How unit 4 is examined
This unit covers polymer basics, polymerization mechanisms, plastics versus rubbers, and the named polymers; the marks sit in the preparation, properties and uses of Bakelite, Nylon 6,6, Buna-S, PVC and Urea-Formaldehyde, then thermoplastic versus thermosetting, vulcanization and the free radical and ionic mechanisms.
Introduction
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>
Definition. <mark>A polymer is a giant molecule of very high molecular mass formed by joining thousands of small repeating units called monomers through covalent bonds; the process is polymerization.</mark>
Key points.
- The repeating unit of the chain is the monomer residue, and the number of units in a chain is the degree of polymerization $n$.
- Molecular mass of a polymer equals $n \times$ the mass of the repeating unit, typically $10^4$ to $10^6$.
- Ethylene ($CH_2{=}CH_2$) gives polyethylene $-(CH_2-CH_2)_n-$.
- A monomer must have functionality of at least two, that is, at least two bonding sites (a double bond or two reactive groups).
Types of polymerisation
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>
Definition. <mark>Addition polymerization joins unsaturated monomers by opening double bonds with no by-product; condensation polymerization joins monomers with two reactive groups by eliminating a small molecule such as $H_2O$ or HCl.</mark>
Key points.
- Addition polymers have the same empirical formula as the monomer, for example polythene, PVC and Teflon.
- Condensation polymers lose a small molecule at each linkage, so the polymer mass is less than the sum of monomers, for example nylon and Bakelite.
- Addition polymerization is a chain-growth process and condensation polymerization is a step-growth process.
- Copolymerization is addition polymerization of two different monomers, for example Buna-S.
Classification
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>
Definition. <mark>Polymers are large molecules made of repeating monomer units, classified by source, structure, molecular forces and mode of synthesis.</mark>
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Key points.
- By source, natural polymers occur in nature (starch, proteins, natural rubber), semi-synthetic ones are modified natural polymers (cellulose acetate, rayon) and synthetic ones are man-made (nylon, PVC).
- By structure, chains may be linear (HDPE), branched (LDPE) or cross-linked into a 3D network (Bakelite).
- By molecular forces, elastomers have weak forces, fibres have strong hydrogen bonding, thermoplastics soften on heating and thermosets harden permanently.
- By mode of synthesis, addition polymers form without by-product and condensation polymers release small molecules.
Asked: [7 marks] (Dec 2024) What are polymers? Give their classification with examples.
Mechanism of polymerisation (free radical and ionic)
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Definition. <mark>Chain-growth polymerization proceeds by initiation, propagation and termination through an active centre (free radical, carbocation or carbanion) that adds monomer units one at a time.</mark>
Functionality. It is the number of bonding sites in a monomer: bifunctional monomers give linear polymers and tri- or polyfunctional monomers give branched or cross-linked polymers.
Free radical (example: styrene or ethylene, $R\bullet$ from benzoyl peroxide).
Step 1: Initiation: I -> 2 R. then R. + CH2=CHX -> R-CH2-C.HX
Step 2: Propagation: R-CH2-C.HX + CH2=CHX -> R-CH2-CHX-CH2-C.HX (repeats n times)
Step 3: Termination: coupling (two radicals join) or disproportionation (H transfer gives one saturated and one unsaturated chain)
Ionic. Cationic polymerization uses Lewis acids ($BF_3$ + trace $H_2O$, $AlCl_3$) on electron-rich monomers such as isobutylene, via a carbocation $\sim CH_2-C^+(CH_3)_2$. Anionic polymerization uses bases ($NaNH_2$, $n$-BuLi) on electron-poor monomers such as styrene or acrylonitrile, via a carbanion.
