Some Important Addition Polymers

Polythene — two very different materials from one monomer

Polythenes are long, linear or slightly branched chains built from ethene. Because they soften on heating and re-harden on cooling, they are thermoplastics. Depending on how the polymerisation is run, you get two quite different products:

• Low-density polythene (LDP) — made by polymerising ethene under very high pressure (1000–2000 atm) at 350–570 K, with a trace of dioxygen or a peroxide initiator, via the free-radical route (with H-atom abstraction along the way). This produces a highly branched structure — a long, wavy main chain with short zig-zag side branches. It is chemically inert, tough yet flexible, and a poor conductor of electricity — which is exactly why it is used to insulate electrical wires, and also to make squeeze bottles, toys, and flexible pipes.
• High-density polythene (HDP) — made in a hydrocarbon solvent using a Ziegler–Natta catalyst (triethylaluminium + titanium tetrachloride) at a milder 333–343 K and 6–7 atm. This gives essentially linear, unbranched chains that pack closely together, so the material is denser — hence high-density. These linear polymers are also chemically inert but tougher and harder than LDP, and are used for buckets, dustbins, bottles, and pipes.

Polytetrafluoroethene (Teflon)

Made by heating tetrafluoroethene with a free-radical or persulphate catalyst under high pressure:

$n\,CF_2{=}CF_2 \xrightarrow{\text{catalyst, high pressure}} {-}(CF_2{-}CF_2){-}_n \;(\text{Teflon})$ …