Initiators induced decomposition

The activation parameters for an initiator can be deterrnined at normal atmospheric pressure by plotting In vs 1/T using initiator decomposition rates obtained in dilute solution (0.2 M or lower) at several temperatures. Rate data from dilute solutions are requited in order to avoid higher order reactions such as induced decompositions. The intercept for the resulting straight line is In and the slope of the line is —E jR therefore both and E can be calculated. 

Two secondary propagating reactions often accompany the initial peroxide decomposition radical-induced decompositions and -scission reactions. Both reactions affect the reactivity and efficiency of the initiation process. Peroxydicarbonates and hydroperoxides are particularly susceptible to radical-induced decompositions. In radical-induced decomposition, a radical in the system reacts with undecomposed peroxide, eg ... 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

The first-order decomposition rates of alkyl peroxycarbamates are strongly influenced by stmcture, eg, electron-donating substituents on nitrogen increase the rate of decomposition, and some substituents increase sensitivity to induced decomposition (20). Alkyl peroxycarbamates have been used to initiate vinyl monomer polymerizations and to cure mbbers (244). They Hberate iodine quantitatively from hydriodic acid solutions. Decomposition products include carbon dioxide, hydrazo and azo compounds, amines, imines, and O-alkyUiydroxylarnines. Many peroxycarbamates are stable at ca 20°C but decompose rapidly and sometimes violently above 80°C (20,44). 

Many of the initiators used in radical polymerization arc susceptible to induced decomposition by various radical species. When the reaction involves the... 

Diacyl peroxides are particularly prone to induced decomposition (Scheme 3.16). Transfer to initiator is of greatest importance for polymerizations taken to high conversion or when the ratio of initiator to monomer is high. It has been shown that, during the polymerization of S initiated by BPO, transfer to initiator can be the major pathway for the termination of chains.7,41... 

Tertiary hyponitrites are not particularly susceptible to induced decomposition. However, the same is not true of primary and secondary hyponitrites. Isopropyl hyponitrite is reported123 to undergo induced decomposition by a mechanism involving initial abstraction of a a-hydrogen (Scheme 3.24). 

Transfer to initiator is generally of lesser importance than with the corresponding diacyl peroxides. They arc, nonetheless, susceptible to the same range of reactions (see 3.3.2.1.4). Radical-induced decomposition usually occurs specifically to give an alkoxy radical and an ester (Scheme 3.34). 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

Some limitations of the method arise due to side reactions involving the nitroxide. However, such problems can usually be avoided by the correct choice of nitroxide and reaction conditions. Nitroxides, while stable in the presence of most monomers, may act as oxidants or rcductants under suitable reaction conditions.516 The induced decomposition of certain initiators (e.g. diacyl peroxides) can be a problem (Scheme 3.94).166 177 There is some evidence that nitroxides may disproportionate with alkoxy radicals bearing a-hydrogens,123 Side reactions with thiols have also been identified.4 18... 

The efficiency of these inhibitors may depend on reaction conditions. For example the reaction of radicals with stable radicals (e.g. nitroxides) may be reversible at elevated temperatures (Section 7.5.3) triphenylmethyl may initiate polymerizations (Section 7.5.2). A further complication is that the products may be capable of undergoing further radical chemistry. In the case of DPPH (22) this is attributed to the fact that the product is an aromatic nitro-compound (Section 5.3.7). Certain adducts may undergo induced decomposition to form a stable radical which can then scavenge further. 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

The accepted kinetic scheme for free radical polymerization reactions (equations 1-M1) has been used as basis for the development of the mathematical equations for the estimation of both, the efficiencies and the rate constants. Induced decomposition reactions (equations 3 and 10) have been Included to generalize the model for initiators such as Benzoyl Peroxide for... 

Depending on the initiator and monomer system secondary decomposition (equation 2), induced decomposition (equations 3,9), primary radical termination (equation 11) or transfer reactions may or may not be important and will have to be considered accordingly in the balance equations. From the above reaction scheme the following equations have been derived under the SSH, the LCA, negligible secondary decomposition and negligible primary radical termination (9,19,20) ... 

Based on the literature data available for styrene polymerized with benzoyl peroxide, (10,12,14) transfer to monomer and termination by disproportionation will be neglected. For the Initiation step, only primary and Induced decomposition reactions will be considered. 

From the reaction schemes investigated, it is clear that induced decomposition and primary radical termination reactions should be considered in the initiator balances in order to account for the observed initiator loadings. This is particularly important when relatively high initiator concentrations are involved. 

For the copper-induced decomposition of diazodiphenylmethane in acetonitrile, a fundamental difference in the catalytic action of Cu C104 and Cu ClO was detected. Whilst with CuC104, intermediary copper carbenoids are believed to be responsible for the mainly formed benzophenone azine402, CufClO initiates a chain reaction, promoted by radical cations and yielding mainly tetraphenylethene... 

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... 

The kinetic study of POOH decay in an inert atmosphere in the presence of another initiator (I). The initiator increases the concentration of macroradicals in the polymer media. If free radicals react with hydroperoxyl groups, one observes the acceleration of POOH decay. The rate of POOH decay in the case of induced decomposition obeys the equation... 

As outlined in Section 2.2.4.1, impurities and catalysts may decrease To significantly a decrease of 100°C is not unusual. The material of construction of the sample cup may act as a catalyst, resulting in surface-induced decomposition which may even be promoted by the sample/surface area ratio in the DSC cup. Therefore, it is important to check if the substance is catalyzed during the DSC experiment and if such catalysis is representative of process conditions. Frequently, substances that are sensitive to catalysis are handled in passivated glass-lined reactors, receptacles, or containers. Another phenomenon to recognize is autocatalytic decomposition. Substances that are susceptible to autocatalytic decomposition have an induction period prior to initiation of rapid decomposition. The same holds for substances that contain inhibitors, which can be depleted. 

When a material balance is performed on the amount of initiator that is decomposed during a polymerization and compared with that which initiates polymerization, it is apparent that the initiator is inefficiently used. There is wastage of initiator due to two reactions. One is the induced decomposition of initiator by the attack of propagating radicals on the initiator, for example... 

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