Half-lives of initiators

In the second step, all the rest of the azo groups are decomposed corresponding to a reaction time of 10 half-lives of initiator decomposition. MMA terminates mainly by disproportionation, therefore ABA block copolymers are principal products. 

After bolus injection, the initial decline in serum concentrations following intravenous administration is rapid for IFN-a, -P, and -y for all species tested with the half-lives of initial distribution on the order of minutes. Serum concentrations initially decrease rapidly, then decline more slowly, with terminal elimination half-lives ranging from minutes (30-40min for IFN-a in mice) [148] to hours (7-12h... 

RADICAL CHAIN POLYMERIZATION TABLE 3-2 Half-Lives of Initiators ... 

Number OF Half- Lives OF Initial Quantity Remaining Quantity Remaining (MG)... 

Equation 3-39 shows that in the first order reaetions, the half-life is independent of the eoneentration of the reaetant. This basis ean be used to test whether a reaetion obeys first order kineties by measuring half-lives of the reaetion at various initial eoneentrations of the reaetant. 

When we carry out conventional studies of solution kinetics, we initiate reactions by mixing solutions. The time required to achieve complete mixing places a limit on the fastest reaction that can be studied in this way. It is not difficult to reduce the mixing time to about 10 s, so a reaction having a half-life of, say, 10 s is about the fastest reaction we can study by conventional techniques. (See Section 4.4 for further discussion of this limit.) The slowest reaction accessible to study depends upon analytical sensitivity and patience let us say that the half-life of a graduate student, 2-2 years, sets an approximate limit. This corresponds to roughly 7 x 10 s. Thus, a range of half-lives of about 10 can be studied by conventional techniques. 

The half-life, f1/2, of a substance is the time needed for its concentration to fall to one-half its initial value. Knowing the half-lives of pollutants such as chlorofluoro-carbons allows us to assess their environmental impact. If their half-lives are short, they may not survive long enough to reach the stratosphere, where they can destroy ozone. Half-lives are also important in planning storage systems for radioactive materials, because the decay of radioactive nuclei is a first-order process. 

The susceptibility of a protein to degradation is expressed as its half-life (t,/2)> the time required to lower its concentration to half the initial value. Half-lives of hver proteins range from under 30 minutes to over 150 hours. Typical housekeeping enzymes have tj/2 values of over 100 hours. By contrast, many key regulatory en-... 

Based on Equation (3), in the case of a system where there is an initial disequilibrium in the chain (namely A,jNj A,2N2), it is generally stated that the system returns to secular equilibrium after -six half-lives of the daughter. The wide variety of parent-daughter pairs allows disequilibria to provide temporal constraints over a wide range in time scales (Fig. 3). 

Number of half-lives elapsed Initial concentration remaining (%) Completeness of process (%)... 

This approach uses a kinetic sequential principle to carry out multicomponent CL-based determinations. In fact, when the half-lives of the CL reactions involved in the determination of the analytes in mixture are appreciably different, the CL intensity-versus-time curve exhibits two peaks that are separate in time (in the case of a binary mixture) this allows both analytes to be directly determined from their corresponding calibration plots. In general, commercially available chemiluminometers have been used in these determinations, so the CL reaction was initially started by addition of one or two reaction ingredients. Thus, in the analysis of binary mixtures of cysteine and gluthatione, appropriate time-resolved response curves were obtained provided that equal volumes of peroxidase and luminol were mixed and saturated with oxygen and that copper(H) and aminothiol solutions were simultaneously injected [62, 63],... 

Myelin components exhibit great heterogeneity of metabolic turnover. One of the novel characteristics of myelin demonstrated in early biochemical studies was that its overall rate of metabolic turnover is substantially slower than that of other neural membranes [1]. A standard type of experiment was to evaluate lipid or protein turnover by injecting rat brains with a radioactive metabolic precursor and then follow loss of radioactivity from individual components as a function of time. Structural lipid components of myelin, notably cholesterol, cerebro-side and sulfatide, as well as proteins of compact myelin, are relatively stable, with half-lives of the order of many months. One complication in interpreting these studies is that the metabolic turnover of individual myelin components is multiphasic - consisting of an initial rapid loss of radioactivity followed by a much longer slower loss. 

For example, initially MBP and PLP exhibit half-lives of 2-3 weeks, but later their half-lives are too long to be calculated accurately. A possible interpretation of these data is that some of the newly formed myelin remains in outer layers or near cytoplasmic pockets (incisures and lateral loops) where it is accessible for catabolism - thus accounting for the rapidly turning-over pool. The more stable metabolic pool would consist of deeper layers of myelin less accessible for metabolic turnover. 

Both hydrocarbon and fluorocarbon organic peroxides were used to initiate polymerization. The half-lives of several that were used are shown in Table 6.3. The perfluoro-organic peroxides were prepared at temperatures below 0°C by the reaction of the corresponding acyl chloride and sodium peroxide (Scheme 2). Sodium peroxide was formed from an aqueous mixture of sodium hydroxide and hydrogen peroxide. 

Chemical/Physical. Under alkaline conditions, diethyl phthalate will initially hydrolyze to ethyl hydrogen phthalate and ethanol. The monoester will undergo hydrolysis forming o-phthalic acid and ethanol (Kollig, 1993). A second-order rate constant of 2.5 x lO /M-sec was reported for the hydrolysis of diethyl phthalate at 30 °C and pH 8 (Wolfe et al, 1980). At 30 °C, hydrolysis half-lives of 8.8 and 18 yr were reported at pH values 9 and 10-12, respectively (Callahan et al., 1979). 

Chemical/Physical. When dicofol was exposed to sunlight for 20 d, a 10% yield of 4,4 -dichlorobenzophenone was obtained. Solvents containing dicofol and exposed to UV light resulted in the formation of chlorobenzilic acid esters (Vaidyanathaswamy et ah, 1981). Hydrolyzes in water forming p,jO -dichlorobenzophenone (Walsh and Hites, 1979). The hydrolysis half-lives of dicofol (400 mg/L initial concentration) at pH 8.2 and 10.2 were 1 h and 3 min, respectively (Walsh and Hites, 1979). 

The concentration of carbon tetrachloride was measured in the expired air of a person who swallowed a large amount of carbon tetrachloride (Stewart et al. 1963). Excretion in expired air was found to decrease exponentially in a biphasic or multiphasic fashion, but no quantitative estimate of the elimination half-life of carbon tetrachloride or of the fraction of the dose excreted by this pathway was provided. Visual inspection of their graphed data suggests very approximate half-lives of less than several hours initially, 40 hours (75-150 hours post exposure), and 85 hours (300-400 hours post exposure). 

That is, the higher the initial concentration, the shorter the half-life This counterintuitive result is due to the reaction rate being proportional to the square of the concentration, meaning that the rate increases more rapidly than the concentration itself. Nonetheless, for [A] to reach 0.01 M, it takes a longer time starting from 0.2 M than starting from 0.1 M by the extra time for [A] to attain from 0.2 to 0.1 M. The half-lives of various reactions are listed in Table 1-2. 

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