Chain termination
Chain termination is any chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 that ceases the formation of reactive intermediates in a chain propagation
Chain propagation
Chain propagation is a process in which a reactive intermediate is continuously regenerated during the course of a chemical reaction. In polymerization reaction, the reactive end-groups of a polymer chain react in each propagation step with a new monomer molecule transferring the reactive group to...

 step in the course of a polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

, effectively bringing it to a halt.

Mechanisms of Termination

In Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 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
A polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...

 because it will react with the isocyanate
Isocyanate is the functional group of elements –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more...

functionality R-N=C=O to produce R-(N-H)-(C=O)-O-R' which is then no longer reactive with the polyol.
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