| Basis | Free radical | Ionic |
|---|---|---|
| Initiator | Peroxides, azo compounds | Lewis acids (cationic), bases (anionic) |
| Active species | Neutral radical | Carbocation or carbanion |
| Temperature | Works at 60-100 C | Low temperature, often below 0 C |
| Solvent | Insensitive, water can be used | Very sensitive, dry non-polar or polar solvent |
| Termination | Coupling, disproportionation | Combination with counter-ion or solvent, transfer; anionic can be "living" |
| Basis | Chain growth | Step growth |
| --- | --- | --- |
| Growth | Only active chain ends add monomer | Any two molecules react |
| Molecular mass | High from the start | Rises slowly, high only near the end |
| By-product | None | Small molecule released |
| Example | PVC, polystyrene | Nylon 6,6, Bakelite |
Answer frame. Open with the definition of chain-growth polymerization; give the chain vs step table first, then the three free radical steps with equations, then cationic and anionic with one initiator each, and close with the free radical vs ionic table.
Asked: [7 marks] (Nov 2022) What is the difference between free radical and ionic polymerization? Asked: [7 marks] (Jun 2023) Write a short note on (i) Functionality (ii) free radical chain polymerization (iii) ionic polymerization. Asked: [7 marks] (Jun 2025) Differentiate chain growth and step growth polymerization. Explain free radical and ionic polymerization with suitable example.
Thermoplastic and thermosetting polymers
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Definition. <mark>Thermoplastics soften on heating and harden on cooling reversibly because their chains are linear or branched; thermosets set permanently on heating because they form a cross-linked 3D network.</mark>
| Basis | Thermoplastic | Thermosetting |
|---|---|---|
| Structure | Linear or branched chains | Cross-linked 3D network |
| Action of heat | Soften, can be remoulded again and again | Set permanently, char on strong heating |
| Formation | Addition, weak forces between chains | Condensation, strong covalent cross-links |
| Recycling | Recyclable | Not recyclable |
| Solubility | Soluble in suitable solvents | Insoluble, only swell |
| Strength | Soft, less brittle | Hard, rigid, brittle |
| Examples | PVC, polythene, nylon | Bakelite, urea-formaldehyde |
Galvanic series vs electrochemical series (part a of the same 14-mark question).
| Basis | Galvanic series | Electrochemical series |
|---|---|---|
| Basis | Potentials measured in a practical medium such as sea water | Standard electrode potentials at unit activity, 25 C |
| Materials | Metals and alloys | Pure metals and ions only |
| Use | Predicts which metal corrodes in real service | Predicts reducing power and cell EMF |
| Order | Can differ from the other series | Fixed standard order |
Answer frame. Open with the definition of each class; draw the table with at least six rows and end each column with two examples; for part (a) give the four-row table; close with "thermoplastics are recyclable, thermosets are not".
Asked: [14 marks] (Nov 2022) Differentiate (a) Galvanic series and Electrochemical series (b) Thermosetting polymers and Thermoplastic polymers, with suitable examples.
Biodegradable polymers
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>
Definition. <mark>Biodegradable polymers are polymers that microorganisms (bacteria, fungi) break down into $CO_2$, water and biomass under natural conditions.</mark>
Key points.
- Examples are PLA (polylactic acid), PHBV (poly-3-hydroxybutyrate-co-3-hydroxyvalerate) and starch-based plastics.
- PHBV is a copolymer of 3-hydroxybutanoic and 3-hydroxypentanoic acids; PLA is made from lactic acid.
- They contain ester or amide links that enzymes can hydrolyse, so they avoid plastic waste.
- Uses include packaging, disposable cutlery, surgical stitches and controlled drug delivery capsules.
Preparation, properties & uses of the following polymers- PVC, PMMA, Teflon, Nylon 6, Nylon 6:6, Polyester phenol formaldehyde, Urea- Formaldehyde, Buna N, Buna S
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>
Definition. <mark>Elastomers are polymers with weak intermolecular forces and coiled chains that can stretch to many times their length and return to shape; Buna-S and Buna-N are synthetic elastomers.</mark>
Key points (one line each: preparation, properties, uses).
