Formation of propagating radical

Initiation is the generation of the primary radical or initiator radical. The formation of propagating radicals in polymerisations by addition of initiator radicals to double bonds, so-called primary radical reactions, is discussed in Section 10.3. Initiation is essential for most radical reactions and therefore becomes a key but simple diagnostic tool in determining mechanism. The most basic test for a radical mechanism is to carry out blank reactions under identical conditions with all the reactants present except the initiator. A zero or very low yield of product in these blank reactions represents excellent evidence for a radical reaction. 

Table 16. Delocalization stabilization for complex formation of propagating radicals with aromatic solvents...

Becanse PMMA has a low ceiling temperature, the formation of propagating radicals on radiolysis at 453 K, with G(S) = 10, Charlesby and Moore (40) found spontaneons depolymerization of the polymer to monomer at this temperature. In a later article, David and co-workers (239) showed that depol5mierization also occnrred when PMMA was irradiated at ambient temperatnre and then heated to 433 K, and that the jdeld of monomer was independent of the absorbed dose and the initial molecular weight of the PMMA. 

Fig. 3. Radicals observed by ESR. (a) Formation of propagating radicals during a standard free radical polymerization, (b) Generation of model radicals from radical precursors prepared by ATRP.

Tlie formation of initiator radicals is not the only process that determines the concentration of free radicals in a polymerization system. Polymer propagation itself does not change the radical concentration it merely changes one radical to another. Termination steps also occur, however, and these remove radicals from the system. We shall discuss combination and disproportionation reactions as modes of termination. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

As the quinone stabilizer is consumed, the peroxy radicals initiate the addition chain propagation reactions through the formation of styryl radicals. In dilute solutions, the reaction between styrene and fumarate ester foUows an alternating sequence. However, in concentrated resin solutions, the alternating addition reaction is impeded at the onset of the physical gel. The Hquid resin forms an intractable gel when only 2% of the fumarate unsaturation is cross-linked with styrene. The gel is initiated through small micelles (12) that form the nuclei for the expansion of the cross-linked network. 

Dithiols and dienes may react spontaneously to afford dithiols or dienes depending on the monomer dithiol ratio.221 However, the precise mechanism of radical formation is not known. More commonly, pholoinilialion or conventional radical initiators are employed. The initiation process requires formation of a radical to abstract from thiol or add to the diene then propagation can occur according to the steps shown in Scheme 7.17 until termination occurs by radical-radical reaction. Termination is usually written as involving the monomer-derived radicals. The process is remarkably tolerant of oxygen and impurities. The kinetics of the tbiol-ene photopolymerizalion have been studied by Bowman and... 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

Mechanism II. Reaction R02 + RH occurs slowly and tertiary peroxyl radicals react more rapidly with the formation of alkoxyl radicals. Chain propagation includes the following steps ... 

The kinetic analysis proves that formation of very active radical from intermediate product can increase the reaction rate not more than twice. However, the formation of inactive radical can principally stop the chain reaction [77], Besides the rate, the change of composition of chain propagating radicals can influence the rate of formation and decay of intermediates in the oxidized hydrocarbon. In its turn, the concentrations of intermediates (alcohols, ketones, aldehydes, etc.) influence autoinitiation and the rate of autoxidation of the hydrocarbon (see Chapter 4). 

Another factor complicating the situation in composition of peroxyl radicals propagating chain oxidation of alcohol is the production of carbonyl compounds due to alcohol oxidation. As a result of alcohol oxidation, ketones are formed from the secondary alcohol oxidation and aldehydes from the primary alcohols [8,9], Hydroperoxide radicals are added to carbonyl compounds with the formation of alkylhydroxyperoxyl radical. This addition is reversible. 

These reactions, if of sufficient magnitude, would result in the formation of non-radical, nitrogen-containing species, thereby stopping the propagation cycle of lipid peroxidation. 

Scheme 4.8 shows that the CH2 of cyclohexadiene moiety acts as the H donor with formation of cyclohexadienyl radical as the intermediate, which rapidly ejects the silyl radical upon re-aromatization. The silyl radical is able to propagate the chain by reaction with a starting halide. The hydrogen donation of silylated cyclohexadienes toward primary alkyl radicals is reported to be 1 X 10 M s at 70 °C [120], which is in accord with the reported range of 10 -10" s at room temperature for the reaction of primary and second-... 

A radical mechanism sequence requires three distinct types of process initiation, propagation, and termination. Initiation is the formation of two radical species by bond fission, whereas propagation involves reaction of a radical with a neutral molecule, a process that leads to generation of a new radical. Because radicals are so reactive, the propagation process may continue as long as reagent molecules are available. Finally, the reaction is brought to a... 

