Polycondensation to a polyester (PET): mechanism and explanation
A dicarboxylic acid and a diol esterify again and again at both ends: a long chain of ester bonds forms. Terephthalic acid and ethylene glycol give PET (drink bottles, polyester fibres).
Example
Each ester bond forms as in the Fischer esterification, releasing one water per bond. Because every monomer has two functional groups, the chain can grow in both directions.
Mechanism step by step
- 1
One COOH group is protonated (acid-catalysed) and an OH of the diol attacks.
- 2
Water is eliminated and the first ester bond forms; both chain ends still carry a reactive group.
- 3
Dimers, trimers and longer pieces keep linking up (step growth). High chain lengths are only reached if water is removed continuously.
To read
This topic is part of the syllabus but not in the training: polymers consist of repeating units and biomolecules react through enzymes in the cell. There is no clear-cut task with exactly one product for it.
Go to the trainer with 157 reactions to practiseFrequently asked questions
Why exactly two functional groups per monomer?
With one group the chain stops after one reaction. Two give a linear chain; three (e.g. glycerol) give cross-linked resins.
How is PET recycled chemically?
By hydrolysis or transesterification (e.g. with methanol) back to the monomers, i.e. the reverse of the polycondensation.