Cage reaction initiator decomposition

The decomposition of an initiator seldom produces a quantitative yield of initiating radicals. Most thermal and photochemical initiators generate radicals in pairs. The self-reaction of these radicals is often the major pathway for the direct conversion of primary radicals to non-radical products in solution, bulk or suspension polymerization. This cage reaction is substantial even in bulk polymerization at low conversion when the medium is essentially monomer. The importance of the process depends on the rate of diffusion of these species away from one another. 

In other cases, the cage reaction may simply lead to reformation of the initiator. This process is known as cage return and is important during the decomposition of BPO (Section 3.3.2.1.1) and DTBP (Section 3.3.2.4). Cage return lowers the rate of radical generation but does not directly yield byproducts. It is one factor contributing to the solvent and viscosity dependence of kA and can lead to a reduced at high conversion. 

The yield of cage reaction products increases with increasing viscosity of the solvent. The decomposition of diacyl peroxides was the object of intensive study. The values of rate constants of diacyl peroxides (diacetyl and dibenzoyl) decomposition (kf and initiation (ki = 2ekd) are collected in Tables 3.4 and Table 3.5. The values of e are collected in the Handbook of Radical Initiators [4]. 

Deviations from the predicted dependence of Rp on [M] and [I] are not unusual. The initiation rate and the initiator efficiency / may depend on [M] if primary radicals escape from their solvent cage (Section 6.5.5) by reaction with the nearest monomer molecules. At high initiation rates, some of the primary radicals from initiator decomposition may terminate kinetic chains. This primary termination causes the observed Rp to depend on [M] to a power greater than one and reduces the dependence of Rp on [I] to less than the power 0.5. 

A significant proportion of primary radicals that are produced by the initiator decomposition in a reaction system do not actually react with the monomer to form chain radicals, and the initiator efficiency / in Eq. (6.11) usually lies in the range 0.3-0.8. A major cause of this low / is the wastage of primary radicals by the so-called cage reactions. To illustrate, the decomposition of diacetyl... 

As stated earlier, not all free radicals that form, however, initiate polymerizations. Some are lost to side reactions. Thus, for instance, some free radicals that form can recombine inside or outside the solvent cage, where the decompositions take place, to yield either tetramethylsuccinonitrile or a ketenimine [4, 5] ... 

However, since the initiator molecule dissociates in a cage surrounding monomer and solvent molecules, some of the newly formed radicals recombine immediately. Due to this cage effect, only a fraction / of initiator radicals P react with monomer in the start reaction P + M IM (st). The rate of radical formation is thus Rr = 2fka [/]. Monomer radicals are generated with a rate Pj which is much greater than the rate Ra of initiator decomposition ... 

The initiator decomposition rate must be reasonably constant during the polymerization reaction. The cage effect (recombination of initiator radicals before starting a polymer chain) should be small, which is generally more the case with azo compounds than with peroxides. 

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