What you'll learn
Polymers are large molecules made from many smaller repeating units, and they're everywhere in modern life—from plastic bottles to clothing fibres. This revision guide covers the formation of polymers through addition and condensation reactions, their molecular structure, and how their properties make them suitable for different applications. You'll learn to recognise polymer structures, understand the differences between thermoplastics and thermosetting polymers, and explain how properties relate to structure at a molecular level.
Key terms and definitions
Monomer — A small molecule that can bond with other identical molecules to form a polymer chain
Polymer — A very large molecule made up of repeating units (monomers) joined together by covalent bonds
Addition polymerisation — A reaction where many unsaturated monomer molecules (containing C=C double bonds) join together to form a polymer chain with no other products
Condensation polymerisation — A reaction where monomers join together to form a polymer chain, releasing small molecules (usually water) as a by-product
Thermosoftening polymer — A polymer that softens when heated and hardens when cooled, which can be repeated because the polymer chains can slide over each other
Thermosetting polymer — A polymer that doesn't soften when heated because it contains strong cross-links (chemical bonds) between polymer chains
Repeating unit — The smallest part of a polymer chain that repeats many times along the chain
Cross-links — Strong covalent bonds that connect different polymer chains together
Core concepts
Addition polymers and their formation
Addition polymerisation occurs when many small monomer molecules containing carbon-carbon double bonds (C=C) join together to form a long chain polymer. The most common monomers are alkenes derived from crude oil.
During addition polymerisation:
- The C=C double bond in each monomer breaks open
- The monomers join together through single covalent bonds
- No other products are formed—only the polymer
- The polymer formed contains only single C-C bonds in the chain
Common addition polymers you need to know:
Poly(ethene) (polythene)
- Monomer: ethene (C₂H₄)
- Uses: plastic bags, bottles, cling film
- The most widely produced plastic globally
Poly(propene) (polypropylene)
- Monomer: propene (C₃H₆)
- Uses: crates, ropes, carpet fibres
- Higher melting point than poly(ethene)
Poly(chloroethene) or PVC
- Monomer: chloroethene (vinyl chloride)
- Uses: window frames, pipes, clothing
- Can be made flexible with plasticisers or rigid
Poly(tetrafluoroethene) or PTFE
- Monomer: tetrafluoroethene
- Uses: non-stick coatings (Teflon), waterproof clothing
- Very unreactive due to strong C-F bonds
Drawing polymer structures:
When representing polymers, you must show:
- The repeating unit in square brackets
- A bond extending from each end of the repeating unit
- The letter 'n' outside the bracket (representing a large number)
For example, ethene (H₂C=CH₂) becomes poly(ethene) with repeating unit: —(CH₂—CH₂)—ₙ
Condensation polymers and their formation
Condensation polymerisation differs from addition polymerisation because small molecules (usually water) are eliminated when monomers join together. This requires monomers with two functional groups—one at each end of the molecule.
Key features of condensation polymers:
- Monomers must have functional groups at both ends
- A small molecule (typically H₂O) is released when each bond forms
- The polymer chain contains different atoms in the main chain (not just carbon)
- Many condensation polymers are biodegradable, unlike addition polymers
Common types you need to know:
Polyesters:
- Formed from dicarboxylic acids and diols
- Contain the ester link: —COO— in the polymer chain
- Example: PET (polyethylene terephthalate) used in drinks bottles and polyester clothing
Polyamides:
- Formed from dicarboxylic acids and diamines
- Contain the amide link: —CONH— in the polymer chain
- Example: nylon used in clothing, ropes, and carpets
Natural condensation polymers:
- Proteins (made from amino acid monomers)
- Starch and cellulose (made from glucose monomers)
- DNA (made from nucleotide monomers)
The reaction between a dicarboxylic acid and a diol produces a polyester plus water:
n(HOOC—R—COOH) + n(HO—R'—OH) → —(OC—R—COO—R'—O)—ₙ + 2nH₂O
Structure and properties of polymers
The properties of polymers depend on:
- What monomers they're made from
- The conditions used during polymerisation
- The arrangement of polymer chains
Intermolecular forces in polymers:
Polymer chains are held together by intermolecular forces, NOT covalent bonds between chains (except in thermosetting polymers). These forces are typically:
- Weak van der Waals forces in simple polymers like poly(ethene)
- Stronger permanent dipole-dipole forces in polymers containing polar groups
- Very strong hydrogen bonds in polymers like nylon
Chain length and properties:
Longer polymer chains generally produce:
- Higher melting points (more intermolecular forces to overcome)
- Greater strength (chains become more tangled)
- Increased viscosity in the molten state
Crystallinity:
Some polymer chains can pack together in regular, ordered arrangements called crystalline regions:
- Increases density
- Increases strength
- Increases melting point
- Makes the polymer less flexible
Branched polymers have more difficulty forming crystalline regions than straight-chain polymers.
