Last updated: 24 August 2026
A plastic bag, a nylon fibre and a protein molecule may look completely different, but chemically they share one important idea: each is built from many smaller units joined together into a much larger molecule. Once you learn to recognise the repeating pattern in a polymer chain, apparently complicated structures become much easier to interpret.
The central question is therefore not simply “What is the polymer called?” but how were its monomers joined? Addition polymerisation and condensation polymerisation use different functional groups, create different links and lead to different chemical behaviour. This distinction also explains why some polymers are extremely resistant to chemical attack while others can be broken down by hydrolysis.
Try these five questions before reading the chapter. Focus on the functional groups in the monomers and the type of link formed in the polymer chain.
- 5 diagnostic questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
What is a polymer?
A polymer is a very large molecule made by joining many smaller molecules called monomers. Because a polymer contains a large number of atoms joined by covalent bonds, it is also described as a macromolecule.
In the syllabus definition, a substance can be regarded as a polymer or macromolecule if it has an average relative molecular mass, Mᵣ, of at least 1000, or contains at least 100 repeat units.
A monomer is the small starting molecule. A repeat unit is the smallest structural section that repeats along the polymer chain.
Monomer, polymer and repeat unit
These terms are related but should not be treated as interchangeable. The monomer is the molecule used to make the polymer, while the repeat unit describes the recurring pattern within the final chain.
Monomer: the relatively small molecule that reacts repeatedly during polymerisation.
Repeat unit: the smallest section of the polymer backbone that can be repeated to represent the whole chain.
Why the brackets and n matter
A polymer structure is normally written by placing one repeat unit inside brackets and writing n outside the brackets. The bonds must pass through the brackets because the chain continues in both directions.
For polyethene, the repeat unit is written as [–CH₂–CH₂–]ₙ. The subscript n does not represent a fixed number: it tells us that the unit is repeated many times.
Addition polymerisation
Addition polymers are commonly made from alkene monomers. Each monomer contains a C=C double bond. During polymerisation, the π bond of each C=C is lost and new C–C single bonds form between neighbouring monomers.
The carbon atoms themselves remain in the polymer. No small molecule is eliminated, which is why the process is called addition polymerisation: the monomers effectively add together.
What changes when an alkene becomes a polymer?
The easiest way to follow the change is to focus on the two carbon atoms of the original C=C bond. In the monomer they are double-bonded. In the polymer they become part of a chain containing only C–C single bonds.
Any substituent already attached to one of those carbon atoms stays attached to the same carbon. For example, propene gives poly(propene), in which a CH₃ group remains attached to every alternate carbon of the polymer backbone.
| Monomer | Polymer repeat unit | Main structural change |
|---|---|---|
| CH₂=CH₂ | [–CH₂–CH₂–]ₙ | C=C becomes part of a C–C single-bonded chain |
| CH₂=CHCH₃ | [–CH₂–CH(CH₃)–]ₙ | CH₃ remains attached to the polymer backbone |
| CH₂=CHCl | [–CH₂–CHCl–]ₙ | Cl remains attached to the polymer backbone |
Working backwards from an addition polymer
Examination questions often give the repeat unit and ask for the monomer. Work backwards by identifying the two backbone carbon atoms that originally came from the alkene and restoring a C=C bond between them.
For an addition polymer, remove the polymer continuation bonds and restore the C=C bond of the alkene monomer. Keep all substituents attached to the same carbon atoms.
If the repeat unit is [–CH₂–CH(CH₃)–]ₙ, restoring a double bond between the two backbone carbon atoms gives CH₂=CHCH₃. The monomer is therefore propene.
Condensation polymerisation
Condensation polymerisation follows a different pattern. Instead of opening C=C bonds, it involves reactions between functional groups. The monomers normally contain at least two reactive functional groups so that each molecule can form a bond at one end and then continue reacting at the other.
The small molecule produced depends on the functional groups involved. Reactions involving carboxylic acids commonly eliminate H₂O, while reactions involving acyl chlorides can eliminate HCl.
Addition polymerisation: the monomer skeleton is incorporated into the chain without elimination of a small molecule.
Condensation polymerisation: functional groups react repeatedly and a small molecule is eliminated as each new link forms.
Why must condensation monomers be bifunctional?
Suppose a molecule contained only one reactive –OH group. Once that –OH group had reacted, the molecule would have no second reactive end available for further chain growth. Long chains therefore require monomers with two or more suitable reactive functional groups.
