Radicalar polymerizations

Radicalar polymerizations are very commonly used. In this case, the active site is an atom bearing a unpaired electron (free bond). The initiator is an organic material which can spontaneously split by homolytic breaking and in this way can produce radicals able to attack the monomer and thus to initiate the process. Actually, as initiators, one uses peroxides (R -O-O-R ), hydroperoxides (R -O-OH) (for instance hydrogen peroxide H202, or cumene hydroperoxide) and aliphatic azoics (R -N=R ). For each initiator, there is a domain of temperature often narrow (about 10 degrees) in which the homolytic decomposition occurs with an acceptable velocity. 

One of the most common initiators is benzoic anhydride (benzoyl peroxide) with the formula 

For instance, this initiator is used for the preparation of polystyrene. Around 70 C, benzoyl peroxide decomposes into primary radicals. 

The chain length depends on the relative quantities of polymer and initiator that are used but also on the transfer frequency. The mass polydispersion of the produced polymer is always large. The lifetime of a radical varies from 10 1 to 10 seconds. The polymerization reaction stops spontaneously by deactivation. 

Finally, let us note that with this method, it is possible to prepare statistical copolymers from mixtures of two species of monomers. 

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A chain polymerization can be radicalar, anionic, cationic, or stereospecific according to the type of initiator used. The reaction may occur in bulk, in suspension, in emulsion, as in solution. It is necessary to choose adequate temperatures for a good control of the processes. 

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