Polymerization with Peroxo Compounds as Initiators

A peroxo compound that is frequently used (concentration 0.1-1 wt% with respect to monomer) is dibenzoyl peroxide (see Example 3.4). It decomposes in solution at temperatures of about 50-80°C, mainly into benzoyloxy radicals at higher temperatures phenyl radicals are formed to an increasing extent by elimination of carbon dioxide, so that the end groups of the resulting polymer are either hydrolyzable benzoic ester groups or non-hydrolyzable phenyl groups  

In practice fert-butyl peroxobenzoate is often used as a thermally decomposing radical initiator. It decomposes into benzoyloxy and methyl radicals, and acetone  

The kinetic relationship, according to which the rate of polymerization increases and the average degree of polymerization decreases with increasing initiator concentration, is satisfied by most mraiomers when either unsubstituted or substituted dibenzoyl peroxides are used as initiators. 

Ammonium peroxodisulfate is more soluble in water than the potassium salt furthermore, it dissolves in more polar organic solvents (e.g., DMF), so that it is sometimes also used for initiating polymerizations in organic media. In polymerizations initiated by peroxodisulfates the reaction medium is liable to become acidic, so that buffering is generally necessary (see Example 3.2). 

A list of some peroxo compounds that generate free radicals is given in Table 3.3, extensive information can be found in the literature. The initiators are selected according to their thermal half-lives to ensure that at the polymerization temperature they provide a source of free radicals. The rate equation for the thermal half-life is given by tj/2 = 0.693-kd, where kd is the rate constant for the thermal decomposition. In technical applications one often uses the temperature at which within a certain time interval one half of the initiator is decomposed (e.g., quoted as 10 h half-life temperature). 

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