You already met addition polymers from alkenes — one monomer, nothing lost. This topic adds condensation polymers, where two functional groups join and a small molecule (usually water) is lost at every link. Those links are ester or amide groups — which means, unlike an inert polyalkene, the chain can be hydrolysed straight back to its monomers.
Condensation polymers
A condensation polymer forms when each monomer has two reactive groups, so the chain can grow from both ends. Every new link expels a small molecule — water (or HCl if an acyl chloride is used). Two families matter:
| Polymer | Monomers | Linkage | Examples & uses |
|---|---|---|---|
| Polyester | dicarboxylic acid + diol | ester link –COO– | Terylene — clothing fibres, bottles |
| Polyamide | dicarboxylic acid + diamine (or amino acids) | amide link –CONH– | nylon 6,6 (fibres, ropes); Kevlar (body armour) |
Join one of each monomer, remove the small molecule at the new link, and put the repeat unit in brackets with a trailing bond at each end (these show the chain continues). You should be able to work in both directions: monomers → repeat unit, and a section of chain → the monomers that made it.
🧪 Exam-style questions
Which compound can form a polymer with the diacyl chloride ClOC(CH2)8COCl?
An equation shows nylon 4,6 forming from five molecules of butane-1,4-diamine and five molecules of hexanedioic acid, releasing y molecules of water and giving a chain of x repeating units. Deduce x and y.
Show answer
x = 5 1 mark
y = 9 1 mark
(Five diamine + five diacid give five repeat units; joining ten monomers into one chain makes nine links, each losing one water.)
Source: AQA A-Level Chemistry past papers.
Forces between chains
A polymer’s strength and melting point come from the intermolecular forces between its chains — and the linkage decides how strong those are:
- Polyamides have N–H and C=O groups, so chains hydrogen-bond to each other (N–H⋯O=C). These are the strongest forces here, giving high melting points and tough fibres — this is why Kevlar and nylon are so strong.
- Polyesters have polar C=O and C–O bonds, so chains attract by permanent dipole–dipole forces — weaker than hydrogen bonds.
- Polyalkenes (poly(ethene), poly(propene)) are non-polar, with only weak van der Waals forces between chains.
🧪 Exam-style questions
Which polymer has hydrogen bonding between its chains?
Source: AQA A-Level Chemistry past papers.
Breaking the links — hydrolysis
Here is the key skill. Because a condensation polymer is held together by ester or amide links, water can break each one — the same hydrolysis you met with simple esters and amides, now applied at every link along the chain. There is one reliable way to do it on paper:
- Find each ester link (–CO–O–) or amide link (–CO–NH–).
- Break the bond between the carbonyl carbon and the O (ester) or N (amide).
- Add –OH to the carbonyl-carbon side — this remakes the –COOH of the carboxylic acid.
- Add –H to the other side — this remakes the –OH of the alcohol (from a polyester) or the –NH2 of the amine (from a polyamide).
Do that at every link and you are back to the monomers: a polyester gives its dicarboxylic acid + diol; a polyamide gives its dicarboxylic acid + diamine.
Hydrolysis can be driven by hot aqueous acid or hot aqueous alkali (NaOH). Under acid, a polyamide gives the diammonium salt and the diacid; under alkali, it gives the diamine and the dicarboxylate salt — but the “break the bond, add OH and H” picture gets you the fragments every time.
A poly(alkene) chain is nothing but non-polar C–C and C–H bonds. There is no Cδ+ for water to attack, so the chain cannot be hydrolysed — which is exactly why poly(ethene) and poly(propene) are inert and non-biodegradable, while polyesters and polyamides break down over time.
Disposal & biodegradability
Because they can be hydrolysed, polyesters and polyamides are biodegradable; polyalkenes are not. Disposal routes each trade off differently: recycling saves raw materials and energy but needs sorting and cleaning; incineration recovers energy but can release toxic gases and CO2; landfill is cheap but uses land and, for polyalkenes, lasts almost indefinitely.
🧪 Exam-style questions
Which type of polymer is not hydrolysed by heating with concentrated aqueous sodium hydroxide?
Which statement concerning nylon-6,6 is correct?
Explain why polyesters are biodegradable but polyalkenes are not.
Show answer
Polyesters contain polar C=O / C–O bonds (the ester link), so they can be attacked by water / nucleophiles and hydrolysed. 1 mark
Polyalkenes have only non-polar C–C bonds, which cannot be hydrolysed / are not attacked. 1 mark
Source: AQA A-Level Chemistry past papers.
Monomers ↔ repeat unit
Most polymer marks come down to reading structures in both directions and hydrolysing cleanly. Given monomers, join one of each and remove the small molecule to get the repeat unit; given a chain, cut it at the links and add back the –OH and –H to recover the monomers. The same “break the bond, add OH and H” move does both the hydrolysis and the monomer-recovery.
- Repeat units without trailing bonds at each end — they must show the chain continues.
- Forgetting the small molecule lost (water, or HCl with an acyl chloride) in a condensation equation.
- Hydrolysing to the wrong fragments — the –OH goes on the carbonyl carbon (making –COOH), the –H on the O or N.
- Drawing hydrogen bonds badly: they run N–H⋯O=C as straight dashed lines, with a lone pair on the oxygen.
Examiner reports note that hydrolysis products are often incomplete — students break the link but forget to add the –OH and –H, leaving fragments that are not the real monomers. Add both every time.
- Addition polymers: from alkenes (C=C), one monomer, nothing lost. Condensation polymers: two functional groups react, a small molecule (H2O or HCl) lost at each link.
- Polyester = diacid + diol (ester link –COO–, e.g. Terylene). Polyamide = diacid + diamine or amino acids (amide link –CONH–, e.g. nylon 6,6, Kevlar).
- Forces between chains: polyamides hydrogen-bond (N–H⋯O=C) → strong, high melting; polyesters have dipole–dipole; polyalkenes only weak van der Waals.
- Hydrolysis: break each ester/amide link, add –OH to the C=O side and –H to the other — polyester → diacid + diol; polyamide → diacid + diamine. Polyalkenes cannot be hydrolysed (non-polar C–C).
- Disposal: polyesters/polyamides are biodegradable (hydrolysable); polyalkenes are inert. Options: recycle, incinerate (energy, but pollutants), landfill (space, non-degrading).