There are two types of polymerisation. In addition polymerisation the monomers contain carbon-carbon double bonds (C=C) that open so the monomers add together with no other product; the repeating unit has the same formula as the monomer.
In condensation polymerisation each monomer has two functional groups and a small molecule, usually water, is eliminated each time two monomers join.
This page covers content standard 12.2 of the Form 5 Polymers chapter: types of polymerisation. Once you know what a polymer is, the natural next question is how the monomers actually join. The syllabus wants you to compare the two mechanisms, addition and condensation, recognise which monomers undergo which, and be able to reason about the repeating unit and the by-product. This is one of the most predictable comparison questions in the chapter.
The two types
Polymerisation is the joining of many monomers into a polymer. It happens in two distinct ways:
- Addition polymerisation, monomers with a carbon–carbon double bond (C=C) add together directly.
- Condensation polymerisation, monomers with two functional groups join, releasing a small molecule each time.
Addition polymerisation
In addition polymerisation the monomer must be unsaturated, meaning it contains a C=C double bond. When polymerisation happens, each double bond opens and the monomers link one after another into a long chain. Crucially, no other product is formed, every atom of the monomer ends up in the polymer.
The key features. (1) Monomers contain C=C. (2) The double bond breaks and becomes a single bond. (3) No small molecule is eliminated. (4) The repeating unit has the same molecular formula as the monomer.
Examples you should know:
| Monomer | Polymer |
|---|---|
| Ethene, CH₂=CH₂ | Polythene, (, CH₂, CH₂, )ₙ |
| Propene, CH₂=CHCH₃ | Polypropene |
| Chloroethene, CH₂=CHCl | Poly(chloroethene), PVC |
| Tetrafluoroethene, CF₂=CF₂ | Poly(tetrafluoroethene), PTFE / Teflon |
| Phenylethene (styrene) | Polystyrene |
Condensation polymerisation
In condensation polymerisation, each monomer carries two reactive functional groups, one at each end. When two monomers join, a small molecule, usually water (H₂O), is eliminated. This repeats many times, building a long chain while releasing many small molecules.
The key features. (1) Monomers have two functional groups each (or two different monomers, each with two groups). (2) A small molecule is eliminated (water, sometimes hydrogen chloride). (3) The repeating unit does not have the same formula as the monomers, because atoms are lost as the small molecule.
Examples you should know:
- Nylon, made from a diamine and a dicarboxylic acid; water is eliminated.
- Terylene (a polyester), made from a diol and a dicarboxylic acid; water is eliminated.
- Proteins, made from amino acids joining with loss of water.
- Starch and cellulose, made from glucose units joining with loss of water.
Comparing the two
| Feature | Addition | Condensation |
|---|---|---|
| Monomer | Has C=C double bond | Has two functional groups |
| Double bond | Opens | Not required |
| By-product | None | Small molecule (water) |
| Repeating unit vs monomer | Same formula | Different (atoms lost) |
| Examples | Polythene, PVC | Nylon, terylene |
Worked example
Question. Chloroethene, CH₂=CHCl, undergoes addition polymerisation to form poly(chloroethene), PVC. (a) State the type of polymerisation and explain how you know. (b) A molecule of PVC has a relative molecular mass of 25 000. Calculate the degree of polymerisation, n. [Relative atomic masses: H = 1, C = 12, Cl = 35.5]
Step 1, Identify the type (part a). The monomer chloroethene contains a C=C double bond, and no small molecule is lost when it polymerises, so this is addition polymerisation. The double bond opens and identical monomers add together.
Step 2, Find the Mr of the repeating unit (part b). In addition polymerisation the repeating unit has the same formula as the monomer, CH₂CHCl (C₂H₃Cl).
Mr = (2 × 12) + (3 × 1) + 35.5 = 24 + 3 + 35.5 = 62.5Step 3, Calculate n.
n = Mr of polymer ÷ Mr of repeating unit
n = 25 000 ÷ 62.5
n = 400Answer. (a) Addition polymerisation, shown by the C=C double bond and the absence of a by-product. (b) The degree of polymerisation is n = 400.
Practice question
Question. Nylon is manufactured by reacting a diamine with a dicarboxylic acid. State the type of polymerisation involved and name the small molecule that is eliminated. Explain why this differs from the way polythene is made.
Answer. Nylon is made by condensation polymerisation, and the small molecule eliminated is water (H₂O). This differs from polythene, which is made by addition polymerisation of ethene: ethene has a C=C double bond that opens with no by-product, whereas the nylon monomers each have two functional groups and lose water each time they join.
Exam tip
The examiner nearly always tests the difference between the two types, so keep three contrasts ready: the monomer (C=C double bond versus two functional groups), the by-product (none versus a small molecule such as water), and the repeating unit (same formula as the monomer for addition, different for condensation). A frequent slip is to say addition polymerisation “removes a double bond and releases hydrogen”, nothing is released in addition. Another is forgetting that the repeating unit of an addition polymer keeps the same molecular formula as the monomer, which is exactly what lets you calculate the degree of polymerisation. When identifying the type from a monomer, look first for a C=C: if it is there, it is addition.
Where this fits
This is content standard 12.2 of the Polymers chapter. It follows directly from natural and synthetic polymers (12.1), which introduced monomers and repeating units, and it prepares you for natural rubber (12.3), which is itself an addition polymer of isoprene. Work through more repeating-unit and degree-of-polymerisation questions on the chapter worked examples page, and review the classic mix-ups on the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English from RM50/hr, a one-to-one teacher can drill the addition-versus-condensation comparison until you can produce all three contrasts from memory.
Quick recap
- Addition: monomers have C=C; double bond opens; no by-product; repeating unit = same formula as monomer. Example: ethene → polythene.
- Condensation: monomers have two functional groups; a small molecule (water) is eliminated. Example: nylon, terylene.
- Identify the type by looking for a C=C double bond in the monomer.
- The repeating unit of an addition polymer lets you find n = Mr(polymer) ÷ Mr(repeating unit).
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