Monomers and polymers
Monomer: a small molecule that can join to others like it. Polymer: a very large molecule (a macromolecule) made of thousands of monomer units joined by covalent bonds.
The part that repeats is the repeating unit. We write it in brackets with n: –(CH₂–CH₂)ₙ– for poly(ethene). n can be 1,000 to 100,000 or more.
Naming: put "poly" in front of the monomer name: ethene → poly(ethene), propene → poly(propene), chloroethene → poly(chloroethene) = PVC, tetrafluoroethene → PTFE (non-stick coating).
Addition polymerisation
Monomers must have a C=C double bond (alkenes). With heat, high pressure and a catalyst, one bond of the double bond breaks. Each carbon now has a free bond, which links to the next monomer. This repeats thousands of times.
n CH₂=CH₂ → –(CH₂–CH₂)ₙ–
- Only one product: the polymer. No small molecule is lost.
- The mass of the polymer = n × mass of the monomer.
- To draw the repeating unit: take the monomer, change C=C to C–C, and add a bond sticking out on each side.
Examples: poly(ethene) (bags, bottles), poly(propene) (ropes, crates), PVC (pipes, cable covers), polystyrene (packaging), PTFE (non-stick pans).
Condensation polymerisation
Here each monomer has two reactive groups, one at each end (for example –OH, –COOH or –NH₂). When two groups react, the monomers join and a small molecule (usually water, H₂O, sometimes HCl) is given off.
- Polyester: a diol (two –OH) + a dicarboxylic acid (two –COOH) → ester links (–COO–) + water. Example: PET, used for bottles and clothes fibres.
- Polyamide: a diamine (two –NH₂) + a dicarboxylic acid → amide links (–CONH–) + water. Example: nylon, used for ropes, tooth-brush bristles and parachutes.
| Addition | Condensation | |
|---|---|---|
| Monomer | has C=C | has 2 reactive groups |
| By-product | none | small molecule (H₂O) |
| Examples | poly(ethene), PVC | nylon, polyester, proteins |
Natural polymers
- Starch (energy store in rice, wheat, potato) and cellulose (plant cell walls, cotton, paper): chains of glucose. They form by condensation, releasing water.
- Proteins (muscle, hair, enzymes, silk, wool): chains of about 20 different amino acids joined by peptide (amide) links.
- DNA: chains of four kinds of nucleotide, which store genetic information.
- Natural rubber: an addition polymer of isoprene from the latex of rubber trees (Kerala grows a lot of it). Heating rubber with sulfur (vulcanisation) adds cross-links and makes it tougher, as in tyres.
Semi-synthetic polymers are natural polymers changed by chemistry, like rayon (made from cellulose).
Structure, properties and uses
How chains are arranged decides how a polymer behaves:
- Thermoplastics: separate chains with weak forces between them. They soften when heated and can be moulded again and again, so they can be recycled. Examples: poly(ethene), PVC, PET, polystyrene.
- Thermosets: chains joined by strong cross-links. Once set, they do not melt; strong heat chars them. Examples: Bakelite (switches, pan handles), melamine (plates), epoxy glue.
- Elastomers: a few cross-links let chains stretch and spring back (rubber).
- Fibres: chains lined up side by side give high strength along one direction (nylon, polyester).
Longer chains tangle more, so the material is stronger and melts at a higher temperature. Low-density poly(ethene) has branched chains (soft bags); high-density poly(ethene) has straight chains that pack closely (stiff bottles, crates).
New materials: Kevlar (bullet-proof vests), conducting polymers and hydrogels are designed by changing monomers and chain structure.
Plastics and the environment
Most synthetic polymers are non-biodegradable: microbes cannot break them down, so they stay for hundreds of years. Plastic waste blocks drains, harms cows and sea animals, and breaks into microplastics. Burning some plastics (like PVC) releases toxic gases.
Solutions:
- Reduce and reuse: cloth bags, steel bottles.
- Recycle thermoplastics: sort by the resin code (1 = PET, 2 = HDPE, 3 = PVC, 4 = LDPE, 5 = PP, 6 = PS).
- Biodegradable polymers such as PLA (from corn starch) and PHBV that microbes can break down.
- Energy recovery and chemical recycling back to monomers.
Most monomers come from crude oil, a limited resource, which is another reason to use polymers carefully.
Key formulas and definitions
- n CH₂=CH₂ → –(CH₂–CH₂)ₙ– (addition)
- diol + dicarboxylic acid → polyester + water (condensation)
- diamine + dicarboxylic acid → polyamide (nylon) + water
- Molar mass of polymer ≈ n × molar mass of repeating unit
- Monomer: ethene C₂H₄ (28 g/mol); propene C₃H₆ (42); chloroethene C₂H₃Cl (62.5)
Worked examples
1. Draw the repeating unit of poly(propene) from propene CH₂=CH–CH₃.
Open the double bond: –(CH₂–CH(CH₃))ₙ–. The CH₃ group hangs off the chain.
2. A poly(ethene) molecule has n = 5000. Find its molar mass.
5000 × 28 g/mol = 140,000 g/mol.
3. Why is nylon a condensation polymer?
Its monomers (a diamine and a dicarboxylic acid) join by amide links and give off water at each link.
4. Why can a PET bottle be recycled but a Bakelite switch cannot?
PET is a thermoplastic (separate chains, melts and remoulds). Bakelite is a thermoset (cross-linked, does not melt).
Common mistakes
- Thinking any monomer can do addition polymerisation. It needs a C=C double bond.
- Forgetting the small molecule (water) in condensation polymerisation equations.
- Keeping the double bond in the repeating unit. In the polymer it becomes a single bond.
- Thinking all polymers are man-made plastics. Starch, proteins, DNA and cotton are polymers too.