In
polymer chemistry
Polymer chemistry is a sub-discipline of chemistry that focuses on the structures, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry are also applic ...
, chain termination is any
chemical reaction
A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...
that ceases the formation of
reactive intermediates in a
chain propagation
In chemistry, chain propagation (sometimes just referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a Chain reaction#Chemical chain reactions, chemical chain reaction. For exa ...
step in the course of a
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
, effectively bringing it to a halt.
Mechanisms of termination
In
polymer chemistry
Polymer chemistry is a sub-discipline of chemistry that focuses on the structures, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry are also applic ...
, 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
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
reactions is the depletion of
monomer
A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
Classification
Chemis ...
. 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
Alcohol may refer to:
Common uses
* Alcohol (chemistry), a class of compounds
* Ethanol, one of several alcohols, commonly known as alcohol in everyday life
** Alcohol (drug), intoxicant found in alcoholic beverages
** Alcoholic beverage, an alco ...
can be used to stop a reaction between a
polyisocyanate and a
polyol because it will react with the
isocyanate functionality to produce 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''.
[ Harry R. Allcock and Frederick W. Lampe ''Contemporary Polymer Chemistry'' (3rd ed., Prentice Hall 2003), p.70-72 ] 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,