Addition polymerisation
Addition polymerisation
- In addition polymerisation, many small molecules join into one long chain — with no other product.
- Each small molecule is a monomer; it must have a C=C double bond.
- The long chain is the polymer.
Practice
In addition polymerisation, the monomers must have:
The C=C double bond opens up to let the monomers link into a long chain, with no other product.
Repeat units
- The repeat unit is the small part copied along the chain. To find it: change the C=C to a single C–C and draw bonds out at each end.
$$n\,\text{CH}_2{=}\text{CH}_2 \rightarrow -(\text{CH}_2{-}\text{CH}_2)_n-$$
- Examples: poly(ethene) from ethene; poly(chloroethene) (PVC) from chloroethene.
Practice
To draw the repeat unit from a monomer, you:
Open the double bond (C=C → C–C) and put a bond out at each end to show the chain continues.
Finding the monomer
- Go the other way: take one repeat unit and put the C=C double bond back.
- That gives you the monomer.
Practice
To find the monomer from a polymer's repeat unit, you:
Reversing the process, restore the C=C double bond in one repeat unit to get the monomer.
The disposal problem
- Poly(alkene)s are non-biodegradable — microbes can't break them down, so they linger for a very long time.
- Burning them can release harmful gases — e.g. burning PVC gives off toxic hydrogen chloride.
Practice
Why are poly(alkene)s hard to dispose of?
Microbes cannot break them down (non-biodegradable); burning PVC releases toxic hydrogen chloride.
You've got it
Key idea
- addition polymerisation: many C=C monomers join into one polymer (no other product)
- repeat unit: change the C=C to a single bond, bonds out at each end
- find the monomer by putting the C=C back
- poly(alkene)s are non-biodegradable; burning PVC releases toxic HCl