MHT-CET Chemistry · Introduction to Polymer Chemistry

Properties and Applications of Polymers

Low-density polythene is branched, amorphous and made at 1000 to 2000 atm with an oxygen or peroxide initiator; high-density polythene is linear, crystalline and made at low pressure over a Ziegler–Natta catalyst; rubber is vulcanised with sulphur and an accelerator; and each commercial polymer has a household use the paper asks by name — PVC for pipes and floor tiles, polystyrene for disposable cups, polypropylene for straws, nylon 6 for tyre cords, nylon 6,6 for bristles and sutures, Teflon for oil seals, glyptal for paints.

All Introduction to Polymer Chemistry notes MHT-CET Chemistry strategy

Why this matters

26 PYQs, none HARD. Nineteen are uses — PVC (pipes, floor tiles), polystyrene (cups and plates, twice, and the carcinogen it leaches), polypropylene (straws), HDPE (toys), nylon 6 (tyre cords), nylon 6,6 (bristles, sutures), PAN (wool substitute, twice), dacron (terycot), glyptal (paints, twice), Teflon (oil seals), Buna-N (belts, shoe soles), perspex (LCD), melamine (dinner ware); seven are properties — the LDPE and HDPE statements (four times), what is NOT true of a thermoplastic or of natural rubber, and the vulcanisation accelerator. Two cards.

Concept 1 of 2

LDPE against HDPE, and Vulcanisation

Intuition

Same monomer, two processes, two materials. Free-radical polymerisation of ethene at very high pressure (1000–2000 atm, 350–570 K, O₂ or peroxide initiator) lets chains branch: LOW-density polythene, amorphous, flexible, a poor conductor, moisture-resistant, made into films and bags and cable insulation. Ziegler–Natta catalysis at low pressure (6–7 atm, 333–343 K) gives unbranched chains that pack: HIGH-density polythene, crystalline, tougher, higher melting, made into buckets, bottles, toys, pipes. Rubber is made useful the same way steel is hardened: heating with sulphur (373–415 K) and an accelerator cross-links the chains — vulcanisation.

Definition

LDPE: branched, amorphous (NOT crystalline), 1000–2000 atm, 350–570 K, O₂ or peroxide initiator; flexible films, squeeze bottles, bags, insulation of cables. 'Needs low pressure' is the false statement.
HDPE: linear, crystalline, low pressure (6–7 atm), 333–343 K, Ziegler–Natta catalyst; harder, higher m.p. than LDPE; buckets, dustbins, bottles, pipes, toys. 'Needs 1000–2000 atm' is the false statement.
Thermoplastics: soften on heating, moderately strong forces (between elastomers and fibres), easily moulded and remoulded — NOT extensively cross-linked.
Vulcanisation: natural rubber + sulphur, 373–415 K, accelerator zinc butyl xanthate (or ZnO) → sulphur cross-links; harder, less tacky, elastic over a wider range.
Natural rubber: addition polymer of isoprene, linear, cis C=C — not butadiene + styrene.

Two polythenes

\text{LDPE: } 10^3\text{–}2\times10^3\ \text{atm, O}_2/\text{peroxide, branched};\quad \text{HDPE: } 6\text{–}7\ \text{atm, Ziegler–Natta, linear}

Worked example

Two samples of polythene: one melts at 105 °C and is used for carrier bags, the other at 130 °C and is used for buckets. Assign LDPE/HDPE and give each one's catalyst or initiator.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Introduction to Polymer ChemistryEASY

Which among the following statements is NOT true about high density polythene?

[Q54 · 10th May Shift 2 · 2024]

Reading '6–7 atm' as LDPE

Low density needs HIGH pressure; the low-pressure Ziegler–Natta process gives the HIGH-density polymer. The paper plants each pressure under the wrong polymer.

Drill 6 more on ldpe against hdpe, and vulcanisation

Concept 2 of 2

Which Polymer Makes Which Article

Intuition

Match the property to the job. Rigid and chemically inert: PVC for pipes and floor tiles, Teflon for oil seals and non-stick coatings. Cheap, light and foamable: polystyrene for disposable cups and plates and thermocol packaging (it can leach styrene, a suspected carcinogen, into food). Heat-resistant and stiff: polypropylene for straws and microwavable trays. Strong threads: nylon 6 for tyre cords, nylon 6,6 for bristles, sutures and ropes, dacron blended with cotton as terycot, PAN as synthetic wool. Oil-resistant rubber: Buna-N for belts, hoses, shoe soles. Hard thermosets: melamine dinner ware, bakelite switches. Clear: perspex for LCD screens and lenses. Coatings: glyptal for paints.

