Think of polymer chains like trains. A monomer is a single train carriage. Polymerisation is coupling thousands of carriages end-to-end. The type of carriage (monomer structure) and how they connect determines everything — from the flexibility of a shopping bag to the strength of a bulletproof vest. Understanding polymer structure is a direct application of the bonding and IMF principles you've already built up.
Core Content
Wrong: Addition polymers and condensation polymers both release a small molecule during formation.
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.
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 |
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.
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.
Formed from a diamine (2 × –NH₂ groups) + a dicarboxylic acid. At each junction, an amide linkage (–CO–NH–) forms and water is released.
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.
Worked Examples
Step 1 — Identify the repeat unit
Repeat unit: –CH₂–CHCl–
Each repeat unit contains 2 carbons. Single bonds throughout in the chain.
No ester or amide linkages present in the repeat unit.
Step 2 — Work backwards to find monomer (addition)
For addition polymerisation: monomer = repeat unit with one C=C bond replacing the two C–C bonds that form during polymerisation.
Monomer: CH₂=CHCl (vinyl chloride)
Step 3 — Classify polymerisation type
No small molecule byproduct, no ester/amide linkages, C=C double bond in monomer → Addition polymerisation
Step 4 — Explain relative stiffness
PVC has –Cl substituent. Chlorine is electronegative (χ = 3.0) → C–Cl bond is polar → each PVC chain has permanent dipoles (δ+ and δ−) along its length.
Dipole-dipole forces act between adjacent PVC chains.
Polyethylene has only –H substituents → non-polar → only dispersion forces between chains.
Dipole-dipole forces > dispersion forces → PVC chains are held more tightly → greater resistance to chain sliding → PVC is stiffer than PE.
Step 1 — Analyse A's properties
MP 130°C (relatively high for a polymer) AND insoluble in all solvents.
Does not dissolve = cannot be separated by any solvent → suggests covalent cross-linking between chains (solvents can't penetrate to break apart IMFs because there are covalent bonds between chains).
High MP + rigid = strong resistance to chain movement.
Conclusion: A is likely a thermosetting/cross-linked polymer.
Step 2 — Analyse B's properties
MP 65°C (low) AND dissolves in organic solvents.
Dissolves = only IMFs between chains (no covalent cross-links) → solvent molecules can displace chain-chain interactions.
Flexible + low MP = weak IMFs, chains can slide easily.
Conclusion: B is likely a thermoplastic polymer (short chain or non-polar substituents).
Step 3 — Attribute differences
Rigidity/non-solubility of A: covalent cross-links between chains lock structure. No solvent disrupts covalent bonds.
Flexibility/solubility of B: only dispersion/dipole-dipole IMFs hold chains together; solvent molecules can compete for these interactions and cause dissolution; heating above 65°C provides energy to overcome IMFs.
Activities
1 The repeat unit of polypropylene is –[CH₂–CH(CH₃)]ₙ–. (a) Write the structural formula of the monomer. (b) Name the type of polymerisation. (c) Predict whether polypropylene or polyethylene would have a higher melting point. Justify with reference to IMFs.
2 Nylon-6,6 has the repeat unit –[NH–(CH₂)₆–NH–CO–(CH₂)₄–CO]ₙ–. (a) What type of polymerisation produced nylon? (b) Identify the linkage that connects the monomers. (c) What small molecule was released at each step?
3 Explain why nylon has a higher melting point than polyethylene, even though both are thermoplastic polymers.
| Polymer type | Structure | Density (g/cm³) | Tensile strength (MPa) | Melting point (°C) |
|---|---|---|---|---|
| LDPE (low density) | Highly branched chains | 0.91–0.94 | 8–20 | 105–115 |
| HDPE (high density) | Linear chains, minimal branching | 0.94–0.97 | 20–37 | 120–140 |
| UHMWPE (ultra high MW) | Very long linear chains | 0.93–0.94 | ~150 | 130–135 |
A Explain why HDPE has a higher density and tensile strength than LDPE, using molecular structural reasoning.
B UHMWPE has similar density to LDPE but much higher tensile strength. What structural feature explains this? Why would UHMWPE be difficult to process by injection moulding?
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Multiple Choice
5 random questions from a replayable lesson bank — feedback shown immediately
Short Answer
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
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).
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.
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).
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).
Return to your Think First response. You should now be able to explain the LDPE vs HDPE difference:
Climb platforms, hit checkpoints, and answer questions on addition and condensation polymers, monomers and structural properties. Quick recall from lessons 1–11.
🎮 Play Science Jump →Tick when you've finished all activities and checked your answers.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
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?
Before reading on, write your best answer. Think about how the polymer chains might be arranged differently in the two materials.