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Module 1 · L11 of 21 30 min ⚡ +50 XP in Learn · +25 to complete Year 11 · Module 1 · IQ4

Polymers: Structure and Properties

Today's hook, A supermarket bag and a bulletproof vest are both long chains of repeating units. One rips on a sharp corner. The other stops a bullet.

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Warm up and recall

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Recall, your gut answer first

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Plastic bags made from low-density polyethylene (LDPE) are soft and stretchy, while plastic milk bottles made from high-density polyethylene (HDPE) are much more rigid. Both are made from the same monomer, ethene. What could explain why the same monomer produces such different physical properties?

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What you'll master, and the words for it

What you'll master

Know

Key facts

The difference between monomers and polymers
Addition and condensation polymerisation
Common polymers and their uses (polyethylene, PVC, nylon, polyester)

Understand

Concepts

How monomer structure determines polymer properties
How chain length, branching, and cross-linking affect properties
Why IMFs between polymer chains matter for physical properties

Can do

Skills

Identify monomers from polymer structures and vice versa
Compare addition and condensation polymerisation mechanisms
Relate polymer structural features to observed physical properties

Beyond the syllabus. This whole lesson is extension. Module 1 does not name polymer chemistry as a requirement, so addition polymers, condensation polymers and the thermoplastic/thermoset split will not be examined as topics in their own right, and none of it is a mastery gate for the module. What the lesson is good for: it is the clearest worked example of the bonding and intermolecular-force ideas that are assessed. Every "why is this polymer stiffer, or higher-melting" answer below turns out to be an IMF answer. Do it for that. If you are short of time, Lesson 10, Intermolecular Forces and Physical Properties, is the one to prioritise.

Addition Polymerisation

core concept

In addition polymerisation, alkene monomers (containing C=C) react through a chain reaction mechanism. Each C=C bond opens, and the carbons form new single bonds to adjacent monomers. No atoms are lost every atom in the monomers appears in the polymer.

General equation: n(CH₂=CHR) →,[CH₂–CHR]ₙ, where R is the substituent group that varies between polymers.

Monomer	Polymer	Substituent R	Key property / use
Ethylene (CH₂=CH₂)	Polyethylene (PE)	–H	Flexible, chemical resistant; bags, bottles, piping
Propylene (CH₂=CHCH₃)	Polypropylene (PP)	–CH₃	Stiffer than PE; food containers, carpet fibre
Vinyl chloride (CH₂=CHCl)	PVC	–Cl	Rigid or flexible (with plasticiser); pipes, electrical insulation
Styrene (CH₂=CHC₆H₅)	Polystyrene (PS)	–C₆H₅	Rigid or foamed; packaging, insulation
Tetrafluoroethylene (CF₂=CF₂)	PTFE (Teflon)	–F	Non-stick, high MP; cookware, bearings

Why does,Cl make PVC stiffer than PE? The C–Cl bond is polar (Cl is electronegative) → PVC chains have permanent dipoles → dipole-dipole forces between chains → stronger IMFs than the dispersion-only PE → higher resistance to deformation. Adding plasticisers (small molecules that wedge between chains) disrupts these IMFs, making PVC flexible.

Addition polymerisation: alkene monomers (with C=C) join in a chain reaction, no byproduct, 100% atom economy. General equation: n(CH₂=CHR) →,[CH₂–CHR]ₙ–. Common polymers: PE (R=H), PP (R=CH₃), PVC (R=Cl), PS (R=C₆H₅). The R group changes properties: PVC is stiffer than PE because polar C–Cl bonds add dipole–dipole forces between chains.

Pause, copy the highlighted definition into your book before moving on.

Match: connect each monomer to the polymer it forms by addition polymerisation.

Ethylene (CH₂=CH₂)
Propylene (CH₂=CHCH₃)
Vinyl chloride (CH₂=CHCl)
Tetrafluoroethylene (CF₂=CF₂)

PVC
PTFE (Teflon)
Polyethylene (PE)
Polypropylene (PP)

Condensation Polymerisation

core concept

Condensation polymerisation requires monomers with two reactive functional groups (bifunctional monomers). Each reaction between functional groups releases a small molecule, usually water (H₂O) or HCl. This reaction is called a condensation reaction.

