Propagation in chain

The values of the rate constants of the chain propagation in chain reactions of ROOH decomposition are collected in Table 4.23. 

To iterate water vapour and air are pressurised by the compression stroke, partial electrolysis occurs, partial H2 ignition occurs, causing heat, which changes some of the vapour to steam, which elevates pressure, which increases temperature, which forms more steam, which continues to propagate in chain reaction during (perhaps the first 10% of) the power stroke. Is it true that s1r9 s HP output cannot be explained thus - especially in view of the unprecedented application water in a 4-stroke cycle ... 

Note 3 Propagation in chain polymerization usually occurs without the formation of small molecules. However, cases exist where a low molar-mass by-product is formed, as in the polymerization of oxazolidine-2,5-diones derived from amino acids (commonly termed amino acid A-carboxy anhydrides). When a low-molar-mass by-product is formed, the adjective condensative is recommended to give the term condensative chain polymerization. 

Series of linked elementary reactions that propagate in chain cycles -> A reaction intermediate is produced in an initiation step. 

Chain Propagation (in chain polymerization)—chemical reaction between a chain carrier and a monomer that results in the growth of a polymer chain and the regeneration of at least one chain carrier. 

FIGURE 1111 Cham propagation in polymerization of styrene The growing polymer chain has a free radical site at the benzylic carbon It adds to a molecule of styrene to extend the chain by one styrene unit The new polymer chain is also a benzylic radical it attacks another molecule of styrene and the process repeats over and over again... 

The mechanism of these reactions places addition polymerizations in the kinetic category of chain reactions, with either free radicals or ionic groups responsible for propagating the chain reaction. 

The monometallic mechanism is illustrated in Fig. 7.13a. It involves the monomer coordinating with an alkylated titanium atom. The insertion of the monomer into the titanium-carbon bond propagates the chain. As shown in... 

Propa.ga.tlon, The tertiary THF oxonium ion undergoes propagation by an S. mechanism as a result of a bimolecular colHsion with THF monomer. Only colHsions at the ring a-carbon atoms of the oxonium ion result in chain growth. Depropagation results from an intramolecular nucleophilic attack of the penultimate chain oxygen atom at the exocycHc a-carbon atom of the oxonium ion, followed by expulsion of a monomer molecule. 

The and e values of the aHyl group in DAP have been estimated as 0.029 and 0.04, respectively, suggesting that DAP acts as a fairly typical unconjugated, bifunctional monomer (42). Cyclization affects copolymerization, since cyclized radicals are less reactive in chain propagation. Thus DAP is less reactive in copolymerization than DAIP or DATP where cyclization is stericaHy hindered. Particular comonomers affect cyclization, chain transfer, and residual unsaturation in the copolymer products. DiaHyl tetrachloro- and tetrabromophthalates are low in reactivity. 

Although the main uses for benzoic acid are as a chemical raw material, it also has numerous direct uses. Benzoic acid is used in substantial quantities to improve the properties of various alkyd resin coating formulations, where it tends to improve gloss, adhesion, hardness, and chemical resistance. Benzoic acid terminates chain propagation in alkyd resins (qv) and promotes crystallinity in the final product. 

The step in which the reactive intermediate, in this case A-, is generated is called the initiation step. In the next four equations in the example above, a sequence of two reactions is repeated this is the propagation phase. Chain reactions are characterized by a chain length, which is the number of propagation steps that take place per initiation step. Finally, there are termination steps, which include any reactions that destroy one of the reactive intermediates necessary for the propagation of the chain. Clearly, the greater the frequency of termination steps, the lower the chain length will be. 

The mechanism proposed by Emmons thus corresponds in part to the decomposition of the trialkyl-oxaziranes by ferrous salts. By radical attack on the 7V-alkyl group of the oxazirane, the radical 32 is formed which rearranges with ring opening to 33. Radical 33 propagates the chain by attack on a further molecule of oxazirane. It takes up an H-atom and is decomposed to ketone and ammonia. The aldehyde produced from the M-alkyl group is converted to tar. 

The increase in the temperature reduces the viscosity of the polymerization medium which increases the termination reactions. This is attributed to an increase in chain transfer reactions higher than that of propagation reactions [16,51]. Consequently, the weight-average molecular weight of the formed polymer decreases. 

Intermolecular chain transfer reactions may occur between two propagating polymer chains and result in the termination of one of the chains. Alternatively, these reactions take place by an intramolecular reaction by the coiling of a long chain. Intramolecular chain transfer normally results in short branches ... 

The two nuclei on the right side are just two of the many possible products of the fission process. Since more than one neutron is released in each process, the fission reaction is a self-propagating, or chain reaction. Neutrons released by one fission event may induce other fissions. When fission reactions are run under controlled conditions in a nuclear reactor, the energy released by... 

In termination, unsaturated and saturated ends are formed when the propagating species undergo disproportionation, head-to-head linkages when they combine, and other functional groups may be introduced by reactions with inhibitors or transfer agents (Scheme 1.2). In-chain defect structures (within the polymer molecule) can also arise by copolymerization of the unsaturated byproducts of initiation or termination. 

The basic Hammett scheme often does not offer a perfect correlation and a number of variants on this scheme have been proposed to better explain reactivities in radical reactions.-0 However, none of these has achieved widespread acceptance. It should also be noted that linear free energy relationships are the basis of the Q-e and Patterns of Reactivity schemes for understanding reactivities of propagating species in chain transfer and copolymerization. 

The ends of polymer chains are often not representative of the overall chain composition. This arises because the initiator and transfer agent-derived radicals can show a high degree of selectivity for reaction with a particular monomer type (Section 3.4). Similarly, there is specificity in chain tennination. Transfer agents show a marked preference for particular propagating species (Section 6.2.2 and 6.2.3). The kinetics of copolymerization are such that the probability for termination of a given chain by radical-radical reaction also has a marked dependence on the nature of the last added units (Section 7.4.3). 

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