Azobisisobutyronitrile decomposition

Burnett et al.63,64 observed an anomalous rate increase in the polymerization of methyl methacrylate in halobenzene. Although the experimental data did not indicate any solvent effect on the rate of decomposition of azobisisobutyronitrile, the efficiency of initiation varied with solvents. Since an enhanced rate of incorporation of initiator fragments and incorporation of solvent fragments into the polymer were not observed, a mechanism describing the increase in the initiator efficiency through the participation of an initiator-solvent-monomer complex was postulated [Eq. (2.4)]. Henrici-Olive et al.65) reported, however, that the rate of the azobisisobutyronitrile decomposition at 50 °C, measured spectroscopically, is higher in halo-benzene than in benzene. Burnett et al.66) found a similar enhanced rate effect of halobenzenes with other initiators, supporting his mechanism. 

The f-butoxy radical can be produced from the radical decomposition of f-butylhypochlorite using azobisisobutyronitrile (AIBN) as catalyst(33) ... 

The various initiators are used at different temperatures depending on their rates of decomposition. Thus azobisisobutyronitrile (AIBN) is commonly used at 50-70°C, acetyl peroxide at 70-90°C, benzoyl peroxide at 80-95°C, and dicumyl or di-t-butyl peroxide at 120-140°C. The value of the decomposition rate constant kj varies in the range... 

For the decomposition of azobisisobutyronitrile in styrene monomer Howard and Ingold (7) give the equation ... 

This decomposition usually shows little dependence on solvent, so if Ed for decomposition in chloroprene is likewise 30.9 kcal. per mole, then since Eox = 25.1 kcal. per mole Ep = 9.6 kcal. per mole, assuming termination to require no energy of activation. This is 1.2 kcal. per mole larger than kp for styrene oxidation (8). Values of e for azobisisobutyronitrile in oxidation systems usually lie in the range 0.6 to 0.8 if e = 0.7, the above equation for the decomposition of the azonitrile and that given earlier for the initiated oxidation of chloroprene permit calculation of kp/kt1/2 for chloroprene and also the kinetic chain lengths of the oxidations (Table IV). 

Current views on polymerization of acrylonitrile in homogeneous solution are illustrated by a description of the reaction in N,N-dimethyl-formamide (DMF) as initiated by azobisisobutyronitrile (AIBN) at about SO to 60°. Primary radicals from the decomposition of AIBN react with monomer to start a growing chain. About one-half of the primary radicals are effective, the others being lost in side reactions not leading to polymer. Bevington and Eaves (32) estimated initiator efficiency by use of AIBN labelled with C-14, whereas Bamford, Jenkins and Johnson (13) used the FeCls termination technique. Both of these methods require that the rate of AIBN decomposition be known, and the numerical value of this rate has undergone a number of revisions that require recalculation of efficiency results. From recently proposed rate expressions for AIBN decomposition at 60° (22, 136) one calculates an efficiency of about 40% by the tracer technique and 60—65% by the FeCl3 method. 

Vazo" or 2,2 -azobisisobutyronitrile catalyst is preferred over the peroxides because of its low decomposition temperature and its non-oxidizing nature. Vazo will not bleach dyes dissolved in the monomer during polymerization. 

The key feature of Inisurfs is their surfactant behavior. They form micelles and are adsorbed at interfaces, and as such they are characterized by a critical micelle concentration (CMC) and an area/molecule in the adsorbed state. This influences both the decomposition behavior and the radical efficiency, which are much lower than those for conventional, low molecular weight initiators. Tauer and Kosmella [4] have observed that in the emulsion polymerization of styrene, using an Inisurf concentration above the CMC resulted in an increase in the rate constant of the production of free radicals. This was attributed to micellar catalysis effects as described, for example, by Rieger [5]. Conversely, if the Inisurf concentration was below the CMC the rate constant of the production of free radicals decreased with an increase in the Inisurf concentration, which was attributed to enhanced radical recombination. Also note that a similar effect of the dependence of initiator efficiency on concentration was reported by Van Hook and Tobolsky for azobisisobutyronitrile (AIBN) [6]. 

The most frequently used initiator of this type is 2,2 -azobisisobutyronitrile (AIBN). It was first prepared by Thiele and Hauser [51] who, at the same time, correctly identified its decomposition products. Analysis of the products was refined by Bickel and Waters [53]. According to them, the resulting compounds are the tetramethyldinitrile of succinic acid (84%), isobutyroni-trile (3%) and 2,3,5-tricyanohexane (9%). 

He polymerized methacrylonitrile by means of 2,2 -azobisisobutyronitrile. The isobutyronitrile radical generated by initiator decomposition is structurally almost identical with the macroradical end group. Under these conditions, the rate constant of primary radical termination should be very near to the rate constant of termination between oligomeric and polymeric radicals. The studied polymerizations were carried out in dimethylformamide, which is a poor solvent for polymethacrylonitrile. In poor solvents, the change in termination rate with the length of the growing chain should not depend on the excluded volume [10],... 

