Polymerisation: Addition and Condensation
What Is Polymerisation?
Polymerisation is the process of joining many small molecules (monomers) together to form a very large molecule called a polymer. The word polymer means "many parts" (poly = many, mer = part).
There are two types of polymerisation at GCSE: addition and condensation.
Addition Polymerisation
Addition polymerisation occurs when many alkene monomers (containing C=C double bonds) join together. The double bond in each monomer "opens up" and the monomers link to form a long chain. No other product is formed — all atoms from the monomers end up in the polymer.
How It Works
1. The C=C double bond in each alkene monomer breaks open
2. Each carbon forms a new bond to the next monomer
3. A long chain with only C−C single bonds is formed
General Representation
n CH₂=CH₂ → −(CH₂−CH₂)ₙ−
The repeating unit is shown in brackets with the subscript n (representing a large number).
Common Addition Polymers
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene (CH₂=CH₂) | Poly(ethene) / polyethylene | Plastic bags, bottles, cling film |
| Propene (CH₃CH=CH₂) | Poly(propene) / polypropylene | Ropes, crates, carpet |
| Chloroethene (CH₂=CHCl) | Poly(chloroethene) / PVC | Window frames, pipes, flooring |
| Tetrafluoroethene (CF₂=CF₂) | PTFE (Teflon) | Non-stick coatings |
| Styrene (C₆H₅CH=CH₂) | Polystyrene | Packaging, insulation |
Drawing Addition Polymers
To draw the repeating unit from the monomer:
1. Remove the C=C double bond (make it C−C single)
2. Add extension bonds from each carbon to show the chain continues
3. Put brackets around the unit with n subscript
To identify the monomer from a polymer:
1. Find the repeating unit (two carbons in the backbone)
2. Change the single C−C bond back to C=C double bond
3. This gives you the monomer
Condensation Polymerisation
In condensation polymerisation, monomers join together with the loss of a small molecule (usually water, H₂O). Each monomer must have two functional groups (one at each end) so it can react at both ends and form a chain.
Types of Condensation Polymers
Polyesters — formed from a dicarboxylic acid (two −COOH groups) and a diol (two −OH groups):
The −COOH on the acid reacts with the −OH on the diol, forming an ester link (−COO−) and releasing water.
The repeating unit contains the ester linkage: −O−CO−
Example: PET (polyethylene terephthalate) — used in drinks bottles and clothing (polyester fibre).
Polyamides — formed from a dicarboxylic acid (two −COOH groups) and a diamine (two −NH₂ groups):
The −COOH on the acid reacts with the −NH₂ on the amine, forming an amide link (−CONH−) and releasing water.
Example: Nylon — used in ropes, clothing, carpets.
Key Differences
| Feature | Addition | Condensation |
|---|---|---|
| Monomer type | Alkenes (C=C double bond) | Two functional groups (e.g. −COOH + −OH) |
| Other product | None | Small molecule (usually water) |
| Link formed | C−C single bond | Ester (−COO−) or amide (−CONH−) |
| Atom economy | 100% | Less than 100% |
| Examples | Poly(ethene), PVC, PTFE | Polyester (PET), nylon |
Naturally Occurring Condensation Polymers
Many biological molecules are condensation polymers:
- Proteins are polyamides — amino acids link by amide (peptide) bonds, releasing water
- Starch and cellulose are polysaccharides — glucose monomers link by condensation
- DNA is a polymer of nucleotides
Disposing of Polymers
Most synthetic polymers are not biodegradable — microorganisms cannot break them down. This creates problems:
- Landfill: takes up space, does not decompose for hundreds of years
- Incineration: burning produces CO₂ (greenhouse gas) and can release toxic gases (e.g. HCl from PVC)
- Litter: harms wildlife, pollutes oceans
Solutions:
- Recycling — thermosoftening polymers can be melted and remoulded
- Biodegradable polymers — made from plant-based materials (starch, cellulose) that decompose naturally
- Reduce and reuse — using less plastic and reusing items
Exam Tips
- Addition polymers come from alkenes; condensation polymers come from monomers with two functional groups
- In condensation polymerisation, water is always produced as a by-product
- Be able to draw the repeating unit from the monomer (addition) or identify the link type (condensation)
- Know that condensation polymers include the ester link (−COO−) in polyesters and the amide link (−CONH−) in polyamides
- Relate disposal problems to the non-biodegradable nature of most synthetic polymers