Thermosoftening vs thermosetting polymers
Thermosoftening polymers (also called thermoplastics):
Structure:
- Individual polymer chains
- No cross-links between chains
- Held together only by weak intermolecular forces
Behaviour when heated:
- Softens and eventually melts when heated
- Hardens again when cooled
- Can be remoulded repeatedly
- Process is reversible
Examples:
- Poly(ethene) — plastic bags
- Poly(propene) — food containers
- Polystyrene — packaging foam
- PVC — pipes and window frames
Thermosetting polymers (thermosets):
Structure:
- Polymer chains connected by strong covalent cross-links
- Creates a giant three-dimensional network structure
- Cross-links cannot be broken by heating
Behaviour when heated:
- Do NOT soften when heated
- Char or burn if heated strongly
- Cannot be remoulded
- Once set, the shape is permanent
Examples:
- Melamine resin — kitchen worktops, plates
- Epoxy resin — adhesives, boat construction
- Vulcanised rubber — car tyres (sulphur cross-links added)
The key difference is the presence of cross-links. Thermosetting polymers sacrifice the ability to be remoulded for greater strength, rigidity and heat resistance.
Uses of polymers based on their properties
Polymers are chosen for specific applications based on their properties:
Packaging materials:
- Poly(ethene): flexible, waterproof, transparent—cling film, carrier bags
- Polystyrene: lightweight, insulating—protective packaging, disposable cups
- PET: strong, transparent, inert—drinks bottles, food containers
Clothing and textiles:
- Polyester: strong, durable, doesn't absorb water—sportswear, carpets
- Nylon: strong, flexible, resistant to abrasion—stockings, ropes, parachutes
- PTFE-coated fabrics: waterproof, breathable—outdoor clothing (Gore-Tex)
Construction and engineering:
- PVC: rigid, waterproof, doesn't corrode—pipes, window frames, guttering
- Epoxy resins: strong, heat-resistant, adhesive—boat hulls, circuit boards
- Polycarbonate: transparent, impact-resistant—safety screens, eyeglass lenses
Non-stick and low-friction applications:
- PTFE: very unreactive, low friction coefficient—non-stick pans, bearings
The versatility of polymers comes from the ability to modify their properties by:
- Changing the monomer used
- Adjusting chain length
- Adding plasticisers (small molecules that separate chains, making them more flexible)
- Creating cross-links
- Forming composites with other materials
Environmental issues with polymers
Most addition polymers are:
- Non-biodegradable (microorganisms cannot break them down)
- Made from crude oil (a finite resource)
- Difficult to dispose of sustainably
Disposal methods:
Landfill:
- Advantages: cheap, easy
- Disadvantages: waste of resources, takes up land space, polymers persist for centuries
Incineration:
- Advantages: produces energy, reduces volume of waste
- Disadvantages: produces CO₂ (greenhouse gas), toxic gases if chlorine-containing polymers burn (e.g., PVC produces HCl)
Recycling:
- Advantages: conserves crude oil, reduces landfill, less energy than making new polymers
- Disadvantages: polymers must be sorted by type, can be expensive, recycled polymer is often lower quality
Developing biodegradable polymers:
Scientists are developing polymers that can be broken down by microorganisms:
- Polylactic acid (PLA) from corn starch
- Starch-based polymers
- Polyhydroxybutyrate (PHB) from bacteria
These help reduce plastic pollution but often have limitations in strength or water resistance compared to traditional polymers.
Worked examples
Example 1: Drawing the repeating unit of a polymer
Question: Chloroethene has the structural formula shown below. Draw the repeating unit of poly(chloroethene).
H₂C=CHCl
Solution:
Step 1: Identify that chloroethene contains a C=C double bond, so this is addition polymerisation.
