Polymers • Topic 2 of 3

Types of Polymerisation

Two broad mechanisms build polymers: addition (chain-growth) and condensation (step-growth) polymerisation.

Addition (chain-growth) polymerisation. Unsaturated monomers (containing C=C double bonds) add to one another repeatedly, so the polymer has the same empirical formula as the monomer and no small molecule is eliminated. It proceeds by a chain mechanism through reactive intermediates that may be free radicals, cations or anions.

Free-radical mechanism (e.g. ethene → polythene with a peroxide initiator) has three stages:

(i) Initiation. A peroxide, R—O—O—R, breaks homolytically on heating to give two free radicals R•. A radical adds to the C=C of a monomer, generating a new, larger radical: R• + CH2=CH2 → R—CH2—CH2•.

(ii) Propagation. The growing radical adds successive monomer molecules, the radical centre always moving to the chain end, so the chain grows rapidly: R—CH2—CH2• + CH2=CH2 → R—CH2—CH2—CH2—CH2•, and so on.

(iii) Termination. Two radical chains combine (coupling) or one abstracts a hydrogen from the other (disproportionation), removing the unpaired electrons and ending growth: 2 (~CH2—CH2•) → ~CH2—CH2—CH2—CH2~.

Condensation (step-growth) polymerisation. Bi- or poly-functional monomers (carrying —COOH, —OH, —NH2 or —COCl groups) react in repeated condensation steps, each eliminating a small molecule such as water, HCl or methanol. Growth occurs in steps between any two species (monomer, dimer, trimer…), so high molecular mass builds only late in the reaction. Examples: nylon-6,6 from hexamethylenediamine and adipic acid (loses water), and terylene from ethylene glycol and terephthalic acid.

Copolymerisation. When two or more different monomers are polymerised together, the product is a copolymer whose chain contains units of each monomer. Copolymers often combine useful properties of both: Buna-S (styrene-butadiene rubber) is a copolymer of 1,3-butadiene and styrene, and is tougher and more resistant to wear than rubber made from either alone.

Key differences. In chain growth, monomer is consumed steadily, high polymer forms early, and only monomer adds to the chain end; in step growth, monomer disappears early, high molecular mass appears only near completion, and any two oligomers can combine, with loss of a small molecule each step.

Free-radical addition polymerisation: initiation, propagation and terminationFree-radical addition polymerisation1. InitiationR-O-O-R → 2 R• (homolysis)R• + CH2=CH2 → R-CH2-CH2•2. PropagationR-CH2-CH2• + CH2=CH2 →R-CH2-CH2-CH2-CH2• (chain grows)3. Termination2 (~CH2-CH2•) →~CH2-CH2-CH2-CH2~ (radicals paired)Net: n CH2=CH2 → -(CH2-CH2)n- (polythene)
Solved Examples
1
Worked Example
Distinguish between addition and condensation polymerisation.
Solution
  1. Addition polymerisation joins unsaturated monomers (C=C) by a chain mechanism with no loss of any small molecule, e.g. ethene → polythene.
  2. Condensation polymerisation joins bi-functional monomers by repeated condensation, eliminating a small molecule such as water or HCl each step, e.g. nylon-6,6.
  3. Hence the addition polymer has the same empirical formula as its monomer, but the condensation polymer does not.

Answer: Addition = chain growth, no small molecule lost; condensation = step growth, a small molecule eliminated.

2
Worked Example
Write the three steps of the free-radical mechanism for the polymerisation of ethene.
Solution
  1. Initiation: a peroxide gives radicals, R-O-O-R → 2 R•, then R• + CH2=CH2 → R-CH2-CH2•.
  2. Propagation: the radical adds successive monomers, R-CH2-CH2• + CH2=CH2 → R-CH2-CH2-CH2-CH2•, repeatedly.
  3. Termination: two chains combine, 2(~CH2-CH2•) → ~CH2-CH2-CH2-CH2~.

Answer: Initiation (radical generation and addition), propagation (chain growth) and termination (radical pairing) give polythene.

3
Worked Example
What is a copolymer? Give one example with its monomers.
Solution
  1. A copolymer is a polymer formed by polymerising two or more different monomers together so the chain contains units of each.
  2. Buna-S is a copolymer of 1,3-butadiene and styrene.
  3. Combining the two units gives a rubber tougher and more wear-resistant than the homopolymer of either.

Answer: A copolymer contains more than one type of monomer unit; e.g. Buna-S from 1,3-butadiene and styrene.

4
Worked Example
Identify the monomers of nylon-6,6 and state the type of polymerisation and the molecule eliminated.
Solution
  1. Nylon-6,6 is made from hexamethylenediamine, H2N-(CH2)6-NH2, and adipic acid, HOOC-(CH2)4-COOH.
  2. The -COOH and -NH2 groups condense to form amide (-CONH-) linkages.
  3. Each amide bond formed releases one molecule of water.

Answer: Monomers are hexamethylenediamine and adipic acid; it is condensation (step-growth) polymerisation eliminating water.

5
Worked Example
Why does high molecular mass form only late in step-growth polymerisation but early in chain-growth?
Solution
  1. In chain growth, each active centre adds many monomers rapidly, so full-length chains appear from the start while unreacted monomer remains.
  2. In step growth, any two species (monomer, dimer, trimer) react, so the average chain length rises slowly.
  3. Monomers couple into dimers, dimers into tetramers and so on, so very long chains require almost complete reaction.

Answer: Chain growth builds full chains immediately at active centres, while step growth raises chain length gradually, so high mass appears only near completion.

6
Worked Example
Classify each as addition or condensation polymer: polythene, terylene, PVC, bakelite, Teflon.
Solution
  1. Polythene forms by addition of ethene → addition.
  2. Terylene forms from ethylene glycol and terephthalic acid with loss of water → condensation.
  3. PVC forms by addition of vinyl chloride → addition.
  4. Bakelite forms from phenol and formaldehyde with loss of water → condensation.
  5. Teflon forms by addition of tetrafluoroethene → addition.

Answer: Addition — polythene, PVC, Teflon; condensation — terylene, bakelite.

Key Points

  • Addition (chain-growth) polymerisation joins unsaturated monomers with no loss of a small molecule; the polymer has the same empirical formula as the monomer.
  • The free-radical mechanism has three steps: initiation (peroxide gives radicals that add to the C=C), propagation (the radical adds successive monomers) and termination (two radical chains pair).
  • Condensation (step-growth) polymerisation joins bi-functional monomers in repeated steps, eliminating a small molecule (H2O, HCl) each time, e.g. nylon-6,6 and terylene.
  • A copolymer contains units of two or more different monomers, e.g. Buna-S from 1,3-butadiene and styrene.
  • Chain growth: monomer is consumed steadily and high polymer forms early; step growth: monomer disappears early but high molecular mass forms only near completion.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.In free-radical addition polymerisation, the step in which the peroxide breaks into radicals is called:
Explanation: Homolysis of the peroxide to form radicals that begin the chain is the initiation step.
Q2.Which of the following forms by condensation polymerisation?
Explanation: Nylon-6,6 forms by condensation of a diamine and a diacid with elimination of water; the others are addition polymers.
Q3.Buna-S is a copolymer of:
Explanation: Buna-S (SBR) is a copolymer of 1,3-butadiene and styrene; Buna-N uses acrylonitrile instead of styrene.
Q4.In the propagation step of chain-growth polymerisation, the growing radical:
Explanation: During propagation the active radical centre adds monomer after monomer, extending the chain.
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