Reaction mechanisms branching-chain

By repetition of the same reaction mechanism further chain branching may occur and the block copolymer may progressively change into a graft copolymer ... 

Chain reactions, 181 branching, 189 initiation step, 182 propagation steps, 182 rate laws for, 188 termination step, 182 well-behaved, 187 Chemical mechanism, 9 Chemical relaxation, 255-260 Coalescence temperature, 262 Col, 170... 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

The reaction with sulfides occurs efficiently only when the resulting carbon-centered radicals are further stabilized by a a-heteroatom. Indeed, (TMSfsSiH can induce the efficient radical chain monoreduction of 1,3-dithiolane, 1,3-dithiane, 1,3-oxathiolane, 1,3-oxathiolanone, and 1,3-thiazolidine derivatives. Three examples are outlined in Reaction (12). The reaction of benzothiazole sulfenamide with (TMS)3SiH, initiated by the decomposition of AIBN at 76 °C, is an efficient chain process producing the corresponding dialkylamine quantitatively. However, the mechanism of this chain reaction is complex as it is also an example of a degenerate-branched chain process. 

The generalized mechanism by which branched chain reactions proceed provides a basis for a semiquantitative understanding of explosions resulting from chain branching. ... 

However, there is no reason to believe that a x-allyl or x-olefin mechanism for C—C bond rupture is limited to ring opening. Rather it should be a general process occurring in straight and branch-chain aliphatics as well. For the purpose of illustration, we write reactions (22) and (23) as generalizations of the reverse of (11) and (13), respectively. We shall refer to mechanisms such as (22) and (23) x-olefin/allyl hydrogenolysis... 

The readsorption and incorporation of reaction products such as 1-alkenes, alcohols, and aldehydes followed by subsequent chain growth is a remarkable property of Fischer-Tropsch (FT) synthesis. Therefore, a large number of co-feeding experiments are discussed in detail in order to contribute to the elucidation of the reaction mechanism. Great interest was focused on co-feeding CH2N2, which on the catalyst surface dissociates to CH2 and dinitrogen. Furthermore, interest was focused on the selectivity of branched hydrocarbons and on the promoter effect of alkali on product distribution. All these effects are discussed in detail on the basis... 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

In a branched-chain mechanism, there are elementary reactions which produce more than one chain carrier for each chain carrier reacted. An example of such an elementary reaction is involved in the hydrogen-oxygen reaction ... 

In the examples in Sections 7.1 and 7.2.1, explicit analytical expressions for rate laws are obtained from proposed mechanisms (except branched-chain mechanisms), with the aid of the SSH applied to reactive intermediates. In a particular case, a rate law obtained in this way can be used, if the Arrhenius parameters are known, to simulate or model the reaction in a specified reactor context. For example, it can be used to determine the concentration-(residence) time profiles for the various species in a BR or PFR, and hence the product distribution. It may be necessary to use a computer-implemented numerical procedure for integration of the resulting differential equations. The software package E-Z Solve can be used for this purpose. 

The hydrogen-chlorine chain reaction has proved to be one of the most controversial systems yet studied. After thirty years of investigation Bodenstein43 was able to say in 1931 that every worker on the photochemical synthesis of HC1 had produced his own mechanism even as late as 1940 little positive information had been obtained. However, the accumulated techniques and experience had firmly established the importance of atom chain reactions. The mechanism of photo-initiation and propagation is the same as for the hydrogen bromide photosynthesis, a non-branching chain reaction... 

Figure 2. Proposed reaction mechanism for (a) insertion, (b) chain termination and (c) chain branching in the case of the Brookhart Ni-bis-imine polymerization catalyst. Large bulky substituents have been removed for clarity...

The sequence [Eqs. (17)—(20)] is of great importance in the oxidation reaction mechanisms of any hydrocarbon in that it provides the essential chain branching and propagating steps as well as the radical pool for fast reaction. 

The shift of curves, as shown in Fig. 3.9, is unsurprising since the larger fuel molecules and their intermediates tend to break down more readily to form radicals that initiate fast reactions. The shape of the propane curve suggests that branched chain mechanisms are possible for hydrocarbons. One can conclude that the character of the propane mechanism is different from that of the H2—02 reaction when one compares this explosion curve with the H2—02 pressure peninsula. The island in the propane-air curve drops and goes slightly to the left for higher-order paraffins for example, for hexane it occurs at 1 atm. For the reaction of propane with pure oxygen, the curve drops to about 0.5 atm. 

The essential point is that the initiation steps provide H atoms that react with the oxygen in the system to begin the chain branching propagating sequence that nourishes the radical reservoir of OH, O, and H that is, the reaction sequences for the complete H2—02 system must be included in any high-temperature hydrocarbon mechanism. Similarly, when CO forms, its reaction mechanism must be included as well. 

Even though there have been appreciably more studies of CS2, COS is known to exist as an intermediate in CS2 flames. Thus it appears logical to analyze the COS oxidation mechanism first. Both substances show explosion limit curves that indicate that branched-chain mechanisms exist. Most of the reaction studies used flash photolysis hence very little information exists on what the chain-initiating mechanism for thermal conditions would be. 

The mechanism of the reaction depicted in Scheme 4.6 differs from the Sf.,1 or Sf.,2 mechanism in that it involves the stage of one-electron oxidation-reduction. The impetus of this stage may be the easy detachment of the bromine anion followed by the formation of fluorenyl radical. The latter is unsaturated at position 9 near three benzene rings that stabilize the radical center. The radical formed is intercepted by the phenylthiolate ion. This leads to the anion-radical of the substitution product. Further electron exchange produces the substrate anion-radical and final product in its neutral state. The reaction consists of radical (R)-nucleophilic (N) monomolecular (1) substitution (S), with the combined symbol Sj j l. Reactions of Sj j l type can have both branch-chain and nonchain characters. 

The mechanism of chain branching in the later stages of oxidation is more complex. For example, it is being demonstrated that intermolecu-lar hydrogen bonds substantially influence the process by forming hydroperoxides and oxidation products among the molecules and by accumulating these in the reaction mixture. 

Such reactions have been used to explain the three limits found in some oxidation reactions, such as those of hydrogen or of carbon monoxide with oxygen, with an "explosion peninsula between the lower and the second limit. However, the phenomenon of the explosion limit itself is not a criterion for a choice between the critical reaction rate of the thermal theory and the critical chain-branching coefficient of the isothermal-chain-reaction theory (See Ref). For exothermic reactions, the temperature rise of the reacting system due to the heat evolved accelerates the reaction rate. In view of the subsequent modification of the Arrhenius factor during the development of the reaction, the evolution of the system is quite similar to that of the branched-chain reactions, even if the system obeys a simple kinetic law. It is necessary in each individual case to determine the reaction mechanism from the whole... 

It was found that the strength of the fuel/air blast thus initiated depends primarily on the effectiveness and rate of free radical source dispersement thruout the vapor cloud. All these observations are not in accord with the claimed universality of the SWACER mechanism. A broad-brush reaction mechanism based on competition of chain branching and chain breaking is presented... 

I he occurrence of a spontaneous explosion in a chemically reacting system is a complicated process. However, the events that lead to explosion can be characterized as being either of a branching chain or of a thermal nature. Branching-chain explosions occur in systems that react by a chain mechanism, the details of which allow the chain carrier concentration, and hence, the over-all reaction rate to increase without limit, even under isothermal conditions. Such a condition is possible only if one or more of the steps in the reaction chain results in a multiplication of chain carriers—i.c., X + A — Y + Z + , where X, F, and Z arc chain carriers. 

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