You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Condensation polymerisation is a process in which monomers join together with the loss of a small molecule -- usually water -- for each new bond formed. This is fundamentally different from addition polymerisation, where monomers simply add together without losing anything. Condensation polymers include some of the most commercially important materials: polyesters and polyamides.
A polyester is formed when a diol (a molecule with two -OH groups) reacts with a dicarboxylic acid (a molecule with two -COOH groups). Each ester bond formed releases one molecule of water.
The monomers must be bifunctional -- they must have a reactive functional group at each end of the molecule so that the chain can continue growing in both directions.
Example: PET (polyethylene terephthalate) / Terylene
Monomers:
The -OH from the diol reacts with the -COOH from the dicarboxylic acid to form an ester bond (-COO-) and release water. This repeats at both ends of each monomer, building a long chain.
PET is one of the most widely used plastics in the world -- it is used for drinks bottles, food packaging, and synthetic fibres (marketed as Terylene or Dacron in textiles).
PLA can be formed from lactic acid (2-hydroxypropanoic acid), which has both an -OH group and a -COOH group on the same molecule. This means a single monomer can form a polyester through condensation with itself -- each molecule reacts with the next, forming ester bonds and releasing water.
PLA is important as a biodegradable plastic derived from renewable resources (corn starch, sugar cane). It is used in compostable packaging, 3D printing filaments, and medical sutures.
A polyamide is formed when a diamine (a molecule with two -NH2 groups) reacts with a dicarboxylic acid. Each amide bond formed releases one molecule of water.
Example: Nylon 6,6
Monomers:
The -NH2 from the diamine reacts with the -COOH from the dicarboxylic acid to form an amide bond (-CONH-) and release water. The "6,6" in the name refers to the six carbon atoms in each monomer.
Nylon 6,6 is used in carpets, ropes, clothing, and engineering plastics. It is strong, flexible, and resistant to abrasion.
Example: Nylon 6
Nylon 6 is formed from a single monomer -- 6-aminohexanoic acid (H2N(CH2)5COOH) -- which has both an -NH2 group and a -COOH group. Like PLA, a single bifunctional monomer can polymerise with itself.
Proteins are natural polyamides -- they are formed by condensation polymerisation of amino acids, with peptide bonds (which are amide bonds) linking the monomers. The same chemistry that produces nylon in a factory produces proteins in living cells.
Silk and wool are natural polyamide fibres -- they are proteins composed of amino acid monomers linked by peptide bonds.
| Feature | Addition Polymerisation | Condensation Polymerisation |
|---|---|---|
| Monomers | Contain C=C double bonds | Contain two functional groups (bifunctional) |
| Small molecule lost | None | Water (or HCl in some cases) |
| Bond formed in polymer | C-C single bonds | Ester (-COO-) or amide (-CONH-) bonds |
| Repeat unit vs monomer | Same empirical formula | Different (lost atoms in water) |
| Atom economy | 100% | Less than 100% (water is a by-product) |
| Biodegradable? | Generally no | Generally yes (hydrolysable bonds) |
| Backbone structure | Non-polar C-C chain | Polar ester/amide bonds in chain |
| Examples | Polyethene, PVC, polystyrene, PTFE | PET, nylon 6,6, proteins, PLA |
A critical distinction: in addition polymerisation, the repeat unit has the same molecular formula as the monomer (just with the double bond opened). In condensation polymerisation, the repeat unit has a different molecular formula from the monomers because atoms have been lost as water.
flowchart TD
A[Polymers] --> B[Addition Polymers]
A --> C[Condensation Polymers]
B --> D[Polyethene<br>Monomers: Ethene<br>Bond: C-C<br>Biodegradable: No]
B --> E[PVC<br>Monomers: Chloroethene<br>Bond: C-C<br>Biodegradable: No]
C --> F[Polyesters]
C --> G[Polyamides]
F --> H[PET<br>Monomers: Diol + Diacid<br>Bond: Ester -COO-<br>Biodegradable: Yes]
F --> I[PLA<br>Monomer: Lactic acid<br>Bond: Ester -COO-<br>Biodegradable: Yes]
G --> J[Nylon 6,6<br>Monomers: Diamine + Diacid<br>Bond: Amide -CONH-<br>Biodegradable: Yes]
G --> K[Proteins<br>Monomers: Amino acids<br>Bond: Peptide -CONH-<br>Biodegradable: Yes]
A common exam skill is identifying the monomers from the structure of a condensation polymer. This is essentially "reverse engineering" the condensation reaction.
Given the repeat unit: -NH(CH2)6NHCO(CH2)4CO-
The environmental impact of polymers is an important topic:
Addition polymers (like polyethene and PVC) are generally non-biodegradable. They have strong, non-polar C-C backbones that are resistant to attack by water and biological organisms. They persist in the environment for hundreds of years.
Condensation polymers are biodegradable because their ester or amide bonds can be broken by hydrolysis. Water (assisted by acids, bases, or enzymes) can break the links between monomers, eventually decomposing the polymer into small molecules that can be metabolised by organisms.
| Bond Type | Polarity | Susceptible to Water? | Enzyme Attack? |
|---|---|---|---|
| C-C (addition polymer backbone) | Non-polar | No -- water cannot attack a non-polar bond | No suitable enzymes evolved |
| Ester -COO- | Polar (delta+ on C) | Yes -- water nucleophile attacks delta+ C | Esterases/lipases |
| Amide -CONH- | Polar (delta+ on C) | Yes -- water nucleophile attacks delta+ C | Proteases/amidases |
This biodegradability is one reason why research into condensation polymers like PLA (polylactic acid, derived from plant sugars) is so active -- they offer an alternative to persistent addition polymers.
Condensation polymers can be hydrolysed back to their monomers:
The ester bonds are broken by heating with dilute acid, regenerating the diol and dicarboxylic acid.
