9701 Chemistry · Topics 20 & 34 · AS + A Level
Polymerisation Cheat Sheet — A Level Chemistry 9701
Polymer questions are usually ‘draw the repeat unit’ or ‘deduce the monomers’, and both are procedural once you know what to look for. This sheet covers addition polymerisation from alkenes, condensation polymerisation to polyesters and polyamides, the trick for cutting a chain at the right bond to find the monomers, and the environmental chemistry of polymer disposal.
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What’s on this cheat sheet
Polymerisation
01 · The two types
| Addition | Condensation | |
|---|---|---|
| monomer | C=C | two functional groups |
| small molecule lost | none | Hâ‚‚O or HCl |
| backbone | C–C only | contains O or N |
| hydrolysable | no | yes |
02 · Addition polymers
The π bond opens and the monomers join end to end: n CH₂=CHR → –(CH₂–CHR)n–.
Common ones: poly(ethene) for bags and bottles, poly(propene) for crates and rope, poly(chloroethene) or PVC for pipes and cable, poly(phenylethene) for packaging, PTFE for non‑stick coatings.
03 · Drawing a repeat unit
1 · Open the C=C into a single bond.
2 · Draw bonds extending through the brackets on both sides.
3 · Write n outside the bracket.
From polymer to monomer
Find the repeating pattern, take one unit, and put the double bond back between the two backbone carbons.
04 · Polyesters A2
Made from a diol + a dicarboxylic acid (or its diacyl chloride), losing water at each link. The link is –COO–.
Terylene (PET) comes from ethane‑1,2‑diol and benzene‑1,4‑dicarboxylic acid. A single monomer carrying both an –OH and a –COOH can also self‑polymerise.
05 · Polyamides A2
Made from a diamine + a dicarboxylic acid, losing water. The link is the amide –CONH–.
Nylon‑6,6 comes from 1,6‑diaminohexane and hexanedioic acid; Kevlar uses aromatic monomers, and its flat, hydrogen‑bonded, aligned chains make it exceptionally strong for its mass. Proteins are natural polyamides.
06 · Identifying monomers A2
Ester link –CO–O– → break between C and O, add –OH to each side.
Amide link –CO–NH– → break between C and N, give the acid an –OH and the amine an –H.
07 · Hydrolysis of condensation polymers
The ester and amide links can be broken by acid or alkali, so these polymers are degradable — the basis of both biodegradable plastics and chemical recycling back to monomers.
Addition polymers have an unreactive C–C backbone with no polar bond for a nucleophile to attack, so they persist in the environment for centuries.
08 · Properties from structure
Longer, unbranched chains pack closely, so London forces are stronger and the polymer is denser, stiffer and higher melting — high‑density poly(ethene) against the low‑density branched form.
Polar links allow hydrogen bonding between chains, which is why nylon and Kevlar are strong fibres while poly(ethene) is not.
09 · Disposal
Landfill — cheap but wasteful of land and of the oil the plastic came from. Incineration — recovers energy, but PVC releases HCl and incomplete burning releases CO and dioxins, so the gases must be scrubbed.
Recycling — mechanical (sort, melt, remould) or chemical (break back to monomers or feedstock). Sorting mixed plastics is the practical difficulty.
10 · Worked example — deduce the monomers
–NH(CH₂)₆NHCO(CH₂)₄CO–
Two amide links → polyamide → cut each –CO–NH–
Amine fragment: H₂N(CH₂)₆NH₂
Acid fragment: HOOC(CHâ‚‚)â‚„COOH
→ 1,6‑diaminohexane and hexanedioic acid: nylon‑6,6.
11 · Worked example — polyester
Join –OH to –COOH, losing H₂O at each end:
–O–CH₂CH₂–O–CO–C₆H₄–CO–
Two water molecules are lost per repeat unit, and bonds must extend through the brackets on both sides.
12 · Polymers to know
| Polymer | Type | Monomer |
|---|---|---|
| poly(ethene) | addition | ethene |
| PVC | addition | chloroethene |
| PTFE | addition | tetrafluoroethene |
| Terylene | polyester | diol + dioic acid |
| nylon‑6,6 | polyamide | diamine + dioic acid |
| protein | polyamide | amino acids |
13 · Hydrolysis equations
→ the diol + the sodium salt of the diacid
Polyamide + dilute HCl
→ the dioic acid + the ammonium salt of the diamine
Acid and alkali give different forms of the same two fragments, so read the conditions before answering.
Marks lost here
— Leaving the double bond in an addition repeat unit, or forgetting the bonds through the brackets.
— Cutting a condensation polymer in the wrong place, so the fragments come out missing –OH or –H.
— Saying addition polymerisation releases water.
— Describing addition polymers as biodegradable.
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Polymerisation — Frequently Asked Questions
How do you draw the repeat unit of an addition polymer?
Open the C=C double bond, draw the two carbons joined by a single bond with all four substituents in place, and put extended bonds through the brackets on each side with n outside the bracket.
How do you deduce the monomers of a condensation polymer?
Cut the chain at every ester or amide linkage. Add –OH to each carbonyl carbon and –H to each oxygen or nitrogen. A polyester gives a diol and a dicarboxylic acid; a polyamide gives a diamine and a dicarboxylic acid.
Why are addition polymers difficult to dispose of?
The chain is a saturated hydrocarbon backbone with only strong, non-polar C–C and C–H bonds and no sites for enzyme or nucleophile attack, so they are chemically inert and non-biodegradable. Condensation polymers contain hydrolysable ester or amide links and break down far more readily.
