Addition Polymerisation

· Addition polymerisation = many alkene monomers join together to form a long-chain polymer.
· Monomers must contain a C=C double bond.
· During polymerisation, the C=C double bond opens and forms new C–C single bonds between monomers.
· No small molecule is eliminated; all atoms in the monomer are retained in the polymer.
· Addition polymers are named as poly(monomer name), e.g. ethene → poly(ethene).
· Key examples for CIE: poly(ethene) and poly(chloroethene), PVC.

This diagram shows how ethene monomers join together during polymerisation. It helps visualise the conversion of C=C bonds into C–C single bonds in the polymer chain. Source

Poly(ethene)

· Monomer: ethene, CH₂=CH₂.
· Polymer: poly(ethene).
· Equation: nCH₂=CH₂ → [–CH₂–CH₂–]ₙ.
· The repeat unit is –CH₂–CH₂–.
· In the polymer, the original C=C double bond has become a C–C single bond.
· The n means a very large number of repeat units.
· Exam tip: draw the repeat unit inside square brackets with continuation bonds passing through the brackets.

This image shows the repeat unit of poly(ethene). It is useful for checking that the polymer chain has only C–C single bonds and that the repeat unit is correctly placed in brackets. Source

Poly(chloroethene), PVC

· Monomer: chloroethene, CH₂=CHCl.
· Polymer: poly(chloroethene), commonly called PVC.
· Equation: nCH₂=CHCl → [–CH₂–CHCl–]ₙ.
· The repeat unit is –CH₂–CHCl–.
· The Cl atom remains attached to one carbon in each repeat unit.
· Exam tip: do not remove atoms from the monomer; addition polymerisation retains all atoms.

This diagram shows chloroethene monomers forming PVC. It clearly shows that the C=C bond opens while the chlorine atom remains as a side group on the polymer chain. Source

Deducing the Repeat Unit from a Monomer

· Identify the C=C double bond in the monomer.
· Change the C=C into a C–C single bond.
· Keep all atoms/groups attached to the same carbon atoms.
· Put the repeat unit inside square brackets.
· Add continuation bonds through the brackets and write n outside.
· Example: CH₂=CHCl → [–CH₂–CHCl–]ₙ.

Identifying the Monomer from a Polymer Section

· Find the smallest repeating pattern in the polymer chain.
· For an addition polymer, the repeat unit usually contains two carbon atoms from the original alkene.
· Remove the square brackets or repeating section.
· Change the C–C single bond between the two backbone carbons back into a C=C double bond.
· Keep all side groups attached to the same carbon atoms.
· Example: [–CH₂–CHCl–]ₙ → CH₂=CHCl.

Disposal Problems of Poly(alkene)s

· Poly(alkene)s are non-biodegradable because their strong C–C backbone is difficult for microorganisms to break down.
· They can persist in the environment for a long time, causing plastic waste accumulation.
· Combustion can produce harmful gases depending on the polymer.
· Burning hydrocarbons may produce CO₂ and, if incomplete, CO and carbon particles.
· Burning PVC can produce harmful acidic gases such as HCl due to the chlorine atoms.
· Disposal methods include recycling, controlled combustion, and reducing single-use plastics, but each has limitations.

Common Exam Mistakes

· Do not draw a C=C double bond inside the polymer repeat unit; it should become C–C.
· Do not forget the n outside the brackets.
· Do not omit side groups, e.g. Cl in PVC.
· Do not draw brackets without continuation bonds passing through them.
· Do not confuse addition polymerisation with condensation polymerisation; addition polymerisation forms no small molecule by-product.