In polymer chemistry, chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.
Chemical reaction in which a chain carrier is converted irreversibly into
a non-propagating species without the formation of a new chain carrier.[1]
YouTube Encyclopedic
-
1/3Views:32 96530 79291 552
-
Initiation Propagation Termination - Free Radical Reactions
-
Sanger sequencing method - dideoxy sequencing of DNA
-
Maxam gilbert DNA sequencing method
Transcription
Mechanisms of termination
In polymer chemistry, there are several mechanisms by which a polymerization reaction can terminate depending on the mechanism and circumstances of the reaction. A method of termination that applies to all polymer reactions is the depletion of monomer. In chain growth polymerization, two growing chains can collide head to head causing the growth of both of the chains to stop. In the case of radical or anionic polymerization, chain transfer can occur where the radical at the end of the growing chain can be transferred from the chain to an individual monomer unit causing a new chain to start growing and the previous chain to stop growing. With step-growth polymerization, the reaction can be terminated by adding a monofunctional species containing the same functionality as one or more of the types of monomer used in the reaction. For example, an alcohol R'−OH can be used to stop a reaction between a 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.[2] In a recombination step, two growing chain radicals (denoted by •) form a covalent bond in a single stable molecule. For the example of a vinyl polymer,
![{\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:
![{\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
- ^ Jenkins, Aubrey D.; Jones, Richard G.; Moad, Graeme (2010). "Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization (IUPAC Recommendations 2010)". Pure and Applied Chemistry. 82 (2): 483–491. doi:10.1351/PAC-REP-08-04-03. S2CID 98243937.
- ^ a b c Harry R. Allcock and Frederick W. Lampe Contemporary Polymer Chemistry (3rd ed., Prentice Hall 2003), p.70-72 ISBN 0-13-065056-0
External links
- IUPAC Gold Book definition: Termination
This page was last edited on 20 October 2023, at 02:59