Step 2: Break the double bond and extend bonds from each carbon:
Step 3: Draw the repeating unit with bonds extending outward and brackets:
H Cl
| |
—( C — C )—
| |
H H ₙ
Mark scheme points (2 marks):
- Correct structure of repeating unit with H, Cl positioned correctly (1 mark)
- Extended bonds shown outside brackets with 'n' subscript (1 mark)
Example 2: Explaining polymer properties
Question: Explain why thermosetting polymers do not melt when heated, whereas thermosoftening polymers do. (4 marks)
Solution:
Thermosetting polymers have strong covalent bonds (cross-links) between polymer chains (1 mark). These cross-links hold the chains rigidly in place (1 mark). When heated, these covalent bonds do not break, so the polymer cannot soften or melt (1 mark).
Thermosoftening polymers only have weak intermolecular forces between separate polymer chains (1 mark). When heated, these weak forces are overcome, allowing chains to slide over each other, so the polymer softens and melts (accept: can be remoulded) (1 mark).
Note: Maximum 4 marks—must show comparison between both types.
Example 3: Identifying condensation polymerisation
Question: A condensation polymer is made from the monomers hexanedioic acid (HOOC(CH₂)₄COOH) and hexane-1,6-diamine (H₂N(CH₂)₆NH₂).
(a) What type of polymer is formed? (1 mark) (b) Name the small molecule released during polymerisation. (1 mark) (c) Draw the link formed between two monomers. (2 marks)
Solution:
(a) Polyamide (or nylon) (1 mark)
(b) Water / H₂O (1 mark)
(c) The amide link: —CO—NH— (2 marks)
- Correct structure (1 mark)
- Bonds shown correctly on both sides (1 mark)
Alternative acceptable answer: Show structural formula of link between two specific monomers with water molecule shown as product.
Common mistakes and how to avoid them
Forgetting to show extended bonds and 'n' when drawing repeating units. Always draw bonds coming out of the brackets on both sides and include the subscript 'n' to show many repeating units.
Confusing addition and condensation polymerisation. Remember: addition uses monomers with C=C double bonds and produces ONLY the polymer; condensation uses monomers with two functional groups and ALWAYS produces water (or another small molecule) as well.
Saying thermosetting polymers have "stronger bonds" without specifying cross-links. Be precise—thermosetting polymers have covalent cross-links between chains; both types have strong covalent bonds within chains.
Thinking all polymers are synthetic. Remember that starch, cellulose, proteins and DNA are natural condensation polymers. The AQA specification expects you to recognise these.
Not recognising that polymer properties depend on intermolecular forces, not covalent bonds. The covalent bonds within polymer chains are all strong; differences in properties (like melting point) come from the strength of forces between chains.
Confusing biodegradable with recyclable. Biodegradable means broken down by microorganisms; recyclable means can be melted down and reformed (which requires thermosoftening polymers). Most addition polymers are recyclable but NOT biodegradable.
Exam technique for "Bonding, structure and properties of matter: polymers and their properties"
"Explain" questions require you to give reasons WHY something happens, not just describe WHAT happens. For polymer properties, link structure (cross-links, chain length, intermolecular forces) to behaviour (melting, flexibility, strength). Expect 3-4 marks for detailed explanations.
Drawing repeating units: Always use brackets with extended bonds and subscript 'n'. Show all atoms clearly—don't use skeletal formulae at GCSE. Check you've maintained the correct number of bonds to each carbon (4 bonds total). Typically worth 2 marks.
Command word "compare" means you must give similarities AND differences. For thermosoftening vs thermosetting, mention both types in your answer and use comparative language ("whereas," "however," "in contrast").
Extended response questions (6 marks) may ask you to evaluate disposal methods or explain why specific polymers are chosen for particular uses. Structure your answer logically, use scientific terminology precisely, and provide balanced arguments showing advantages and disadvantages.
Quick revision summary
Polymers are large molecules formed from many monomers. Addition polymers form from alkene monomers with no by-products; condensation polymers form from monomers with two functional groups, releasing water. Thermosoftening polymers have separate chains held by weak forces and can be remoulded; thermosetting polymers have covalent cross-links and cannot melt. Polymer properties depend on chain length, intermolecular forces, and crystallinity. Most synthetic polymers are non-biodegradable, creating environmental challenges. Choose polymers based on required properties for specific applications.