Predicting the Type of Polymerisation
Specification Reference Organic Chemistry, Polymerisation 35.2
Quick Notes
- Addition polymerisation occurs when alkenes or molecules with C=C bonds join together.
- Condensation polymerisation occurs when monomers with two functional groups (e.g. –OH, –COOH, –NH2) react, eliminating a small molecule (like water or HCl).
- You should be able to:
- Identify the type of polymerisation from the monomer(s).
- Deduce the polymerisation type by looking at a section of a polymer.
Full Notes
There are two main types of polymerisation reactions in organic chemistry, addition and condensation. (see addition polymerisation and condensation polymerisation for more detail).
Addition Polymerisation
- Involves alkenes or other molecules with C=C double bonds.
- The double bond opens up and links many monomers into a long chain.
- No small molecules are eliminated, all atoms from the monomer are retained in the polymer.
Example: Ethene (CH2=CH2) to Poly(ethene)
Condensation Polymerisation
- Involves monomers with two functional groups (like –COOH, –OH, –NH2).
- Each time a bond is formed, a small molecule is lost (usually water or HCl).
- Forms polyesters, polyamides, or proteins depending on the groups involved.
Example: A diol and a dicarboxylic acid can form a polyester + H2O
How to Predict the Type of Polymerisation
When given monomers, ask yourself:
- Does the monomer have a C=C bond?
Yes → Addition polymerisation - Does the monomer have two different functional groups or is there a pair of monomers with –OH, –COOH, –NH2?
Yes → Condensation polymerisation - Will a small molecule be eliminated?
Yes → Condensation
No → Addition
Deducing the Polymerisation Type from a Polymer Chain
If you’re given a section of a polymer, look for the following clues:
- Signs of Addition Polymerisation:
- Carbon-only backbone.
- No atoms missing (no small molecule loss).
- Repeating units look like modified alkenes.
- Signs of Condensation Polymerisation:
- Look for linkages such as:
- Ester bonds (–COO–)
- Amide bonds (–CONH–)
- Peptide bonds (proteins)
- There are signs that water or HCl was lost during formation.
Key Functional Groups to Look Out For
| Polymerisation Type | Clues in Monomer/Polymer | Examples |
|---|---|---|
| Addition | C=C double bond present, no small molecule lost | Poly(ethene), Poly(propene) |
| Condensation | Two functional groups, loss of H2O or HCl, ester/amide linkages | Polyesters, Polyamides, Proteins |
Summary
- Addition polymerisation uses alkenes or C=C bonds, no atoms lost.
- Condensation polymerisation uses monomers with two functional groups, losing small molecules like H2O or HCl.
- To identify the type:
- Check for C=C → Addition.
- Check for 2 functional groups and small molecule loss → Condensation.
- Look for characteristic linkages (ester, amide, peptide) to spot condensation polymers.