Last updated: 13 October 2026
A plastic bag, a polyester shirt, a fishing line and a drinks bottle may look completely different, yet they all have something important in common: their materials contain extremely large molecules called polymers. These giant molecules are built by joining many smaller molecules together.
Polymer Chemistry becomes much easier when you stop thinking of each polymer as a separate structure to memorise. Instead, follow what happens to the monomer. Which bond or functional group allows it to react? Which atoms remain in the polymer? Is anything removed when the chain forms? Those questions tell you whether you are looking at addition or condensation polymerisation.
The same reasoning also lets you work backwards. If an examination question gives you a repeating unit, you can often reconstruct the monomer or monomers by reversing the bonding changes that occurred during polymerisation.
Diagnostic quiz
Try these five questions before reading the chapter. They check the ideas that make polymer structures much easier to understand.
- 5 short questions
- Instant feedback for every option
- Spot common misconceptions early
From monomers to polymers
A monomer is a small molecule that can join to other monomer molecules. A polymer is the very large molecule produced when many monomers join.
Because a polymer molecule may contain thousands of atoms, it is also described as a macromolecule.
Monomer, repeating unit and polymer
Three terms are easily confused:
- A monomer is the small starting molecule used in polymerisation.
- A repeating unit is the smallest section of the polymer chain that repeats.
- A polymer is the complete long-chain molecule containing many repeating units.
The atoms contributed by a monomer may still be present in the repeating unit, but their bonding can change during polymerisation. This is especially important in addition polymerisation, where C=C becomes C–C.
You can think of a repeating unit as the pattern that appears again and again along the chain. The brackets and the letter n tell us that this unit occurs a very large number of times.
Addition polymerisation
Addition polymerisation occurs when many unsaturated monomers join together. At O-Level, these monomers are usually alkenes containing a carbon–carbon double bond, C=C.
The essential change is simple:
As this happens, each monomer forms new bonds to neighbouring monomer units, creating a long carbon chain.
Example: ethene forms poly(ethene)
Ethene has the structural formula:
Many ethene molecules can join:
Notice what has changed. The two carbon atoms are still present, and all four hydrogen atoms are still present. The C=C bond has become C–C, and the two carbon atoms now form bonds that continue into neighbouring repeating units.
Every atom in the alkene monomers appears in the polymer. No small molecule such as H₂O is formed, so the addition polymer is the only product.
How to draw an addition polymer from its monomer
Consider propene:
Start by identifying the two carbon atoms in the C=C bond. These become part of the main polymer chain.
- Change C=C into C–C.
- Draw one bond extending from each of these carbon atoms towards the neighbouring repeating units.
- Keep every atom and side group attached to its original carbon atom.
- Place the repeating unit inside brackets.
- Write n outside the brackets.
The CH₃ group has not disappeared. It remains attached to the same carbon atom that carried it in the original propene molecule.
Another example: poly(chloroethene)
Chloroethene has the structure:
Its repeating unit is:
Again, only the C=C part changes into the polymer backbone. The chlorine atom remains attached to the same carbon.
How to deduce an alkene monomer from its polymer
Examination questions also ask you to work backwards. Suppose the repeating unit is:
Reverse the polymerisation:
- Take one repeating unit out of the brackets.
- Remove the two bonds that continued into neighbouring units.
- Place a C=C bond between the two carbon atoms of the polymer backbone.
- Keep all side groups attached to the same carbon atoms.
The monomer is therefore propene.
Useful check: For an addition polymer, the monomer and its corresponding repeating unit contain the same atoms. If you have accidentally removed a CH₃ group, Cl atom or hydrogen atom, the structure is wrong.
Why must the bonds pass through the brackets?
A repeating unit is only one small section of a much longer chain. The bonds passing through the left and right brackets show that the displayed atoms are covalently bonded to more repeating units on either side.
Therefore:
correctly shows an extending polymer chain. Simply drawing CH₂–CH₂ inside a closed box would not clearly show how the chain continues.
Condensation polymerisation
Condensation polymerisation follows a different pattern. Instead of using a C=C bond to connect monomers, functional groups on neighbouring monomers react with one another.
Each time a new linkage is formed, a small molecule is also produced. For the polyester and polyamide reactions studied at this level, the small molecule is usually water, H₂O.
Why must the monomers have two reactive groups?
To build a long chain, a monomer must be able to connect at both ends.
Imagine a molecule containing only one reactive functional group. Once that group has reacted, there is no second reactive end through which the chain can continue. Condensation-polymer monomers therefore contain two reactive functional groups.