The potential of transformation reactions for synthesizing a wider range of block copolymers has not been realized because either the reactions are not quantitiative or deterimental side reactions occur. Thus coupling of two propagating carbanions by one phosgene competes with the 1 1 transformation in Eq. 5-123. The anionic-to-radical transformation in Eq. 5-124 involves the formation of trimethyllead radical, which initiates homopolymerization of monomer B. 

With the formation of free radicals having been initiated, these radicals react with oxygen (Reaction 3) to begin the propagation of the radical chains in forming a peroxy radical. The peroxy radical then attacks the 10-carbon-hydrogen bond to form the hydroperoxide radical (Reaction 4). [The possibility of such an intramolecular attack has been demonstrated in an aliphatic system where two reactive hydrogen atoms are located in the favorable 1,4-positions (9)]. 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

Mechanistic information from these reactions points to the initial formation of a radical anion of the aromatic compound, followed by loss of halide ion (3.15) subsequent attack by a second enolateanion and electron transfer to a second molecule of aryl halide provides the substitution product, and the reaction is propagated. The operation of a chain mechanism is indicated by the observation that quantum... 

The formation of ion radicals from monomers by charge transfer from the matrices is clearly evidenced by the observed spectra nitroethylene anion radicals in 2-methyltetrahydrofuran, n-butylvinylether cation radicals in 3-methylpentane and styrene anion radicals and cation radicals in 2-methyltetrahydrofuran and n-butylchloride, respectively. Such a nature of monomers agrees well with their behavior in radiation-induced ionic polymerization, anionic or cationic. These observations suggest that the ion radicals of monomers play an important role in the initiation process of radiation-induced ionic polymerization, being precursors of the propagating carbanion or carbonium ion. On the basis of the above electron spin resonance studies, the initiation process is discussed briefly. 

An important example of a complex reaction is a chain reaction in which free radicals are involved. These reactions consist of three essential steps Formation of free radicals or initiation propagation and termination. 

These findings imply that the use of probabilities for i-ad formation at a given temperature in a given solvent is insufficient to describe the monomer constitutions influence on the stereocontrol in free radical polymerizations. The lack of correlation is either the result of the combined action of more than one parameter (size of substituent, resonance stabilization and/or structure of propagating radicals, etc.) or the result of noncomparable experimental conditions. 

Phosphites are known to act by a preventive mechanism, i.e. preventing the formation of initiating radicals from hydroperoxides by reducing the latter to alcohols, see reaction 4 [20]. In addition to their peroxidolytic activity (PD), sterically hindered aromatic phosphites, e.g. Ultranox U626, act also by chain breaking (CB) mechanism. These phosphites react with the propagating alkylperoxyl (ROO ), reactions 5, and alkoxyl (RO) radicals, reactions... 

We have also been able to obtain an explicit analytic solution to eqn (4), and hence to the general time-dependent Smith-Ewart differential difference equations, for the case where the rate of formation of new radicals in the external phase is zero, i.e., cr = 0. Of course, if no radicals ever have been generated within the external phase of the reaction system, then the problem becomes trivial and admits of an obvious and simple solution, namely, that all loci are at all times devoid of propagating radicals, and the rate of polymerisation is always zero. This solution is clearly of no interest. The case which is of interest is that of a reaction system in which radicals have been generated within the external phase, so that a certain rate of polymerisa-... 

The second possibility is that CH3S could be regenerated due to formation of OH radicals. Balia and Heicklen investigated the dependence of the SO2 yield on O2 pressure in the C.W. photolysis of DMDS-O2 mixtures (12). They postulated a chain mechanism propagated by a reaction between CH3S and O2 whose overall effect was to give OH radicals. 

The formation of oxyl radicals in reaction of alkyl radicals with oxygen is important for the possible occurrence of branching of oxidation reaction due to oxygen atoms. This aspect is not, however, obvious from the experiments performed. It seems that oxyl radicals in HDPE are not formed in propagation reaction of alkyl radicals and oxygen but as the product of termination of two peroxyl radicals. 

Equation (13.10) indicates that the total concentration of macroradicals depends on the ratio of initiation to termination rates, but not on the propagation rate. In numerous studies only the initiator decomposition is considered as rate limiting for the formation of first radicals, and therefore we may write ... 

PMMA has been known, since the 1950s, to show the ESR spectrum with five intense and four weak hyperfine lines (see the insert of Fig. 9), when it is irradiated with y-rays at room temperature. After a long history of the study on this anomalous ESR spectrum [32-34], the interpretation is now almost settled that it is due to the propagating-type radical -CH2-C(CH3)COOCH3 (the radical expected to form during the radical polymerization of methyl methacrylate) [35, 36], Although the formation of this radical is a definite proof of the radiation-induced scission of the PMMA main-chain, the previous ESR studies have failed to elucidate the mechanism for the formation of this radical. 

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