Chemical chain reactions

Because this kind of reaction takes place at low temperatures, thermal oscillations do not essentially contribute to backbone stretching. In fact, Ea is zero in this case. When Ea of bond scission is less than that required to form the active particles, cracking exhibits the character of a mechanically activated chemical reaction—chain scission in active particles takes place. There is a cause-and-effect relationship between the strain processes (which are cumulative in the deformed fragment to activate the backbone) and the destruction processes. 

E4 First-Order Chemical Reactions Chain multipole (irregular) CG... 

IV. FLAMES SUPPORTED BY COMPLEX SYSTEMS OF CHEMICAL REACTION CHAINS... 

Fig. 8.17. Mechano-chemical reaction chain after Zakrevskii and Zhiurkov (17, 20, 551 (a) stress-induced chain scission, (b) formation of chain-end radicals, (c) radical reaction leading to main-chain radicals, (d) scission of radicalized chains, (e) formation of a submicrocrack by repetition of the steps (c) and (d) chain-end radical e.g. -CH2-CH2, x main-chain radical e.g. -CH2-CH-CH2-, stable end groups e.g. -CH2-CH3.

Fig. 8.18. Scheme of energy levels of steps fl-c in the mechano-chemical reaction chain. 

Section 11 10 Chemical reactions of arenes can take place on the ring itself or on a side chain Reactions that take place on the side chain are strongly influ enced by the stability of benzylic radicals and benzylic carbocations... 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

In a cross-linked polymer, the junction units are different kinds of monomers than the chain repeat units, so these molecules might be considered to be still another comonomer. While the chemical reactions which yield such cross-linked substances are copolymerizations, the products are described as cross-linked rather than as copolymers. In this instance, the behavior due to cross-linking takes precedence over the presence of an additional type of monomer in the structure. 

Chain reactions do not go on forever. The fog may clear and the improved visibility ends the succession of accidents. Neutron-scavenging control rods may be inserted to shut down a nuclear reactor. The chemical reactions which terminate polymer chain reactions are also an important part of the polymerization mechanism. Killing off the reactive intermediate that keeps the chain going is the essence of these termination reactions. Some unusual polymers can be formed without this termination these are called living polymers. 

Chemical Factors. Because knock is caused by chemical reactions in the engine, it is reasonable to assume that chemical stmcture plays an important role in determining the resistance of a particular compound to knock. Reactions that produce knock are generally free-radical chain-type reactions which are different from those that occur in the body of the flame the former occur at lower temperatures and are called cool flame reactions. 

Collectors ndFrothers. Collectors play a critical role ia flotation (41). These are heteropolar organic molecules characterized by a polar functional group that has a high affinity for the desired mineral, and a hydrocarbon group, usually a simple 2—18 carbon atom hydrocarbon chain, that imparts hydrophobicity to the minerals surface after the molecule has adsorbed. Most collectors are weak acids or bases or their salts, and are either ionic or neutral. The mode of iateraction between the functional group and the mineral surface may iavolve a chemical reaction, for example, chemisorption, or a physical iateraction such as electrostatic attraction. 

X-Amino acids ate ampholytic compounds. The chemical reactions of amino acids can be classified according to their carboxyl, amino, and side-chain groups. Most of the reactions have been well known for a long time the details of these reactions have been reviewed (77). 

The side groups of the amino acids vary markedly in size and chemical nature and play an important role in the chemical reactions of the fiber. For example, the basic groups (hisidine, arginine, and lysine) can attract acid (anionic) dyes, and in addition the side chains of lysine and hisidine are important sites for the attachment of reactive dyes. The sulfur-containing amino acid cysteine plays a very important role, because almost all of the cysteine residues in the fiber are linked in pairs to form cystine residues, which provide a disulfide bridge —S—S— between different polypeptide molecules or between segments of the same molecules as shown ... 

Chemical Reactions of Dyes. Decolorization is important for cyanines used ia imaging materials. Understanding decolorization provides clues to dye reactions that may cause degradation of imaging materials duting preparation and storage. For many dyes, protonation of the methine chain occurs readily and reversibly (64). Highly basic carbocyanine dyes like those from benzimidazole (eg, 36) protonate so readily that this provides a practical decolorization method. 

In many gaseous state reactions of technological importance, short-lived intermediate molecules which are formed by die decomposition of reacting species play a significant role in die reaction kinetics. Thus reactions involving die mediane molecule, CH4, show die presence of a well-defined dissociation product, CH3, die mediyl radical, which has a finite lifetime as a separate entity and which plays an important part in a sequence or chain of chemical reactions. 

Chain uncoiling, and the converse process of coiling, is conveniently considered as a unimolecular chemical reaction. It is assumed that the rate of uncoiling at any time after application of a stress is proportional to the molecules still coiled. The deformation Dhe(0 at tinie t after application of stress can be shown to be related to the equilibrium deformation Dhe( ) by the equation... 

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