Chain process termination

Most ethers are potentially ha2ardous chemicals because, in the presence of atmospheric oxygen, a radical-chain process can occur, resulting in the formation of peroxides that are unstable, explosion-prone compounds (7). The reaction maybe generalized in terms of the following steps involving initiation, propagation, and termination. 

The overall rate of a chain process is determined by the rates of initiation, propagation, and termination reactions. Analysis of the kinetics of chain reactions normally depends on application of the steady-state approximation (see Section 4.2) to the radical intermediates. Such intermediates are highly reactive, and their concentrations are low and nearly constant throughout the course of the reaction ... 

Because dideoxynucleotides lack 3 -OH groups, these nucleotides cannot serve as acceptors for 5 -nucleotide addition in the polymerization reaction, and thus the chain is terminated where they become incorporated. The concentrations of the four deoxynucleotides and the single dideoxynucleotide in each reaction mixture are adjusted so that the dideoxynucleotide is incorporated infrequently. Therefore, base-specific premature chain termination is only a random, occasional event, and a population of new strands of varying length is synthesized. Four reactions are run, one for each dideoxynucleotide, so that termination, although random, can occur everywhere in the sequence. In each mixture, each newly synthesized strand has a dideoxynucleotide at its 3 -end, and its presence at that position demonstrates that a base of that particular kind was specified by the template. A radioactively labeled dNTP is included in each reaction mixture to provide a tracer for the products of the polymerization process. 

B Termination The chain process is eventually ended by a reaction that consumes the radical. Combination of two growing chains is one possible chainterminating reaction. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

The chain process of the Meerwein reaction can be visualized as shown in Scheme 10-57. There are at least two likely termination reactions for the chain process, namely the addition of a chlorine atom from CuCl2 to the aryl radical (Scheme 10-58) or reaction of the aryl radical with a hydrogen atom of acetone, followed by the formation of chloroacetone (Scheme 10-59). 

Mathematical expressions can be derived to describe the kinetics of chain processes building on the ideas already discussed that such reactions take place in the well defined stages of initiation, propagation, and termination. 

Considering only termination by reaction (72) and assuming that stationary conditions are attained, the rate of reduction of peroxydisulphate in the chain process is given by... 

Bisphenol A, whose official chemical name is 2,2-bis(4-hydroxyphenyl)propane, is a difunctional monomer with two reactive hydroxyl groups, as shown in Fig. 20,2. It polymerizes svith dicarbonyl organic monomers, such as phosgene or diphenyl carbonate, which are illustrated in Fig. 20.3. During polymerization, shown in Fig. 20.4, the hydroxyl groups of the bisphenol A deprotonate in the presence of a base. After deprotonation, the oxygen atoms on the bisphenol A residue form ester bonds with the dicarbonyl compounds. The polymerization process terminates when a monohydric phenol reacts with the growing chain end. 

If the loss of solubility of these initially linear polymers takes place through a free-radical-chain process in which the cross-linking reaction represents the termination step, one may be hesitant at first to explain the fact that a free radical, generated at one point on a relatively sluggish polymer chain, can find a radical on a neighboring chain with which to terminate. However, perhaps reactions of the type R + R H --- RH + R or R00 + R H-- ... 

It is also worth emphasizing that the initiation and termination steps are not included in the central chain process. For instance, in metal hydride-promoted domino reactions the initial halogen abstraction (or SePh displacement, etc.) and the final hydrogen abstraction from R MH are not classified as part of the domino sequence. More precisely, only the propagation steps within the mechanism of this process will be considered as a strict integral part of the domino reaction. 

Calorimetric results demonstrate that the chain process is inhibited and terminated by oxygen. The inhibition period depends on oxygen, the light intensity and the type of photoinitiator. The measured values vary from 40 to 11 sec (variation of the light intensity (I0 = 4.15. .. 1.0 mW/cm2), p(air) = 1000 mbar), from 40 to 7 sec (variation of the air pressure (p(air) = 1000. .. 6 mbar, Ie = 1.0 mW/cm2)), and from 3 to 30 sec (variation of the initiator). Using values of the inhibition time and reaction rate one can estimate the relative efficiency of several radicals in the chain process. 

A chain reaction can proceed if the rate of propagation is higher than the rate of chain termination. Hence, the necessary condition of a chain process is that the radicals generated in the system preferentially undergo reactions with conservation of free valence [2],... 

Typically, a chain reaction involves a number of steps which, depending on their role in the overall chain process, are classified as chain initiation, chain propagation, and chain termination reactions. 

The introduction of an inhibitor to oxidized hydrocarbon (v = const.) slows down the chain process due to the intensification of chain termination. The inhibitory efficiency can be characterized empirically by the ratio of the rates of chain termination with and without inhibitor (see earlier). In the absence of the inhibitor, chains are terminated via the... 

It can be seen that complete termination of the chain process is ensured only by the three-component system. The very efficient retardation of hydrocarbon oxidation by such a triple system can be explained by the following mechanism [45,46] ... 

More complicated mechanisms of the same category are encountered in SrnI reactions (Section 2.5.6) where the electrocatalytic reaction, which corresponds to a zero-electron stoichiometry, is opposed to two-electron consuming side reactions (termination step in the chain process). 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

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