- PVC. Free radical addition of vinyl chloride, $nCH_2{=}CHCl \xrightarrow{\text{peroxide}} -(CH_2-CHCl)_n-$. Hard, rigid, chemically resistant, brittle unless plasticised. Uses: pipes, cable insulation, flooring, raincoats.
- PMMA (Perspex). Addition of methyl methacrylate with peroxide. Transparent, light, tough, weather resistant. Uses: lenses, aircraft windows, signboards.
- Teflon. Tetrafluoroethylene $CF_2{=}CF_2$ with ammonium persulphate under pressure gives $-(CF_2-CF_2)_n-$. Very inert, non-stick, low friction, stable to about $300\,^\circ C$. Uses: non-stick pans, gaskets, seals.
- Nylon 6. Ring-opening of caprolactam with water at about $250\,^\circ C$. Strong, elastic, abrasion resistant. Uses: fibres, ropes, tyre cords.
- Nylon 6,6. Condensation of adipic acid $HOOC(CH_2)_4COOH$ with hexamethylenediamine $H_2N(CH_2)_6NH_2$ at about $270\,^\circ C$ with loss of water: $-[NH(CH_2)_6NHCO(CH_2)_4CO]_n-$. High tensile strength, abrasion resistance, melting point about $265\,^\circ C$. Uses: fibres, gears, ropes, bearings.
- Polyester (Terylene, PET). Ethylene glycol with terephthalic acid, losing water: strong, crease resistant. Uses: fabrics, bottles.
- Bakelite (phenol-formaldehyde). Phenol and formaldehyde with an acid catalyst (excess phenol) give o- and p-hydroxybenzyl alcohols, which condense into linear Novolac. Heating Novolac with hexamethylenetetramine ($-CH_2-$ cross-links) gives the 3D thermoset Bakelite. With a base and excess HCHO the first product is resol. Hard, rigid, scratch resistant, electrical insulator, heat and chemical resistant. Uses: switches, plugs, handles of cookware, brake linings.
- Urea-formaldehyde. Urea with formaldehyde forms mono- and dimethylolurea, which condense with loss of water into a cross-linked resin. Colourless, hard, brittle, poor water resistance. Uses: adhesives for plywood, buttons, electrical fittings.
- Buna-S (SBR). Copolymerization of 75% 1,3-butadiene with 25% styrene in emulsion with peroxide at about $5\,^\circ C$: $nCH_2{=}CH{-}CH{=}CH_2 + nC_6H_5CH{=}CH_2 \rightarrow -[CH_2CH{=}CHCH_2-CH_2CH(C_6H_5)]_n-$. High abrasion resistance and load bearing, poor oil resistance. Uses: tyres, conveyor belts, shoe soles.
- Buna-N (NBR). Butadiene with acrylonitrile in emulsion. Excellent oil and petrol resistance. Uses: oil seals, hoses, fuel tanks, gaskets.
Living polymers. Polymers whose chain ends stay active because there is no termination or chain transfer (anionic polymerization of styrene with $n$-BuLi); they resume growth on adding monomer and give narrow molecular mass and block copolymers. PVC itself is not living.
Answer frame. For each polymer write: monomer, equation, conditions; then properties (3), uses (3); for Bakelite draw the Novolac then cross-link scheme; for the Buna question begin with the elastomer definition; for the PVC question start with the living polymer definition.
Asked: [7 marks] (Jun 2022) What are living polymers? Give preparation, properties and uses of PVC. Asked: [7 marks] (Jun 2022, Dec 2023) What is Elastomers? Give the preparation and properties and uses of BUNA's. Write a short note on synthetic rubber with a suitable example. Asked: [7 marks] (Nov 2022) Write a note on preparation, properties and uses of (i) Bakelite (ii) Nylon 6,6. Asked: [7 marks] (Dec 2023, Dec 2024) Explain the preparation, properties and uses of Bakelite. Asked: [7 marks] (Jun 2023) Write a note on preparation, properties and uses of (i) Phenol-formaldehyde resin (ii) Buna-S. Asked: [7 marks] (Jun 2025) Distinguish between thermoplastic and thermosetting resins. Give preparation and application of Urea-Formaldehyde and PVC.
