Polyaddition to a polyurethane: mechanism and explanation
Diisocyanates and diols add to each other without a by-product: each OH group adds to an N=C=O group, forming a urethane bond (–NH–CO–O–).
Example
The isocyanate carbon is strongly electrophilic. The alcohol oxygen attacks it and the H moves to the nitrogen. Polyurethanes are foams, coatings and elastane fibres.
Mechanism step by step
- 1
The diol oxygen attacks the central carbon of the N=C=O group.
- 2
The proton moves from O to N: a urethane group forms.
- 3
Both ends of every monomer keep reacting and the chain grows without releasing a by-product.
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
How does the foam form?
A little water reacts with isocyanate to a carbamic acid, which releases CO₂. The gas inflates the polymer.
Polyaddition, polycondensation, polymerisation?
Polymerisation: C=C monomers, chain growth. Polycondensation: step growth with a by-product (e.g. water). Polyaddition: step growth without a by-product (e.g. polyurethane, epoxy resin).