Polymers Class 12: Types, Polymerisation and Everyday Uses (School-Assessed)

What Polymers is about

In real board papers: not asked in any of the 8 board papers we hold (2023 to 2026). See the questions

Key concepts

What polymers are and their classification by source

NCERT section CBSE reading material: Polymers (introduction); 15.1 Classification of Polymers

A polymer is a very large molecule (molecular mass about 10³–10⁷ u), also called a macromolecule, made by joining repeating structural units on a large scale. The repeating units come from simple, reactive molecules called monomers, linked by covalent bonds. Forming a polymer from monomers is polymerisation.

Classification by source:

• Natural polymers: found in plants and animals: proteins, cellulose, starch, some resins and rubber.
• Semi-synthetic polymers: made by modifying natural polymers: cellulose derivatives such as cellulose acetate (rayon) and cellulose nitrate.
• Synthetic polymers: man-made: plastics (polythene), synthetic fibres (nylon 6,6) and synthetic rubbers (Buna-S).

Polymers can also be classified by their structure, by the forces between chains, or by the type of polymerisation.

Addition polymerisation; homopolymers and copolymers

NCERT section CBSE reading material: 15.2 Types of Polymerisation Reactions; 15.2.1 Addition Polymerisation; 15.2.5 Copolymerisation

There are two broad types of polymerisation: addition (chain growth) and condensation (step growth).

In addition polymerisation, molecules of the same or different monomers add together on a large scale. The monomers are unsaturated compounds: alkenes, alkadienes and their derivatives. The chain grows through free radicals or ionic species; the free-radical route is the most common.

• A homopolymer is formed from a single monomer species: polythene from ethene.
• A copolymer is formed from two different monomers. Buna-S, from buta-1,3-diene and styrene, is a copolymer made by addition polymerisation.

Copolymerisation can happen by chain growth or by step growth. A copolymer contains many units of each monomer in the same chain, and its properties can be quite different from those of the homopolymers. Butadiene–styrene copolymer is tough and a good substitute for natural rubber, used for auto tyres, floor tiles, footwear components and cable insulation.

Important addition polymers

NCERT section CBSE reading material: 15.2.2 Some Important Addition Polymers

Polythene: linear or slightly branched long chains that soften on heating and harden on cooling again and again: thermoplastic polymers.

• Low density polythene (LDP): ethene polymerised at 1000–2000 atm and 350–570 K with traces of oxygen or a peroxide initiator, by free-radical addition. It is highly branched, chemically inert, tough but flexible and a poor conductor of electricity. Uses: insulation of electric wires, squeeze bottles, toys, flexible pipes.
• High density polythene (HDP): ethene polymerised in a hydrocarbon solvent with a Ziegler–Natta catalyst (triethylaluminium and titanium tetrachloride) at 333–343 K and 6–7 atm. Its chains are linear and pack closely, so its density is high. It is inert, tougher and harder. Uses: buckets, dustbins, bottles, pipes.

Teflon (polytetrafluoroethene): made by heating tetrafluoroethene with a free-radical or persulphate catalyst at high pressure. It is chemically inert and resists corrosive reagents. Uses: oil seals, gaskets, non-stick coatings on utensils.

Polyacrylonitrile: addition polymer of acrylonitrile with a peroxide catalyst; a substitute for wool in fibres such as orlon or acrilan.

Condensation polymerisation and important condensation polymers

NCERT section CBSE reading material: 15.2.3 Condensation Polymerisation; 15.2.4 Some Important Condensation Polymers

In condensation (step growth) polymerisation, bi-functional or tri-functional monomers react repeatedly, often losing small molecules such as water, alcohol or HCl. Each step gives a new bi-functional product, and each step is independent, hence 'step growth'.

Polyamides (nylons): contain amide links; made from diamines and dicarboxylic acids, or from amino acids or their lactams.

• Nylon 6,6: from hexamethylenediamine and adipic acid at high pressure and temperature. A fibre-forming solid with high tensile strength, because hydrogen bonding between chains packs them closely (crystalline nature). Uses: sheets, bristles for brushes, textiles.
• Nylon 6: from heating caprolactam with water at a high temperature. Uses: tyre cords, fabrics, ropes.

Polyesters: from dicarboxylic acids and diols. Terylene (dacron) is made by heating ethylene glycol and terephthalic acid at 420–460 K with a zinc acetate–antimony trioxide catalyst. It is crease-resistant; used in blends with cotton and wool and as glass reinforcing material in safety helmets.

Phenol–formaldehyde polymers (the oldest synthetic polymers): phenol and formaldehyde, with an acid or base catalyst, first give o- and/or p-hydroxymethylphenols, which join through –CH₂– groups. The linear product novolac is used in paints. Heating novolac with more formaldehyde cross-links the chains into bakelite, a thermosetting polymer that cannot be remoulded. Uses: combs, phonograph records, electrical switches, handles of utensils.

