Peroxy-type radicals

The molecular oxygen 02(3-Tg) found in nature is paramagnetic. This fact indicates that the spins of the two electrons are parallel and thus the molecule shows the characteristics of a bi-radical. This is extremely important in oxidation reactions, since the oxygen molecule very rapidly combines with free radicals forming peroxy type radicals... 

Under aerobic conditions, during a phase known as propagation, a dioxygen molecule reacts with the alkyl radical to form peroxy-type radicals (AOO ) ... 

This free radical (1) exists as three resonance contributors. It reacts with oxygen to yield a conjugated peroxy free radical such as (2). In a molecule, like trilinolein with its multiple functionaHty, there are many possible hydroperoxy products that are formed (21) radical (2) iUustrates the principal type of stmcture formed. 

On the other hand, radicals are undoubtedly involved in the photodegradation of PVC under some experimental conditions. Recent ESR studies have provided evidence for the formation of alkyl and allyl-type radicals during the low-temperature UV irradiation of the polymer (43,72,87). Peroxy radicals were also observed when molecular oxygen was present (43,87). Other ESR work has shown convincingly that the radical -CHCI-CH2-CH-CH2-CHCI- results from the irradiation of PVC at liquid-nitrogen temperature (61,93) and is converted into a -CH2-CC1-CH2- radical at -110°C (93). 

Allyl Free Radicals. Ayscough and Evans (3) have recently studied, by ESR measurements, the types of allylic free radicals produced by gamma-irradiation of several monomeric olefins. In irradiated polyethylene the allyl free radical is quite stable, persisting for several months at room temperature (31). The presence of these allyl free radicals is most noticeable in the case of high density polyethylene, and this type of free radical is undoubtedly the cause of the slow oxidation of polyethylene at room temperature, which lasts for 40 or more days after irradiation (10). Williams and Dole (40) could observe little if any oxidation of low density polyethylene when it was exposed to air after irradiation. By oxidation we mean formation of carbonyl groups as detected by infrared absorption studies at 1725 cm"1. Parenthetically, it should be noted that adding an oxygen. molecule to a free radical produces initially another type of free radical, a peroxy free radical, but in this paper we shall not discuss free radicals of this or any other types except those of hydrocarbons. 

According to these results the macroradical —CF2—CH—CF2— plays the major role in cross-linking of VDF-HFP copolymer. The dehydrofluorination is witnessed by the findings of polyenyl-type radicals. In the presence of oxygen, the alkyl radicals react with 02 to form peroxy radicals, which intervene both in cross-linking and in chain-scission reactions. 

Free-radical initiated polymerization is normally done by using peroxy-type initiators such as benzoyl peroxide, tert-butyl perbenzoate, or difunctional initiators. The mechanism of these radical polymerizations proceeds via the following sequence ... 

This is still too high an activation energy to account for a simple one-step reaction, which has been reported. On the other hand, if the net increment in solvation energy is as large as 12 kcal., then Step 4 could conceivably be an initiation step in a reasonably long chain mechanism. If, however, a chain reaction is involved, the most likely next steps would be the attack of R radicals on either oxygen or on ozone. The oxygen reaction is likely to be diffusion controlled and would lead to peroxy-type products. However, attack on ozone is more likely to lead to formation of free acid and smaller alkyl radicals, and these are not observed as major products. 

During irradiation of lipids in the presence of oxygen, the alkyl-type radicals formed on the fatty-acid chain quickly react with O2 forming peroxy radicals. By abstracting an H atom from another fatty-acid molecule, the alkyl-type radical is regenerated and the peroxy radical is... 

Fig. 3. Free Radical Decomposition Mechanism for Hydroxylamines. R = alkyl (R ), alkoxy (RO ), or peroxy (ROO ) type radicals.

The initiation process constitutes the first reaction step in free radical polymerization, leading to the generation of (primary) radicals. The kinetics of the initiation process, ie its rate and effectiveness, are of fundamental importance in both theoretical studies and commercial applications. Commercial procedures mainly rely on the formation of primary radicals via thermal decomposition processes using azo- and peroxy-type compounds. Investigative kinetic studies are— to a large extent—carried out using photoinitiators, which decompose upon irradiation with UV or visible light. The main reason for this choice is the possibility to define exact start and end times of the initiation and subsequently the polymerization process. 

The decay behavior of allenes (1,2-propadienes) is quite different from that of the conjugated 1,3-dienes. Figure 8 shows the decay of ArS in cyclohexane for the reaction with methyl-substituted allenes [46]. By adding allene, the decay of ArS is accelerated even in the degassed solution, suggesting that the reaction proceeds irreversibly. Such irreversibility occurs when the incipient C atom-centered radical becomes a resonance stable allyl-type radical by rotation of the C-C bond, as shown in Scheme 9. In the aerated solution, the decay of ArS is further accelerated, indicating that the irreversibility due to the rotation is not completely established the addition of O2 further shifts the equilibrium to the peroxy radical side by trapping the incipient short-lived C atom-centered radicals. 

An exhaustive study on the reaction of mechanically degraded vulcanizates has been carried out [40]. Six different inhibitors were added in various percentages (5-25%) and combinations to rubber vulcanizates gelled in benzene. The solutions were degraded at 80°K in glass ampoules. The effectivenes of each stabilizer was determined by examining the ESR spectra for the degraded vulcanizates with each of the inhibitors as a function of temperature. Phenols were found to be the best inhibitors for radicals of the peroxy type. The most universal acceptor compound was found to be trichlorothio-phenol. Its effectiveness was questionable only in the case of alkyl sulfide radicals in this study of both sulfur and peroxide cross-linked vulcanizates. 

Antioxidants and stabilizers are added to most polymer systems to improve durability. Antioxidants fall into two major categories, primary and secondary, (Petrie 2004) and within primary antioxidants there are two main chemical types, hindered phenolics and aromatic amines. Primary antioxidants function by donating their reactive hydrogen to the peroxy free radicals so that the propagation of subsequent free radicals does not occur. 

Process 4, conversion of peroxy radicals to hydroperoxides can be interrupted by traditional primary antioxidants (see Fig. 16). The fastest reacting primary antioxidants are the aromatic amines (e.g. Naugard 445). However, these materials yellow upon exposure to UV light which restricts their applieations. More common in adhesives are the hindered phenol types of which numerous types are available, with Irganox 1010 the most common choice for adhesives. 

The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionizing radiation in material is to produce active radical sites. A material s sensitivity to radiation ionization is reflected in its G value, which represents the number of radicals in a specific type (e.g., peroxy or allyl) produced in the material per 100 eV of energy absorbed. For example, the G value of poly(vinyl chloride) is 10-15, of PE is 6-8, and of polystyrene is 1.5-3. Regarding monomers, the G value of methyl methacrylate is 11.5, of acrylonitrile is 5.6, and of styrene is >0.69. 

When applied to ketones, this is called Norrish Type / cleavage or often just Type I cleavage. In a secondary process, the acyl radical R —CO can then lose CO to give R radicals. Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the 0—0 bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R. The latter is an important source of radicals R , since the other product is the very stable N2. 

The above-mentioned mode of reactions changes when the irradiation is carried out in the presence of gases such as oxygen. In this case, energy transfer, the reaction of oxygen with polymer radicals [32] (leading to the formation of peroxy radicals) and other reactions may affect the type and concentration of products formed [33]. The same can be said for certain additives mixed into the elastomer for one or the other purpose. 

First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrish-type I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after a-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the investigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical. 

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