PolymersEdexcel GCSE Chemistry: Revision notes
Section 1
What is a polymer?
A polymer is a substance of high average relative molecular mass, made up of many small repeating units called monomers, joined together in long chains.
DNA, starch and proteins are all natural polymers built from different monomers:
- DNA is a polymer made from four different monomers called nucleotides
- Starch is a polymer based on sugar monomers
- Proteins are polymers based on amino acid monomers
Section 2
How are addition polymers formed?
Addition polymerisation occurs when many monomer molecules, each containing a C=C double bond, join together to form one long chain — the double bonds open up and link the monomers, with no other product formed.
Example: many ethene molecules combine to form the addition polymer poly(ethene).
Other addition polymers, all made from a monomer containing C=C:
| Polymer | Monomer |
|---|---|
| Poly(ethene) | Ethene |
| Poly(propene) | Propene |
| Poly(chloroethene) (PVC) | Chloroethene |
| Poly(tetrafluoroethene) (PTFE) | Tetrafluoroethene |
You should be able to deduce the structure of a monomer from its polymer (and vice versa) — the repeating unit of the polymer, when its bond to neighbouring units is replaced with a C=C double bond, gives the monomer structure.
To deduce a monomer from a polymer's repeating unit, break the bond to each neighbouring unit and reform the C=C double bond in its place — examiners specifically test this skill.
Section 4
How are condensation polymers such as polyesters formed?
Polyesters are condensation polymers. A polyester forms when a monomer containing two carboxylic acid groups reacts with a monomer containing two alcohol groups.
Each time an ester link forms between the two monomers, a molecule of water is also produced — this loss of a small molecule (water) during joining is what makes it a condensation reaction, unlike addition polymerisation where nothing else is formed.
The monomers alternate in the chain, connected by ester links, releasing one water molecule per link formed.
A common error is describing polyester formation as addition polymerisation — remember the defining feature of a condensation polymer is that water (or another small molecule) is released at each join.
Section 5
What problems and considerations are associated with polymers?
Problems associated with polymers include:
- Availability of starting materials — most polymers are made from crude oil, a finite resource
- Persistence in landfill — many polymers are non-biodegradable and do not break down, so they persist for a very long time
- Gases produced during disposal by combustion — burning polymers can release toxic or harmful gases
- Need to sort polymers before recycling, so that only the same type is melted and reformed together
Recycling polymers has advantages and disadvantages to evaluate:
- Advantages: saves crude oil (a finite resource), reduces landfill use, can reduce costs and environmental impact of making new plastic
- Disadvantages: sorting different polymer types is labour-intensive and costly; recycling processes still use energy and may produce emissions; recycled plastic quality can be lower
For 'evaluate' questions on recycling, always give both an advantage and a disadvantage, weighed against the specific data or context given in the question.
Must Know
- A polymer is a large molecule made of many repeating monomer units; DNA (nucleotides), starch (sugars) and proteins (amino acids) are natural examples
- Addition polymers form from monomers containing C=C, with no other product; e.g. poly(ethene), poly(propene), PVC, PTFE
- You must be able to deduce monomer structure from polymer structure and vice versa
- Polyesters are condensation polymers, formed from a dicarboxylic acid + a dialcohol monomer, releasing water at each ester link
- Polymer problems: finite raw materials, non-biodegradable landfill persistence, harmful gases from combustion, need to sort before recycling
- Recycling polymers: weigh economic, resource and environmental advantages against sorting/processing disadvantages
That's the notes covered.
Carry on to the next subtopic.