There are two common possibilities. Two different bifunctional monomers may react with each other, or one type of monomer may contain two different functional groups within the same molecule.
Polyesters
A polyester contains repeated ester linkages, –COO–, along its chain. One common route uses a diol and a dicarboxylic acid.
Each –OH group of the alcohol can react with a –COOH group of the carboxylic acid. Because both monomers have two reactive groups, the reaction can continue at both ends and produce a long chain.
When identifying a polyester, look for the ester linkage –CO–O– within the polymer backbone.
Finding the monomers from a polyester
To reverse the condensation process, imagine cutting the polymer at each ester link. Restore an –OH group to the alcohol-derived side and a –COOH group to the acid-derived side.
A chain containing –O–CH₂–CH₂–O–CO–C₆H₄–CO– can be traced back to the monomers HO–CH₂–CH₂–OH and HOOC–C₆H₄–COOH.
Polyamides
Polyamides contain repeated amide linkages, –CO–NH–. One route uses a diamine together with a dicarboxylic acid.
The amino groups provide –NH₂ groups while the carboxylic acid provides –COOH groups. Repeated condensation between these groups forms the amide links that become part of the polymer backbone.
The quickest structural clue for a polyamide is the –CO–NH– linkage in the polymer chain.
Polyesters and polyamides compared
| Polymer type | Characteristic link | Typical monomers |
|---|---|---|
| Polyester | –CO–O– | Diol + dicarboxylic acid |
| Polyamide | –CO–NH– | Diamine + dicarboxylic acid |
| Protein | –CO–NH– peptide links | α-amino acids |
Proteins are natural condensation polymers
Proteins provide an important biological example of condensation polymerisation. Their monomers are α-amino acids. An α-amino acid contains an amino group, –NH₂, and a carboxyl group, –COOH, attached to the same carbon atom.
The group represented by R varies from one amino acid to another. The –NH₂ and –COOH groups are the parts that allow amino acids to join together.
Formation of a peptide bond
When two amino acids undergo condensation, the carboxyl group of one reacts with the amino group of the other. The elements of H₂O are removed and an amide linkage is formed.
In proteins, this particular amide linkage is called a peptide bond. Repeating the process many times gives a polypeptide chain.
A peptide bond is an amide linkage, –CO–NH–, formed between amino acid residues.
How much water is removed?
Joining two amino acid molecules requires one peptide bond and therefore eliminates one H₂O molecule. Joining three amino acid molecules in a single linear chain requires two peptide bonds and eliminates two H₂O molecules.
In general, if n amino acid molecules form one linear chain, n − 1 peptide bonds are formed and n − 1 molecules of water are eliminated.
Ten amino acid molecules joining to form one linear peptide chain produce nine peptide bonds. Therefore, nine H₂O molecules are eliminated during condensation.
Hydrolysis of condensation polymers
Condensation polymerisation creates ester or amide bonds by eliminating a small molecule. Hydrolysis is conceptually the reverse: water is used to break these links.
Ester and amide groups contain polar bonds and can undergo hydrolysis under suitable conditions. This is an important chemical difference from the largely hydrocarbon backbone of a poly(alkene).
Hydrolysis of proteins
Proteins can be hydrolysed by heating with aqueous acid or aqueous alkali. The peptide bonds are broken and the chain is converted into much smaller units derived from the original amino acids.
Condensation: functional groups join and a small molecule is removed.
Hydrolysis: water is used to break the linkage formed during condensation.
Polymer structure and biodegradability
Whether a polymer can be broken down chemically depends strongly on the bonds and functional groups present in its backbone.
Why poly(alkenes) are difficult to break down
Poly(alkenes), such as polyethene and poly(propene), contain backbones made mainly from strong C–C bonds with many C–H bonds. These chains contain no ester or amide group that can readily be attacked by water in a hydrolysis reaction.
As a result, many common addition polymers are chemically inert and persist for long periods in the environment.
Why condensation polymers can be more susceptible to breakdown
Polyesters and polyamides contain polar ester or amide linkages. These links can undergo hydrolysis, so condensation polymers have chemical points at which their chains can potentially be broken.
This does not mean that every polyester or polyamide rapidly biodegrades under ordinary environmental conditions. The actual rate depends on factors such as polymer structure, crystallinity, temperature, moisture and the availability of suitable microorganisms or enzymes. The important chemical comparison is that ester and amide links are hydrolysable, whereas the C–C backbone of a typical poly(alkene) is much more resistant.
