Self-propagating chain reaction

Both classes of polymers were attacked simultaneously, so that free-radical-initiated, self-propagating chain reactions and slow, endothermic step reactions were studied side by side. After the first results were attained, a grand strategy for practical applications developed quite naturally the vinyl- and diene-type addition polymers were pursued with the ultimate aim being the production of a synthetic rubber. The signals coming from the... 

A few months after the discovery of fission, it was clear that neutrons were set free in the process (von Halban jun et al. 1939). This initiation opened the way to speculations for a self-propagating chain reaction that would produce almost unlimited amounts of energy for peaceful and military uses (Fliigge 1939). With the beginning of World War II, this had the consequence that from 1940 to the end of the war results on the fission process were kept secret and could not be published. Meanwhile, enormous efforts and money were invested in the development of nuclear reactors and nuclear weapons, particularly in the USA and somewhat later also in the Soviet Union, in Great Britain, France, China, and in other countries. These developments will not be covered here. Further information may be found in O Chap. 1 of this Volume. The rest of this chapter will de dedicated to the physics of the fission process, i.e., to the experimental methods used, to the results, and their implications on the structure and behavior of nuclear matter. 

Clyne and co-workers [23] considered the NF2 + H reaction as an efficient, self-propagating chain reaction where the initiating step (1) is followed by rapid secondary steps that are capable of recycling H atoms with a chain length 20. Major steps in the chain were believed to be steps (1) and (6) and F + H2 HF + H. Steps (5) and (7) and wall recombination of H atoms were thought to play a minor role. Characteristic reaction profiles for N( D) and N( S) atoms showed N( D) to be an initial and H( S) to be a subsequent product. These profiles were simulated on the basis of the following simplified reaction scheme (adopted from [24]) ... 

The second method makes use of the fact that when a subcritical quantity of an appropriate isotope, that is, 239Pu (or 235U), is strongly compressed, it can become critical or supercritical. The reason for this is that compressing the fissionable material, that is, increasing its density increases the rate of production of neutrons by fission relative to the rate of loss by escape. The surface area (or neutron escape area) is decreased, while the mass (upon which the rate of propagation of fission depends) remains constant. A self-sustaining chain reaction may then become possible with the same mass that was subcritical in the uncompressed state. 

As noted, the transient nature of most free radical species is a major consideration in ESR studies of free radicals. Free radical chemistry [77] involves an initiation step in which the free radicals are formed, often followed by one or more propagation (chain) reactions before termination. Because most radical-radical termination reactions are fast, the majority of free radicals decay rapidly by self reaction, i.e., they are transient even in the absence of another species. (In non-transient, i.e., persistent, radicals the radical center is sterically hindered, thereby inhibiting self-reaction.) A comment on terminology may be appropriate at this point many transient radicals are frequently described as stable or unreactive, which can lead to some confusion. The source of this confusion is that reactivity and stability are often used to denote... 

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... 

Due to the relative ease of carrying out the reaction and the versatility of the process, the hydrosilylation reaction has been used in a number of interesting extensions and applications. Here several of them are highlighted. In one report, Lop-inski and coworkers used the same concept of the radical-initiated hydrosilylation reaction on the Si(100)-2 x 1 surface to induce self-directed growth of molecular wires on the surface [141]. On the Si(100)-2 x 1 surface, the radical chain reaction propagates primarily along the direction of the dimer row, leading to lines of... 

Perhaps the most striking feature of chain reactions is that some of them can result in detonation. If the propagation mechanism includes a step or steps that produce more chain carriers than they consume, the reaction is self-accelerating. This is called chain branching. The result may be an event much more violent than a thermal explosion, in which self-acceleration stems from temperature increase owing to the inability of heat transfer to keep up with the heat production of the reaction. The detonation of a nuclear bomb can be viewed as a chain reaction with neutrons as carriers and with chain branching. 

FIGURE 19.10 In a self-propagating nuclear chain reaction, the number of neutrons grows exponentially during fission. 

Chain reaction A reaction that is self-perpetuating, e.g. the photochlorination of methane. The product quantum yield is greater than one, which means that more molecules of product are formed than quanta of light are absorbed by the reagent. There is typically an initiation step, followed by a large number of propagation steps, ending with a series of termination steps. 

Figure 26-11 A self-propagating nuclear chain reaction. A stray neutron induces a single fission, liberating more neutrons. Each of them induces another fission, each of which is accompanied by release of two or three neutrons. The chain continues to branch in this way, very quickly resulting in an explosive rate of fission.

Four chain extension reactions are possible when monomers Mj and M2 are present in a polymerization reaction mixture. Two of these steps are self-propagating steps (Equations 26 and 28) and two are cross-propagating steps (Equations 27 and 29). The difference in the reactivity of the monomers can be expressed in terms of reactivity ratios that are ratios of the propagating rate constant where 2 22/ 21 ... 

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