Definition

PVC: water pipes, floor tiles, raincoats. LDPE: bags, films, cable insulation. HDPE: toys, buckets, bottles. PP: drinking straws, microwavable food trays. PS: disposable cups and plates, thermocol; leaches styrene (carcinogen).
Nylon 6: tyre cords, fabrics. Nylon 6,6: bristles for brushes, surgical sutures, ropes, carpets. PAN: wool substitute (orlon). Dacron: terycot with cotton, fabrics.
Teflon: oil seals, gaskets, non-stick cookware. Buna-N: rubber belts, hoses for gasoline, shoe soles. Neoprene: gasoline hose pipes, conveyor belts.
Glyptal: paints and lacquers. Perspex: LCD screens, lenses, windows. Polycarbonate: bullet-proof glass, CDs. Melamine-formaldehyde: unbreakable dinner ware. Bakelite: electrical switches, handles. Polyacrylamide: electrophoresis gels.

Article	Polymer	Why
Water pipes, floor tiles	PVC	Rigid, chemically resistant
Disposable cups and plates	Polystyrene	Cheap, foamable; leaches styrene
Drinking straws	Polypropylene	Rigid, heat-resistant
Toys, buckets	HDPE	Tough, crystalline
Tyre cords	Nylon 6	High tensile strength Nylon 6 → tyre cords; nylon 6,6 → bristles and sutures.
Brush bristles, surgical sutures	Nylon 6,6	Stiff, strong polyamide
Wool substitute	Polyacrylonitrile	Soft, warm fibre
Terycot	Dacron + cotton	Wrinkle-free blend
Oil seals, gaskets	Teflon	Inert, low friction
Rubber belts, shoe soles	Buna-N	Oil- and abrasion-resistant
Paints	Glyptal	Alkyd resin
LCD screens	Perspex	Transparent PMMA
Plastic dinner ware	Melamine-formaldehyde	Hard thermoset

The two nylons split the jobs: 6 for tyre cords, 6,6 for bristles and sutures.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Introduction to Polymer ChemistryMODERATE

Which from following polymers is used to obtain bristles for brushes?

[Q99 · Shift 1 · 2022]

Nylon 6 for bristles

Both nylons are strong, but the paper's split is fixed: nylon 6 → tyre cords, nylon 6,6 → bristles and sutures. Swapping them is the standard wrong answer.

Drill 18 more on which polymer makes which article

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

LDPE against HDPE, and Vulcanisation
Two polythenes
$\text{LDPE: } 10^3\text{–}2\times10^3\ \text{atm, O}_2/\text{peroxide, branched};\quad \text{HDPE: } 6\text{–}7\ \text{atm, Ziegler–Natta, linear}$

Reference tables (1)

Which Polymer Makes Which Article13 rows

Article	Polymer	Why
Water pipes, floor tiles	PVC	Rigid, chemically resistant
Disposable cups and plates	Polystyrene	Cheap, foamable; leaches styrene
Drinking straws	Polypropylene	Rigid, heat-resistant
Toys, buckets	HDPE	Tough, crystalline
Tyre cords	Nylon 6	High tensile strength Nylon 6 → tyre cords; nylon 6,6 → bristles and sutures.
Brush bristles, surgical sutures	Nylon 6,6	Stiff, strong polyamide
Wool substitute	Polyacrylonitrile	Soft, warm fibre
Terycot	Dacron + cotton	Wrinkle-free blend
Oil seals, gaskets	Teflon	Inert, low friction
Rubber belts, shoe soles	Buna-N	Oil- and abrasion-resistant
Paints	Glyptal	Alkyd resin
LCD screens	Perspex	Transparent PMMA
Plastic dinner ware	Melamine-formaldehyde	Hard thermoset

The two nylons split the jobs: 6 for tyre cords, 6,6 for bristles and sutures.

Watch out for (2)

Reading '6–7 atm' as LDPE
→ LDPE against HDPE, and Vulcanisation
Nylon 6 for bristles
→ Which Polymer Makes Which Article

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