2.4.4—Polymers
- Syllabus
- First assessment 2025
- Objective
- 2.4.4
- Level
- SL
Polymer Structure and Properties
A polymer is a macromolecule built from repeating monomer-derived units. Chain structure and cross-links influence plastic properties.
| Chain feature | Molecular-motion or packing effect | Typical qualitative consequence |
|---|---|---|
| Long, relatively linear chains | can pack more closely when chain chemistry permits | stronger intermolecular contact and often greater strength/density |
| More branching | can hinder close, regular packing | often lowers packing efficiency and can increase flexibility |
| Few/no cross-links | chains can move past one another more readily on heating | thermoplastic softening and reshaping |
| Dense cross-linking | strongly restricts chain movement | rigid thermoset behaviour; does not simply melt and reshape |
These are conditional structure–property trends: functional groups, chain length and processing history also matter.
Compare chain mobility: weakly interacting, unlinked chains can soften and be reshaped, whereas extensive cross-linking restricts movement and gives thermoset behaviour. A useful structure–property explanation names the repeat-chain feature, the permitted molecular motion and the resulting macroscopic response.
Cellulose is a natural polymer, whereas polyethene is synthetic. Both are macromolecules with repeating units, but origin alone does not determine a plastic's properties or biodegradability: chain structure, functional groups, intermolecular attractions, branching and cross-linking control packing and molecular motion.
Describing Polymer Properties
Assessment in practice
Representative question
Question 1
[Maximum number: 4]
Contrast the physical properties of polymers with extensive covalently bonded cross-links to polymers which only have a few of these links, giving an example of each.
| Physical properties | Example | |
|---|---|---|
| Extensive covalent cross-links: | ________________ | □ ____________ |
| Few covalent cross-links: | ________ □ | □ |
i
Example
Accept any correct example. eg. billiard balls for thermoset.
Accept "resins" for thermoset.
Accept other valid examples.
Accept "polyester" for either thermoset or thermoplastic for both.
Do not accept same physical property argument for both eg. higher mp for thermoset, lower mp for thermoplastics.
«thermoset»
Bakelite/HDPE/epoxies/
polyurethane
Few covalent
cross-links:
flexible/able to return to shape/can be
recycled
«thermoplastics» rubber/PVC/polystyrene/nylon/ polypropene/polyethene
Models to Materials Summary
Retrieve the pathway: locate bonding contributions, connect them to material properties, distinguish alloy lattice disruption, and construct addition or condensation polymer repeating units from monomer evidence.
Check that the bonding description matches the material, the property explanation names the structural cause, and every polymer substituent, continuation bond, and released small molecule is represented.