Radical site reaction initiation

Radical site reaction initiation (a-cleavage) involves the tendency for electron pairing. The unpaired electron participates in the formation of a new bond to an adjacent atom. Another bond of this a-atom cleaves (a-cleavage). Three general variants of a-cleavage (Scheme 5.15) are illustrated with real examples (in parentheses). 

As the G(S) was found to equal to G(OH), the average number of radical site per initial polymer can easily exceed unity at absorption dose of few krad. Under these experimental conditions, the fractions of peroxy radicals may undergo intramolecular instead of intermolecular reactions. 

In many free-radical polymerizations, the molecular weight of the polymer produced is lower than that predicted from Eq. (6-64). This is because the growth of macroradicals in these systems was terminated by transfer of an atom to the macroradical from some other species in the reaction mixture. The donor species itself becomes a radical in the process, and the kinetic chain is not terminated if this new radical can add monomer. Although the rate of monomer consumption may not be altered by this change of radical site, the initial macroradical will have ceased to grow and its size is less than it would have been in the absence of the atom transfer process. These reactions are called chain transfer processes. They can be classified as varieties of propagation reactions (Section 6.3.2). 

The driving force is like that underlying the high reactivity of neutral radicals in processes such as dimerization and hydrogen abstraction. The tendency for the radical site to initiate a reaction in competition with the charge site generally parallels the radical site s tendency to donate electrons N > S, O, rc, R - > Cl, Br > H, where n signifies an unsaturated site and R- an alkyl radical. (This... 

Like most other engineering thermoplastics, acetal resins are susceptible to photooxidation by oxidative radical chain reactions. Carbon—hydrogen bonds in the methylene groups are principal sites for initial attack. Photooxidative degradation is typically first manifested as chalking on the surfaces of parts. 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

Chain polymerization involves three steps. To start the reaction, a catalyst that can generate an active site, such as a free radical (R ), is used. In the initiation step, the radical adds to the double bond, and the radical site is moved to the end carbon. This new radical reacts with another molecule to give a larger radical, and the propagation reaction is imderway. Usually, the number of monomers in the chain is greater than 1000. In the above formulae. 

Possible pathways of the degradation reaction may be visualized for a linear hydrocarbon chain in which the reaction centre ( ) is formed by the effect of initiation (heat, light, oxygen, shear stress, etc.), see Scheme la. A complementary reaction site is denoted as (-). For example, when ( ) is a free radical site, (-) is also a free radical site, if ( ) is a cation, then (-) is an anion, etc. The three stages of the reaction depicted in Scheme la, are initiation, propagation and termination, respectively. The dissociation energies of bonds situated in a /(-position to the reaction site ( ) are considerably lower than those... 

The tendency for the fragmentation initiation with the radical site is parallel to the donor properties of this site. The most spectacular examples involve the processes triggered by the removal of a nitrogen n-electron. Halogens are the least active in these reactions. 

The hydroxyl radicals formed may abstract hydrogen from the cellulose fiber substrate which gives grafting sites and subsequently grafted polymer with monomer present. The HO- radicals may also initiate homopolymerization. This means that reaction (17) is not specific for initiation of grafting. Another disadvantage is that the Fe + ions formed - if not carefully removed -may cause discoloration of the resulting product. 

The core first method starts from multifunctional initiators and simultaneously grows all the polymer arms from the central core. The method is not useful in the preparation of model star polymers by anionic polymerization. This is due to the difficulties in preparing pure multifunctional organometallic compounds and because of their limited solubility. Nevertheless, considerable effort has been expended in the preparation of controlled divinyl- and diisopropenylbenzene living cores for anionic initiation. The core first method has recently been used successfully in both cationic and living radical polymerization reactions. Also, multiple initiation sites can be easily created along linear and branched polymers, where site isolation avoids many problems. 

If no substituents block the very reactive 5,7-positions in the allyl radicals, other reactions may readily ensue at these sites. Reduction of 1,4-diphenyldihydrodiazepinium perchlorate in aqueous ethanol led to the formation of 1,4-diphenylhexahydrodiazepine and 1,4-dianilinoethane (UP4), but reductions of a number of phenyl-substituted dihydrodiazepi-nium salts in N,N-dimethylformamide provided unexpected products arising from further reactions undergone by the initial reduction products [78CC499 79BSB113 80JCS(P2)1441 81JCS(P2)801]. 

FIGURE 10.22 Direct mutagenicity of ambient particles (mutagen density, rev m-3, TA98, —S9) as a function of ambient concentrations of 2-nitropyrene, a directly mutagenic product of a gas-phase atmospheric reaction initiated by OH radical attack on pyrene. Samples collected at six sites in California with different types of emissions ( ) Glendora (O) Yuba City ( ) Concord ( ) Mammoth Lakes ( a ) Oildale ( ) Reseda (see Fig. 10.22) (adapted from Atkinson et al., 1988a). 

Since the triplet reactions occur at rates much faster than any measured radiationless decay, a likely remaining process which could lower the quantum yield is the reverse of the initial biradical forming reaction, in effect a disproportionation between the two radical sites. 

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