Dibenzoyl peroxide decomposition mechanism

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

Numerous lists are available in the literature that give the decomposition temperatures or the half-lives at certain elevated temperatures of many initiators. Decompositions of peroxides may proceed via concerted mechanisms and the rates are structure-dependent. This can be illustrated on benzoyl peroxide. The benzoyl groups, the two halves of the molecule, are dipoles. They are attached, yet they repel each other. Rupture of the peroxide link releases the electrostatic repulsion between the two dipoles. Presence of electron donating groups in the para position increases the repulsion, lowers the decomposition temperature, and increases the decomposition rate. The opposite can be expected from electron attracting groups in the same position. The effect of substituents on the rate of spontaneous cleavage of dibenzoyl peroxide was expressed in terms of the Hammett equation, log K/Ko) = pa. This is shown in Table 2.3. 

The decomposition of dibenzoyl peroxide in dimethylaniline is extremely fast, which is also due to induced decomposition. The reaction mechanism involves the formation of radical cation and a subsequent transformation into radicals and stable species. In polymer synthesis, small quantities of dimethylaniline are sometimes added to BPO to promote radical generation ... 

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