Polymers

Polymers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your NEET UG exam.

Polymers are high molecular mass macromolecules built from repeating monomer units joined by covalent bonds through polymerisation. The count of repeating units (n) is the degree of polymerisation, given by n = M̄ / M₀, where M̄ is the average molecular mass of the polymer and M₀ is the molecular mass of one monomer.

Two polymerisation modes dominate NEET questions:

• Addition (chain-growth): no by-product. Examples: polythene, PVC, Teflon, polypropene.
• Condensation (step-growth): small molecules such as H₂O, NH₃, or HCl are eliminated. Examples: nylon-6,6, terylene (Dacron), bakelite.

High-yield recall: Natural rubber is cis-1,4-polyisoprene; vulcanisation with sulphur adds –S–S– cross-links that harden and strengthen rubber while keeping it elastic.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months to revise.

Classification of Polymers

Basis	Type	Example
Source	Natural	Starch, cellulose, proteins, natural rubber
Source	Semi-synthetic	Cellulose nitrate (gun cotton), rayon
Source	Synthetic	PVC, polythene, nylon-6,6, bakelite
Structure	Linear	HDPE, PVC, nylon
Structure	Branched	LDPE
Structure	Cross-linked	Bakelite, vulcanised rubber
Polymerisation	Addition (chain-growth)	Polythene, Teflon, PVC
Polymerisation	Condensation (step-growth)	Nylon-6,6, terylene, bakelite
Molecular forces	Elastomer	Neoprene, vulcanised rubber
Molecular forces	Fibre	Terylene, nylon-6,6
Molecular forces	Thermoplastic	Polythene, PVC
Molecular forces	Thermosetting	Bakelite, melamine–formaldehyde resin

Free Radical Mechanism of Addition Polymerisation

Vinyl monomers (CH₂=CHX) polymerise by a three-step free radical pathway, typically initiated by benzoyl peroxide:

1. Initiation: The peroxide O–O bond homolytically cleaves to give two phenyl radicals, which add to a monomer to form the chain-initiating radical.
2. Propagation: The active radical successively adds monomer units, shifting the radical site to the chain end. Each step grows the chain by one –CH₂–CHX– unit.
3. Termination: Two growing chains end by either combination (radical–radical coupling) or disproportionation (H-atom transfer), giving a dead polymer chain.

Monomer Identification (NEET Trap Zone)

• Nylon-6,6 ← hexamethylenediamine (H₂N–(CH₂)₆–NH₂) + adipic acid (HOOC–(CH₂)₄–COOH) with elimination of H₂O at every –NH₂/–COOH pair.
• Terylene (Dacron) ← ethylene glycol (HOCH₂CH₂OH) + terephthalic acid (HOOC–C₆H₄–COOH) → (–OCH₂CH₂O–CO–C₆H₄–CO–)ₙ, eliminating H₂O.
• Bakelite ← phenol + formaldehyde (HCHO); cross-linked via –CH₂– bridges, eliminating H₂O.
• Neoprene ← chloroprene (2-chloro-1,3-butadiene); vulcanised with metal oxides (ZnO/MgO) rather than sulphur.

Memory hook: Condensation polymers lose atoms — write the monomer pair, cancel H from –NH₂/–OH with OH from –COOH, and count what leaves (always a small neutral molecule).

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Molecular Mass Averages and PDI

Real polymer samples contain chains of varying lengths. Two averages are tested:

Quantity	Formula	Meaning
Number-average (M̄ₙ)	Σ Nᵢ Mᵢ / Σ Nᵢ	Weighted by count of chains
Weight-average (M̄w)	Σ Nᵢ Mᵢ² / Σ Nᵢ Mᵢ	Weighted by mass of chains
Polydispersity index	PDI = M̄w / M̄n	PDI = 1 → monodisperse; PDI > 1 → polydisperse

PDI is dimensionless. Synthetic polymers almost always have PDI > 1.5; natural polymers (proteins) are nearly monodisperse.

Tacticity and Crystallinity

In vinyl polymers (–CH₂–CHR–)ₙ, the spatial placement of R groups along the backbone controls packing:

• Isotactic: all R groups on the same side → regular, crystalline, high density (e.g., isotactic polypropylene).
• Syndiotactic: R groups alternate sides → regular, can crystallise.
• Isotactic and syndiotactic polymers are produced using Ziegler–Natta catalysts (TiCl₄ + Al(C₂H₅)₃).
• Atactic: random R group placement → amorphous, soft (e.g., atactic polystyrene).

Copolymer Architectures

• Random (statistical): –AABABABB– (e.g., SBR rubber from styrene + butadiene).
• Alternating: –ABABABAB–.
• Block: –AAAA–BBBB–AAAA– (e.g., SBS thermoplastic elastomer).
• Graft: backbone of one monomer with branches of another (e.g., HIPS, high-impact polystyrene, isoprene grafted on polystyrene).

Biodegradable Polymers

These break down by microbial enzymatic action:

• PHBV — poly-β-hydroxybutyrate-co-β-hydroxyvalerate; used in orthopaedic devices and controlled drug release.
• Nylon-2,6 — glycine + adipic acid (alternating α-amino acid units).
• Polyglycolic acid (PGA) and polylactic acid (PLA) for surgical sutures.

Exam trap: PHBV is sometimes confused with PHB (single monomer) — NEET has used both, so remember the co- prefix indicates two hydroxyl-acid monomers in the chain.

Common Mistakes to Avoid

• Writing a by-product (H₂O/NH₃) for addition polymers like PVC or polythene — addition polymerisation is clean.
• Drawing trans-1,4-polyisoprene as natural rubber — the natural isomer is cis-1,4.
• Calling bakelite a thermoplastic — it is thermosetting because the cross-linked network cannot flow on heating.
• Confusing vulcanisation with full hardening; sulphur cross-links increase strength and elasticity, they do not make rubber rigid like a thermoset.

Practice Prompts

1. Identify the monomer(s) and write the repeating unit of the polymer formed when adipic acid reacts with hexamethylenediamine. State the by-product and the type of polymerisation.
2. A polymer sample has M̄ₙ = 50 000 g mol⁻¹ and PDI = 1.8. If the monomer molar mass is 50 g mol⁻¹, calculate the average degree of polymerisation and the weight-average molecular mass.

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