A Level Chemistry: Condensation Polymerisation — Revision Notes

Condensed for the final weeks. For the full explanation, use the Condensation Polymerisation study guide.

The reaction

Monomers join with the elimination of a small molecule — usually water, or HCl if an acyl chloride is used.

Each monomer needs two functional groups, one at each end, so the chain can extend in both directions. A molecule with only one reactive group would terminate the chain.

Two routes:

1. Two different monomers, each with two of the same group — a diacid plus a diol, or a diacid plus a diamine.
2. One monomer with two different groups — such as an amino acid, or a hydroxy acid.

Atom economy is below 100%, because the eliminated water is a second product. This is the key contrast with addition polymerisation.

Polyesters

dicarboxylic acid + diol  ->  polyester + water

The linkage is the ester group –COO–. Benzene-1,4-dicarboxylic acid and ethane-1,2-diol give this type of polyester (commercially known as Terylene, or PET) — the syllabus works from given monomers or a given repeat unit, not from recall of commercial polymer names, so the name is background context rather than something to memorise.

Polyamides

dicarboxylic acid + diamine  ->  polyamide + water

The linkage is the amide group –CONH–.

• Hexanedioic acid + 1,6-diaminohexane gives this type of polyamide, commercially known as Nylon-6,6 (the numbers give the carbons in each monomer).
• Benzene-1,4-dicarboxylic acid + 1,4-diaminobenzene gives this type of polyamide, commercially known as Kevlar.

As above, the commercial names are background, not required recall — 9701 gives the monomers or repeat unit and asks you to deduce the polymer’s structure and properties from those, not to name a specific commercial product.

Kevlar’s strength comes from structure, not chemistry: the rigid aromatic rings hold the chains straight and allow them to align, and extensive hydrogen bonding between the amide groups on adjacent chains locks them together. Its strength-to-weight ratio exceeds steel’s.

Polyamides are stronger than polyesters because amide groups form hydrogen bonds between chains, whereas ester groups can only manage permanent dipole–dipole attraction.

Worked example: repeat unit

Deduce the repeat unit from ethane-1,2-diol, HOCH₂CH₂OH, and benzene-1,4-dicarboxylic acid, HOOCC₆H₄COOH.

Each ester linkage forms between one –OH of the diol and one –COOH of the diacid, losing H₂O each time:

–OCH₂CH₂OOCC₆H₄CO–

This is the repeat unit of PET, the polyester used in drinks bottles. Running the process in reverse – from a given polymer section back to its monomers – means spotting the repeating ester (–COO–) or amide (–CONH–) linkage, then adding back an H₂O (or HCl) at each one broken to regenerate the diol/diamine and diacid.

Predicting the type of polymerisation

Given an unfamiliar monomer or pair, the functional groups present decide which polymerisation type applies:

• A C=C double bond, with no other reactive group needed → addition polymerisation, no small molecule lost.
• Two matching reactive groups per monomer – –OH/–COOH or –COCl pairs, or –NH₂/–COOH or –COCl pairs → condensation polymerisation, losing H₂O or HCl at each linkage.

The same logic runs in reverse for a polymer section: an unbroken, saturated carbon backbone signals addition polymerisation; repeating ester or amide linkages signal condensation, and which one tells you whether the monomers were diol/diacid-type or diamine/diacid-type.

Photodegradable polymers are a separate mechanism from hydrolysis-based biodegradability: certain bonds or additives absorb UV radiation and break, fragmenting the material – but fragmentation isn’t necessarily full breakdown into harmless small molecules, and a polymer can be biodegradable, photodegradable, both, or neither. Don’t confuse the two mechanisms when a question names one specifically.

Hydrolysis and biodegradability

Condensation polymers can be hydrolysed — the ester or amide link is broken by water under acid or alkaline conditions, regenerating the monomers.

This is why they are biodegradable and addition polymers are not. The polar C–O and C–N bonds in the linkage can be attacked by water and by enzymes; the non-polar C–C backbone of an addition polymer cannot. Stating that comparison is the highest-value point in the topic.

Condition	Polyester gives	Polyamide gives
Acid hydrolysis	Carboxylic acid + alcohol	Carboxylic acid + ammonium salt
Alkaline hydrolysis	Carboxylate salt + alcohol	Carboxylate salt + amine

Note the difference: under acid, the amine is protonated to a salt; under alkali, the acid is deprotonated to a salt. Getting the right product for the right conditions is a routine mark.

Comparison

	Addition	Condensation
Monomer	Alkene (C=C)	Two functional groups
Small molecule eliminated	None	Water or HCl
Atom economy	100%	Below 100%
Backbone	C–C, non-polar	Contains polar C–O or C–N
Biodegradable	No	Yes

Exam traps

• Forgetting to show the eliminated water.
• Drawing monomers with only one functional group.
• Saying polyesters hydrogen bond between chains.
• Giving the wrong hydrolysis product for the conditions specified.
• Explaining Kevlar’s strength without hydrogen bonding and chain alignment.
• Claiming condensation polymerisation has 100% atom economy.

Self-test

1. Why must each monomer have two functional groups?
2. Why is the atom economy below 100%?
3. Why are polyamides stronger than polyesters?
4. Why are condensation polymers biodegradable when addition polymers are not?
5. What does alkaline hydrolysis of a polyamide produce?

Answers: 1. So the chain can extend at both ends; a monomer with a single reactive group would terminate the chain. 2. A small molecule such as water is eliminated as a second product, so not all the reactant mass ends up in the polymer. 3. Amide groups form hydrogen bonds between adjacent chains, whereas ester groups can only form weaker permanent dipole–dipole attractions. 4. The linkage contains polar C–O or C–N bonds that can be hydrolysed by water and attacked by enzymes, whereas the non-polar C–C backbone of an addition polymer cannot. 5. A carboxylate salt and a free amine.