Polyester (e.g. PET, polyethylene terephthalate)

Formed from a diol (2 ×,OH groups) + a dicarboxylic acid (2 ×,COOH groups). At each junction, an ester linkage (–COO–) forms and water is released.

Monomers → PET: Ethylene glycol (HO–CH₂CH₂–OH) + Terephthalic acid (HOOC–C₆H₄–COOH) →,[–OC–C₆H₄–CO–O–CH₂CH₂–O–]ₙ, + n H₂O
Uses: Plastic drink bottles, polyester clothing fibre.

Polyamide (e.g. Nylon-6,6)

Formed from a diamine (2 ×,NH₂ groups) + a dicarboxylic acid. At each junction, an amide linkage (–CO–NH–) forms and water is released.

Monomers → Nylon-6,6: Hexamethylenediamine (H₂N–(CH₂)₆–NH₂) + Adipic acid (HOOC–(CH₂)₄–COOH) →,[–NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–]ₙ, + n H₂O
Uses: Clothing fibre, rope, toothbrush bristles, engineering components.

Addition

C=C double bond

None (100% atom economy)

C–C bond (chain)

PE, PVC, polystyrene, PTFE

Condensation

2 functional groups (bifunctional)

Small molecule released (H₂O, HCl)

Ester (–COO–), amide (–CONH–), etc.

Nylon, polyester (PET), polycarbonate

We just saw that addition polymerisation joins alkene monomers with no byproduct. That raises a question: how are condensation polymers like nylon and PET made, and how do they differ from addition polymers? This card answers it → condensation uses bifunctional monomers and releases a small molecule as a byproduct (often water, but it can be HCl, methanol or another small molecule, depending on the monomers).

Condensation polymerisation: bifunctional monomers (two reactive groups) join while releasing a small molecule (usually H₂O). Polyester (PET) = diol + dicarboxylic acid → ester linkage (–COO–) + H₂O. Polyamide (Nylon-6,6) = diamine + dicarboxylic acid → amide linkage (–CO–NH–) + H₂O. Key distinction: condensation always releases a small byproduct; addition releases none.

Add the highlighted distinction to your notes before the check below.

Write it out: in one or two sentences, describe what is released when nylon-6,6 forms and where it comes from.

Structural Features and Physical Properties

core concept

As you learned in L10, IMF strength determines physical properties. For polymers, the same rules apply, but the sheer size of polymer chains and how they interact also matters enormously.

Effect on properties

↑ chain length → ↑ total IMF surface → ↑ MP/viscosity/tensile strength

↑ branching → chains can't pack closely → ↓ density, ↓ MP, ↑ flexibility

Covalent bonds between chains → rigid, insoluble, thermosetting

↑ polarity → ↑ dipole-dipole or H-bonding between chains → ↑ MP, ↑ stiffness

Example

Wax (short PE) vs. HDPE plastic (long PE)

LDPE (branched, flexible bags) vs. HDPE (linear, rigid pipes)

Vulcanised rubber, bakelite, epoxy resin

Nylon (N–H···O H-bonds) vs. polyethylene (dispersion only)

Thermoplastic vs. thermosetting: Thermoplastics (PE, PVC, nylon) soften on heating, chains can slide past each other because only IMFs hold them together. Thermosetting polymers (bakelite, epoxy) are cross-linked with covalent bonds, heating cannot soften them; they char or decompose. This determines whether a polymer can be remelted: thermoplastics can be remelted and remoulded; thermosets cannot.

Environmental reality check. Two ideas here are easy to overstate. (1) Addition polymerisation has about 100% atom economy, but that figure describes only the polymerisation reaction; the real environmental footprint also depends on initiators and catalysts, additives and plasticisers, the energy used, the reaction yield, the (usually fossil) feedstock, and what happens at end-of-life. (2) Being a thermoplastic means a polymer is technically remeltable, which is not the same as being recycled in practice: real recycling also needs collection, sorting, freedom from contamination and a market for the recyclate, and the polymer can degrade a little each time it is remelted. Treat "100% atom economy" and "recyclable" as the start of a lifecycle evaluation, not the conclusion.

