Initiation sites

This mechanism not only accounts for the substitution of the more labile chlorine atom on the polymer chain, it also results in the elimination of a new potential initiation site by moving the double bond out of conjugation with any adjacent chlorine atoms. The newly formed C—O or C—S bonds, with AH > 484 kJ/mol (100 kcal/mol), are significantly more thermally stable than even the normal C—Cl bonds in PVC at about 411 kj/mol (85 kcal/mol) (11). 

Pig. 3. Representation of promoter sites on the pro-enkephalin gene. The numbers represent the distance in nucleotides from the pro-enkephalin initiation codon the arrow indicates the direction of transcription. The TATA promoter box occurs immediately before the pro-enkephalin initiation site the AP-2 site, which binds immediate-early gene products, is 70 nucleotides upstream, and the CRE site, which binds a regulatory protein involved in cAMP induction of mRNA synthesis, is 107 nucleotides upstream from the initiation codon. The expanded section shows that the CRE site actually consists of two elements, ENKCRE-1 and ENKCRE-2, which separately confer cAMP sensitivity to pro-enkephalin mRNA synthesis. 

If the initiation reaction is much faster than the propagation reaction, then all chains start to grow at the same time. Because there is no inherent termination step, the statistical distribution of chain lengths is very narrow. The average molecular weight is calculated from the mole ratio of monomer-to-initiator sites. Chain termination is usually accompHshed by adding proton donors, eg, water or alcohols, or electrophiles such as carbon dioxide. 

The most likely places for SCC to be initiated are crevices or areas where the flow of water is restricted. This is due to the buildup of corrodent concentrations in these areas. For example, chloride can concentrate from 100 ppm in the bulk water to as high as 10,000 ppm (1%) in a crevice. Deposits are initiating sites because of crevices formed beneath them. The low water velocities in shell-side cooling are also detrimental. 

A number of patents describe accelerators that will reduce the time required for stabilization (24,27,28). The accelerators are often inherent to the polymer or precursor but may be added to the gas phase during stabilization. For example, it is common to have an acid group present as comonomer such as itaconic acid [97-65-4] or methacrylic acid [79-41-4]. The acid groups provide initiation sites for cyclization. Alternatively, the stabilization atmosphere composition can be modified to accelerate stabilization (29). 

Separate the metal from the environment with a physical barrier. Many corrosion inhibitors make use of this principal to protect metals. Proper use of an appropriate inhibitor may reduce or eliminate pitting. Pits are frequently initiation sites for corrosion-fatigue cracks. The effectiveness of inhibitors depends upon their application to clean metal surfaces. An example of this method is the use of zinc coatings on steel to stifle pit formation. 

The orientation of the cracks indicates that cyclic bending stresses or cyclic axial stresses generated by thermal expansion and contraction provided the responsible stresses. The large number of crack initiation sites and the tightness of the cracks indicate high-level stresses. 

Such defects result from abnormal manufacturing operations such as insufficient lubrication between the metal and the mandrel during the tube-forming process. The lubricant may have been contaminated. Measurement indicated that some of these defects penetrated 8% of the tube wall thickness. Defects of this type can act as corrosion-initiation sites in a sufficiently aggressive environment. 

In a study where the uniform nucleation rate was varied over several values at a fixed strain rate of = 10 /s, the fragment number results shown in Fig. 8.21 were obtained. At lower nucleation rates nearly all fracture initiation sites grow to completion and provide the reduced number of fragments shown. 

Figure 9.1 The transcriptional elements of a eucaryotic structural gene extend over a large region of DNA. The regulatory sequences can be divided into three main regions (1) the basal promoter elements such as the TATA box, (2) the promoter proximal elements close to the initiation site, and (3) distal enhancer elements far from the initiation site.

Systemie poisons attaek organs other than the initial site of eontaet. The eritieal organs are the kidneys, liver, blood and bone maiTow. 

Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

Two mechanisms for acid-catalyzed hydrolysis can be imagined depending on the initial site of protonation. One mechanism begins widi protonation at X this makes X a better leaving group. The alternative is protonation of the carbonyl oxygen. 

Thermogravimetric analysis and other studies made on low-molecular weight model compounds such as 1,3, 5,-trichlorohexane [7,8] corresponding to the idealized head-to-tail structure of PVC show these structures to be considerably more stable than the polymer. This abnormal instability of the polymer is attributed to structural irregularities or defects in the polymer chain, which serve as initiation sites for degradation. 

In earlier investigations chain ends were suggested to be important initiation sites for dehydrochlorination. Provided there are no transfer reactions during polymerization, at least half the polymer chain ends will carry initiator fragments. In practice, transfer reactions swamp the normal termination processes and <30% of the chain ends carry initiator residues [59]. 

Head-to-head units formed in a molecule have not only been considered as initiation sites for the dehydrochlorination but also as termination points for the growing polyene sequences [19,66,68]. Head-to-head units can either be formed through termination by combination [Eq. (19)] or by head-to-head addition during propagation [Eq. (20)]. 

With respect to the initiation of cationic chain polymerizations, the reaction of chlorine-terminated azo compounds with various silver salts has been thoroughly studied. ACPC, a compound often used in condensation type reactions discussed previously, was reacted with Ag X , X, being BF4 [10,61] or SbFa [11,62]. This reaction resulted in two oxocarbenium cations, being very suitable initiating sites for cationic polymerization. Thus, poly(tetrahydrofuran) with Mn between 3 x 10 and 4 x lO containing exactly one central azo group per molecule was synthesized [62a]. Furthermore, N-... 

Berenson and Wranglen have suggested that the presence of magnesium sulphide inclusions in ductile cast irons may act as corrosion-initiating sites and may therefore make ductile iron more liable to corrosion than grey iron but this view has been contested by Collins . 

For noiiadditive rulas, can no longer be obtained by merely looking at the evolution of the initial difference state. A fairly typical nonadditive behavior is that of rule R126. We see that, apart from small fluctuations, H t) tends to steadily increase in a ronghly linear fashion. This means that as time increases, the values of particular sites will depend on an over increasing set of initial sites he., space-time patterns arc scnsitivr.ly dependent on the initial conditions. We will pick up this theme in our diseus.sion of chaos in continuous systems in chapter 4. 

Both the complexity and the sensitivity to initial states of the patterns generally increases as the number of initial sites with nonzero amplitude increases. As the number of nonzero sites increases, it also becomes increasingly likely that a change in the value of one of those sites (or a move in its location either to the left or right) will significantly alter the resulting pattern. 

Recognizable regularity in probability patterns, starting from randomly selected initial site-states, generally appears only when the measurement threshold is significantly less than certainty. 

The grafting reaction requires primary or secondary aliphatic hydroxyl functions as initiation sites and uses Ce4+ ions as the catalyst. This type of grafting was first developed by Mino and Kaizerman and applied to preparation of the polyacrylamide-polyvinylalcohol copolymer [102],... 

An analysis of the rate of release of adsorbed atoms from sites with a continuous energy spectrum for the case of an arbitrary distribution function of initial site populations was given by Carter (32). The rate equation for the t th desorption process with x = 1 and negligible readsorption is... 

Find the translation and transcription initiation sites, find promoter sites, define open reading rames (ORF)... 

Iversen (1991) stresses the need for some in vivo testing for neurotoxicity and emphasizes the value of sensitive behavioral tests. Behavioral tests are described for mice and rats, which provide measures of mood, posture, CNS excitation, motor coordination, sedation, exploration, responsiveness, learning, and memory function. Such assays can function as primary screens for neurotoxicity before adopting a stepwise scheme of in vitro tests to discover more about the initial site of action of neurotoxic compounds. It is argued that the requirement for animal testing can be drastically reduced by adopting structured in vitro protocols such as these. 

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