Radical polymerization

Radical polymerization is a type of polymerization in which the reactive center of a polymer chain consists of a radical.

Related Topics:
Polymerization - Reactive center - Radical

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The polymerization reaction is initiated by three classes of free-radical initiators:

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• certain compounds that can be broken down in two radicals at temperatures just above room temperature. Such compounds include organic peroxides such as Benzoyl peroxide and certain azo compounds such as AIBN.

:: RO-OR ----> 2 RO.

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:: R2.CN.CN = NC.CN.R2 -----> 2 R2C.CN + N2

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• photosensitive molecules, which under the influence of light, get into an excited state or react with other molecules, forming radicals.
• a redox--system with transfer of one electron during the reaction. This often involves a metal-ion such as in the reaction of a ferrous ion with hydrogen peroxide to a ferric ion in which a hydroxyl radical is formed.

Emulsion polymerization is a special radical polymerization technique and features yet another initiation method.

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Taking the polymerization of ethene as an example, the free radical reaction mechanism can be divided in to three stages: initiation, chain propagation and chain termination.

Related Topics:
Polymerization - Ethene - Reaction mechanism - Initiation - Chain propagation - Chain termination

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Initiation is the creation of free radicals necessary for propagation. The radicals can be created from radical initiators, such as organic peroxide molecules, or other molecules containing an O-O single bond or by reacting oxygen with ethene. The products formed are unstable and easily break down into two radicals. In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule in to another radical.

Related Topics:
Radical initiator - Peroxide - Oxygen - Ethene - Sigma bond - Pi bond

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Propagation is the rapid reaction of this radicalised ethene molecule with another ethene monomer, and the subsequent repetition to create the repeating chain.

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Termination occurs when a radical reacts in a way that prevents further propagation. The most common method of termination is by coupling where two radical species react with each other forming a single molecule. Another, less common method of termination is chain disproportionation where two radicals meet, but instead of coupling, they exchange a proton, which gives two terminated chains, one saturated and the other with a terminal double bond. A chain transfer reaction is also a side-reaction in radical polymerization.

Related Topics:
Chain disproportionation - Saturated - Double bond - Chain transfer reaction

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Free radical addition polymerization of ethylene must take place at high temperatures and pressures, approximately 300 °C and 2000 At. While most other free radical polymerizations do not require such extreme temperatures and pressures, they do tend to lack control. One effect of this lack of control is a high degree of branching. Also, as termination occurs randomly, when two chains collide, it is impossible to control the length of individual chains.

Related Topics:
°C - At

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