We just saw how condensation polymers form. That raises a question: what determines whether a polymer is rigid or flexible, tough or soft? This card answers it → chain length, degree of branching, and the type of IMFs between chains all control mechanical properties.

Polymer physical properties are controlled by chain structure: longer chains → more IMF contact area → higher MP and tensile strength. More branching → chains cannot pack closely → lower density, lower MP, more flexible. Polar groups (N–H, O–H, C=O) → H-bonding or dipole–dipole between chains → higher MP and stiffness. Thermoplastics (only IMFs between chains) can be remelted; thermosetting polymers (covalent cross-links) char when heated.

Pause, write the highlighted structure-property rule into your book.

Quick check: which property is characteristic of a thermosetting polymer such as bakelite or epoxy?

Polymer Chain Branching

core concept

Branched LDPE chains cannot pack closely; straight HDPE chains pack tightly for stronger forces.

LDPE (low-density PE) = heavily branched → chains cannot pack closely → lower density, flexible → used for shopping bags. HDPE (high-density PE) = mostly linear → chains pack closely → higher density, rigid → used for pipes and bottles. Same monomer (ethylene); different processing conditions create the branching difference. Greater chain contact area → stronger total dispersion forces → harder to deform.

Add the highlighted LDPE/HDPE comparison to your notes before the check below.

Match: connect each structural feature of polyethylene to the property it produces.

Many short side-branches (LDPE)
Long, mostly linear chains (HDPE)
Covalent cross-links between chains
Very short polymer chains

Chains pack closely → strong IMFs → rigid, high-density material
Low total IMF → low MP, soft, wax-like solid
Chains cannot pack closely → flexible, lower-density bag material
Cannot melt, heating chars or decomposes the material (thermoset)

Short Answer Questions

core concept

6. Compare addition and condensation polymerisation. In your answer, describe the monomer requirements, the mechanism, and the products (including any byproducts) for each. Provide one named example of each type. 5 MARKS

✏️ Answer in your book

7. LDPE (low-density polyethylene) is used for flexible shopping bags while HDPE (high-density polyethylene) is used for rigid pipes. Both are made from the same monomer (ethylene). Explain how the difference in chain structure leads to these different applications. 4 MARKS

✏️ Answer in your book

8. Nylon-6,6 has a higher melting point (265°C) than polyethylene (~130°C for HDPE), despite both being addition-type synthetic polymers. Explain the molecular basis for this difference, referencing IMF types. 3 MARKS

✏️ Answer in your book

We just saw the LDPE/HDPE branching comparison. That raises a question: how do you structure written exam answers comparing different polymers or polymer types? This card answers it → identify the monomer type, the polymerisation type, and the dominant IMF between chains, then connect each to the property asked about.

For exam answers: addition needs C=C monomer, no byproduct; condensation needs bifunctional monomers, releases a small molecule (usually water, sometimes HCl or methanol). For LDPE vs HDPE: branched → bags (flexible); linear → pipes (rigid). For nylon vs PE MP comparison: nylon has,CO–NH, → N–H···O=C H-bonding between chains; PE has only dispersion → nylon MP (265°C) ≫ HDPE (~130°C). Always state "more energy required to overcome stronger IMFs".

Pause, copy the highlighted comparison strategy into your book before moving on.

Write it out: in one or two sentences, explain why nylon-6,6 (MP 265°C) melts at a higher temperature than HDPE (MP ~130°C).

Worked examples

Worked examples · reveal as you go

Worked example +5 XP on full reveal

The following repeat unit is found in a common polymer:,[CH₂–CHCl]ₙ–. (a) Identify the monomer. (b) Classify the polymerisation type (addition or condensation). (c) Explain why this polymer is stiffer than polyethylene.

Repeat unit:,CH₂–CHCl–. Single bonds throughout the chain, no ester or amide linkages present.

Identify the repeat unit by counting atoms between chain bonds.

Monomer: CH₂=CHCl (vinyl chloride). Replace the two C–C bonds that form during polymerisation with one C=C double bond.

In addition polymerisation, the monomer has the C=C bond that opens during reaction.

No small molecule byproduct. No ester/amide linkages. C=C double bond in monomer.

