Polymers
Poly(ethene), nylon and Terylene: addition and condensation polymerisation, monomer deductions, plastic pollution and recycling.
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The core idea
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Learning objectives
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- describe polymers as large molecules built up from small units (monomers), different polymers having different units and/or different linkages
- describe the formation of poly(ethene) as an example of addition polymerisation of ethene as the monomer (see also 11.2(j))
- state some uses of poly(ethene) as a typical plastic, e.g. plastic bags; clingfilm
- deduce the structure of the polymer product from a given monomer and vice versa
- describe nylon, a polyamide, and Terylene, a polyester, as condensation polymers, the partial structure of nylon being represented as and the partial structure of Terylene as (details of manufacture and mechanisms of these polymerisations are not required)
- state some typical uses of man-made fibres such as nylon and Terylene, e.g. clothing; curtain materials; fishing line; parachutes; sleeping bags
- describe the pollution problems caused by the disposal of non-biodegradable plastics
- describe two methods of recycling plastics as
- physical method (exemplified by melting small pieces of poly(ethene) waste into pellets)
- chemical method (exemplified by depolymerisation and cracking of plastic waste into chemical feedstock and fuel respectively) — describe depolymerisation as a process in which polymers are broken down into their monomers, exemplified by hydrolysis of polyesters using acid as a catalyst (details of mechanisms are not required)
- discuss the social, economic and environmental issues of recycling plastics.
Learn to read the repeating pattern first. That lets you connect monomers to addition polymers, recognise the linkages in nylon and Terylene, and explain how physical and chemical recycling treat polymer chains differently.
1. Definition
A. Polymer
A polymer is a large molecule made of many repeating units.
B. Monomer
A monomer is a small molecule that joins to form a polymer.
C. Polymerisation
Polymerisation is the process of joining many monomers to form a polymer.
D. Addition polymerisation
Addition polymerisation is polymerisation of an unsaturated monomer (contains C = C) where monomers join and no other product is formed.
E. Condensation polymerisation
Condensation polymerisation is polymerisation where monomers with two functional groups join and a small molecule is eliminated (usually H₂O).
2. Key Ideas
- Addition polymerisation: alkene monomer (C = C), no byproduct.
- Condensation polymerisation: two functional groups, small byproduct (often H₂O).
- Name format:
poly(monomer)(e.g. poly(ethene), poly(propene)). - Monomer questions: find the repeating unit, then add the double bond back.
- Nylon contains amide linkages; Terylene contains ester linkages.
- Chemical recycling includes cracking and depolymerisation; acid-catalysed hydrolysis can return a polyester to its monomers.
- Plastics: useful because they are inert/durable; harmful because many are non-biodegradable.
Alkenes (unsaturation): Alkenes
Carboxylic acids + esters (ester link): Carboxylic acids and esters
Air pollution: Air pollutants
Acid rain: Acid rain chemistry and control
3. Detailed Explanations
- Polymer = many repeating units; monomer = small molecule that joins to form the polymer.
- Addition polymerisation uses an alkene monomer (C = C) and forms no byproduct.
- Condensation polymerisation uses monomers with two functional groups and forms a small byproduct (often H₂O).
- Monomer from repeating unit (addition): find the repeat, then add the C = C back.
- Environment keywords: many plastics are non-biodegradable; recycling saves resources but needs collection, sorting and energy.
A. Addition polymerisation (alkenes)
The C = C bond opens and becomes C-C single bonds linking many monomers into a chain.
Example: ethene → poly(ethene): n(CH₂ = CH₂) → [-CH₂-CH₂-]ₙ
In addition polymerisation, no atoms are lost, so the polymer has the same empirical formula as the monomer.
Common addition polymers you should recognise:
| Monomer | Polymer | Typical use (exam-safe) |
|---|---|---|
| ethene, CH₂ = CH₂ | poly(ethene) | plastic bags; packaging |
| propene, CH₂ = CHCH₃ | poly(propene) | food containers; ropes |
B. Condensation polymerisation (two functional groups)
Monomers have two functional groups, so they join end-to-end. A small molecule (often H₂O) is produced.
