Free radical polymerization follows the same three steps mentiond in radical reactions:
- Initiation
- Propagation
- Termination
Initiation
The Initiation process in free radical polymerization reactions has two stages:
- Decomposition - and formation of free radicals.
- Addition - of a monomer molecule to free radical.
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Decomposition of initiator
The initiator is commonly a molecule with two mirror-images parts joined by a single or double bond. The bond is usually broken thermally ( thermal decomposition ) but it can also be broken by light ( photochemical decomposition ).
For example benzoyl peroxide
and 2,2-azobisisobutyronitrile ( called AIBN )
Addition of radical to monomer
Once the free radical is formed a monomer molecule
if we represent the free radical ( formed the initiator I ) as 
and the monomer molecule as 
, then the complete initiation process can be represented as folloes:
Mechanism
Propagation
The chain radical formed in the initiation setp is capable od adding successive monomers to propagate the chain ( make the chain longer ).
Termination
Termination of a polymerization reaction can occur by two mechanisms:
- Combination
- Disproportionation
Termination – By Combination
Termination – By Disproportionation
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This type op polymerisation involves chain reactions, where a reactive species is formed containing an unpaired electron ( called a free-radical ) or a charged particle ( called an ion ). Monomers then add themselves to the reactive species to form a long polymer chain.
Basic features of chain reactions can be show in the reaction between hydrogen and chlorine. If chlorine molecules are exposed to light and then exposed to a mixture of chlorine and hydrogen, tow rapid reactions occur ( see equations below ).
Initiation
mechanism
The UV light breaks the covalent bond in the chlorine molecule to form chlorine atoms with unpaired electrons. The unpaired electrons make the chlorine atoms very reactive. The chlorine atoms are called free-radicals.
Propagation
In these reactions a molecule of chlrine and a molecule of hydrogen have been combined to form two molecules of hydrogen chloride. But note that a chlorine atom has been formed. Thus chlorine atom ( a free-radical ) can initiate further reactions of hydrogen and chlorine and hence, these reactions are called propagation reactions.
Therefore, the addition of a few free-radical chlorine atoms has started a whole chain of reactions which could carry on until the hydrogen or chlorine is used up. The propagationstep of free-radical reactions is extremely fast and generates a large amount of heat ( very exothermic ). this series of reactions is known as a chain reaction.
Termination
The reaction may stop before all the reactions are used up by the following reactions which remove the free-radicals by reacting the them with themselves. This process is known ad termination:
In summary, free-radical reactions have three stages: initiation, propagation and termination. It is important to understand these processes as free-radical polymerization reactions follow the same three stages, as we shall see.
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