A diol has two –OH groups. A diamine has two –NH₂ groups. A dicarboxylic acid has two –COOH groups. Their two reactive ends allow molecules to connect repeatedly and produce a long chain.
Polyesters and Terylene
A polyester can be formed when a diol reacts with a dicarboxylic acid.
- A diol contains two –OH groups.
- A dicarboxylic acid contains two –COOH groups.
When an alcohol group reacts with a carboxylic acid group, an ester linkage forms and water is removed.
The characteristic ester linkage is:
Depending on the direction in which the chain is drawn, you may also see it written as –OCO–. It represents the same ester connection.
Terylene
Terylene is a polyester. It can be made from ethane-1,2-diol:
and benzene-1,4-dicarboxylic acid:
A repeating unit can be represented as:
You do not need to regard this as a collection of symbols to memorise. Look for the –COO– ester linkages. Those linkages tell you that the polymer is a polyester.
Where does the water come from?
When a –COOH group reacts with an –OH group, an –OH from the carboxylic acid and an H from the alcohol group are removed. Together they form H₂O.
The atoms left behind form the ester linkage connecting the two molecules. Repeating the same process at both ends builds the polymer chain.
Polyamides and nylon
A polyamide can be formed when a diamine reacts with a dicarboxylic acid.
- A diamine contains two –NH₂ groups.
- A dicarboxylic acid contains two –COOH groups.
The characteristic amide linkage is:
It may also be written as –NHCO– when the chain is viewed in the opposite direction.
Nylon
Nylon is a polyamide. A commonly shown example, nylon-6,6, can be made from the diamine:
and the dicarboxylic acid:
A repeating unit can be written as:
The important feature is the amide linkage, –CONH–. Once you recognise this linkage, you can identify the polymer as a polyamide.
Where does the water come from in polyamide formation?
An –OH from a carboxylic acid group and an H from an –NH₂ group are removed. These form H₂O, while the remaining atoms form an amide linkage.
Addition and condensation polymerisation compared
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| Typical monomers | Unsaturated monomers containing C=C | Monomers containing two reactive functional groups |
| Important bonding change | C=C becomes C–C as monomers join | Functional groups react to form new linkages |
| Atoms from monomers | All retained in the polymer | Some atoms leave as a small molecule |
| Other product | None | A small molecule, usually H₂O in the examples studied |
| Examples | Poly(ethene), poly(propene), poly(chloroethene) | Terylene and nylon |
Fast recognition rule: If the monomer contains C=C and the polymer is formed by changing that double bond into single bonds that connect neighbouring units, think addition polymerisation. If monomers react through functional groups at both ends while a small molecule is removed, think condensation polymerisation.
Deducing polymer and monomer structures
Polymer questions often test whether you can move in both directions:
The method depends on whether the polymer is an addition polymer or a condensation polymer.
For an addition polymer: find the two-carbon backbone section
If the repeating unit contains a two-carbon section such as:
remove the outward bonds and restore a double bond between those two carbon atoms:
This gives chloroethene.
For a polyester: locate the ester linkage
Look for:
To work backwards conceptually, break the ester connection and restore the functional groups from which it formed:
- one side becomes part of a –COOH group;
- the other side becomes part of an –OH group.
Because the chain contains repeating ester links, the monomer or monomers must provide reactive groups at both ends so that chain growth can continue.
For a polyamide: locate the amide linkage
Look for:
When working backwards, restore:
- a –COOH group on the carboxylic-acid-derived part; and
- an –NH₂ group on the amine-derived part.
This is the reverse of the condensation step that originally formed the amide linkage and water.
In addition polymerisation, no atoms disappear. In condensation polymerisation, account for the atoms that leave in the small molecule. Following the atoms is a much safer method than trying to reproduce a polymer structure from memory.
Why polymers are useful materials
Different monomers produce different polymer structures, and different polymers can therefore have different physical properties. Depending on the polymer and how it is manufactured, a material may be flexible, strong, light, tough or resistant to water and chemicals.
This explains why polymer materials are used in such a wide range of products.
| Polymer | Type | Examples of uses |
|---|---|---|
| Poly(ethene) | Addition polymer | Plastic bags, packaging, bottles and electrical insulation |
| Poly(propene) | Addition polymer | Containers, fibres, ropes and household products |
| Nylon | Polyamide | Clothing, fishing lines, ropes and strong fibres |
| Terylene | Polyester | Clothing fibres, fabrics and thread |
A polymer name therefore tells you about more than the finished object. It tells you something about the molecular structure from which the material is built.
