is used as a non-stick coating on frying pans.
You can also reason backwards. A job that needs a slippery, unreactive surface calls for PTFE, and a job that needs a light, flexible, cheap film calls for poly(ethene).