Melamine–formaldehyde polymer: from melamine and formaldehyde; used for unbreakable crockery.

Rubber

NCERT section CBSE reading material: 15.2.6 Rubber; 15.2.7 Polymers of Commercial Importance

Natural rubber is a natural polymer with elastic properties: an elastomer. Its chains are held by weak intermolecular forces, which let it stretch; a few cross-links between chains pull it back to its original shape when the force is removed.

It is made from latex, a colloidal dispersion of rubber in water obtained from the rubber tree (grown in India, Sri Lanka, Indonesia, Malaysia and South America).

Natural rubber is a linear polymer of isoprene (2-methyl-1,3-butadiene) and is called cis-1,4-polyisoprene.

Synthetic rubbers such as Buna-S (a copolymer of buta-1,3-diene and styrene) are tough substitutes for natural rubber.

Chemistry in real life: Polymers

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Classification, Addition Polymerisation and Copolymers
• Condensation Polymers and Rubber

Polymers formula sheet

Polythene (addition, homopolymer)

$nC{H}_2=C{H}_2\stackrel{\text{peroxide or Ziegler-Natta}}{\to }-(C{H}_2-C{H}_2{)}_n-$

LDP: free radical, high pressure; HDP: Ziegler–Natta, low pressure.

Teflon

$nC{F}_2=C{F}_2\stackrel{\text{high pressure, catalyst}}{\to }-(C{F}_2-C{F}_2{)}_n-$

Chemically inert; non-stick coatings.

Nylon 6,6 (condensation)

$\text{hexamethylenediamine}+\text{adipic acid}\to \text{nylon 6,6}+\text{water}$

Amide links; strong hydrogen bonding.

Terylene (condensation)

$\text{ethylene glycol}+\text{terephthalic acid}\to \text{terylene}+\text{water}$

420–460 K, zinc acetate–antimony trioxide catalyst.

Buna-S (addition copolymer)

$\text{buta-1,3-diene}+\text{styrene}\to \text{Buna-S}$

Copolymer: two monomers in one chain.

Natural rubber

$\text{isoprene (2-methyl-1,3-butadiene)}\to \text{cis-1,4-polyisoprene}$

Elastomer with a few cross-links.

Tricks to remember Polymers

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Addition Adds, Condensation Cuts

  Addition polymers add whole monomers (no loss); condensation polymers cut out small molecules like water as they link.

  Use it when: Telling the two types apart.

• Nylon 6,6 = 6 + 6

  Hexamethylenediamine has 6 carbons and adipic acid has 6 carbons, hence nylon 6,6. Nylon 6 comes from one 6-carbon monomer, caprolactam.

  Use it when: Remembering nylon monomers.

• Buna-S: Butadiene + Styrene

  Bu for butadiene, S for styrene: two monomers in one chain, so it is a copolymer.

  Use it when: Copolymer example.

• Low density = Lots of branches

  LDP: L for Lots of branches, flexible; HDP: H for Highly packed linear chains, hard.

  Use it when: Comparing the two polythenes.

Polymers: previous year questions

• Not asked in any of the 8 board papers we hold (2023 to 2026).

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define polymer and monomer.Show answer

   A polymer is a macromolecule of high molecular mass made of repeating units joined by covalent bonds; a monomer is the simple reactive molecule from which these units come.

2. 2.Give one example each of natural, semi-synthetic and synthetic polymers.Show answer

   Natural: starch or rubber; semi-synthetic: cellulose acetate (rayon); synthetic: polythene or nylon 6,6.

3. 3.What is the difference between a homopolymer and a copolymer?Show answer

   A homopolymer is made from one kind of monomer; a copolymer from two or more different monomers.

4. 4.Which catalyst is used for high density polythene?Show answer

   Ziegler–Natta catalyst (triethylaluminium and titanium tetrachloride).

5. 5.What are the monomers of terylene?Show answer

   Ethylene glycol and terephthalic acid.

6. 6.What is novolac?Show answer

   The linear phenol–formaldehyde polymer that is cross-linked with more formaldehyde to give bakelite.

7. 7.Why does nylon 6,6 have high tensile strength?Show answer

   Strong hydrogen bonding between chains holds them closely packed.

8. 8.What is natural rubber chemically?Show answer

   cis-1,4-Polyisoprene, a polymer of isoprene (2-methyl-1,3-butadiene).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give three examples each of natural and synthetic polymers?
• ☐Can you explain the difference between addition and condensation polymerisation?
• ☐Can you define homopolymer and copolymer with one example each?
• ☐Can you compare LDP and HDP in preparation, structure and uses?
• ☐Can you name the monomers of nylon 6,6, nylon 6, terylene and Buna-S?
• ☐Can you explain how novolac becomes bakelite?
• ☐Can you explain why rubber is elastic?

Polymers: frequently asked questions