Addition polymers form when unsaturated monomers join by opening their double bonds, producing no by-products. Condensation polymers form when monomers with two functional groups react, releasing a small molecule — usually water — at each step. Poly(ethene) is the most common addition polymer; nylon and polyesters are condensation polymers.
What is a polymer?
A polymer is a very large molecule built from many small, repeating units called monomers. The process of joining monomers together is called polymerisation. Polymers have very high relative formula masses — often millions — and their properties depend on:
- The identity of the monomer
- The length of the polymer chain
- Whether chains are cross-linked or branched
There are two fundamentally different types of polymerisation at GCSE: addition and condensation.
How does addition polymerisation work?
Addition polymerisation occurs when monomer molecules containing a carbon–carbon double bond (C=C) join together. The double bond opens (one bond breaks), and each monomer joins to the next to form a long chain. No atoms are lost — the polymer has exactly the same atoms as all the monomers put together:
n × (monomer) → polymer chain
The general pattern: the C=C in each monomer becomes a C–C single bond as the chain forms.
Key features:
- All monomers must contain at least one C=C double bond (they are unsaturated)
- Only one product is formed (the polymer itself)
- No small molecule is released
The name of an addition polymer is written as poly(monomer name):
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene (CH₂=CH₂) | Poly(ethene) [LDPE/HDPE] | Plastic bags, bottles, pipes |
| Propene (CH₂=CHCH₃) | Poly(propene) | Food containers, rope, carpet |
| Chloroethene (CH₂=CHCl) | Poly(chloroethene) / PVC | Window frames, pipes, electrical cable insulation |
| Styrene (CH₂=CHC₆H₅) | Poly(styrene) | Packaging, cups, insulation |
| Tetrafluoroethene (CF₂=CF₂) | PTFE (Teflon) | Non-stick coatings, waterproof fabric |
How does condensation polymerisation work?
Condensation polymerisation occurs when two different monomers, each with two functional groups, react together. Each time a bond forms between monomers, the functional groups react and a small molecule is expelled — usually water (H₂O) or occasionally hydrogen chloride (HCl). Two products are always formed: the polymer and the small molecule.
The functional group pairs that react together:
| Functional groups reacting | Small molecule released | Polymer type |
|---|---|---|
| –COOH (carboxylic acid) + –NH₂ (amine) | H₂O | Polyamide (e.g. nylon) |
| –COOH (carboxylic acid) + –OH (alcohol/diol) | H₂O | Polyester (e.g. PET) |
Each monomer must have two of the relevant functional groups (one at each end) so that the chain can grow in both directions.
What are the main condensation polymers?
Nylon (a polyamide)
Nylon-6,6 is formed from two monomers:
- Hexane-1,6-diamine (contains two –NH₂ amine groups)
- Hexanedioic acid (contains two –COOH carboxylic acid groups)
Each –COOH reacts with each –NH₂ to form a peptide (amide) bond (–CO–NH–), releasing a molecule of water at each step. The long chain produced is nylon.
Uses: clothing, ropes, parachute fabric, toothbrush bristles, fishing line.
Polyester (e.g. PET)
PET (polyethylene terephthalate) is formed from:
- Ethane-1,2-diol (a diol with two –OH groups)
- Benzene-1,4-dicarboxylic acid (terephthalic acid, with two –COOH groups)
Each –COOH reacts with each –OH to form an ester bond (–CO–O–), releasing water.
Uses: drinks bottles, polyester clothing, food packaging, carpet fibre.
Natural condensation polymers
Many biological polymers are condensation polymers:
- Proteins are polyamides built from amino acids (each amino acid has both –COOH and –NH₂ groups)
- Starch and cellulose are polyesters-like structures built from glucose units
- DNA is built from nucleotide monomers joined by condensation reactions
How do addition and condensation polymers compare?
| Feature | Addition polymer | Condensation polymer |
|---|---|---|
| Monomer type | Unsaturated; one type of monomer (with C=C) | Two types of monomer, each with two functional groups |
| By-product | None — only the polymer is made | Small molecule released at each step (usually H₂O) |
| Bond formed | C–C single bond (from opening C=C) | Ester bond (–CO–O–) or amide bond (–CO–NH–) |
| Examples | Poly(ethene), poly(propene), PVC | Nylon, polyester (PET), proteins, DNA |
| Natural examples? | Very rare | Very common (proteins, nucleic acids, polysaccharides) |
| Recyclable? | Many are recyclable (HDPE, PET) | Some (PET); others difficult |
Frequently asked questions
Why must addition polymerisation monomers have a double bond?
The C=C double bond is essential because it provides the mechanism by which monomers join. The double bond consists of a sigma bond and a pi bond; the pi bond is weaker and more reactive. During polymerisation, the pi bond breaks, freeing up one electron on each carbon atom. These electrons form new bonds with adjacent monomers on each side, extending the chain. Without the double bond, there is no mechanism for the monomers to link — they would simply remain as separate, stable molecules.
How can you tell from a structural formula whether a polymer is an addition or condensation polymer?
Look for two clues. First, check the repeating unit: if it contains only C and H (with possible halogen substituents) and shows no C=C double bond in the backbone, it is an addition polymer (the double bond was converted to a single bond during polymerisation). Second, look for characteristic bonds: if the backbone contains –CO–O– (ester linkage) or –CO–NH– (amide/peptide linkage), the polymer is a condensation polymer, and a small molecule (water) was released when each of these bonds formed.
What is the connection between nylon and proteins?
Both are polyamides: they are condensation polymers in which amino groups (–NH₂) react with carboxylic acid groups (–COOH) to form amide (peptide) bonds, releasing water. Proteins are built from amino acid monomers, each of which has both a –COOH group and a –NH₂ group on the same molecule. Nylon-6,6 uses two different monomers — one with two amine groups and one with two carboxylic acid groups — to build a similar –CO–NH– backbone. The chemistry of bond formation is identical; the biological specificity and the sequence of different amino acids in proteins are what distinguish them from synthetic nylons.
Why are most addition polymers difficult to recycle but PET (a condensation polymer) is recycled widely?
Most addition polymers are chemically stable — the C–C backbone is hard to break chemically under normal conditions, so they persist in the environment and are difficult to chemically recycle (break back into monomers). Recycling them mechanically (melting and re-moulding) is possible but degrades the polymer with each cycle. PET, a condensation polymer, can be hydrolysed (the ester bonds broken by reaction with water under acid or base conditions) back into the original diol and dicarboxylic acid monomers — a genuine chemical recycling route. Enzymatic hydrolysis of PET using engineered enzymes (like PETase) is an active area of research that could make PET recycling far more efficient.
For Socratic GCSE chemistry with Professor Curie — starting from functional group reactions and building up to long-chain polymer structures — visit aitutors.me.