AIBN, decomposition

The byproducts of decomposition of certain dialkyldiazcncs can be a concern. Consider the case of AIBN decomposition (Scheme 3.13). The major byproduct is the ketenimine (lO).61 100"102 This compound is itself thermally labile and reverts to cyanoisopropyl radicals at a rate constant similar lo that for AIBN thermolysis.59,60 102 This complicates any analysis of the kinetics of initiation/2,60... 

Irotn AIBN decomposition 76-7 from combination of cyanoalky] radicals 37-9 from cyanoisopropyl radicals 76, I 16, 257 from dimeric PMAN 257-8 thermal stability 257... 

The linear dependence between the activation energy of decomposition of the azocompounds RN2R and the BDE of the R—H bond (D(R—H)) was established [3], The rate constants of the decomposition of azo-compounds in the gas phase and hydrocarbon solvents have close values. The mean value of the rate constant of AIBN decomposition in hydrocarbon and aromatic solutions was recommended to bekd= 1015 x exp(— 127.5/RT) s-1 [2], The values of the activation energies and the rate constants of the decomposition of azo-compounds in the gas and liquid phases can be found in the Handbook of Radical Initiators [4],... 

Initiator decomposition studies of AIBN in supercritical C02 carried out by DeSimone et al. showed that there is kinetic deviation from the traditionally studied solvent systems.16 These studies indicated a measurable decrease in the thermal decomposition of AIBN in supercritical C02 over decomposition rates measured in benzene. Kirkwood correlation plots indicate that the slower rates in supercritical C02 emanate from the overall lower dielectric constant (e) of C02 relative to that ofbenzene. Similar studies have shown an analogous trend in the decomposition kinetics ofperfluoroalkyl acyl peroxides in liquid and supercritical C02.17 Rate decreases of as much as 30% have been seen compared to decomposition measured in 1,1,2-trichlorotrifluoroethane. These studies also served to show that while initiator decomposition is in general slower in supercritical C02, overall initiation is more efficient. Uv-visual studies incorporating radical scavengers concluded that primary geminate radicals formed during thermal decomposition in supercritical C02 are not hindered to the same extent by cage effects as are those in traditional solvents such as benzene. This effect noted in AIBN decomposition in C02 is ascribed to the substantially lower viscosity of supercritical C02 compared to that ofbenzene.18... 

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. 

The second reason is closely related to the time scale of the experiment. This merits some kinetic considerations. Arrhenius activation energy of the homolytic decomposition of AIBN in toluene is l43kJmol 1 and log A is 17.33 (32). The Arrhenius parameters are not very sensitive to the solvent (33). Not all radicals produced by AIBN decomposition yield ROO radicals, because of reaction (17)... 

Complications caused by secondary decomposition of AIBN need not be considered. Its initiating effectiveness, /, is practically independent of temperature, but it increases with pressure. The combination rate of the radicals (CH3)2 C (CN) in the toluene cage also increases with pressure the process therefore is evidently diffusion-controlled [55]. AIBN decomposition can be accelerated by the addition of Lewis acids. In this respect some formal similarity therefore exists between the behaviour of peroxy and azo initiators. 

Lakhinov et al. [56] observed an 8.5-fold increase in the rate constant of AIBN decomposition in methyl methacrylate caused by the presence of ZnCl/. The initiation rate increase more than fourfold (see Figs. 1 and 2), and the degree of polymerization falls more slowly with increasing concentration of initiator. The complexes of BF3 with AIBN decompose to radicals 14 times more rapidly than the azo initiator itself [59], thus making possible its use at around room temperature. 

Some radical initiators decomposing by heat can also be decomposed by UV radiation. Photopolymerization of acrylonitrile in the presence of AIBN or hydrogen peroxide, or of other initiators [75-78] has been reported. The quantum yield of AIBN decomposition is 0.4 at 298 K and 0.6 at 318 K. Photopolymerization of methyl methacrylate, styrene, and vinyl acetate can be initiated by tetramethylsilane, methylchlorosilanes, and halides of Group IV metals [79]. We assume that the radicals are formed by homolytic splitting of the covalent bond... 

