Lesson 8 of 12
Building polymer chains
Identify the repeat unit and the link formed between monomers.
A-Level 8873, revised syllabus (2026-2027)
I can recognise polymers as macromolecules built from monomers.Syllabus
Syllabus 8873, 9(h). Recognise polymers as macromolecules built from monomers.
I can distinguish addition and condensation polymers.Syllabus
Syllabus 8873, 9(i). Distinguish addition and condensation polymers.
I can describe proteins as condensation polymers of alpha-amino acids.Syllabus
Syllabus 8873, 9(j). Describe proteins as condensation polymers of alpha-amino acids.
Turn monomers into chains
Identify the repeat unit and the link formed between monomers.
A polymer is a very large molecule built from many monomer units. In this course, a molecule counts as a polymer if its average relative molecular mass is at least 1000 or it has at least 100 repeat units. A polymer sample contains chains of different lengths, so an average is used. The n in the formula does not mean every chain has exactly the same length.
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| Starting structures | Typically monomers with C=C, such as ethene. | Each chain-building monomer needs at least two link-forming sites. A diol has two OH groups and a dicarboxylic acid has two COOH groups; an amino acid has one amino group and one carboxyl group. These allow links to form repeatedly. |
| Bond change | The C=C pi bond is replaced by single bonds connecting monomer units. | New links form between functional groups, commonly ester or amide links. |
| Small product | No small molecule is eliminated in forming the addition chain. | A small molecule is eliminated per link in the examples here, usually water. |
| Repeat unit | Contains all the atoms of its contributing alkene monomer. | Accounts for the atoms left after the small molecule is removed; it may contain fragments of two different monomers. |
The substituent stays on its original carbon
The propene double bond becomes a single bond in the polymer backbone. The methyl substituent remains attached to the second carbon. The repeat-unit brackets include two backbone carbons; continuation bonds extend through the bracket boundaries.
For poly(ethene), nCH2=CH2 → [-CH2-CH2-]n. The bonds on both sides of the repeat unit must go through the brackets. To find the monomer from an addition repeat unit, take two neighbouring backbone carbons, put back their double bond and keep their groups attached. Do not break a C-H bond to make the monomer.
An alpha-amino acid has NH2 and COOH on the same carbon: H2N-CH(R)-COOH. In aminoethanoic acid, R = H. In a protein, the carboxyl group of one amino acid joins the amino group of the next, forming a peptide bond, -C(=O)-NH-. A peptide bond is an amide link. Repeating this condensation builds a chain with a repeating -NH-CH(R)-CO- backbone.
Worked exampleBuild a peptide link and keep the side chains
Show one dipeptide formed with aminoethanoic acid, H2NCH2COOH, on the left and 2-aminopropanoic acid, H2NCH(CH3)COOH, on the right.
Try it first. Then show the worked stepsHide the worked steps
- Remove OH from the left molecule's COOH and H from the right molecule's NH2, accounting for H2O overall.
- Join the left carbonyl carbon to the right nitrogen: H2NCH2CONHCH(CH3)COOH.
- The central -CONH- is the peptide link. One amino end and one carboxyl end remain, allowing further chain extension. The CH3 side chain stays attached to its original alpha carbon.
One water molecule is removed per peptide link in this condensation description. A linear chain of N amino acids contains N - 1 peptide links, not N.
Check your understanding
What monomer gives the repeat unit [-CH2-CHCl-]n?
Show the answer
CH2=CHCl, chloroethene
Restore the C=C between the two backbone carbons. Chlorine stays on its carbon. The polymer is PVC.