Decomposition of azo initiators

The reduction can be effected by either MMA-propagating radicals or other tertiary radicals such as those provided by the decomposition of azo initiators. The reduction in eq 34 is autoaccelerated, indicating involvement of LCoH, most likely through eq 38.304... 

Recent work in the area has concentrated on the reactions of carotenoids with peroxyl radicals, generated mainly by the thermal decomposition of azo-initiators that lead to a variety of products. " Most of these products seem to be apocarotenals or apocarotenons of various chain lengths produced by cleavage of a double bond in the polyene chain, such as P-apo-12 -carotenal, P-apo-14 -carotenal, P-apo-lO-carotenal, and P-apo-13-carolenone. Kennedy and Liebler " reported that 5,6-epoxy-p,p-carotene and 15,15 -epoxy-P,P-carotene and several unidentified polar products were formed by the peroxyl radical oxidation of P-carotene by the peroxyl radicals. 

The type of initiator utilized for a solution polymerization depends on several factors, including the solubiUty of the initiator, the rate of decomposition of the initiator, and the intended use of the polymeric product. The amount of initiator used may vary from a few hundredths to several percent of the monomer weight. As the amount of initiator is decreased, the molecular weight of the polymer is increased as a result of initiating fewer polymer chains per unit weight of monomer, and thus the initiator concentration is often used to control molecular weight. Organic peroxides, hydroperoxides, and azo compounds are the initiators of choice for the preparations of most acryUc solution polymers and copolymers. 

Figure 3 C-nuclear magnetic resonance spectra of polyurethane type II macroazo initiator before (a) and after (b) decomposition of azo group [20].

Determinations based on the rate of decomposition of the initiator assuming /=1 are indicated by I. (Values given in parentheses have been calculated using/=0.60 and 0.80 for styrene-Bz202 and for styrene-azo, respectively, as indicated by other work. -22) Those from the inhibition method are indicated by Inhib., and those from analysis of molecular weights (Sec. 2e) by MW. 

The rate of copolymerization in a binary system depends not only on the rates of the four propagation steps but also on the rates of initiation and termination reactions. To simplify matters the rate of initiation may be made independent of the monomer composition by choosing an initiator which releases primary radicals that combine efficiently with either monomer. The spontaneous decomposition rate of the initiator should be substantially independent of the reaction medium, as otherwise the rate of initiation may vary with the monomer composition. 2-Azo-bis-isobutyronitrile meets these requirements satisfactorily. The rate Ri of initiation of chain radicals of both types Ml and M2 is then fixed and equal to 2//Cd[7], or twice the rate of decomposition of the initiator I if the efficiency / is equal to unity (see Chap. IV). The relative proportion of the two types of chain radicals created at the initiation step is of no real importance, for they wall be converted one into the other by the two cross-propagation reactions of the set (1). Melville, Noble, and Watson presented the first complete theory of copolymerization suitable for handling the problem of the rate. The theory was reduced to a more concise form by Walling, whose procedure is followed here. 

Radicals produced from the initiator either directly attack the organic compound RH (for instance, this is the case during the decomposition of peroxides) or first react with dioxygen, and then, already as peroxyl radicals, attack RH (for instance, this is the case of decomposition of azo-compounds). RH gives rise to alkyl radicals when attacked by these radicals. The reaction of dioxygen addition to an alkyl radical,... 

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],... 

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar... 

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]. 

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

In addition (chain) polymerization, monomers containing an unsaturated (vinyl) bond polymerize in the presence of an initiator, which generates an active site at the end of the chain. Several chemical reactions take place simultaneously in the course of the polymerization. First, an initiation reaction via photo- or heat-decomposition of the initiator occurs to form the active species, which are either peroxides or azo compounds. The active species react with a monomer to generate the active site (i.e., initiation). 

The choice of reaction conditions has much less effect on the behavior of azo initiators. The activation energies for decomposition of azo compounds are similar to those of peroxides although the azo initiators do not contain a weak bond like... 

Azo compounds often decompose with loss of nitrogen. The decomposition of the initiator, AIBN, is an example (see Section 2). 

A conventional peroxide initiator such as BPO (I-42) does not work in a manner similar to that of azo initiators in the homogeneous system for styrene with CuBr2 and L-4.185 In this polymerization, the Cu(I) species is generated via reduction of Cu(II) species by a styryl radical and further acts as an accelerator in the decomposition of BPO. It then reacts with the remaining BPO to form the benzoyloxy radical and inactive copper(II) benzoate salts again. Thus, no polymerization occurs. However, the use of a heterogeneous system and low-temperature initiation at 70 °C followed by the polymerization at 110 °C in fact induces styrene polymerization. The polymers had... 

In order to increase the imprinting efficiency, it is desirable to achieve the polymerization at low temperatures where the non-covalent adducts between atrazine and methacrylic acid are efficiently formed. For this purpose, the initiation radicals should not be produced by thermal decomposition of radical initiator AIBN (the temperature must be 50 °C or higher here). Instead of this thermal process, AIBN (or other azo-type initiators) is decomposed by UV irradiation at 0 °C (Fig. 6.4). Under these conditions, the non-covalent adducts are abundant in the... 

Tin hydride reaction sequences are usually initiated by thermal decomposition of azo-Z>/5-isobutyrylnitrile (AIBN), which has a decomposition half-life of about 1 h at 80 °C. Although other initiators can be used at different temperatures, AIBN is... 

The thermal decomposition of benzoyl peroxide, which takes place between 60 and 90°C, involves the homolytic cleavage of the 0-0 bond to yield benzoyl free radicals that may react to yield phenyl radicals and carbon dioxide. An example of photochemically induced free-radical formation is the decomposition of azo-bisisobutyronitrile by short-wavelength visible light or near-ultraviolet radiation at temperatures as low as 0°C, where no thermal initiation occurs. 

Initiation has two steps, the formation of the free radical and the addition of the first monomer. The initial generation of a free radical is usually accomplished by the decomposition of an initiator, which is a relatively unstable compound such as a peroxide (R-O-O-R ), hydroperoxide (R-O-O-H), or azo compound (R-N=N-R ). Sometimes UV radiation and high temperatures are used to generate free radicals. After the initiator free radical is formed, the addition of the first monomer unit to the chain is accomplished, as shown in Fig. 3.15. 

Investigations into the decomposition of non-peroxide initiators in SCCO2 are rare. The decomposition of azo-hts-isobutyronitrile (AIBN), however, has been... 

Some of the transfer constants Cj can be quite large (Table 20-9). They are affected basically by the induced decomposition of these initiators. Azo-bisisobutyronitrile has practically no transfer action. 

Free radicals may be generated by the chemical decomposition of azo and peroxide compounds thermally and by y-irradiation. In the polymerization reactions (4)—(7), vinyl chloride monomer and free-radical initiator are involved, resulting in polyvinylchloride (PVC) by free-radical polymerization. 

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