Polymers (AQA GCSE Chemistry): Revision Notes
Polymers
What are polymers?
Polymers are very long molecules made up of many smaller units joined together. They can be found in nature or made artificially in factories. The artificial ones made through addition reactions are called addition polymers.
Addition polymerisation
Addition polymerisation is the process where small molecules with double bonds join together to make very long chains.
How it works
The mechanism of addition polymerisation involves several key steps:
- Small molecules called monomers act as building blocks
- These monomers contain a double bond
- Many monomers join together in a chain reaction
- This creates huge molecules called polymers
Key example: Ethene to poly(ethene)
Worked Example: Ethene Polymerisation
Ethene is a simple molecule with the formula C₂H₄. When many ethene molecules join together, they form poly(ethene), which we commonly know as plastic.
The process looks like this:
- Many ethene monomers → One long poly(ethene) chain
- The double bond breaks and new single bonds form between molecules
Another example: Propene to poly(propene)
Worked Example: Propene Polymerisation
Propene molecules can also join together to form poly(propene). This follows the same pattern as ethene polymerisation, where the double bonds break and monomers link together to form long polymer chains.
Drawing polymer structures
Step-by-Step: Drawing Polymer Structures
When showing how a monomer becomes a polymer:
- Draw the monomer with its double bond
- Break the double bond to show single bonds
- Add long lines on each side to show the chain continues
- Put brackets around the repeating unit
- Write 'n' after the bracket to show many repeating units
Three key characteristics of addition polymerisation
-
The monomer has a double bond - This is essential for the joining process
-
The repeating unit has the same atoms as the monomer - No atoms are lost or gained, just rearranged
-
Only the polymer forms - No other products are made in the reaction
Common Mistakes to Avoid:
- Don't forget to break the double bond when drawing the polymer
- Remember to add the continuation lines and brackets
- The repeating unit must have the same number of atoms as the monomer
Key Points to Remember:
- Polymers are long chains made from small monomer units
- Addition polymerisation needs monomers with double bonds
- The double bond breaks so monomers can join together
- Only the polymer is formed - no other products
- Common examples include ethene making poly(ethene) and propene making poly(propene)