Skip to content
Example · Example 2

Q.Explain, with equations, the free-radical addition polymerization mechanism by which ethene is converted into polythene, naming the three stages involved.

West Bengal WbchseTextbookSubjectiveImportance★★★★★
5% · 2/40 Questions
✓ Free question

Initiation. Heating an organic peroxide initiator (e.g. benzoyl peroxide) generates a reactive radical, R∙\text{R}^{\bullet}, which adds across the π\pi bond of one ethene molecule: R∙+CH2=CH2→R–CH2−CH2∙\text{R}^{\bullet}+\text{CH}_2{=}\text{CH}_2 \rightarrow \text{R--CH}_2{-}\text{CH}_2^{\bullet}. Propagation. The new radical adds to a further ethene molecule in the same way, extending the SAME chain by one −CH2–CH2−-\text{CH}_2\text{--CH}_2- unit and regenerating a radical at the new chain end: R–CH2CH2∙+CH2=CH2→R–CH2CH2–CH2CH2∙\text{R--CH}_2\text{CH}_2^{\bullet}+\text{CH}_2{=}\text{CH}_2 \rightarrow \text{R--CH}_2\text{CH}_2\text{--CH}_2\text{CH}_2^{\bullet}; this step repeats thousands of times, building up the long −(CH2–CH2)n−-(\text{CH}_2\text{--CH}_2)_n- backbone. Termination. The chain finally stops growing when two radical chain-ends meet and combine (or disproportionate), consuming both radicals: R1∙+R2∙→R1–R2\text{R}_1^{\bullet}+\text{R}_2^{\bullet} \rightarrow \text{R}_1\text{--R}_2.

✓Final answer

Initiation generates a radical from the initiator, propagation repeatedly adds ethene to the SAME growing chain (regenerating a radical each time), and termination combines two radical chain ends to stop the reaction, giving the polythene repeat unit −(CH2–CH2)n−-(\text{CH}_2\text{--CH}_2)_n-.

Unlock everything free for 14 days

  • Full step-by-step solutions
  • Concept-first explanations
  • Methods, shortcuts & mistakes
  • PYQ mapping + timed mock tests
Start 14-day free trial →See Plans & Pricing →

Full access for 14 days. No credit card required.

←Previous questionExample 1 — Classify the following as natural or synthetic polymers, giving a reas…→Next questionExample 3 — Explain condensation polymerization using the formation of nylon-6,6 f…