Bond scission type radical

A7.1 Two Site Model and Coordinate System of Polyethylene (PE) Chain End Radical, —Cp H 2 (Ca )Hot2 (Bond Scission Type Radical and Propagating Radical)... 

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

There are, however, other possible routes to block copolymers successive addition of units of the reactive monomer to the polymer already present, Reaction 5 termination reactions between polymer molecules —side reactions of unknown nature lead to loss of reactive hydroxyl groups (18) possible reactions are ortho carbon-carbon coupling followed by dimerization, addition of amine or water to the ketal intermediate, etc. Block copolymers might even be formed by polymer-polymer redistribution assuming that such redistribution in polymers of greatly different reactivities (such as DMP and DPP), takes place almost exclusively in one type of polymer sequence—that is, that bond scission in a "mixed ketal such as IV occurs always in the same direction—to produce the aryloxy radical corresponding to the more reactive monomer. None of these possible sources of block copolymer can be ruled out on the basis of available evidence. All could produce homopolymer in addition to block copolymer. All of the polymers produced in this work, except for those characterized as completely random copolymers, probably contained at least small amount of one or both homopolymers. 

The phenyl radical is considered to be one of the most reactive hydrocarbon radicals though the reasons for this high reactivity have not been clear. Three different electronic configurations have been proposed for it. In the first, the electron remains in the s -orbital of the carbon atom at which bond scission has occurred (i.e. it is a or-type radical). In the second, an electron from the ir-system can pair with the unpaired electron to give a lone pair in the p -orbital and leave 5 electrons in the 6-centre 7r-system. In the third, the carbon atom at which scission has occurred becomes divalent and does not participate in the 7r-system this leaves a radical with 5 electrons in a 5-centre w-system. 

It is assumed that this is also a rate determining step for the overall reaction. The activation energy of reaction (4) and the site density of oxygen active centers were the only adjustable parameters of the model. In general, a C-H bond scission for reactants and products of the methane dimerization process occurs by an Eley-Rideal (E-R) type mechanism to form a gas-phase alkyl radical and a hydroxyl surface site (HO ) ... 

The unsaturation level in the polymer is at most 0.1 double bonds per chain and unsaturated oligo-mers are not detected at all. B-Scission and radical disproportionation reactions which occur in Fawcett-type (high-temperature, high pressure) ethylene polymerizations are therefore of minor importance in silver perchlorate solution at 0-40°C. 

In Fig. 47 the proportion of various types of radicals is presented indicating the radiolysis route. The evolution of hydrogen from irradiated polystyrene depends on the position of bond scission 22.3 % result from a-position, 39.2 % come out from 3-position and 38.5 % have the origin from benzene ring [60C1]. 

Many types of peroxides (R-O-O-R ) are also utilized, including diacyl peroxides, peroxydicarbonates, peroxyesters, dialkyl peroxides, and inorganic peroxides such as persulfate, the latter being used mainly in water-based systems. The rate of peroxide decomposition as well as the subsequent reaction pathway is greatly affected by the nature of the peroxide chemical structure, as illustrated for fert-butyl peroxyesters in Scheme 4.2. Pathway (a), the formation of an acyloxy and an alkoxy radical via single bond scission, is favored for structures in which the carbon atom in the a-position to the carbonyl group is primary (for example, terf-butyl peroxyace-tate, R = CHg). Pathway (b), concerted two-bond scission, occurs for secondary and tertiary peroxyesters (for example, terf-butyl peroxypivalate, R = C(CH3)3) [1, 2]. The tert-butoxy radical formed in both pathways may decompose to acetone and a methyl radical, or abstract a hydrogen atom to form tert-butanol. 

A related study has been the elucidation of the crosslink structures induced within polyethylene by high energy radiation. The secondary carbon radicals thus produced by C—H bond scission may diffuse by hydrogen atom abstraction. They have been shown to combine in pairs to form H type junctions, and to create Y type junctions by reactions with the vinyl end groups of the chains and with primary carbon radicals produced by main chain scission. In each case the shifts characteristic of the new structure were identified [32], The shifts of the H junctions are distinct, being 41.1, 31.9 and 28.7 ppm respectively at the (CH) junction and the first and second linked carbons, as is shown in Scheme 1, but the shifts of the Y junctions coincide with those at the roots of long branches, and their formation is recognised only when a careful comparison has been made of the areas of these shifts before and after irradiation. 

Chain scission in polysaccharides is prevented to a certain extent by molecular oxygen, as demonstrated for dextran in Table 5.7. Actually, the inhibitive action of O2 may be related to glycoside bond-deavage involving C-2-type radicals. When O2 reacts with C-l-type radicals, it does not prevent glycoside bond cleavage, as can be seen from Scheme 5.30. 

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