Polymers

Polymers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

A polymer is a high molecular mass macromolecule (10³–10⁷ u) built by linking many small monomers through covalent bonds via polymerisation. The degree of polymerisation (n) is calculated as:
n = Molecular mass of polymer / Molecular mass of repeat unit.
Two main polymerisation routes: addition (chain-growth) — no byproduct, requires an initiator like benzoyl peroxide, e.g., ethene → polythene; and condensation (step-growth) — eliminates a small molecule (usually H₂O or HCl), e.g., hexamethylenediamine + adipic acid → nylon-6,6.
CUET pointers: (1) Match the monomer to the polymer — PVC from vinyl chloride, Teflon from tetrafluoroethene, Bakelite from phenol + formaldehyde. (2) Distinguish thermoplastic (linear, remouldable — polythene) from thermosetting (cross-linked, infusible — Bakelite, melamine). (3) Vulcanisation of natural rubber with ~3–5% sulphur introduces disulphide cross-links that harden and elasticise it.

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification of Polymers

• By source: natural (starch, cellulose, proteins, natural rubber), semi-synthetic (rayon, cellulose nitrate), synthetic (PVC, nylon, Bakelite).
• By structure: linear (HDPE, nylon-6,6), branched (LDPE), cross-linked (Bakelite, vulcanised rubber).
• By molecular forces: elastomers (weak forces, stretchy), fibres (strong H-bonding, high tensile strength), thermoplastics (intermediate, soften on heating), thermosetting (heavily cross-linked, cannot be remoulded).

Addition vs Condensation Polymerisation

Addition polymerisation proceeds via a free-radical chain mechanism in three steps — initiation (benzoyl peroxide → phenyl radical → adds to CH₂=CHX), propagation (radical keeps adding monomers), and termination (coupling of two radicals, or disproportionation). No small molecule is lost.
Condensation polymerisation is step-growth; every bond formed releases H₂O, HCl, or NH₃, so the polymer mass is less than the sum of monomer masses. Examples: nylon-6,6, terylene (DMT + ethylene glycol), phenol-formaldehyde resin.

Worked Relationship

For PVC, the repeat unit is –[CH₂–CHCl]– with mass = (2×12) + (3×1) + 35.5 = 62.5 u. If a PVC sample has M = 62,500 u, then n = 62,500 / 62.5 = 1000 repeat units per chain.

Typical CUET Question Patterns

• “Monomer of ____” matching (Teflon ↔ tetrafluoroethene; Buna-S ↔ 1,3-butadiene + styrene).
• “Identify the condensation polymer.”
• “Natural rubber is cis-1,4-polyisoprene — true/false?”
• Numerical on n or PDI = Mw / Mn.

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Molecular Mass Averages

Polymers are polydisperse — chains have different lengths. Two averages are tested:

• Number-average Mn = Σ Ni Mi / Σ Ni (counts each molecule equally)
• Weight-average Mw = Σ Ni Mi² / Σ Ni Mi (heavier chains contribute more)
• PDI = Mw / Mn; a monodisperse polymer has PDI = 1, while real synthetic polymers show PDI = 1.5–3, and biological polymers can exceed 10.

Natural Rubber and Vulcanisation

Natural rubber is cis-1,4-polyisoprene, (C₅H₈)ₙ, harvested as latex from Hevea brasiliensis. The cis geometry around the C=C gives coiled chains that uncoil under tension — but pure rubber is sticky and weak. Vulcanisation (Charles Goodyear, 1839) heats raw rubber with 3–5% sulphur; sulphur forms short –S–S– disulphide bridges between chains, locking them into a 3-D network. Result: higher elasticity, tensile strength, and hardness; the material no longer softens in summer. The isomer gutta-percha is trans-1,4-polyisoprene — straight, non-elastic, hard — and is used in dentistry and golf balls.

Mechanism Snapshot — Free-Radical Addition

1. Initiator (R–O–O–R′) → 2 R• (homolysis).
2. R• + CH₂=CHX → R–CH₂–ĊHX (propagation, repeated).
3. Termination by coupling: two growing radicals combine, or by disproportionation (H-transfer), ending the chain.

Common Mistakes

• Confusing nylon-6 (caprolactam, one monomer) with nylon-6,6 (two monomers, diamine + diacid).
• Writing PVC monomer as “CH₂=CHCl” but then drawing the repeat unit as –[CH=CHCl]– — the double bond opens and disappears in the chain.
• Calling Bakelite a thermoplastic — it is thermosetting (cross-linked, phenol–CH₂–phenol bridges).
• Assuming polythene always means HDPE: LDPE has branched chains (free-radical, high pressure) and HDPE is linear (Ziegler–Natta catalyst, low pressure).

Practice Prompts

1. A polymer has Mn = 50,000 u and Mw = 1,00,000 u. Calculate PDI and comment on the uniformity of the sample.
2. Write the monomer(s), repeat unit, and classify (addition/condensation, thermoplastic/thermosetting) for: (a) Teflon, (b) terylene, (c) Bakelite, (d) Buna-N.

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.