Poly(alkenes): mainly C–C and C–H bonds → no readily hydrolysable backbone link → generally very resistant to biodegradation. Condensation polymers: ester or amide links → potentially hydrolysable → can be more susceptible to chemical or biological breakdown.
Polymers and the environment
Many synthetic polymers are manufactured from feedstocks ultimately obtained from crude oil. Fossil resources are finite, while polymer waste can remain in landfills or the natural environment for very long periods.
Recycling therefore serves two purposes: it reduces the amount of waste that must be disposed of and decreases the demand for fresh raw materials and energy-intensive polymer production.
Reduce, reuse and recycle
| Approach | Main idea | Why it helps |
|---|---|---|
| Reduce | Use less unnecessary polymer material | Decreases resource consumption and waste generation |
| Reuse | Use an item again rather than immediately discarding it | Extends the useful lifetime of the material |
| Recycle | Recover polymer material and use it again | Reduces demand for virgin feedstocks and lowers waste accumulation |
The chemistry of the polymer matters environmentally: chemically inert polymers may persist for long periods, whereas polymers containing hydrolysable links provide possible routes for chain breakdown.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Assuming every polymerisation reaction eliminates water | Addition polymerisation does not eliminate a small molecule. | First decide whether the monomer reacts through C=C or through functional groups. |
| Drawing the C=C bond unchanged inside an addition polymer repeat unit | The alkene π bond is lost when the chain forms. | Replace the monomer C=C with C–C bonds and show continuation bonds through the brackets. |
| Using monomers with only one reactive group to make a long condensation polymer | Chain growth would stop after the only reactive group has reacted. | Look for bifunctional monomers with two suitable reactive groups. |
| Saying every condensation polymer is automatically biodegradable | Hydrolysable links make breakdown possible, but the actual rate depends on the polymer and environmental conditions. | Explain the structural reason: ester and amide links can hydrolyse, whereas poly(alkene) C–C backbones are much more resistant. |
What you should be able to do
Recognise polymer terminology
Distinguish clearly between a monomer, a repeat unit, a polymer and a macromolecule.
Draw addition polymers
Convert an alkene monomer into the correct polymer repeat unit and work backwards from a repeat unit to its monomer.
Recognise condensation polymers
Identify bifunctional monomers and recognise when a small molecule is eliminated during chain formation.
Identify polymer linkages
Recognise ester links in polyesters and amide links in polyamides and proteins.
Explain protein formation and hydrolysis
Describe peptide-bond formation between α-amino acids and how proteins can be hydrolysed.
Relate structure to environmental behaviour
Explain why poly(alkenes) are resistant to biodegradation and why hydrolysable links change the behaviour of condensation polymers.
These ten questions test whether you can apply the chapter to polymer structures, monomers, condensation reactions and unfamiliar examples.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Questions students often ask about polymers
Polymer questions become much easier once you focus on the functional groups in the monomer and the type of linkage found in the final chain. These questions clarify a few distinctions that commonly cause confusion.
What is the quickest way to distinguish addition from condensation polymerisation?
Look first at the monomers. If an alkene C=C bond opens and the monomers join without eliminating a small molecule, the process is addition polymerisation. If functional groups react repeatedly and a small molecule such as H₂O or HCl is eliminated, it is condensation polymerisation.
How do I find the monomer from an addition polymer repeat unit?
Identify the two carbon atoms that form the main backbone of the repeat unit. Remove the continuation bonds through the polymer brackets and place a C=C bond between those two carbon atoms. Any substituents remain attached to the same carbon atoms.
Does condensation polymerisation always produce water?
No. Water is common when alcohol, carboxylic acid or amino functional groups undergo condensation, but other monomers may eliminate a different small molecule. For example, reactions involving acyl chlorides can eliminate HCl.
Why is a peptide bond also described as an amide bond?
A peptide bond contains the same –CO–NH– structural unit that defines an amide functional group. The term peptide bond is used when this amide link connects amino acid residues in peptides and proteins.
Are all condensation polymers biodegradable?
Not necessarily. Ester and amide links can undergo hydrolysis, so condensation polymers contain possible sites for chain breakdown that typical poly(alkenes) lack. However, whether a particular polymer biodegrades readily depends on its complete structure and the environmental conditions. Hydrolysable does not automatically mean rapidly biodegradable.
Polymer chemistry becomes much less intimidating when you stop looking at the entire giant molecule and instead identify the repeating pattern. Check whether the monomer contains C=C or reactive functional groups, find the linkage formed in the chain, and the distinction between addition polymers, polyesters, polyamides and proteins follows naturally.