Polymers • Topic 3 of 3

Important Polymers

The commercially vital polymers are best learnt by their monomer, method and uses.

Polythene. LDPE (low-density) is made by polymerising ethene at high pressure (1000–2000 atm) with traces of oxygen; it is branched, flexible and a good insulator (squeeze bottles, insulation, bags). HDPE (high-density) is made at low pressure with Ziegler–Natta catalysts (TiCl4 + Al(C2H5)3); it is linear, tough and stiff (buckets, pipes, bottles).

PVC (polyvinyl chloride) is the addition polymer of vinyl chloride, CH2=CHCl; rigid PVC is used for pipes and the plasticised form for raincoats, hose pipes and cable insulation.

Teflon (polytetrafluoroethene) is the addition polymer of tetrafluoroethene, CF2=CF2; its strong C—F bonds make it chemically inert and heat-resistant, used for non-stick cookware coatings and gaskets.

Polyacrylonitrile (Orlon, Acrilan) is the addition polymer of acrylonitrile, CH2=CH—CN; it is a substitute for wool in sweaters and blankets.

Polyesters. Dacron / Terylene is a condensation polymer of ethylene glycol and terephthalic acid; it is crease-resistant and blended with cotton/wool for fabrics, and as film it is used for magnetic recording tape.

Polyamides (nylons). Nylon-6,6 is a condensation polymer of hexamethylenediamine and adipic acid, used in fabrics, ropes, tyre cords and bristles. Nylon-6 is made by heating caprolactam (a cyclic amide), which opens and polymerises; it is used in tyre cords, fabrics and ropes.

Phenol-formaldehyde and amino resins. Bakelite is a condensation polymer of phenol and formaldehyde; linear at first (Novolac), it cross-links to a hard, infusible thermoset used for electrical switches, handles and combs. Melamine-formaldehyde resin gives unbreakable crockery (melamine).

Rubbers. Natural rubber is cis-1,4-polyisoprene, an elastomer with weak forces and poor strength when hot. Vulcanisation — heating rubber with sulphur (about 3–5% for tyres) — introduces sulphur cross-links between chains, greatly increasing strength, elasticity and resistance to wear and temperature. Synthetic rubbers: neoprene (polychloroprene, from chloroprene) resists oils and is used in conveyor belts and gaskets; Buna-N (copolymer of 1,3-butadiene and acrylonitrile) is oil-resistant; Buna-S (copolymer of 1,3-butadiene and styrene) is tough and used for car tyres.

Biodegradable polymers break down by the action of micro-organisms. PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate) is a copolymer of 3-hydroxybutanoic and 3-hydroxypentanoic acids, used in packaging and orthopaedic devices. Nylon-2-nylon-6 is a biodegradable polyamide copolymer of glycine and aminocaproic acid. These reduce the pollution caused by ordinary non-degradable plastics.

Repeating amide linkage in nylon-6,6Nylon-6,6 repeating unit (amide linkage)...NH-(CH2)6-NH-CO-(CH2)4-CO...from diaminefrom adipic acid-NH-CO- = amide (peptide) linkageMonomers: H2N-(CH2)6-NH2 + HOOC-(CH2)4-COOHEach linkage formed releases one H2O (condensation)Strong H-bonding between amide groups makes nylon a fibre
Solved Examples
1
Worked Example
Name the monomer and one use of (a) Teflon and (b) polyacrylonitrile.
Solution
  1. Teflon is the addition polymer of tetrafluoroethene, CF2=CF2.
  2. Its inertness and heat resistance make it ideal for non-stick cookware coatings.
  3. Polyacrylonitrile is the addition polymer of acrylonitrile, CH2=CH-CN.
  4. It is used as a wool substitute in sweaters and blankets.

Answer: (a) Teflon — tetrafluoroethene; non-stick coatings. (b) Polyacrylonitrile — acrylonitrile; synthetic wool.

2
Worked Example
What is vulcanisation of rubber? How does it improve the properties of natural rubber?
Solution
  1. Vulcanisation is heating natural rubber with sulphur (about 3-5% for tyres).
  2. Sulphur forms cross-links (-S-S- bridges) between the polyisoprene chains.
  3. These cross-links restrict chain slippage, so the rubber becomes stronger, more elastic and harder.
  4. It also gains resistance to wear, swelling by solvents and changes of temperature.

