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
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.
| Feature | Addition | Condensation |
|---|---|---|
| Monomer requirement | C=C double bond | 2 functional groups (bifunctional) |
| Byproduct | None (100% atom economy) | Small molecule released (H₂O, HCl) |
| Linkage formed | C–C bond (chain) | Ester (–COO–), amide (–CONH–), etc. |
| Examples | PE, PVC, polystyrene, PTFE | Nylon, polyester (PET), polycarbonate |
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.
| Structural feature | Effect on properties | Example |
|---|---|---|
| Chain length (n) | ↑ chain length → ↑ total IMF surface → ↑ MP/viscosity/tensile strength | Wax (short PE) vs. HDPE plastic (long PE) |
| Branching | ↑ branching → chains can't pack closely → ↓ density, ↓ MP, ↑ flexibility | LDPE (branched, flexible bags) vs. HDPE (linear, rigid pipes) |
| Cross-linking | Covalent bonds between chains → rigid, insoluble, thermosetting | Vulcanised rubber, bakelite, epoxy resin |
| Polar substituents | ↑ polarity → ↑ dipole-dipole or H-bonding between chains → ↑ MP, ↑ stiffness | Nylon (N–H···O H-bonds) vs. polyethylene (dispersion only) |
Worked Examples
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?
Multiple Choice
1. Which feature of a monomer is required for addition polymerisation?
2. Which correctly identifies the small molecule released during polyester formation from a diol and a dicarboxylic acid?
3. An unknown polymer is rigid, does not soften on heating, and is insoluble in all solvents. The most likely structural explanation is:
4. PVC can be made either rigid (for pipes) or flexible (for electrical cable insulation) by adding plasticisers. How do plasticisers increase flexibility?
5. The repeat unit of a polymer is –[NH–(CH₂)₅–CO]ₙ–. This polymer was formed by:
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).
Tick when you've finished all activities and checked your answers.