Ethene to Poly(ethene) — study notes

A plastic bottle and a strand of nylon rope don't look related, yet both are built the same way: thousands of small molecules snapped end to end into one giant chain. Understanding how that chain forms is the key to almost every polymer question on the O-Level paper.

How a Chain Actually Forms

Start with a small molecule called a monomer — the single repeating unit a chain is built from. For poly(ethene), that monomer is ethene, CH₂=CH₂. The whole trick sits in that double bond between the two carbons: given the right conditions — raised pressure plus a suitable catalyst — one of the two bonds joining the carbons breaks open, leaving each carbon with a spare connection. Those spare connections grab the next ethene unit on either side, so the molecules link into an ever-lengthening backbone. Because nothing from the original monomers is discarded along the way, no by-product is released — this is what marks it out as addition polymerisation. Since you can't draw a chain thousands of carbons long, chemists write the repeat unit inside square brackets with a small n outside, and the bonds either side are drawn crossing straight through the brackets to show the pattern continues in both directions.

Worked Example — Finding the Monomer from a Repeating Unit

A polymer's repeating unit is given as —[CH₂—CHF]ₙ—. What monomer was used to make it?

1. Identify the two backbone carbons inside the brackets: CH₂ and CHF.
2. Reverse the addition process by placing a double bond back between those two carbons, since that's the bond that opened up when the chain first formed.
3. Remove the brackets and the n, leaving a single small molecule: CH₂=CHF, fluoroethene.

Notice the F atom simply rides along unchanged on the second carbon — that bond was untouched throughout, so it plays no role in the joining step at all.

That's the addition side of the story; O-Level papers also test a second route — condensation polymerisation, which builds nylon and Terylene by releasing water at every join instead. The full lesson below walks through both linkages, the amide-versus-ester distinction, a listen-along audio walkthrough and the practice worksheet.