B.1 Polyamides (nylon)
Nylon is a condensation polymer with amide linkages, -CONH⁻.
A representative partial structure of nylon is: [-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-]ₙ
Trace along the chain and look for the repeated -CO-NH⁻ amide linkages.
Uses: ropes, fishing nets, parachutes.
B.2 Polyesters (Terylene)
Terylene is a condensation polymer with ester linkages, -COO⁻.
A representative partial structure of Terylene is: [-O-CH₂-CH₂-O-CO-C₆H₄-CO-]ₙ
The -CO-O⁻ connections are ester linkages. You need to recognise the partial structures and linkages; manufacturing mechanisms are not required.
Uses: polyester clothing; fillings (e.g. sleeping bags).
C. Addition vs condensation (summary)
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| monomer requirement | must be unsaturated (C = C) | two functional groups |
| byproduct | none | small molecule (often H₂O) |
| linkage | C-C single bonds | amide (-CONH⁻) or ester (-COO⁻) |
| examples | poly(ethene), poly(propene) | nylon, Terylene |
D. Plastics and the environment
Many synthetic polymers are plastics. They are useful because they are cheap and inert, but disposal causes environmental problems.
| Issue | What to write (keywords) |
|---|---|
| non-biodegradable | plastics not broken down by bacteria → build up in landfill/oceans |
| litter + microplastics | harm animals (choking/entanglement); enter food chains |
| incineration | produces carbon dioxide and may produce other air pollutants |
Solutions (exam-safe):
- reduce and reuse,
- recycling,
- biodegradable plastics (still depends on disposal conditions).
E. Recycling plastics (two methods)
The syllabus expects two main recycling methods:
1) Physical recycling (melting/remoulding)
- sort by polymer type and colour,
- wash/shred,
- melt and remould into new products.
Key idea: the polymer is not broken into monomers, so quality can drop if plastics are mixed or contaminated.
2) Chemical recycling
- depolymerisation breaks suitable polymers back into monomers that can be purified and reused;
- cracking breaks plastic waste into smaller molecules used as chemical feedstock or fuel.
Both routes change the polymer molecules rather than merely reshaping the plastic.
F. Depolymerisation
Depolymerisation breaks a polymer back into its monomers. The required example is the acid-catalysed hydrolysis of a polyester:
polyester + water → [acid\ catalyst] diol + dicarboxylic\ acid
Hydrolysis breaks the ester linkages by adding the components of water. The diol and dicarboxylic acid monomers can then be recovered. You do not need the reaction mechanism.
G. Social, economic and environmental issues of recycling
| Issue type | What to write (exam-safe) |
|---|---|
| social | people must sort waste; contamination (food/labels) reduces recycling rates |
| economic | collection/sorting plants cost money; recycled plastic may be more expensive than making new plastic |
| environmental | recycling uses energy and transport (emissions), but reduces landfill and litter when done well |
4. Common Mistakes
- Saying condensation polymerisation has “no byproduct” (wrong; that is addition polymerisation).
- Calling every polymer a plastic (plastics are usually synthetic polymers; polymers can be natural too).
- Forgetting to add the double bond back when finding a monomer from a repeating unit.
- Writing bromine water as a test for the polymer instead of the alkene monomer.
- Describing chemical recycling as melting: melting and remoulding are physical recycling because the polymer chains remain polymers.
- Defining depolymerisation without naming the required example of acid-catalysed polyester hydrolysis.
5. Exam Tips
- Identify the repeating unit.
- Add the double bond back between the two carbon atoms.
- Name the monomer and write
poly(monomer).
Addition: alkene monomer, no byproduct. Condensation: two functional groups, small byproduct (often H₂O).
Depolymerisation can recover monomers; cracking produces smaller chemical feedstocks or fuels. For polyesters, state acid-catalysed hydrolysis as the depolymerisation example.
