Polymers: Structure and Properties
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Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Key facts
- The difference between monomers and polymers
- Addition and condensation polymerisation
- Common polymers and their uses (polyethylene, PVC, nylon, polyester)
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
Skills
- Identify monomers from polymer structures and vice versa
- Compare addition and condensation polymerisation mechanisms
- Relate polymer structural features to observed physical properties
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.
| 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 |
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 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.
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.
Uses: Clothing fibre, rope, toothbrush bristles, engineering components.
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 always releases H₂O as a byproduct.
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.
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.
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?
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)
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
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
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
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 monomer, releases H₂O. 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 · reveal as you go
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.
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.
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.
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?
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Pick your answer, then rate your confidence — that tells the system what to drill next.
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.
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.
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.
📖 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
1. B — Addition polymerisation requires C=C. Bifunctional monomers with –COOH, –OH, or –NH₂ are for condensation polymerisation.
2. C — Ester formation (diol + dicarboxylic acid): –OH + HOOC– → –COO– + H₂O. Water is always the byproduct for diol/diacid condensation.
3. A — 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.
4. D — 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.
5. B — 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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