Predicting the Type of Polymerisation

· Core exam skill: decide whether a monomer or polymer forms by addition polymerisation or condensation polymerisation.
· Look for the functional groups in the monomer(s), then link them to the polymer type.
· Addition polymerisation usually uses alkene monomers containing a C=C double bond.
· Condensation polymerisation uses monomers with two reactive functional groups and eliminates a small molecule, usually H₂O or HCl.
· In exams, always identify: monomer functional group(s) → type of polymerisation → repeat unit / linkage.

Addition Polymerisation: how to recognise it

· Monomer contains a C=C double bond.
· The C=C opens up to form a polymer chain with a C–C backbone.
· No small molecule is lost: all atoms in the monomer are retained in the polymer.
· Typical monomers: ethene, propene, chloroethene, other substituted alkenes.
· Polymer repeat unit contains the same substituents as the monomer, but the double bond becomes a single bond.
· Exam clue: if the monomer is an alkene, predict addition polymerisation.

This diagram shows addition polymerisation of ethene. The C=C double bond opens and forms a long-chain polymer with C–C single bonds. No small molecule is eliminated. Source

Condensation Polymerisation: how to recognise it

· Monomers must have two reactive functional groups so they can form chains.
· A small molecule is eliminated, usually H₂O from reactions involving carboxylic acids, alcohols, amines or amino acids.
· If using acyl chlorides / dioyl chlorides, the eliminated molecule is usually HCl.
· Condensation polymers usually contain ester links or amide links.
· Exam clue: if monomers are di-functional and form ester / amide bonds, predict condensation polymerisation.
· Common condensation polymer types: polyesters and polyamides.

This diagram helps distinguish broad polymerisation types. For CIE, focus on identifying addition polymerisation from alkene monomers and condensation polymerisation from monomers with two reactive functional groups. Source

Predicting from monomers

· One alkene monomer with C=C → addition polymerisation.
· Diol + dicarboxylic acid → condensation polymerisation forming a polyester + H₂O.
· Diol + dioyl chloride → condensation polymerisation forming a polyester + HCl.
· Diamine + dicarboxylic acid → condensation polymerisation forming a polyamide + H₂O.
· Diamine + dioyl chloride → condensation polymerisation forming a polyamide + HCl.
· Hydroxycarboxylic acid → condensation polymerisation forming a polyester.
· Aminocarboxylic acid / amino acid → condensation polymerisation forming a polyamide / polypeptide.

This diagram shows a diol + dicarboxylic acid forming a polyester. The key exam feature is the formation of ester links with elimination of water. Source

This diagram shows a diamine + dicarboxylic acid forming a polyamide. The key exam feature is the formation of amide links with elimination of water. Source

Predicting from a polymer section

· Polymer chain with a C–C backbone only and no ester/amide links → likely addition polymerisation.
· Polymer section containing –COO– links → condensation polymerisation, specifically a polyester.
· Polymer section containing –CONH– links → condensation polymerisation, specifically a polyamide.
· For addition polymers, work backwards by placing a C=C double bond between the two carbons of the repeat unit.
· For condensation polymers, split the polymer at the ester or amide links to identify the original monomer(s).
· If the polymer has alternating residues from two monomers, expect a pair of monomers.
· If the polymer repeat unit could come from one monomer with two functional groups, expect a single bifunctional monomer.

Quick decision rules

· C=C in monomer? → Addition polymerisation.
· Two –OH groups + two –COOH / –COCl groups? → Condensation polymerisation, polyester.
· Two –NH₂ groups + two –COOH / –COCl groups? → Condensation polymerisation, polyamide.
· –COO– in polymer? → made by condensation polymerisation.
· –CONH– in polymer? → made by condensation polymerisation.
· No small molecule lost? → usually addition polymerisation.
· H₂O or HCl eliminated? → condensation polymerisation.

Common exam traps

· Do not call every polymer made from one monomer an addition polymer; hydroxycarboxylic acids and aminocarboxylic acids can form condensation polymers alone.
· Do not forget that dioyl chlorides form condensation polymers with loss of HCl, not water.
· A polymer with C–C backbone may still contain side groups; focus on whether the backbone contains ester / amide links.
· When deducing monomers from condensation polymers, break at the –COO– or –CONH– linkage, then restore the correct functional groups.
· For addition polymers, the repeat unit must show single bonds in the chain, not the original C=C.