Polymers

Polymers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Polymers are high molecular mass macromolecules (10³–10⁷ u) built from repeating monomers linked by covalent bonds. The number of repeat units (–mer) is the degree of polymerisation (DP) = M(polymer)/M(monomer). Two routes form them: addition (chain-growth) polymerisation of alkenes (no by-product; e.g., polythene, PVC, Teflon, polystyrene) and condensation (step-growth) polymerisation with elimination of H₂O or HCl (e.g., nylon-6,6, terylene, bakelite). Natural rubber is cis-1,4-polyisoprene; vulcanisation with sulphur cross-links the chains, raising elasticity and strength. Bakelite (phenol + formaldehyde) is a thermosetting resin that cannot be remoulded, whereas polythene and PVC are thermoplastics that soften on heating. JEE tests classification (source/structure/molecular forces), the four named synthetic rubbers, and the vulcanisation equation. Remember: homopolymer = one monomer; copolymer = two or more monomers.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification by source and structure

• Natural: starch, cellulose, proteins, nucleic acids, natural rubber.
• Semi-synthetic: cellulose nitrate (gun-cotton), rayon (regenerated cellulose).
• Synthetic: polythene, PVC, nylon, bakelite, terylene, neoprene.
• Linear (HDPE, nylon) pack tightly → fibres; branched (LDPE) lowers density; cross-linked (vulcanised rubber, bakelite) form 3-D networks.

Classification by molecular forces

Type	Forces	Behaviour	Examples
Elastomers	Weakest	Elastic, reversible stretching	Natural rubber, neoprene
Fibres	Strong H-bonds / dipoles	High tensile strength, crystalline	Nylon-6,6, terylene
Thermoplastics	Intermediate	Soften on heating, remouldable	Polythene, PVC, polystyrene
Thermosetting	Extensive cross-links	Set permanently, cannot remould	Bakelite, urea-formaldehyde

Addition polymerisation — free radical mechanism

1. Initiation: Benzoyl peroxide → phenyl radical → adds to CH₂=CHR giving R–CH₂–ĊHR.
2. Propagation: Radical keeps adding monomers, chain grows.
3. Termination: Combination (R–R) or disproportionation ends the chain. Monomers: ethylene → polythene, vinyl chloride → PVC, tetrafluoroethylene → Teflon, styrene → polystyrene.

Condensation polymerisation

Each step ejects a small molecule (H₂O or HCl). Examples:

• Nylon-6,6: hexamethylenediamine + adipic acid → –[NH(CH₂)₆NHCO(CH₂)₄CO]ₙ– + n H₂O.
• Nylon-6: caprolactam ring-opening (H₂O initiator) → –[NH(CH₂)₅CO]ₙ–.
• Terylene (Dacron): ethylene glycol + terephthalic acid → –[OCH₂CH₂OOC-C₆H₄-CO]ₙ– + n H₂O.
• Bakelite: phenol + HCHO; acidic medium → linear novolac, basic medium → cross-linked resole.

Synthetic rubbers

• Neoprene: polychloroprene, from CH₂=CCl–CH=CH₂.
• Buna-S (SBR): 1,3-butadiene + styrene (75 : 25).
• Buna-N (NBR): 1,3-butadiene + acrylonitrile; oil-resistant.

Vulcanisation

Natural rubber + ~3–5% S at 100–150 °C → –CH₂–CH=C(CH₃)–CH₂– + S → cross-linked vulcanised rubber: harder, less plastic, more elastic.

Exam pointers

JEE Main frequently asks one MCQ on (i) identifying monomer from polymer structure, (ii) classification category, (iii) the vulcanisation reagent, and (iv) Buna-S vs Buna-N monomer pairs.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Molecular-weight distributions

Real polymers are polydisperse. Mₙ (number-average) = Σ NᵢMᵢ / Σ Nᵢ weights each chain equally; M_w (weight-average) = Σ NᵢMᵢ² / Σ NᵢMᵢ weights heavier chains more. The polydispersity index PDI = M_w / Mₙ equals 1.0 for a perfectly monodisperse sample and exceeds 1 for all real polymers (≈1.5–3 for radical addition polymers, ≈2 for many commercial resins). A high PDI means broad chain-length spread, which broadens melting behaviour and affects mechanical strength.

Tacticity and stereoregularity

For –[CH₂–CH(R)]ₙ– chains (e.g., polystyrene, polypropylene), pendant groups arrange in three ways: isotactic (all on one side, regular → partially crystalline, high Tg), syndiotactic (alternating sides, regular → crystalline), atactic (random → amorphous, soft). Ziegler–Natta catalysts (TiCl₄ + Al(C₂H₅)₃) force stereoregular growth — this is why HDPE is linear and crystalline while LDPE (free-radical, branched) is not.

Glass transition temperature (Tg) and crystallinity

Below Tg the polymer is glassy and brittle; above Tg amorphous regions gain mobility. Cross-linking raises Tg sharply (rubber: Tg ≈ –70 °C; ebonite, fully cross-linked: hardens). Thermoplastics have moderate Tg; thermosets decompose before they flow because of covalent networks.

Common mistakes to avoid

• Monomer vs repeat unit: PVC repeat unit is –CH₂–CHCl–, but the monomer is CH₂=CHCl (one double bond opened).
• Addition polymerisation has NO by-product; condensation ALWAYS ejects H₂O or HCl.
• Natural rubber = cis-1,4-polyisoprene; gutta-percha is the trans isomer and is hard, not elastic.
• Buna-S contains styrene, not styrene oxide; Buna-N contains acrylonitrile (CH₂=CH–CN), not acrylic acid.
• Novolac is linear (acid catalyst, needs further HCHO to harden), while resole is already cross-linked (base catalyst, sets on heating) — many students swap them.

Practice prompts

1. Identify the monomer and predict the classification (thermoplastic/thermoset/elastomer/fibre) for the polymer –[O–CH₂–CH₂–O–CO–C₆H₄–CO]ₙ–.
2. A polymer has Mₙ = 50 000 g mol⁻¹ and PDI = 2.4. If the repeat unit is –CH₂–CHCl– (M = 62.5), calculate the average DP and estimate M_w.

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