Radical polymerisation: mechanism and explanation
A radical adds to the C=C bond of a monomer, the new radical adds to the next monomer, and the chain grows. Styrene becomes polystyrene, ethene polyethylene, vinyl chloride PVC.
Example
The drawing shows the repeating unit (the asterisks mark where the chain continues). The radicals always add so that the more stable radical forms, which is why the phenyl groups sit on every second carbon (head-to-tail linkage).
Mechanism step by step
- 1
Initiation: on heating the initiator splits into radicals (R·), which add to the C=C bond of a monomer.
- 2
Propagation: the radical formed adds to the next monomer, thousands of times in a row. With styrene the stable benzyl radical forms each time.
- 3
Termination: two chain radicals meet and combine (or disproportionate); the chain is finished.
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
What is the difference between polymerisation and polycondensation?
In polymerisation monomers with C=C bonds join without a by-product (chain growth). In polycondensation two functional groups react and a small molecule (usually water) is released (step growth).
Are there ionic polymerisations too?
Yes: cationic (started with acid, for electron-rich monomers such as isobutene) and anionic (started with butyllithium, for monomers with an acceptor such as acrylonitrile). Ziegler–Natta catalysts give especially regular polypropylene.