Polymers and the environment
One reason synthetic polymers are so useful is that many of them are durable. They do not rot easily and can survive exposure to water and many chemicals.
Unfortunately, the same durability becomes a problem when a plastic product is thrown away.
Why many synthetic polymers are non-biodegradable
A biodegradable material can be broken down by microorganisms through biological processes. Many common synthetic polymers are non-biodegradable because microorganisms cannot readily break down their polymer structures.
The waste may therefore remain in the environment for a long time.
Non-biodegradable does not mean that a piece of plastic can never change. Sunlight, heat and physical wear can break a large object into smaller fragments. However, producing smaller pieces is not the same as biological decomposition.
Problems caused by plastic waste
Poorly managed plastic waste can:
- occupy landfill space for long periods;
- accumulate as litter on land and in waterways;
- harm animals that swallow it or become trapped in it; and
- break into very small plastic fragments that are difficult to remove from the environment.
What about burning plastics?
Incineration reduces the volume of plastic waste and can be used to recover energy. However, it is not environmentally free.
Burning carbon-containing polymers produces carbon dioxide when combustion is complete. If combustion is incomplete, carbon monoxide and soot may also form. Some polymers containing other elements can produce additional harmful gases, so controlled incineration requires suitable treatment of the exhaust gases.
Be precise. The products depend on the polymer and the conditions of combustion. A clear environmental disadvantage that applies widely is that combustion of carbon-containing plastics contributes CO₂ to the atmosphere.
Reusing and recycling polymers
Reducing unnecessary plastic use, reusing suitable products and recycling plastics can reduce the amount of new material required and the amount of waste sent for disposal.
Mechanical recycling
In mechanical recycling, waste plastic is usually:
- collected;
- sorted into suitable polymer types;
- cleaned;
- shredded;
- melted; and
- reshaped into new products.
The important point is that the polymer is mainly being physically reprocessed. It is not deliberately converted back into its original monomers.
Chemical recycling
Chemical recycling uses chemical reactions to break polymer molecules into smaller useful molecules. In suitable cases, these may include monomers that can be used to manufacture new materials.
Condensation polymers contain linkages that can be broken chemically. For example, reactions involving water can split ester or amide linkages. The essential O-Level idea is that chemical recycling changes the polymer chemically, unlike simply melting and reshaping it.
Why can plastics not simply be mixed together for recycling?
Different polymers have different physical properties and soften or melt under different conditions. Mixing incompatible plastics can therefore produce a recycled material with poor or unpredictable properties.
This is why sorting is an important part of many recycling processes.
| Approach | What happens? | Important limitation |
|---|---|---|
| Reuse | The same product is used again | The product must remain suitable and safe to reuse |
| Mechanical recycling | Plastic is sorted, melted and reshaped | Contamination and mixtures of different polymers cause problems |
| Chemical recycling | Polymer molecules are chemically broken into smaller molecules | Processes may require energy, chemicals and specialised equipment |
| Incineration with energy recovery | The polymer is burned and some released energy is recovered | The material is destroyed and CO₂ is produced |
Recycling is useful, but it is not “free”. Plastic still has to be collected, transported, sorted, cleaned and processed. Reducing unnecessary consumption and reusing suitable products can therefore be important alongside recycling.
Common polymer mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Calling the repeating unit the monomer | The monomer is the starting molecule; its bonding may change during polymerisation | Distinguish the original monomer from the unit found inside the polymer chain |
| Leaving C=C inside an addition polymer | The C=C bond is used when the polymer chain forms | Change C=C to C–C in the repeating unit |
| Removing a CH₃ group, Cl atom or another side group | Addition polymerisation retains all the atoms of the monomer | Keep every side group attached to its original carbon |
| Forgetting the bonds through the brackets | The structure no longer clearly shows how the repeating unit connects to neighbouring units | Show a bond passing through each side of the brackets |
| Forgetting the small n | The drawing does not clearly represent many repeating units | Write n outside the brackets |
| Saying addition polymerisation produces water | No atoms are removed during addition polymerisation | State that the addition polymer is the only product |
| Confusing nylon and Terylene | Nylon and Terylene contain different characteristic linkages | Remember nylon: –CONH–; Terylene: –COO– |
| Using monomers with only one reactive end to make a condensation polymer | The chain would not be able to continue growing at both ends | Look for monomers with two reactive functional groups |
| Saying non-biodegradable means the plastic can never break apart | Plastic may fragment physically even when microorganisms cannot readily biodegrade it | Describe biodegradability in terms of breakdown by microorganisms |
| Saying recycling has no environmental cost | Collection, sorting, transport and processing all require resources and energy | Recognise both the benefits and limitations of recycling |
Polymers checklist
Core ideas
- Can I define monomer and polymer?
