Polymers: Addition, Condensation and Natural
Polymers
Polymers are very large molecules made by linking together thousands of small molecules called monomers. The process of joining monomers together is called polymerisation. For separate chemistry, you need to understand two types: addition polymerisation and condensation polymerisation.
Addition Polymers
Addition polymerisation occurs when many unsaturated monomer molecules (alkenes) join together. The double bond (\(\text{C}=\text{C}\)) in each monomer “opens up” and bonds to the next monomer, forming a long saturated chain. No other product is formed — the only product is the polymer.
In addition polymerisation, the monomer must contain a \(\text{C}=\text{C}\) double bond. The polymer has a \(\text{C}-\text{C}\) single bond backbone. The atom economy is 100% because no by-products are formed. Example: ethene (\(\text{C}_2\text{H}_4\)) polymerises to form poly(ethene).
Common addition polymers include:
- Poly(ethene) — from ethene. Used for plastic bags, bottles, and packaging.
- Poly(propene) — from propene. Used for ropes, carpets, and food containers.
- Poly(chloroethene) (PVC) — from chloroethene. Used for window frames, pipes, and flooring.
- PTFE (Teflon) — from tetrafluoroethene. Used for non-stick coatings on pans.
Condensation Polymers
Condensation polymerisation is different from addition polymerisation. It occurs when monomers with two functional groups join together, and a small molecule (usually water) is released at each link.
Condensation polymerisation produces a polymer plus a small molecule (usually \(\text{H}_2\text{O}\)). This means the atom economy is less than 100%. The two main types of condensation polymer are polyesters (from a dicarboxylic acid + a diol) and polyamides (from a dicarboxylic acid + a diamine).
Polyesters are formed when a dicarboxylic acid (with two \(-\text{COOH}\) groups) reacts with a diol (with two \(-\text{OH}\) groups). An ester link (\(-\text{COO}-\)) forms between each pair, and water is released. Example: PET (polyethylene terephthalate), used in plastic bottles and clothing fibres.
Polyamides are formed when a dicarboxylic acid reacts with a diamine (with two \(-\text{NH}_2\) groups). An amide link (\(-\text{CONH}-\)) forms between each pair, and water is released. Example: nylon, used in ropes, clothing, and engineering plastics.
Natural Polymers
Many biological molecules are natural polymers formed by condensation polymerisation:
Proteins are natural polyamides. They are made from amino acid monomers joined by peptide bonds (a type of amide link). Each amino acid has both an amine group (\(-\text{NH}_2\)) and a carboxyl group (\(-\text{COOH}\)), so they can form long chains through condensation reactions.
DNA is a natural polymer made from nucleotide monomers. Each nucleotide contains a sugar, a phosphate group, and a base. The nucleotides are linked by condensation reactions between the sugar of one nucleotide and the phosphate of the next.
Starch and cellulose are natural polymers of glucose (a sugar monomer). Starch is used for energy storage in plants, while cellulose provides structural support in plant cell walls.
Addition: Monomers must have \(\text{C}=\text{C}\) double bonds. Only one product (the polymer). 100% atom economy. Examples: poly(ethene), PVC.
Condensation: Monomers must have two functional groups. Two products (polymer + water). Less than 100% atom economy. Examples: polyester (PET), nylon, proteins, DNA.
You can identify the type of polymerisation from the polymer structure: if the polymer backbone contains only \(\text{C}-\text{C}\) bonds, it is an addition polymer. If the backbone contains ester links (\(-\text{COO}-\)) or amide links (\(-\text{CONH}-\)), it is a condensation polymer.