Addition Polymerisation T
Alkenes’ biggest job is making polymers. In addition polymerisation, thousands of small molecules (monomers) join together into one very large molecule (the polymer) — and because it’s an addition reaction, nothing else is produced. The monomers must be alkenes: it’s the C=C double bond that opens up to link each monomer to its neighbours. That’s also the whole answer to “why do alkenes form polymers but alkanes don’t” — alkanes have no double bond to open.
n ethene molecules → poly(ethene). The repeating unit contains exactly the same atoms as the monomer — nothing else is formed.
Going monomer → repeating unit:
- Replace the C=C with a single C–C bond.
- Keep every other atom and group exactly where it was — in poly(propene), the CH3 stays as a side group on one carbon.
- Draw square brackets with the two end bonds extending through them, and write n at the bottom right. Balance the equation with n on the left: n C2H4 → (C2H4)n.
Going polymer → monomer (the reverse question, often asked with PVC): find the repeating two-carbon unit in the chain, redraw those two carbons with a double bond, and keep their side groups — for poly(chloroethene), that’s chloroethene, a C=C with one Cl. The repeating unit and the monomer always contain the same atoms.
- Leaving a C=C inside the brackets. The polymer backbone is all single bonds — the double bond was spent joining the chain.
- Stopping the end bonds at the brackets. They must poke through — they show the unit repeats on both sides.
- Forgetting the n — or losing side groups. Atom-count the repeating unit against the monomer: they must match.
🧪 Exam-style questions
Which compound could be a monomer for addition polymerisation? Tick (✓) one box.
Explain why alkenes form polymers but alkanes do not. Tick (✓) one box.
A polymer chain has the repeating unit –CH2–CHCl– (one chlorine side group). What is its monomer? Tick (✓) one box.
In addition polymerisation, why does the repeating unit contain the same atoms as the monomer? Tick (✓) one box.
Condensation Polymerisation & Amino Acids T H
Addition polymerisation needs a C=C. Condensation polymerisation works on a completely different principle: the monomers carry two functional groups each, one at each end of the molecule. When the functional groups react, the monomers join end-to-end — and each new link kicks out a small molecule, usually water. That lost water is what the name “condensation” records, and it’s the headline difference from addition polymerisation, where nothing is lost.
The simplest case uses two different monomers, each carrying two of the same group: a diol (an –OH at both ends, e.g. ethanediol) and a dicarboxylic acid (a –COOH at both ends, e.g. hexanedioic acid). An –OH and a –COOH react to form an ester link (section 8’s chemistry, working overtime), so the product is a polyester:
n is the number of repeating units; two ester links form per unit, so 2n water molecules are lost in total.
| Addition | Condensation | |
|---|---|---|
| Monomer(s) | One type of monomer | Usually two different monomers |
| Functional group(s) | A C=C double bond | Two functional groups per monomer |
| Products | The polymer only | Polymer + small molecule (usually water) |
| Repeating unit | Same atoms as the monomer | Fewer atoms than the monomers (water has left) |
“Compare the two types of polymerisation” is a textbook 4-marker: one mark per row, as long as each point is a genuine comparison (“addition has X whereas condensation has Y”).
Amino acids — one monomer, two different groups
The diol/diacid pair needs two different monomers because each carries two of the same group. An amino acid does the job single-handedly: it carries two different functional groups in one molecule — a basic amine group (–NH2) at one end and an acidic carboxylic acid group (–COOH) at the other. The –NH2 of one molecule reacts with the –COOH of the next, water is lost, and the chain grows by condensation polymerisation into a polypeptide.
The named example to learn is glycine, H2NCH2COOH:
The bond joining each unit is an amide, or peptide, link (–CO–NH–) — the amino acid brings both groups itself, so it polymerises single‑handedly.
Check the bookkeeping against the figure: each repeating unit has lost one H from the –NH2 and an OH from the –COOH — exactly one H2O per link, which is why n water molecules appear on the right. And when different amino acids are combined in the same chain, the product is a protein — the bridge into section 11.
🧪 Exam-style questions
Which pair of monomers could undergo condensation polymerisation to form a polyester? Tick (✓) one box.
In condensation polymerisation, what is produced as well as the polymer? Tick (✓) one box.
Amino acids can polymerise on their own, without a second type of monomer. Why? Tick (✓) one box.
Glycine polymerises: n H2NCH2COOH → (–NHCH2CO–)n + ? What completes the equation? Tick (✓) one box.
DNA & Natural Polymers T
Polymer chemistry didn’t start in a refinery — nature got there first. This short final section (back to all tiers of Triple, not just Higher) asks for one idea: the big molecules of biology are polymers, and you should be able to name the monomers each one is made from.
DNA (deoxyribonucleic acid) is a large molecule essential for life: it encodes genetic instructions for the development and functioning of living organisms and viruses. Structurally, most DNA molecules are two polymer chains wound into the famous double helix, and each chain is built from just four different monomers, called nucleotides.
DNA: two polymer chains (sugar–phosphate backbones) wound into a double helix and built from four different monomers (nucleotides). The strands run off each end — the chain continues well beyond this short section.
| Natural polymer | Monomer(s) |
|---|---|
| DNA | Nucleotides (four different ones) |
| Proteins | Amino acids |
| Starch | Sugars (glucose) |
| Cellulose | Sugars (glucose) |
“Name the type of monomer that makes [polymer]” is the whole question — one row of this table per mark. Starch and cellulose share an answer: both are polymers of sugars.
🧪 Exam-style questions
What are the monomers that make up DNA called? Tick (✓) one box.
Which row correctly matches a naturally occurring polymer with its monomer? Tick (✓) one box.
Which two statements about DNA are correct? Tick (✓) two boxes, then press Check.