- Can I identify a repeating unit?
- Can I explain why a polymer is a macromolecule?
- Can I distinguish a monomer from its repeating unit?
Addition polymerisation
- Can I recognise a suitable alkene monomer from C=C?
- Can I change C=C into C–C correctly?
- Can I keep all side groups in the correct positions?
- Can I use brackets, outward bonds and n correctly?
Condensation polymerisation
- Can I recognise monomers with two functional groups?
- Can I explain why a small molecule is also formed?
- Can I identify –COO– as an ester linkage?
- Can I identify –CONH– as an amide linkage?
Structures and environment
- Can I deduce an addition monomer from its repeating unit?
- Can I distinguish nylon from Terylene?
- Can I explain why many synthetic polymers persist in the environment?
- Can I compare reuse, recycling and incineration?
Mastery quiz
Now apply what you have learned to structures, unfamiliar monomers, condensation linkages and environmental reasoning.
- 10 application questions
- Structural-formula practice
- Explanation for every answer choice
Frequently asked questions
Questions students often ask about polymers
These questions connect the structural rules to some of the details that often cause confusion when students first learn polymerisation.
Why does the C=C bond disappear during addition polymerisation?
The C=C bond changes into C–C so that the two carbon atoms can form new bonds to neighbouring monomer units. The carbon atoms themselves are not lost; their bonding changes as the long chain forms.
Is the repeating unit exactly the same thing as the monomer?
No. The monomer is the starting molecule before polymerisation. The repeating unit is the section found repeatedly in the finished polymer. For example, ethene contains C=C, while its repeating unit in poly(ethene) contains C–C.
Why does the repeating unit need bonds passing through the brackets?
Those bonds show that the displayed repeating unit continues into neighbouring units. A polymer is not an isolated molecule containing only the atoms inside one set of brackets.
Why do condensation-polymer monomers need two functional groups?
A long chain can grow only if each monomer can connect at both ends. Once one functional group reacts, the second functional group allows the chain to continue connecting to another molecule.
Does condensation polymerisation always produce water?
In the polyester and polyamide reactions normally studied at O-Level, water is the small molecule formed when the monomers join. Other condensation reactions can produce different small molecules, but those are not needed to understand the examples in this chapter.
How can I tell whether a polymer is a polyester or a polyamide?
Look at the linkage in the polymer chain. A polyester contains –COO– ester linkages, while a polyamide contains –CONH– amide linkages.
Why is nylon called a polyamide?
Nylon contains many amide linkages, –CONH–, along its polymer chain. The prefix “poly-” means that this type of linkage occurs repeatedly throughout the macromolecule.
Why is Terylene called a polyester?
Terylene contains many ester linkages, –COO–, along its chain. It can be formed by condensation polymerisation between a diol and a dicarboxylic acid.
Are all plastics completely non-biodegradable?
No. Different polymers behave differently. However, many widely used synthetic plastics are not readily broken down by microorganisms and can therefore persist in the environment for long periods.
If a plastic breaks into tiny pieces, has it biodegraded?
Not necessarily. Physical fragmentation can turn a large plastic object into smaller pieces without chemically or biologically decomposing the polymer. Biodegradation specifically involves biological breakdown, usually by microorganisms.
Why is sorting plastics important before mechanical recycling?
Different polymers have different physical properties and processing conditions. If unsuitable plastics are mixed, the recycled material may have poor or unpredictable properties.
Is recycling always better than simply making new plastic?
Recycling can conserve raw materials and reduce waste, but it still requires collection, transport, sorting, cleaning and processing. Environmental decisions therefore consider the whole process rather than assuming that recycling has no cost.
Polymers become much easier to understand once you learn to follow the atoms. In addition polymerisation, the C=C bond of an alkene becomes part of a long carbon chain without any atoms being removed. In condensation polymerisation, functional groups react repeatedly to form linkages such as –COO– and –CONH– while a small molecule is produced. The same structural reasoning lets you move from monomers to polymers, work backwards from repeating units and understand why these useful materials also create important environmental challenges.