chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
that ceases the formation of
reactive intermediates
In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these co ...
in a
chain propagation
Chain propagation (sometimes referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a chemical chain reaction. For example, in the chlorination of methane, there is a two-step ...
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 f ...
, 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 of chemicals, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry ar ...
, 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 (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
reactions is the depletion of
monomer
In chemistry, a monomer ( ; '' mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
...
. 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
Radical may refer to:
Politics and ideology Politics
* Radical politics, the political intent of fundamental societal change
*Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe an ...
chain transfer
Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule.
:P• + XR' → PX + R'•
Chain transfer reactions reduce the average molecular weight of the final polymer. Ch ...
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
Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally occurring ...
, 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 most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
R'-OH can be used to stop a reaction between a polyisocyanate and a
polyol
In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups (). The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, thr ...
because it will react with the
isocyanate
In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...
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''.
Harry R. Allcock
Harry R. Allcock (born April 8, 1932, Loughborough, England) is Evan Pugh Professor of chemistry at Pennsylvania State University.recombination step, two growing chain radicals form a covalent bond in a single stable molecule. For the example of a
vinyl polymer
In polymer chemistry, vinyl polymers are a group of polymers derived from substituted vinyl () monomers. Their backbone is an extended alkane chain . In popular usage, "vinyl" refers only to polyvinyl chloride (PVC).
Examples
Vinyl polymers ...
,
:-------CH2–C°HX + -------CH2–C°HX → -------CH2–CHX–CHX–CH2-------
Termination by recombination increases the chain length and therefore the molecular weight of the final polymer.
In a
disproportionation
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...
step, one radical transfers a hydrogen atom to the other to form two stable molecules:
:-------CH2–C°HX + -------CH2–C°HX → -------CH2–CHX + -------CH=CHX
Termination by disproportionation usually has a higher
activation energy
In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
since it involves breaking of one bond, and therefore is more important at higher energy.
References
{{reflist
External links
* IUPAC
Gold Book
The International Union of Pure and Applied Chemistry publishes many books which contain its complete list of definitions. The definitions are divided into seven "colour books": Gold, Green, Blue, Purple, Orange, White, and Red. There is also an e ...