1. Polymers and their classification
Polymer: a macromolecule (about 10³–10⁷ g mol⁻¹) of many repeating structural units from monomers; polymerisation joins them. Degree of polymerisation = number of repeating units = polymer molar mass ÷ repeating-unit mass.
- Source: natural (proteins, cellulose, starch, natural rubber); semi-synthetic (cellulose acetate/rayon, cellulose nitrate, vulcanised rubber); synthetic (polythene, nylon, Buna-S).
- Structure: linear (HDPE, PVC), branched (LDPE), cross-linked/network (Bakelite, melamine).
- Mode of polymerisation: addition, condensation.
- Molecular forces: elastomers (weakest; rubber, Buna-S, neoprene), fibres (strongest, H-bonds; nylon 6,6, terylene), thermoplastics (intermediate; polythene, PVC, polystyrene), thermosetting (cross-link on heating, cannot be remelted; Bakelite, melamine).
- Homopolymer: one monomer (polythene, PVC, nylon 6). Copolymer: two or more monomers (Buna-S, Buna-N, nylon 6,6, terylene).
Formulas
- Degree of polymerisation
Key terms
- Monomer
A small molecule that joins with many others to form a polymer.
- Degree of polymerisation
The number of repeating units in a polymer chain.
- Thermosetting polymer
A polymer that becomes permanently hard on heating because of extensive cross-linking.
- Copolymer
A polymer formed from two or more different monomers.
2. Addition and condensation polymerisation
Addition (chain-growth): unsaturated monomers add without loss of any molecule; repeating unit = monomer composition. Free radical mechanism (initiator: benzoyl peroxide): initiation (R• adds to C=C), propagation (radical adds monomer after monomer), termination (two radicals combine).
Condensation (step-growth): bifunctional monomers react with loss of a small molecule (H₂O, HCl, CH₃OH): nylon 6,6, terylene, Bakelite.
| Addition | Condensation | |
|---|---|---|
| Monomer | Unsaturated | Bifunctional |
| By-product | None | H₂O, HCl, etc. |
| Growth | Chain | Step |
| Examples | Polythene, PVC, Teflon, PAN | Nylon 6,6, terylene, Bakelite |
| Step | What happens |
|---|---|
| Initiation | Peroxide → radicals; radical adds to monomer |
| Propagation | Radical adds monomer after monomer |
| Termination | Two radicals combine |
Key terms
- Addition polymerisation
Repeated addition of unsaturated monomers without loss of any small molecule.
- Condensation polymerisation
Repeated reaction of bifunctional monomers with loss of a small molecule such as water.
- Initiator
A substance, such as benzoyl peroxide, that forms free radicals to start chain polymerisation.
3. Important addition and condensation polymers
- LDPE: ethene, 1000–2000 atm, 350–570 K, peroxide initiator; branched; carry bags, squeeze bottles, insulation.
- HDPE: ethene, 333–343 K, 6–7 atm, Ziegler–Natta catalyst (triethylaluminium + TiCl₄); linear; buckets, pipes.
- Teflon: CF₂=CF₂, free radical or persulfate catalyst, high pressure; inert; non-stick coatings.
- Polyacrylonitrile: CH₂=CHCN, peroxide catalyst; acrylic fibres (orlon, acrilan).
- Nylon 6,6: H₂N(CH₂)₆NH₂ + HOOC(CH₂)₄COOH, high temperature; polyamide; bristles, fabrics. Repeating unit C₁₂H₂₂N₂O₂, .
- Nylon 6: caprolactam heated with water; tyre cords, ropes.
- Terylene (Dacron): ethylene glycol + terephthalic acid, 420–460 K, zinc acetate–antimony trioxide catalyst; polyester; fabrics, safety belts.
- Bakelite: phenol + formaldehyde → hydroxymethylphenols → Novolac (linear) → cross-linked Bakelite; combs, switches, handles.
- Melamine–formaldehyde: unbreakable crockery.
| Polymer | Monomer(s) | Mode |
|---|---|---|
| LDPE, HDPE | Ethene | Addition |
| Teflon | Tetrafluoroethene | Addition |
| PAN | Acrylonitrile | Addition |
| Nylon 6,6 | Hexamethylenediamine + adipic acid | Condensation |
| Nylon 6 | Caprolactam | Condensation (ring opening) |
| Terylene | Ethylene glycol + terephthalic acid | Condensation |
| Bakelite | Phenol + formaldehyde | Condensation |
Key terms
- Ziegler–Natta catalyst
Triethylaluminium with titanium tetrachloride, used to make linear high density polythene.
- Polyamide
A polymer whose units are joined by amide links, –CO–NH–, e.g. nylons.
- Polyester
A polymer whose units are joined by ester links, e.g. terylene.
4. Rubber
Natural rubber: cis-1,4-polyisoprene from latex; coiled chains, weak van der Waals forces → elastic. Drawbacks: soft and sticky above about 335 K, brittle below about 283 K, swells in non-polar solvents, poor abrasion resistance.
Vulcanisation: heating raw rubber with sulfur (and an additive such as zinc oxide) at 373–415 K; sulfur cross-links form at the reactive allylic sites, making rubber stiffer, stronger and elastic over a wider temperature range (tyre rubber about 5% sulfur).
Synthetic rubbers: neoprene (polychloroprene; homopolymer; oil resistant: hoses, gaskets); Buna-N (butadiene + acrylonitrile; copolymer; resists petrol: oil seals, tank linings); Buna-S (butadiene + styrene; copolymer; tough: tyres, soles).
Key terms
- Vulcanisation
Heating rubber with sulfur to form sulfur cross-links between chains.
- Elastomer
A rubber-like polymer that can be stretched and returns to its shape.
5. Molecular mass and biodegradable polymers
Polymer samples contain chains of different lengths, so averages are used:
- Number-average (osmotic pressure).
- Weight-average (light scattering, ultracentrifugation).
- PDI : 1 for monodisperse natural polymers; greater than 1 for synthetic polymers.
Biodegradable polymers contain groups (ester, amide) that micro-organisms can hydrolyse: PHBV (3-hydroxybutanoic + 3-hydroxypentanoic acid; packaging, orthopaedic devices, controlled drug release); nylon-2–nylon 6 (glycine + aminocaproic acid).
| Polymer | Biodegradable? | Why |
|---|---|---|
| Polythene | No | Only C–C and C–H bonds |
| PHBV | Yes | Ester links hydrolysed by microbes |
| Nylon-2–nylon 6 | Yes | Amide links from glycine units |
Formulas
- Number-average molar mass
- Weight-average molar mass
- Polydispersity index
Key terms
- Polydispersity index
Ratio of weight-average to number-average molar mass, Mw/Mn.
- Biodegradable polymer
A polymer that micro-organisms can break down in the environment.
Exam tips
Common mistakes
- Calling Bakelite a thermoplastic, or polythene a thermoset.
- Saying every condensation polymer is a copolymer (nylon 6 is a homopolymer).
- Forgetting that two water molecules are lost per nylon 6,6 repeating unit.
- Writing trans-polyisoprene for natural rubber.
- Using Mi instead of Mi² in the numerator of Mw.
How marks are usually awarded
- 1-mark items: monomer(s), type of polymer, a use.
- Preparations (2–3 marks): monomers, conditions or catalyst, and the type of polymerisation.
- Calculations: degree of polymerisation, repeating-unit mass, Mn, Mw and PDI.