ChemistryCBSE Class 12 · Polymers

Addition Polymerisation, Homopolymers and Copolymers: Class 12 Chemistry

Updated 18 October 2026 · Fact-checked

A polymer is a macromolecule of molecular mass about 10^3 to 10^7 u, formed from monomers joined by covalent bonds. By source, polymers are natural, semi-synthetic or synthetic. Addition polymerisation joins unsaturated monomers by a chain (usually free-radical) mechanism; a homopolymer has one monomer, a copolymer two different ones, as in Buna-S.

A homopolymer chain repeats one monomer, while a copolymer like Buna-S has two different monomers, both many times, in the same chain.

Illustration drawn for QuestionBuddy.

Understand Classification, Addition Polymerisation and Copolymers

What polymers are. A polymer is a very large molecule (molecular mass about 10^3 to 10^7 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: proteins, cellulose, starch, some resins and rubber.
Semi-synthetic: modified natural polymers, such as cellulose acetate (rayon) and cellulose nitrate.
Synthetic: polythene (plastic), nylon 6,6 (fibre), Buna-S (rubber).

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

Homopolymer: formed from a single monomer species, for example polythene from ethene.
Copolymer: formed from two different monomers, with many units of each in the same chain. Buna-S, from buta-1,3-diene and styrene, is an addition copolymer, tough and a good substitute for natural rubber, used for auto tyres, floor tiles, footwear components and cable insulation.

Polythene. Linear or slightly branched chains that soften on heating and harden on cooling again and again: thermoplastics.

LDPE: ethene at 1000 to 2000 atm and 350 to 570 K with traces of oxygen or a peroxide initiator (free radical). Highly branched, inert, tough, flexible, poor conductor. Used for wire insulation, squeeze bottles, toys and flexible pipes.
HDPE: ethene in a hydrocarbon solvent with a Ziegler-Natta catalyst (triethylaluminium and titanium tetrachloride) at 333 to 343 K and 6 to 7 atm. Chains are linear and pack closely, so density is high; tougher and harder. Used for buckets, dustbins, bottles and pipes.

Teflon. Tetrafluoroethene heated with a free-radical or persulphate catalyst at high pressure. Chemically inert and resistant to corrosive reagents; used for oil seals, gaskets and non-stick coatings.

Polyacrylonitrile. Acrylonitrile with a peroxide catalyst; a substitute for wool in fibres like orlon or acrilan.

Classification, Addition Polymerisation and Copolymers: formulas and key facts

Source-based classes: Natural protein, cellulose, starch, rubber
Semi-synthetic cellulose acetate, cellulose nitrate
Synthetic polythene, nylon 6,6, Buna-S; Rayon is cellulose acetate.
LDPE versus HDPE: $L D P E : 1000 - 2000 a t m, 350 - 570 K, p er o x i d e, b r an c h e d ∣ H D P E : 6 - 7 a t m, 333 - 343 K, Z i e g l er - N a tt a, l in e a r$; Linear chains pack closely.
Polymers and monomers: $P o l y t h e n e : e t h e n e ∣ T e f l o n : t e t r a f l u or oe t h e n e ∣ P A N : a cr y l o ni t r i l e ∣ B u na - S : b u t a - 1, 3 - d i e n e + s t y r e n e$; Buna-S is the copolymer.

Natural	protein, cellulose, starch, rubber
Semi-synthetic	cellulose acetate, cellulose nitrate
Synthetic	polythene, nylon 6,6, Buna-S

How to solve Classification, Addition Polymerisation and Copolymers questions

Formative (school) assessment only in 2026-27. Questions ask for classification, structures from monomers, and degree-of-polymerisation arithmetic.

1Identify the monomer(s): one monomer means homopolymer; two means copolymer.
2Check for a C=C bond to confirm addition polymerisation.
3Write the repeating unit in brackets with n.
4For degree of polymerisation, divide polymer molar mass by monomer molar mass.
5Compare conditions to separate LDPE from HDPE.

Classification, Addition Polymerisation and Copolymers in real life

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.

Why non-stick pans work

Suppose a frying pan has a thin non-stick layer that food does not stick to even when heated.

Name the polymer and explain the property that makes it suitable.

Try it first, then show the solutionSolution

Teflon (polytetrafluoroethene) is used for non-stick coatings.
It is chemically inert and resists corrosive reagents.

