← Properties of selected organic compounds

Addition polymerisation, in depth

A named topic in nearly every year on record — drawing repeat units correctly and explaining reactivity are both separately assessed.

The mechanism and how to draw it

  • Addition polymerisation joins monomers with a C=C double bond, with NO by-product — the double bond opens, and the two carbons form new single bonds to neighbouring monomer units instead.
  • To draw the REPEAT UNIT: redraw the monomer with the C=C converted to a C-C single bond, and add a bond extending from each of those two carbons (shown as continuation lines "-[...]-n" or explicit bonds into the brackets).
  • Draw AT LEAST the number of repeat units the question specifies (usually two or three) joined end to end, to show the pattern continues.

Explaining WHY the monomer is more reactive than the polymer

  • The monomer has a C=C DOUBLE BOND, which is a region of high electron density that readily reacts with other species (addition reactions).
  • Once polymerised, that double bond is GONE — replaced by single C-C bonds throughout the polymer chain — so the specific reactive site that made the monomer reactive no longer exists.
  • This is why a polymer used in a real product (mask filters, non-stick coatings) is chosen for being UNREACTIVE and stable, while its monomer would be too reactive for the same use.
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Explaining reactivity without naming the actual structural change

Simply stating "the polymer is less reactive than the monomer" without explaining WHY (the loss of the C=C double bond specifically) misses the mechanism the schedule is grading. Always name the double bond explicitly, and state that its disappearance during polymerisation is what removes the reactive site. ℹ

Condensation polymers are outside this standard's scope

Addition polymerisation (monomers with a C=C, no by-product) is the ONLY polymerisation type examined at Level 2 in this standard — condensation polymerisation (which produces a small by-product molecule, such as water, and typically requires two different functional groups reacting together) belongs to a later standard. Do not attempt to describe a condensation mechanism if a question here asks about polymer formation.

Practise just this