Answer: Vulcanisation introduces sulphur cross-links that make rubber stronger, more elastic and more durable than raw rubber.

3
Worked Example
Give the monomers of (a) nylon-6,6 and (b) nylon-6, and the type of polymerisation for each.
Solution
  1. Nylon-6,6 is made from hexamethylenediamine and adipic acid by condensation, eliminating water.
  2. Nylon-6 is made from a single monomer, caprolactam (a cyclic amide).
  3. On heating with a trace of water, caprolactam ring-opens and polymerises into nylon-6.

Answer: (a) Nylon-6,6 — hexamethylenediamine + adipic acid (condensation). (b) Nylon-6 — caprolactam (ring-opening/condensation).

4
Worked Example
Distinguish between Buna-S and Buna-N in terms of monomers and a characteristic property.
Solution
  1. Buna-S is a copolymer of 1,3-butadiene and styrene.
  2. Buna-N is a copolymer of 1,3-butadiene and acrylonitrile.
  3. Buna-S is tough and wear-resistant (used in car tyres); Buna-N is oil- and chemical-resistant.

Answer: Buna-S = butadiene + styrene (wear-resistant); Buna-N = butadiene + acrylonitrile (oil-resistant).

5
Worked Example
What are biodegradable polymers? Give two examples with their monomers.
Solution
  1. Biodegradable polymers are decomposed by the action of micro-organisms, reducing plastic pollution.
  2. PHBV is a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.
  3. Nylon-2-nylon-6 is a copolymer of glycine (amino acetic acid) and aminocaproic acid.

Answer: Microbe-degradable polymers; e.g. PHBV (3-hydroxybutanoic + 3-hydroxypentanoic acids) and nylon-2-nylon-6 (glycine + aminocaproic acid).

6
Worked Example
How are LDPE and HDPE prepared, and how does this explain their different properties?
Solution
  1. LDPE is made by polymerising ethene at high pressure (1000-2000 atm) with traces of oxygen, giving branched chains.
  2. Branching prevents close packing, so LDPE is low-density, flexible and a good insulator.
  3. HDPE is made at low pressure using a Ziegler-Natta catalyst (TiCl4 + triethylaluminium), giving linear chains.
  4. Linear chains pack closely, so HDPE is high-density, tough and stiff.

Answer: LDPE (high-pressure, branched) is flexible and low-density; HDPE (Ziegler-Natta, linear) is tough and high-density.

Key Points

  • Polythene: LDPE (high pressure + O2, branched, flexible, bags/insulation) and HDPE (Ziegler-Natta, linear, tough, buckets/pipes); both are addition polymers of ethene.
  • Addition polymers: PVC from vinyl chloride (pipes, raincoats), Teflon from tetrafluoroethene (non-stick, inert), polyacrylonitrile from acrylonitrile (synthetic wool).
  • Condensation polymers: Dacron/terylene (ethylene glycol + terephthalic acid), nylon-6,6 (hexamethylenediamine + adipic acid), nylon-6 (caprolactam), bakelite and melamine (with formaldehyde).
  • Rubbers: natural rubber is cis-1,4-polyisoprene; vulcanisation with sulphur cross-links the chains, raising strength and elasticity; neoprene, Buna-N and Buna-S are synthetic rubbers.
  • Biodegradable polymers are broken down by micro-organisms: PHBV (3-hydroxybutanoic + 3-hydroxypentanoic acids) and nylon-2-nylon-6 (glycine + aminocaproic acid).
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The monomer of Teflon is:
Explanation: Teflon (PTFE) is the addition polymer of tetrafluoroethene, CF2=CF2.
Q2.Vulcanisation of rubber involves heating it with:
Explanation: Heating rubber with sulphur introduces cross-links that strengthen and toughen it.
Q3.Nylon-6 is obtained by polymerising:
Explanation: Nylon-6 is made by ring-opening polymerisation of the single monomer caprolactam.
Q4.Which of the following is a biodegradable polymer?
Explanation: PHBV is degraded by micro-organisms, unlike polythene, PVC and Teflon.
Practice more MCQs → 📝 Assignment · 30 marks