Chain termination
In
reactive intermediates in a chain propagation step in the course of a polymerization
, effectively bringing it to a halt.
IUPAC definition
Chemical reaction in which a chain carrier is converted irreversibly into
a non-propagating species without the formation of a new chain carrier.[1]
Mechanisms of termination
In
polyisocyanate and a polyol because it will react with the isocyanate
functionality R−N=C=O to produce R−(N−H)−(C=O)−O−R' which is then no longer reactive with the polyol.
Termination of radical polymerization
The termination steps of free radical polymerization steps are of two types: recombination and disproportionation.recombination step, two growing chain radicals (denoted by •) form a covalent bond in a single stable molecule. For the example of a vinyl polymer,
Termination by recombination increases the chain length and therefore the molecular weight of the final polymer.[2]
![{\displaystyle \ {\Bigl [}\!\!\!\!{\ce {-CH2-CHX}}^{\bullet }{\Bigr ]}\quad +\quad {\Bigl [}\!\!\!\!{\ce {-CH2-CHX}}^{\bullet }{\Bigr ]}\quad \longrightarrow \quad {\Bigl [}\!\!\!\!{\ce {-CH2-CHX-CHX-CH2{}-}}\!\!\!\!{\Bigr ]}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/a9512db4f933e97f6501ef22a14900c4498c7d20)
In a disproportionation step, one radical transfers a hydrogen atom to the other to form two stable molecules:
Termination by disproportionation usually has a higher activation energy since it involves breaking of one bond, and therefore is more important at higher energy.[2]
![{\displaystyle \ {\Bigl [}\!\!\!\!{\ce {-CH2-CHX^{\bullet }}}{\Bigr ]}\quad +\quad {\Bigl [}\!\!\!\!{\ce {-CH2-CHX^{\bullet }}}{\Bigr ]}\quad \longrightarrow \quad {\Bigl [}\!\!\!\!{\ce {-CH2-CHX}}{\Bigr ]}\quad +\quad {\Bigl [}\!\!\!\!{\ce {-CH=CHX}}{\Bigr ]}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/117c28af56480ddbbfd284c7d92078c261a47b4d)
References
- S2CID 98243937.
- ^ ISBN 0-13-065056-0
External links
- IUPAC Gold Book definition: Termination