How condensation polymers form
Condensation polymerisation forms long-chain molecules through repeated reactions between functional groups on monomers. Each monomer must be able to react at two positions so that the growing chain can continue reacting.
Formation of a new link normally eliminates a small molecule. Water is produced when carboxylic acids are used, while reactions involving acyl chlorides produce hydrogen chloride.
| Monomers | Polymer | Link formed | Small molecule |
|---|---|---|---|
| Dicarboxylic acid + diol | Polyester | Ester, | |
| Diacyl dichloride + diol | Polyester | Ester, | |
| Dicarboxylic acid + diamine | Polyamide | Amide, | |
| Diacyl dichloride + diamine | Polyamide | Amide, |
Reactive groups at both ends
Diol
Has two alcohol groups available for condensation.
Dicarboxylic acid
Has two carboxylic acid groups.
Diamine
Has two amine groups that can form amide links.
Carboxylic-acid routes to these links are reversible and require acid catalysis. Replacing the acid groups with more reactive acyl chloride groups makes the corresponding reactions proceed effectively to completion without that catalyst, although is then released as a hazardous product.
Two different monomers are common, but one monomer can also form a condensation polymer when it contains both functional groups needed for repeated reaction.
- A monomer containing both and groups can form a polyester.
- An amino acid contains both an amine group and a carboxylic acid group and can form amide links by repeated condensation.
For a finite linear chain made from molecules of each of two bifunctional monomers, the starting molecules become joined by new links. One reactive group remains at each chain end, so small molecules are lost.
Exam Tip: If a question asks for the polymer class, answer polyester or polyamide as appropriate. If it asks for the polymerisation process, state condensation polymerisation.