6. Worked Examples
Modelled example 1
Identify the monomer of poly(ethene)
Problem
Study the worked solution
Locate one repeating unit
Method
Identify the shortest structural section that repeats.Reason
Each repeat retains the two-carbon skeleton from one monomer.Working
Repeating unit: -CH₂-CH₂-.Restore the alkene bond
Method
Remove the outward chain bonds and place C = C between the two carbon atoms.Reason
Addition polymerisation opens the monomer’s double bond to make chain-forming single bonds.Working
Monomer structure: CH₂ = CH₂.Name the monomer
Method
Name CH₂ = CH₂ as ethene.Reason
It is the two-carbon alkene required for poly(ethene).Working
Monomer: ethene, CH₂ = CH₂.
Guided practice 2
Repeating unit from monomer
Problem
Open the double bond and preserve substituents
Hints
Hint 1: backbone
Hint 2: substituent
View solution step by step
Open the double bond
Method
Replace C = C with a C-C single bond.Reason
The additional bonding capacity links each monomer to the units on either side.Working
CH₂ = CHCH₃ → -CH₂-CH(CH₃)⁻.Show continuation
Method
Keep an outward bond at each end of the two-carbon unit.Reason
Those bonds show that the same unit continues through the polymer chain.Working
Repeating unit: -CH₂-CH(CH₃)⁻.
Common misconception 3
Classify the polymerisation type
Learner claim
Use the linkage as evidence
View solution step by step
Inspect the linkage
Method
Find the repeated -CO-NH⁻ connection.Reason
This is an amide linkage.Working
Amide linkage → polyamide.Identify the example
Method
Classify it as condensation polymerisation and identify nylon.Reason
Nylon is the required polyamide example.Working
Condensation polymer; polyamide; nylon.
Examiner practice 4
Read nylon and Terylene structures
Examination question
Match each partial structure to its linkage
View solution step by step
Identify structure A
2 marksMethod
Name nylon and its amide linkage.Reason
Nylon is a polyamide, so its chain contains -CO-NH⁻ connections.Working
A: nylon; amide linkage, -CONH⁻.Identify structure B
2 marksMethod
Name Terylene and its ester linkage.Reason
Terylene is a polyester, so its chain contains -CO-O⁻ connections.Working
B: Terylene; ester linkage, -COO⁻.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark both polymer identities and both linkage names.
Challenge 5
Recover monomers from a polyester
Recycling transfer
Connect the ester linkage to hydrolysis
Hints
Hint 1: linkage
Hint 2: reverse the formation
View solution step by step
Name the chemical method
Method
Select depolymerisation rather than physical recycling.Reason
The aim is to change polymer molecules back into monomers, not merely reshape the material.Working
Method: chemical recycling by depolymerisation.Apply the polyester example
Method
Use water with an acid catalyst to hydrolyse the ester linkages.Reason
Hydrolysis reverses the linkage-forming process and recovers the two monomer types.Working
polyester + water → [acid\ catalyst] diol + dicarboxylic\ acid.
7. Mind Stretchers
Mind stretcher 1: Spot the wrong monomerExtension
Question: A polymer has repeating unit -CH₂-CH(CH₃)⁻. A student says its monomer is propane, CH₃CH₂CH₃. Explain precisely what is wrong and give the correct monomer.
Show Answer
This is an addition polymer, so its monomer must be an alkene with a C = C bond.
CH₃CH₂CH₃ has no C = C, so it cannot form this polymer by addition polymerisation.
The correct monomer is CH₂ = CHCH₃ (propene).
Final: restore the double bond between the two backbone carbons to obtain propene.
Mind stretcher 2: “Biodegradable” overclaimExtension
Question: A student writes: “Biodegradable plastics solve plastic pollution completely.” Explain why “completely” is unsafe.
Show Answer
Breakdown depends on conditions (temperature, oxygen, microbes). In the wrong conditions, biodegradables can still persist.
Also, biodegradable plastics do not stop littering or all microplastic problems.
Final: Biodegradable does not mean instant or guaranteed breakdown in all conditions.
8. Quiz
Test addition and condensation structures, nylon and Terylene linkages, monomer deductions, and physical versus chemical recycling.