Vulcanization of rubber
<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>
Definition. <mark>Vulcanization is heating raw (natural) rubber with sulphur at 100-140 C so that sulphur forms cross-links between the polyisoprene chains and improves its properties.</mark>
Key points.
- Raw rubber is cis-1,4-polyisoprene $-(CH_2-C(CH_3){=}CH-CH_2)_n-$, soft and sticky, weak, with a large permanent set.
- Sulphur, about 3-5%, adds at the double bonds and forms $-S-S-$ bridges between chains; accelerators (ZnO, thiazoles) speed it up.
- Cross-links stop chains sliding, so the rubber becomes stronger, harder and more elastic.
- Tensile strength rises from about 2 MPa to about 20 MPa, and the rubber resists abrasion, heat and solvents better.
- Sulphur content controls hardness: 3-5% gives soft tyre rubber, 30% gives hard ebonite.
- Goodyear discovered the process in 1839.
Degree of freedom (also in the same 14-mark question): the number of variables (temperature, pressure, concentration) that can be changed independently without changing the number of phases, $F = C - P + 2$. Electron affinity belongs to Unit 7. Biodegradable polymers are covered above.
Answer frame. Open with the definition; write the cross-linking scheme (chains joined by $S-S$); develop points 3, 4 then 5; close with "vulcanized rubber is used for tyres and belts".
Asked: [14 marks] (Jun 2022) Write brief note on (any two) (i) Degree of freedom (ii) Biodegradable polymers (iii) Electron affinity (iv) Vulcanization of rubber.
Last-minute revision
- A polymer has $M \approx n \times$ repeating unit mass; addition gives no by-product, condensation loses $H_2O$ or HCl.
- Free radical initiators are benzoyl peroxide and azo compounds; ionic use $BF_3$ (cationic) and $NaNH_2$ (anionic).
- Termination in free radical is coupling or disproportionation.
- Thermoplastics are linear and remould; thermosets are cross-linked and do not.
- Bakelite: phenol + HCHO gives Novolac, then hexa cross-linking.
- Nylon 6,6 is adipic acid + hexamethylenediamine; nylon 6 is caprolactam.
- Buna-S is 75% butadiene + 25% styrene; Buna-N is butadiene + acrylonitrile.
- Teflon is $-(CF_2-CF_2)_n-$ with a non-stick surface.
- Vulcanization is rubber + 3-5% sulphur at 100-140 C.
- Degree of freedom $F = C - P + 2$.
- PLA and PHBV are biodegradable.
Memory hooks
- "Thermo-PLASTIC = PLAys again": remoulds; "thermoSET = SET forever".
- Bakelite: "Phenol + Formaldehyde, Novolac then Net".
- "SBR = Styrene, Butadiene, Road": tyres.
- "Nylon 6,6: 6 carbons diamine, 6 carbons diacid".
- "Sulphur Stitches rubber chains".
Coverage checklist
- Introduction: covered, no past questions.
- types of polymerisation: covered, no past questions.
- Classification: Dec 2024 (7 marks).
- mechanism of polymerisation (Free radical & Ionic polymerization): Nov 2022, Jun 2023, Jun 2025.
- Thermoplastic & Thermosetting polymers: Nov 2022 (14 marks).
- Elementary idea of Biodegradable polymers: covered; part of the Jun 2022 note question.
- preparation, properties & uses of the following polymers- PVC, PMMA, Teflon, Nylon 6, Nylon 6:6, Polyester phenol formaldehyde, Urea- Formaldehyde, Buna N, Buna S: Jun 2022 (two), Nov 2022, Jun 2023, Dec 2023, Dec 2024, Jun 2025.
- Vulcanization of Rubber: Jun 2022 (14 marks).