Heating with NaOH produces the diol and the sodium salt of the dicarboxylic acid. This is irreversible (the salt formation drives the reaction to completion).
Heating with dilute acid breaks the amide bonds, producing the diamine salt and the dicarboxylic acid. With NaOH, the products are the free diamine and the sodium salt of the dicarboxylic acid.
In nature, enzymes catalyse the hydrolysis of natural condensation polymers (proteins, starch) under mild biological conditions.
Claiming polyethene is a condensation polymer. Polyethene is formed by addition polymerisation of ethene. No small molecule is lost. It has a C-C backbone, not ester or amide bonds.
Drawing the repeat unit with the wrong formula. In condensation polymers, the repeat unit is NOT the same as the monomer because water has been lost. Remember to remove H2O from each bond formed.
Forgetting to restore -OH and -H when identifying monomers. When you break an ester or amide bond to find monomers, you must add water back across the bond. Do not just split the repeat unit without adding the atoms.
Confusing nylon 6,6 with nylon 6. Nylon 6,6 uses two different monomers (diamine + diacid), each with 6 carbons. Nylon 6 uses one monomer (aminohexanoic acid) with 6 carbons.
Stating that all condensation polymers need two different monomers. PLA and nylon 6 each use a single monomer that has two different functional groups on the same molecule.
Condensation polymerisation produces polyesters (from diols + dicarboxylic acids) and polyamides (from diamines + dicarboxylic acids), with loss of water for each bond formed. Unlike addition polymers, condensation polymers are biodegradable because their ester and amide bonds are susceptible to hydrolysis. Understanding how to identify monomers from polymer structures and comparing the two types of polymerisation are key exam skills.
Edexcel 9CH0 specification, Topic 18 — Organic nitrogen compounds and condensation polymers, sub-strands 18.8–18.10 covers the formation of polyesters by condensation of dicarboxylic acids (or diacyl chlorides) with diols, with examples including PET (poly(ethylene terephthalate), formed from terephthalic acid + ethane-1,2-diol) and PLA (poly(lactic acid), from lactic acid only — a hydroxy-acid that self-polymerises); the formation of polyamides by condensation of dicarboxylic acids (or diacyl chlorides) with diamines, with examples including nylon-6,6 (hexanedioic acid + 1,6-diaminohexane) and Kevlar (terephthalic acid + 1,4-phenylenediamine); and the biodegradability of condensation polymers compared to addition polymers (poly-alkenes), with the explicit reasoning that ester and amide linkages are hydrolysable while C–C single bonds in addition polymers are not (refer to the official specification document for exact wording). Examined directly on Paper 2; synoptic on Paper 3 with sustainability/green chemistry contexts.
Question (8 marks):
(a) Draw the repeat unit of nylon-6,6, formed from hexanedioic acid (HOOC(CH2)4COOH) and 1,6-diaminohexane (H2N(CH2)6NH2). Show clearly the amide linkages. (3)
(b) Explain why nylon-6,6 is biodegradable while poly(ethene) is not. Refer to the bonds in each polymer. (3)
(c) Suggest, with justification, why diacyl chlorides are sometimes preferred over dicarboxylic acids for polymerisation in the laboratory. (2)
Solution with mark scheme:
(a) Repeat unit of nylon-6,6:
[-NH-(CH2)6-NH-CO-(CH2)4-CO-]n
Drawn with two amide bonds (-CO-NH- and -NH-CO-) per repeat, the C4 chain from hexanedioic acid (-CO-(CH2)4-CO-), and the C6 chain from 1,6-diaminohexane (-NH-(CH2)6-NH-). The hyphens at the ends indicate continuation.
M1 — both amide linkages drawn correctly.
M1 — correct chain lengths for the C4 (from acid) and C6 (from diamine) segments.
A1 — repeat unit enclosed with terminal hyphens or square brackets and "n" subscript.
(b) Step 1 — identify the linkage type.
Nylon-6,6 contains amide linkages (-CO-NH-) along the polymer backbone. Poly(ethene) contains only C–C single bonds along its backbone (formed by addition polymerisation of ethene).
M1 — linkage types correctly identified for each polymer.
Step 2 — explain hydrolysability.
Amide bonds can be hydrolysed by water (slow at neutral pH) or much faster by acid or base catalysis or by enzymes (proteases, microbial degradation). The C=O carbon is δ+ and accessible to nucleophilic attack by water.
C–C single bonds are non-polar, very strong (≈ 348 kJ mol⁻¹) and not susceptible to hydrolysis under any environmentally accessible conditions.
M1 — amide hydrolysability vs C–C inertness contrast.
A1 — link to biodegradability: micro-organisms in soil/landfill produce enzymes that hydrolyse amide (and ester) bonds, so condensation polymers degrade over years; addition polymers persist for centuries.
(c) Step 1 — kinetic argument.
Diacyl chlorides (e.g. CH2(CH2)4(COCl)2) react far more rapidly with diamines than dicarboxylic acids do, because acyl chlorides are much more electrophilic at the carbonyl carbon (chloride is a good leaving group; the C=O carbon is highly δ+).
M1 — reactivity argument.
Step 2 — equilibrium/yield argument.
The reaction is irreversible with diacyl chlorides (HCl byproduct, irreversible elimination) but reversible with dicarboxylic acids (water byproduct, equilibrium). The diacyl chloride route therefore gives high molecular weight polymer at moderate temperature without needing to drive off water.
A1 — irreversibility / yield link.
Total: 8 marks (M5 A3).
Question (8 marks): PET (poly(ethylene terephthalate)) is manufactured from terephthalic acid (1,4-benzenedicarboxylic acid) and ethane-1,2-diol (ethylene glycol).
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.