Topic 8 of 12
Building polymer chains
Identify the repeat unit and the link formed between monomers.
A-Level 8873, revised syllabus (2026-2027)
Turn monomers into chains
Identify the repeat unit and the link formed between monomers.
A polymer is a macromolecule built from many monomer units. For this course, recognise a polymer as having an average relative molecular mass of at least 1000 or at least 100 repeat units. A polymer sample normally contains chains of different lengths, so an average is useful; the subscript n 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 at both sides of the repeat unit must continue through the brackets. To infer an alkene monomer from a simple addition repeat unit, isolate two adjacent backbone carbons, restore their double bond and keep their substituents attached. Do not break a C-H bond to make the monomer.
An alpha-amino acid has NH2 and COOH attached to the same carbon: H2N-CH(R)-COOH. Aminoethanoic acid has R = H. In a protein, the carboxyl group of one amino acid joins the amino group of another, giving a peptide bond, -C(=O)-NH-. A peptide bond is an amide link; repeated condensation builds a chain with a repeating -NH-CH(R)-CO- backbone.
Worked example
Build 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.
- 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.