Answer: The coating is Teflon, made from tetrafluoroethene. Being chemically inert and resistant to corrosive reagents, it does not react with food or cleaners, making it suitable for non-stick coatings.

Memory hook: Teflon is inert.

Bucket versus squeeze bottle

Suppose a factory chooses polythene for a rigid bucket and another type for a soft squeeze bottle.

Which polythene suits each and why?

Try it first, then show the solutionSolution

HDPE has linear chains packing closely: tougher and harder.
LDPE is branched, flexible and tough.

Answer: A rigid bucket suits HDPE because its linear chains pack closely, giving a tougher and harder material. A squeeze bottle suits LDPE, which is highly branched, flexible and tough.

Memory hook: High density, hard; low density, flexible.

Tricks to remember Classification, Addition Polymerisation and Copolymers

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

Homo = one, Co = two
Homopolymer: one monomer; copolymer: two different.
Use it when: Definition.
Ziegler-Natta = HDPE
TiCl4 and triethylaluminium give linear chains.
Use it when: Catalyst for HDPE.
Orlon = wool substitute
PAN replaces wool.
Use it when: Uses.

Classification, Addition Polymerisation and Copolymers: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Identify the copolymer · 1 mark · MCQBoard level

Which of these is a copolymer?
(a) polythene
(b) Teflon
(c) Buna-S
(d) polyacrylonitrile

Try it first, then show the solutionSolution

Buna-S is made from buta-1,3-diene and styrene, two monomers (1 mark).

Answer: (c) Buna-S

Example 2: LDPE conditions (2 marks) · 2 marks · Very short answerBoard level

State the conditions for the preparation of low density polythene and why it is highly branched. (2 marks)

Try it first, then show the solutionSolution

1000 to 2000 atm, 350 to 570 K, traces of oxygen or a peroxide initiator (1 mark).
Free-radical addition gives a highly branched structure (1 mark).

Answer: LDPE is made from ethene at 1000 to 2000 atm and 350 to 570 K using traces of oxygen or a peroxide initiator; free-radical addition gives a highly branched polymer.

Example 3: Degree of polymerisation (4 marks) · 4 marks · Short answerHarder board level

A sample of polythene has molar mass 56,000 u and a sample of Teflon has molar mass 300,000 u. Taking C = 12, H = 1, F = 19, find the degree of polymerisation of each. (4 marks)

Try it first, then show the solutionSolution

Ethene C2H4 = 2 x 12 + 4 x 1 = 28 u (1 mark).
Polythene: 56,000 / 28 = 2000 units (1 mark).
Tetrafluoroethene C2F4 = 24 + 76 = 100 u (1 mark).
Teflon: 300,000 / 100 = 3000 units (1 mark).

Answer: Ethene = 28 u, so polythene has 56,000 / 28 = 2000 units. C2F4 = 100 u, so Teflon has 300,000 / 100 = 3000 units.

Example 4: LDPE versus HDPE (5 marks) · 5 marks · Long answerHarder board level

Compare LDPE and HDPE on preparation conditions, structure, properties and uses. (5 marks)

Try it first, then show the solutionSolution

LDPE: 1000 to 2000 atm, 350 to 570 K, peroxide initiator; HDPE: 6 to 7 atm, 333 to 343 K, Ziegler-Natta catalyst (1 mark).
Structure: LDPE highly branched; HDPE linear (1 mark).
Linear chains pack closely so HDPE density is higher (1 mark).
Properties: LDPE tough and flexible; HDPE tougher and harder (1 mark).
Uses: LDPE wire insulation and squeeze bottles; HDPE buckets, dustbins, pipes (1 mark).

Answer: LDPE is made at 1000 to 2000 atm and 350 to 570 K with a peroxide initiator, is highly branched and flexible, and is used for wire insulation and squeeze bottles. HDPE is made at 6 to 7 atm and 333 to 343 K with a Ziegler-Natta catalyst, has linear chains that pack closely giving a high density, is tougher and harder, and is used for buckets, dustbins and pipes.

Example 5: Semi-synthetic and synthetic · 4 marks · Short answerHOTS / competency

Classify as natural, semi-synthetic or synthetic with a reason: (a) cellulose acetate, (b) starch, (c) Buna-S, and name the monomers of (c). (4 marks)

Try it first, then show the solutionSolution

(a) Semi-synthetic: modified natural cellulose (1 mark).
(b) Natural: found in plants (1 mark).
(c) Synthetic: man-made rubber (1 mark).
Monomers of Buna-S: buta-1,3-diene and styrene (1 mark).

