Expanding chain reaction

The trimethylsilyl ethers 212 of four-membered 1-alkenyl-1-cyclobutanols rearrange to the ring-expanded 0-mercuriocyclopentanones 213. These can be converted into the a-methylenecyclopentanones 214 through elimination or further expanded by one-carbon atom into cyclohexanones 215 via the Bu3SnH-mediated free radical chain reactions [116]. A similar radical intermediate is suggested to be involved in the ring expansion of a-bromomethyl-fi-keto esters [117]. (Scheme 84)... 

A chain reaction is a reaction that sustains itself once it has begun and may even expand. Normally, the limiting reactant is regenerated as a product to maintain the progress of the chain. Nuclear fission processes are considered chain reactions because the number of neutrons produced in the reaction equals or is greater than the number of neutrons absorbed by the fissioning nucleus. 

Polymer blends and interpenetrating polymer networks (IPNs) are different from copolymers but like copolymers are used to bring together the properties of different polymers [Paul et al., 1988]. The total of all polymer blends (produced by both step and chain reactions) is estimated at about 3% of the total polymer production—about 3 billion pounds per year in the United States. There is considerable activity in this area since new products can be obtained and markets expanded by the physical mixing together of existing products. No new polymer need be synthesized. 

The polymerase chain reaction has numerous applications in basic research, and its use as a diagnostic tool is expanding. The main limitation to the method is that, to synthesize appropriate oligonucleotide primers, the sequence of at least part of the molecule to be amplified must be known beforehand. 

In a natural finite setup, some neutrons are lost by diffusion out through the surface. There will be therefore a certain size of say a sphere for which the surface losses of neutrons are just right to still sustain a chain reaction. This radius depends on the density. As the reaction proceeds the material tends to expand, increasing the required minimum size faster than the actual size increases. 

The other dominant technology is the polymerase chain reaction (PCR) system, which is a means of amplifying (i.e., rapidly growing) small quantities of bacterial DNA to reach critical mass for analysis. Since characteristic segments of DNA are a form of fingerprint for a microbe, this system is used to expand the quantity of a test substance to a level at which it can be assayed by DNA-sensing technology. 

Warner JP, Barron LH, Brock DJH. A new polymerase chain reaction (PCR) assay for the trinucleotide repeat that is unstable and expanded on Huntington s disease chromosomes. Molec Cellular Probes 1993 7 235-9. 

The attention paid to the polymer solid state is minimized in favour of the melt and in this chapter the static properties of the polymer are considered, i.e. properties in the absence of an external stress as is required for a consideration of the rheological properties. This is addressed in detail in Chapter 3. The treatment of the melt as the basic system for processing introduces a simplification both in the physics and in the chemistry of the system. In the treatment of melts, the polymer chain experiences a mean field of other nearby chains. This is not the situation in dilute or semi-dilute solutions, where density fluctuations in expanded chains must be addressed. In a similar way the chemical reactions which occur on processing in the melt may be treated through a set of homogeneous reactions, unlike the highly heterogeneous and diffusion-controlled chemical reactions in the solid state. 

Chain reaction A reaction that, once initiated, sustains itself and expands. 

That process would take times of the order of a sizeable part of a millisecond [i.e., a thousandth of a second], and for the whole chain reaction to develop would take several milliseconds once the material got hot enough to vaporize, it would begin to expand and the reaction would be stopped before it got much further. So the thing might blow up like a pile of gunpowder, but no worse, and that wasn t worth the trouble. 

Efficiency appeared to be a serious problem. The reaction will not go to completion in an actual gadget. Untamped, a bomb core even as large as twice the critical mass would completely fission less than 1 percent of its nuclear material before it expanded enough to stop the chain reaction from proceeding. An equally disadvantageous secondary effect also tended to stop the reaction as the pressure builds up it begins to blow off material at the outer edge of the (core]. Tamper always increased efficiency it re-... 

The situation in the core was one in which a small change in the proportion of steam could lead to an increase in core reactivity which generated enough neutrons for the chain reaction to expand without waiting for the delayed neutrons—a prompt critical situation. 

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