These features all point to addition polymerisation, not condensation.

The,Cl is electronegative (χ = 3.0), so the C–Cl bond is polar. This creates permanent dipoles along each PVC chain.

Electronegativity difference causes bond polarity in PVC.

Dipole–dipole forces between PVC chains are stronger than the dispersion forces between PE chains (which has only,H). Stronger IMFs require more energy to overcome, so PVC is stiffer.

Stiffness correlates directly with IMF strength between chains.

Worked example +5 XP on full reveal

Two polymers A and B are tested: A has MP 130°C, is rigid at room temperature, and does not dissolve in any solvent. B has MP 65°C, is flexible at room temperature, and dissolves in some organic solvents. Suggest the structural feature responsible for each difference.

A: MP 130°C (high), rigid, insoluble in all solvents.

Gather the data for polymer A and identify which properties are most important.

Insolubility in all solvents suggests covalent cross-links between chains. Solvents cannot penetrate or break covalent bonds, unlike IMFs which solvents can displace.

Only covalent cross-links explain complete insolubility.

B: MP 65°C (low), flexible, dissolves in organic solvents.

Gather the data for polymer B.

Dissolves in solvents → only IMFs between chains, not covalent bonds. Solvent molecules can compete with chain-chain interactions. Low MP → weak IMFs overall.

Solubility indicates the presence of only IMFs, which can be overcome by solvents.

A is a thermosetting polymer (cross-linked, rigid, non-recyclable). B is a thermoplastic polymer (IMFs only, flexible, recyclable).

The structural differences (cross-links vs. IMFs) explain all the observed property differences.

Misconception to fix

Common errors · the 3 traps that cost marks

Misconception to fix

Wrong: Addition polymers and condensation polymers both release a small molecule during formation.

Misconception to fix

Right: Addition polymers form by monomers adding together with no by-product (e.g., polyethylene from ethene). Condensation polymers form with the loss of a small molecule like water or HCl. The presence or absence of a by-product is the defining distinction between the two polymerisation types.

Confusing thermoplastics with thermosets

Students often write that "all plastics can be melted and remoulded". Only thermoplastics (held by IMFs only, PE, PVC, nylon) can. Thermosetting polymers (bakelite, epoxy, vulcanised rubber) have covalent cross-links between chains, so heating chars or decomposes them, they cannot be reshaped or recycled by melting.

Fix: Identify whether the polymer is held together by IMFs (thermoplastic, recyclable) or covalent cross-links (thermoset, not recyclable) before answering questions about heating.

Drill, then revisit

Quick-fire practice · 5 reps +2 XP per reveal

Name the monomer of polyethylene and write the general equation for its polymerisation.

What small molecule is released when nylon-6,6 forms? Where does it come from?

Classify each as addition or condensation polymer: PVC, PET, polystyrene, nylon-6,6.

Why is LDPE softer and less dense than HDPE, even though they share the same monomer?

Bakelite is insoluble and chars on heating instead of melting. What does this tell you about its structure?

Working · 1

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Revisit your thinking

Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?

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Short answer

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Q1. 6. Compare addition and condensation polymerisation. In your answer, describe the monomer requirements, the mechanism, and the products (including any byproducts) for each. Provide one named example of each type.

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Q2. 7. LDPE (low-density polyethylene) is used for flexible shopping bags while HDPE (high-density polyethylene) is used for rigid pipes. Both are made from the same monomer (ethylene). Explain how the difference in chain structure leads to these different applications.

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Q3. 8. Nylon-6,6 has a higher melting point (265°C) than polyethylene (~130°C for HDPE), despite both being addition-type synthetic polymers. Explain the molecular basis for this difference, referencing IMF types.

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📖 Comprehensive answers (click to reveal)

Activity 1

1. (a) Monomer: CH₂=CHCH₃ (propylene/propene). (b) Addition polymerisation, the repeat unit has no ester or amide linkages; the monomer had a C=C double bond. (c) PP and PE would have similar MPs (both ~130°C) because both have only dispersion forces between chains. The methyl group in PP adds slightly more electrons per repeat unit → marginally stronger dispersion forces → PP is generally slightly stiffer and has a slightly higher MP than LDPE, but similar to HDPE. The key difference is physical: PP is stiffer due to greater chain rigidity from the methyl substituent.

