How do monomers join to form addition and condensation polymers?
Distinguish addition and condensation polymerisation, identify monomers and repeating units, and relate polymer structure to properties.
How addition polymerisation links alkene monomers with no by-product while condensation polymerisation joins monomers with loss of a small molecule, and how to identify monomers and repeating units.
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What this dot point is asking
You must distinguish the two mechanisms, identify the monomer and repeating unit, and link structure to properties.
Addition polymerisation
Ethene polymerises to polyethene:
The repeating unit is . Other addition polymers include polypropene (from propene), PVC (from chloroethene) and polystyrene (from styrene).
Condensation polymerisation
Two important types:
- Polyesters form when a dicarboxylic acid reacts with a diol; the link is an ester (-COO-) group and water is released.
- Polyamides (e.g. nylon) form when a dicarboxylic acid reacts with a diamine; the link is an amide (-CONH-) group and water is released.
Identifying monomers and repeating units
- For an addition polymer, the repeating unit is the monomer with the double bond opened. Reverse this to find the monomer: redraw the C=C.
- For a condensation polymer, the repeating unit contains the linkage (ester or amide); the monomers are recovered by adding back the eliminated water across that linkage.
Structure and properties
The physical properties of a polymer depend on its structure:
- Chain length: longer chains give stronger intermolecular forces and a higher melting point.
- Branching: unbranched chains (HDPE) pack closely, giving a denser, more rigid, higher-melting material; branched chains (LDPE) pack loosely, giving a softer, more flexible, lower-density material.
- Cross-linking: covalent links between chains give rigid, heat-resistant thermosetting plastics.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2023 SACE Stage 24 marksPolystyrene (melting range 170-280 C, hardness 90) and polycarbonate (melting range 280-320 C, hardness 120) are both thermoplastics. With reference to the structures of polystyrene and polycarbonate, explain the difference in the physical properties of the two polymers.Show worked answer →
Relate the structures to the strength of intermolecular forces between chains.
Polystyrene has only carbon and hydrogen in its backbone and bulky non-polar phenyl side groups, so the main forces between chains are weak dispersion forces.
Polycarbonate contains polar carbonate (O-CO-O) groups, so its chains experience stronger dipole-dipole attractions in addition to dispersion forces.
Stronger intermolecular forces in polycarbonate mean more energy is needed to separate the chains, giving a higher melting temperature range.
The stronger attractions and more rigid aromatic/carbonate backbone also make polycarbonate harder. One mark each for the polystyrene forces, the polycarbonate forces, the melting-point link, and the hardness link.
2022 SACE Stage 21 marksThe structural formula of one nylon polymer is shown, containing repeating amide linkages. State the type of polymerisation used to make this polymer.Show worked answer →
Nylon is formed by condensation polymerisation.
The monomers (a diamine and a dicarboxylic acid) join through their functional groups, forming amide linkages and eliminating a small molecule (water) at each join. One mark for "condensation".
2024 SACE Stage 22 marksWhen making a wetsuit, polychloroprene undergoes vulcanisation that forms crosslinks between the polymer chains. Explain why vulcanised polychloroprene is difficult to recycle.Show worked answer →
Crosslinking changes the polymer from thermoplastic to thermoset behaviour.
Vulcanisation forms strong covalent crosslinks (bridges) between adjacent polymer chains, locking them into a rigid three-dimensional network.
To recycle a plastic it normally must be melted and reshaped, but these covalent crosslinks cannot be broken by heating; the material decomposes rather than melting and flowing. It therefore cannot be remoulded, making it difficult to recycle. One mark for the crosslinks, one for explaining it cannot be melted/reshaped.