The first reported polymerization of fluoroolefins in carbon dioxide was by Fukui and coworkers [39,40]. Tetrafluoroethylene, chlorotrifluoroethylene,and other fluoroolefins were polymerized in the presence of CO2 using ionizing radiation [39, 40] and free-radical initiators [40]. DeSimone and coworkers reported the homogeneous telomerization of tetrafluoroethylene [41] and vinylidene fluoride [42] in CO2 using AIBN as an initiator. The kinetics of AIBN decomposition in CO2 is well understood [4]. However, peroxide initiators are preferred over azo initiators for producing stable endgroups in fluoroolefins... 

To solve the presented problem as study object well investigated vinylacetate (VA) polymerization system initiated by AIBN photodecomposition has been choiced.At first let us discuss some literature data concerning AIBN decomposition. According to [1]... 

It has been shown that light (X=366nm) induces AIBN decomposition which initiates VA, as well styrene polymerization [2],... 

Study of AIBN Decomposition Mechanism Induced by Light and by Laser, AIBN as Quencher. The Specificity of Laser Action... 

AIBN is manifold recrystallized from absolute methanol solution and dried at 293K. Chlorobenzene (ChB) is distilled just before it s use. AIBN decomposition rate is studied making use gasometric [21] and spectrophotometric(SPECORD M-40) [3] methods. 

Statistical copolymers of FOA are also surprisingly soluble in SC-CO2. The kinetic parameters of the thermal AIBN decomposition in SC-CO2 have been determined accurately, and 1,1-difluoroethylene was telomerized correspondingly. [22]... 

The size or MW is self-regulated by the termination reaction and the inherent side reactions. Externally, MW can be also adjusted by the reaction conditions. An increase of temperature causes faster AIBN decomposition, resulting in a lower MW because more chains are created on the contrary, higher pressure increases the propagation and inhibits the termination resulting in higher MW. The increase of the AIBN concentration has a similar effect to that of the increase of temperature. 

Effect of some organic solvents [4-6]. The inQience of acetonitrile (ACN) and dimethyl formamide (DMFA) on the radieal polymerization of styrene and (meth) acryhc esters initiated with azoisobutyronitrile (AIBN) or BP was studied. Basic characteristics of the gross kinetios of polymerization in solution have been foimd The reaction order with respeet to initiator eoneentration is always 0.5, which points to bimolecular chain termination. The reaetion order with respect to monomer varies within rather wide limits (above or below unity) depending on the chemical nature of the solvent, monomer, and initiator. The lowest (0.83) and highest (1.6) values were foimd for BP initiated polymerization of MMA in DMFA solution and for AIBN initiated polymerization of styrene in ACN solution, respectively. Sueh a high value of the order with respect to monomer is eaused by the abnormally low rate of AIBN decomposition in ACN, which, in turn, is explained by the donor-aeceptor interaction... 

Table 1. Results of Kinetic Analysis of AIBN Decomposition by Programs KINET and THERM...

When modeling the oxidation of ethylbenzene initiated by AIBN at 60 °C step (1) is replaced by the AIBN decomposition (see Table 7.4) ultimately resulting in two radicals with rate constants 7.28-10 s" (taking into consideration that the parameter e = 0.7, characterizing the yield of radicals from the solvent cage). 

The number of compounds can effect on the AIBN decomposition. It was established [1] that some mercaptans induce the dissociation of AIBN. Mercaptoacetic and mercaptosuccinic adds, 2-mercaptobenzothiazoIe decrease the polymerization of... 

TaUe 6-5 First-Order Rate Constant (ilb) for AIBN Decomposition as a Function of Temperature... 

Determine whether the rate constant for AIBN decomposition obeys the Arrhenius relationship and estimate the value of the activation energy E. 

Figure 6-6 Arrhenius plot for AIBN decomposition rate constants (Example 6-3). The solid line is the best fit obtained by linear least-squares analysis.

Fortunately, powerful nonlinear regression programs now are available. These programs allow us to minimize the sum of the squares of the deviations in any variable we choose, linear or not. Moreover, some of the easier nonlinear regression problems can be solved with a simple spreadsheet. Let s illustrate the use of a spreadsheet to carry out nonlinear regression by reanalyzing the AIBN decomposition data in Table 6-5. 

The Integral Method of Data Analysis 173 Table 6-6 Values of A, E, and ko for AIBN Decomposition in Supercritical Carbon Dioxide... 

Figure 6-7 Values of for AIBN decomposition in supercritical CO2, calculated from the results given in Table 6-6.

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