Generating Radicals

Eventually, the chain is terminated hy steps such as the union of two radicals that consumes hut does not generate radicals ... 

Catalysts and Promoters. The function of catalysts in LPO is not weU understood. Perhaps they are not really catalysts in the classical sense because they do not necessarily speed up the reaction (25). They do seem to be able to alter relative rates and thereby affect product distributions, and they can shorten induction periods. The basic function in shortening induction periods appears to be the decomposition of peroxides to generate radicals (eq. 33). 

The ptincipal commercial initiators used to generate radicals are peroxides and a2o compounds. Lesser amounts of carbon—carbon initiators and photoinitiators, and high energy ionising radiation are also employed commercially to generate radicals. 

Although a variety of methods for generating radicals by one or more of these three methods are reported in the Hterature, commercial initiators are primarily organic and inorganic peroxides, aUphatic a2o compounds, certain organic compounds with labile carbon—carbon bonds, and photoinitiators. 

Transition-metal ions also react with the generated radicals to convert the radicals to ions ... 

There are many chemical methods for generating radicals reported in the hterature that do not involve conventional initiators. Specific examples are included in References 64—79. Most of these radical-generating systems carmot broadly compete with the use of conventional initiators in industrial polymer apphcations owing to cost or efficiency considerations. However, some systems may be weU-suited for initiating specific radical reactions or polymerizations, eg, grafting of monomers to cellulose using ceric ion (80). 

Transition-metal ions also interact with hydroperoxide-generated radicals by converting them into ions, eg ... 

Hydrosdylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro sdylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

Many azo compounds also generate radicals when photolyzed. This ean oeeur by a thermal decomposition of the c -azo compounds that are formed in the photoehemieal... 

Radical cations can be derived from aromatic hydrocarbons or alkenes by one-electron oxidation. Antimony trichloride and pentachloride are among the chemical oxidants that have been used. Photodissociation or y-radiation can generate radical cations from aromatic hydrocarbons. Most radical cations derived from hydrocarbons have limited stability, but EPR spectral parameters have permitted structural characterization. The radical cations can be generated electrochemically, and some oxidation potentials are included in Table 12.1. The potentials correlate with the HOMO levels of the hydrocarbons. The higher the HOMO, the more easily oxidized is the hydrocarbon. 

H2 and H2O2 [64]. The three primary radical species react with monomers to give radicals derived from monomers [63]. All these generated radicals are available to contribute to the chain initiation. This increases the exponent of the monomer concentration in these systems. 

The decomposition of an initiator seldom produces a quantitative yield of initiating radicals. Most thermal and photochemical initiators generate radicals in pairs. The self-reaction of these radicals is often the major pathway for the direct conversion of primary radicals to non-radical products in solution, bulk or suspension polymerization. This cage reaction is substantial even in bulk polymerization at low conversion when the medium is essentially monomer. The importance of the process depends on the rate of diffusion of these species away from one another. 

It was proposed171 that radical addition to 46 or 48 should occur exclusively at the respective methylene group to generate radicals 47 (Scheme 4.30). 

Three types of model study have been performed. The first approach has been to decompose a mixture of two initiators (/.< . one to generate radical A, the other to generate radical B). With this method experimental difficulties arise because the two types of radical may not be generated at the same rate and because homotermination products from cage recombination complicate analysis. 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

ATRP catalysts may be used to generate radicals and thus alkoxyamines can be produced from alkyl halides in high yield (Scheme 9.21).174 The alkoxyaminc 102 was obtained in 92% yield 174 whereas reaction of TEMPO with PMMA under ATRP conditions is reported to provide a macromonomer (Section 9.7.2.1). 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

O. R. Brown u. J. A. Harrison. Reaction of cathodically generated Radicals and Anions, J. Electroanal. Chem. 21, 387 (1969). 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

The further products depend on the site at which the generated radicals attack the starting material ... 

The deposition mechanism in HWCVD of a-Si H can be divided into three spatially separated processes. First, silane is decomposed at the tungsten filament. Second, during the diffusion of the generated radicals (Si, H) from the filament to the substrate, these radicals react with other gas molecules and radicals, and new species will be formed. Third, these species arrive at the substrate and contribute to the deposition of a-Si H. 

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