The dissociation rate of the dimer of the triphenylmethyl radical in 28 solvents was studied by Ziegler el al. [167]. The decomposition rate of azobisisobutyronitrile in 36 solvents was measured by different authors [183-185, 562], Despite the great variety of solvents, the rate constants vary only by a factor of 2... 4. This behaviour is typical for reactions involving isopolar transition states and often indicates, but does not prove, a radical-forming reaction. The lack of any marked solvent effects in most free-radical forming reactions will become more apparent after an examination of some further reactions presented in Table 5-8. 

Cage effects also account for the fact that not all the radicals produced from the decomposition of initiators such as azobisisobutyronitrile (AIBN) are effective in initiating radical polymerizations. In the somewhat simplified reaction Scheme (5-168) depicting the thermolysis of AIBN, two types of cyanopropyl radicals are shown, one still within the solvent cage, whereas the others have reached their statistical separation... 

The effect of pressure on the decomposition of azobisisobutyronitrile has also been investigated and a value of 4 cm. mole obtained for Af. This indicates a considerable stretching ( 10%) of the C-N bond in the activated state, which IS in line with the large value of AS and a loose activated complex. 

Free-radical polymerization is usually initiated by thermal or UV-initiated decomposition (usually homolytic dissociation) of a suitable radical initiator (Scheme 9.10). For example, thermal decomposition of AIBN (azobisisobutyronitrile) gives N2 and carbon radicals, which initiate polymerization of vinyl monomers. 

Only free radical polymerisation, which requires the formation of reactive free radical species to initiate polymerisation, appears to have been used to form MIPs. Free radicals are produced by the decomposition of an initiator species by the action of heat or light. Commonly used initiators are benzoyl peroxide and azobis compounds such as azobisisobutyronitrile (AIBN) or 2,2 -azobis(2,4-dimethylvaleronitrile) (ABDV) Figure 6.20). 

Because most synthetic plastics, elastomers, and fibers are prepared by free-radical chain polymerizations, this method will be discussed here. Initiation can occur through decomposition of an initiator such as azobisisobutyronitrile (AIBN), light, heat, sonics, or other technique to form active free radicals. Here initiation will be considered as occurring by decomposition of an initiator, I, and is described as follows. 

Several types of peroxy compounds that are widely used are listed in Table 6.2. The dissociation of these compounds occurs by reaction of the same type as shown in Eq. (6.32). Besides peroxides, another class of compounds that find extensive use as initiators are the azo compounds By far the most important member of this class of initiators is 2,2 -azobisisobutyronitrile (AIBN) which generates radicals by the decomposition reaction ... 

Azo compounds are widely used as radical initiators in organic synthesis [1], AIBN (2,2 -azobisisobutyronitrile) is the most commonly used initiator because of its high decomposition ability and stability. Azo compounds are decomposed by heat to the corresponding alkyl radicals and nitrogen [1]. As previously described, it is known that they undergo decomposition via the cis form by absorbing light [3]. 

By comparison to peroxides, the azo compounds are generally not susceptible to chemically induced decompositions. It was shown,however, that it is possible to accelerate the decomposition of a,a -azobisisobutyronitrile by reacting it with bis(-)-ephedrine-copper(II) chelate. The mechanism was postulated to involve reductive decyanation of azobisisobutyronitrile through coordination to the chelate. Initiations of polymerizations of vinyl chloride and styrene with a,a -azobisisobutyronitrile coupled to aluminum alkyls were investigated. Gas evolution measurements indicated some accelerated decomposition. Also, additions of large amounts of tin tetrachloride to either a,a -azobisisobutyronitrile or to dimethyl-a,a -azobisis-obutyrate increase the decomposition rates. Molar ratios of [SnCl4]/[AIBN]= 21.65 and [SnCl4]/[MAIB] = 19.53 increase the rates by factors of 4.5 and 17, respectively. Decomposition rates are also enhanced by donor solvents, like ethyl acetate or propionitrile in the presence of tin tetrachloride. ... 

What sources of initiating free radicals do you know Give the chemical equation for thermal decomposition of a,a -azobisisobutyronitrile. Explain how free radicals can recombine to form inert compounds. 

The decomposition rate can also depend on solvent, but the dependence is not as pronounced as in the case of ionic reactions. For example, dibenzoyl peroxide in carbon tetrachloride decomposes to 13% in 60 min at 79.8 C, in benzene to 16%, in cyclohexane to 51%, and in 1,4-dioxane to 82% over the same period and at the same temperature. Decomposition to 95% occurs within 10 min in /-propanol, and the decomposition occurs explosively in amines. In contrast, the decomposition of azobisisobutyronitrile is much less influenced by solvent, as can be seen from times for 5% decomposition 540 min in p-dioxane, 420 min in A/", A -dimethyl formamide and 280 min in styrene. 

---