2. (a) Condensation polymerisation. (b) Amide linkage:,CO–NH, (connecting a carbonyl C to a nitrogen). (c) Water (H₂O), released when,COOH reacts with,NH₂:,COOH + H₂N, →,CO–NH, + H₂O.

3. Nylon has N–H bonds in its amide linkages (–CO–NH–). The N–H group can form hydrogen bonds (N–H···O=C) with adjacent nylon chains because N is electronegative enough to create H-bonding. These hydrogen bonds are far stronger than the dispersion-only forces between PE chains. Overcoming nylon's H-bonds requires more energy → higher MP (265°C vs ~130°C).

Activity 2

A: HDPE's linear chains can pack closely together in a regular arrangement → high packing density → higher physical density. Close packing also maximises the surface area contact between chains → stronger total dispersion forces → chains are harder to separate → higher tensile strength. LDPE's branches prevent close packing: side chains physically block neighbouring chains from approaching → larger gaps between chains → lower density and weaker total dispersion forces → lower tensile strength.

B: UHMWPE has the same monomer and negligible branching as HDPE, but the chains are far longer (molecular weight ~3–6 million g/mol vs ~50,000 for HDPE). Longer chains → more total IMF contact area along each chain → much stronger total adhesion between chains even though the force per unit area is the same (dispersion only) → exceptional tensile strength. Injection moulding difficulty: very long entangled chains have extremely high viscosity even when melted, the polymer barely flows under typical moulding pressures, making processing very difficult.

❓ Multiple Choice

Addition polymerisation requires C=C. Bifunctional monomers with,COOH,,OH, or,NH₂ are for condensation polymerisation.

Ester formation (diol + dicarboxylic acid):,OH + HOOC, →,COO, + H₂O. Water is always the byproduct for diol/diacid condensation.

Insolubility + resistance to heating = covalent cross-links. Long chains alone still dissolve (HDPE does dissolve in hot organic solvents). H-bonds and dispersion forces are broken by solvents.

Plasticisers are small molecules that wedge between PVC chains, disrupting dipole-dipole interactions between chain segments → chains can slide past each other more easily → more flexible. They don't break covalent bonds.

The,CO–NH, linkage is an amide bond = condensation polymerisation product from,COOH +,NH₂ groups → releases H₂O. This is Nylon-6 (one monomer type: 6-aminohexanoic acid or caprolactam ring-opening).

Short Answer Model Answers

Q6 (5 marks): Addition polymerisation: monomer requires a C=C double bond (alkene); the double bond opens and adjacent monomers form new C–C single bonds in a chain reaction; products are only the polymer, no byproduct (100% atom economy); example: polyethylene from ethylene (CH₂=CH₂) (2 marks). Condensation polymerisation: monomer must be bifunctional (two reactive functional groups, e.g.,COOH and,OH, or,COOH and,NH₂); functional groups react to form ester or amide linkages; byproduct (usually H₂O) is released at each junction; products are polymer + small molecule; example: nylon-6,6 from hexamethylenediamine + adipic acid, releasing H₂O (3 marks).

Q7 (4 marks): LDPE has highly branched chains (1 mark). Branches prevent adjacent chains from packing closely → less dense, fewer chain–chain contacts → weaker total dispersion forces between chains (1 mark) → flexible and easy to stretch → suitable for thin flexible bags. HDPE has linear chains with minimal branching (1 mark). Linear chains pack closely in an ordered arrangement → high density → greater chain–chain contact area → stronger total dispersion forces → rigid and strong (1 mark) → suitable for pressure pipes.

Q8 (3 marks): Nylon contains amide linkages (–CO–NH–); the N–H groups can form hydrogen bonds (N–H···O=C) with the carbonyl oxygen on adjacent nylon chains (1 mark). Hydrogen bonding is significantly stronger than dispersion forces (the only IMF in polyethylene, which has only C–H groups) (1 mark). More energy must be supplied to overcome nylon's hydrogen bonds during melting → higher melting point (265°C vs ~130°C) (1 mark).

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