Free radical combinations

Table 6.5 Some Free Radical Combination Reactions Which Yield n-mers and Their Rate Laws...

The free radicals combine to form a carbon-to-carbon bond and give a saturated polymer molecule with initiator fragments on both ends. Termination by disproportionation produces two polymer molecules, one of which will contain a double bond ... 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

This new free radical adds to the double bond of another monomer molecule, growing the polymer chain. The polymerization process ends as the unpaired electrons of two free radicals combine to form a single bond ... 

Selective combination of the secondary geminate radical pairs occurs in the micelle, compared to nonselective free-radical combination reactions in solution. This results from the micelle host effectively constraining the separation of the geminate radical pair. 

In solution, the Norrish type 1 reaction of ketones results in the non-selective free-radical combination reactions to give products AA, AB and BB in the ratio of 1 2 1, whereas photolysis of ketones in zeolites produces ... 

When two free radicals combine, they form a molecule with aU covalent bonds that is a stable and unreactive molecule we call P . It consists of n monomer units (M) linked together with an initiator unit (A) at each end, AM, A. From ethylene, this polymer is the molecule A CH2)2nA. Note that the product molecule is simply a linear alkane with no double bonds remaining. This molecule is totally unreactive, and it is called the dead polymer, ff n is large enough, we neglect the end groups and caU it simply (C2iin)n-... 

Jin F, Leitich J, von Sonntag C (1993) The superoxide radical reacts with tyrosine-derived phenoxyl radicals by addition rather than by electron transfer. J Chem Soc Perkin Trans 2 1583-1588 Jonsson M, Lind J, Reitberger T, Eriksen TE, Merenyi G (1993) Free radical combination reactions involving phenoxyl radicals. J Phys Chem 97 8229-8233 Jovanovic SV, Simic MG (1985) Repair of tryptophan radicals by antioxidants. J Free Rad Biol Med 1 125-129... 

These free radicals combine rapidly with ethylene to initiate growing chains ... 

Molecules or molecular fragments that possess one (or more) unpaired electrons are called free radicals. Free radicals are usually formed through homolytic cleavage of a chemical bond recombination is the process of two free radicals combining to re-form the bond. 

T7ery little has appeared in the literature concerning the radiation chemistry of covalent inorganic compounds in condensed phase. In the search for new, high energy oxidizers, it appears plausible that ion fragmentation, electron capture, ion-molecule reactions, and free radical combination reactions at low temperatures may be utilized. 

Free radicals and random Free radical combinations. 

SCHEME 13.2 Simplified representation of the formation of a geminate radical pair, incage combination of the radical pair (a), progressive separation (and reencounters) of the radicals, and bimolecular random free radical combination reactions taking place at a longer time scale when the rates of diffusion and reaction of A and B are approximately equal. An important aspect of this scheme is that radicals A and B can reencounter each other and form A-B in out-of-cage reactions. 

SCHEME 13.8 Basic processes in a classic Norrish T)fpe I reaction of ACOB in a constraining medium. Events faster that 30 is and much slower (himolecular) free radical combination processes are separated for simplicity. / and / represent any of the radicals shown in the top panel. The scheme involves a number of important assumptions. For details, see Ref. 29a. Reprinted with permission from Ref. 29a. Copyright (2007) American Chemical Society. 

It is also possible that degradation products of these phenols, or free radical combinations, may be present, with potential for chemical interaction with the drug. 

In each step the consumption of a free radical is accompanied by the formation of a new, bigger free radical. Eventually, the reaction chain is terminated by steps that consume but do not form free radicals combination or disproportionation of two free radicals. 

The chlorine/chlorine single covalent bond is broken symmetrically to yield two radicals. In this first step, the overall number of radicals has increased. This is characteristic of an initiation step. Free radicals are, by their very nature, reactive species and so they tend to react quite quickly with any other species that happens to be in the vicinity, which, when the concentration of free radicals is low, will usually be a neutral molecule. The product of this second reaction step is another radical species. Many free radical reactions are characterised by the lack of free radical/free radical combination, and instead display a large number of reactions in which the initial free radical reacts with some other type of molecule in the reaction mixture. 

Initiation reactions are usually started by an active free radical such as peroxide (-0-0-), e.g. benzoyl peroxide is a good inititator for the free radical addition polymerisation of styrene to produce polystyrene AICI3 is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber or azobisiso-butyronitrile compounds (-N=N-) (abbreviated to AIBN). Propagation reactions are the continuing process and, eventually, lead to the termination stage that occurs by combination or disproportionation. This usually occurs when the free radicals combine with themselves and signals the end of the polymerisation process. All polymers formed by this process are thermoplastics. Table 4.1 is a list of common polymers prepared by the addition process. 

A positive effect of fillers may be observed during irradiation crosslinking. It was found that the yield of radicals in polyethylene was increased 50% when a small amount (0.05 %) of aerosil was added [21]. It has been assumed that a higher production of radicals takes place at the interphase aerosil-polyethylene, where macromolecules can be in the nonequilibrium state cf uncompensated strains. With a higher content of a filler, a transfer of energy from the filler to the polymer j se may occur and thus contribute to a higher yidd of free radicals. Combination of irradiation with reactive admixtures may, moreover, affect a localization of crosslinks along the poly-mo chain. 

Resultant free radicals combine to form complex mixtures including block and graft components... 

Buettner GR, Jurkiewicz BA. Catalytic metals, ascorbate and free radicals Combinations to avoid. 

Polymeric chains, bearing free radicals, combine with each other to give branched structures. Addtions of chains with a free radical to double bonds result in formations of crosslinks. ... 

The chain reaction stops when two free radicals combine to form molecules. These steps are called chain termination steps and three operate in this mechanism ... 

The newly generated bromine atom, a free radical, can react with another hydrogen molecule as shown in reaction b, after which the new hydrogen atom can react with another bromine molecule as given in reaction c, and so forth. Such a reaction cycle can continue undiminished until one reactant is virtually depleted, or until two free radicals combine to make a molecule that is relatively unreactive, like each of these ... 

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