Answer: (a) Cellulose acetate is semi-synthetic, as cellulose is chemically modified. (b) Starch is natural, found in plants. (c) Buna-S is synthetic; its monomers are buta-1,3-diene and styrene.

Where students lose marks in Classification, Addition Polymerisation and Copolymers

The slips examiners see most often, and what to write instead.

Mistake: Calling Buna-S a homopolymer
Single name looks like one monomer.
Wrong working
Buna-S is a homopolymer of butadiene.
Right working
Buna-S is a copolymer of buta-1,3-diene and styrene.
Do this: Two monomers, so copolymer.
Usually costs: 1 mark.
Mistake: Swapping LDPE and HDPE conditions
Density sounds like pressure.
Wrong working
HDPE is made at 1000 to 2000 atm.
Right working
LDPE is made at 1000 to 2000 atm; HDPE at 6 to 7 atm with Ziegler-Natta.
Do this: Low density needs high pressure.
Usually costs: 2 marks.
Mistake: Forgetting n in the repeating unit
Drawing only one unit.
Wrong working
CH2-CH2 is polythene.
Right working
-(CH2-CH2)n- is polythene.
Do this: Brackets and n.
Usually costs: 1 mark.

Exam tips

Polymers is formative (school) assessment only in 2026-27.
Always write the repeating unit in brackets with n.
Learn conditions of LDPE and HDPE as a pair.

Practice questions on Classification, Addition Polymerisation and Copolymers

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The catalyst used for HDPE is:
(a) peroxide
(b) Ziegler-Natta catalyst
(c) persulphate
(d) zinc acetate
Hint
Triethylaluminium and TiCl4.
Answer
(b) Ziegler-Natta catalyst
Q2 · 2 marks · Very short answerBoard level
Name two uses of Teflon. (2 marks)
Hint
Seals and coatings.
Answer
Oil seals and gaskets, and non-stick coatings on utensils.
Q3 · 3 marks · Short answerHarder board level
A polythene chain has degree of polymerisation 1500. Find its molar mass (C = 12, H = 1) and the mass of ethene needed for 1 mol of the polymer, assuming no loss. (3 marks)
Hint
28 x 1500.
Answer
Molar mass of ethene = 28 u. Polymer molar mass = 28 x 1500 = 42,000 u, so 1 mol needs 42,000 g (42 kg) of ethene.
Q4 · 5 marks · Long answerHarder board level
Distinguish addition polymerisation from condensation polymerisation using the monomer type, by-products and the name of one polymer in each, and explain why Buna-S is a copolymer formed by addition. (5 marks)
Hint
Unsaturated monomers versus bi-functional.
Answer
Addition (chain growth) uses unsaturated monomers like alkenes and dienes, usually by a free-radical chain, with no small molecules lost; polythene is an example. Condensation (step growth) uses bi-functional or tri-functional monomers and loses small molecules such as water; nylon 6,6 is an example. Buna-S is a copolymer because it is formed from two different monomers, buta-1,3-diene and styrene, which are unsaturated and add together.
Q5 · 1 mark · Assertion-reasonHOTS / competency
Assertion (A): HDPE is harder than LDPE.
Reason (R): HDPE has linear chains that pack closely.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Hint
Packing and density.
Answer
(a) Both A and R are true, and R is the correct explanation of A.

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.What are monomers?Show answer
Simple reactive molecules that join to form a polymer.
2.Name a semi-synthetic polymer.Show answer
Cellulose acetate (rayon) or cellulose nitrate.
3.Which fibre substitutes wool?Show answer
Polyacrylonitrile (orlon, acrilan).
4.What are thermoplastics?Show answer
Polymers that soften on heating and harden on cooling repeatedly.

Classification, Addition Polymerisation and Copolymers: frequently asked questions

What is the difference between a homopolymer and a copolymer?

A homopolymer has one monomer; a copolymer has two different monomers.

Which monomer gives Teflon?

Tetrafluoroethene.

What is Ziegler-Natta catalyst?

A catalyst of triethylaluminium and titanium tetrachloride used for HDPE.