Half-life values

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

Table 3. Common Initiators and Their Half-Life Values...

The substitution on the C-atom is by contrast of great effect on the rate of hydrolysis. Table X gives the half-life values for the fission... 

Perminks et al. (1987) reported that 80% of a single oral dose of dioctyltin dichloride at 2 mg/kg body weight was excreted in the faeces within 2 days. After 3 days, excretion of radioactivity followed first-order kinetics, with a half-life of 8.9 days. After intravenous administration, 66% of the radioactivity was excreted in the faeces, and a half-life value of 8.3 days was obtained, roughly similar to that of oral administration. Percentages of radioactivity excreted in the urine were 11% and 22% following intravenous and oral dosing, respectively. 

The thermal stability of peroxides can be expressed in terms of their half-hfe (ti/a)- Half-life values can be estimated in solution utilizing the technique of differential thermal analysis. These values, or more precisely the temperatures at which their half-life is equivalent, provide an indication of practical vulcanization temperatures [49] (Table 14.29). 

Endosulfan undergoes hydrolysis to endosulfan diol in surface water and groundwater. The rate of hydrolysis is influenced by pH. Half-life values reported in the literature vary somewhat. The chemical degradation of a- and P-endosulfan was studied under both anaerobic and aerobic environments. Under aerobic conditions, both hydrolysis and oxidation of endosulfan can occur, while under anaerobic conditions, only hydrolysis can occur. The hydrolytic half-lives for a- and P-endosulfan under anaerobic conditions at pH 7 were 35 and 37 days, respectively (Greve and Wit 1971). At pH 5.5 the half-lives were 151 and 187 days, respectively. Under aerobic conditions, the half-lives decreased. At pH 7, the half-lives of the chemical degradation (hydrolysis and oxidation) of both a- and P-endosulfan were 23 and 25 days, respectively, while at pH 5, the half-lives were 54 and 51 days, respectively. At T=20 and pHs of 5.5 and 8.0, the half-lives of a-endosulfan in distilled water were 11.3 and 5.3 days. 

Table II gives published ( ) half-life data for the two initiators along with values calculated from the optimized values of Yl and Y2. In each case, solvent C data were used to calculate the base activation energies and frequency factors, and the equality of half-life values at Tb illustrates the anchoring of the rate constant for each initiator. Except for initiator 1 at the low temperature, the differences between the optimized and published values are within the range of the differences reported for differing solvents.

These simulations clearly reveal the importance of considering M, in calculating the optimal temperature. is dependent on the heat of polymerization (-AH) as given by Eq. (13). Most monomers have heats of polymerization in the range of 50 to 80 KJ/mol. We thus decided to study the effect of (-AH) on optimal temperature and time for various half-life values of the initiator. The results are shown in Figme 5. 

It is relevant also to compare the results in Fig.5 with previously published data for PAC production under similar environmental conditions, where with higher concentrations of initial benzaldehyde (600 mM), pyruvate (400 mM) and PDC activity (8.4 U ml ) a similar maximum concentration of PAC of 330 mM was produced [6]. PDC stability was similar in both processes with half life values of approximately 27h. However, PAC production was much faster in the benzaldehyde emulsion system, presumably due to higher initial enzyme concentration. 

Note. In a recent paper, Miller and Stein have provided values for both C-C and C-0 bonds for a variety of coal model compounds, including bibenzyl and benzyl phenyl ether (11). Their rate constant for bibenzyl provides half-life values at 335°C and 400°C even larger than those discussed here, and it would seem on the basis of their data that at those low temperatures C-C scission in bibenzyl itself is too slow for thermal scission to be significant. 

Parent isotope Stable daughter product Currently accepted half-life values... 

The half-life of a radionuclide represents the amount of time required for half of the sample to decay. Relative stabilities of radionuclides are indicated by their half-life values. The shorter the half-life, the less stable is the radionuclide. 

Table 13.4 The serum half-life values of some IgG antibody preparations when administered to humans...

Hp calculated by Nyman as 5 days may represent the half-life as a result of the normal catabolism—type (a)—if we ignore the erythrocyte destruction. It is meaningless to speak of half-life values of Hp when discussing Hp turnover absolute amount per unit of time and weight should, instead, be used. 

The half-life values of selected radioisotopes are listed in Table 15.3. 

TABLE 15.3. Half-Life Values of Selected Radioisotopes. ... 

Plant. In plants, phosmet is degraded to nontoxic compounds (Hartley and Kidd, 1987). Dorough et al. (1966) reported the half-life in Bermuda grass was 6.5 d. Half-life values ranged from 1.2 to 6.5 d (Dorough et al, 1966 Leuck and Bowman, 1968 Menzies et al, 1979 Miller et al, 1969). 

The metabolism and elimination of TCAs takes several days to occur, the elimination half-life ranging from 20 hours for amitriptyline to 80 hours for protriptyline. The half-life values for the desmethylated metabolites such as desmethylimipramine and nortriptyline are approximately twice those of the parent compounds imipramine and amitriptyline. It is also well established that the half-life values of the TCAs are considerably greater in the elderly, which predisposes such patients to a greater possibility of severe side effects. 

Thus, if we are comparing half-life values between two groups of patients, or in an... 

Phase I data of XL647 in 31 patients with NSCLC were presented at the 17th AACR-NCI-EORTC conference, November 2005. Patients received oral doses of XL647 ranging from 0.06 to 3.12mg/kg on day 1 and days 4-8 in a 14-day cycle. The terminal half-life value after 5 days consecutive dosing was 70 h. One partial response and seven stable disease states has been achieved... 

Absorption/Distribution - The major portion of IM injections of iron dextran is absorbed within 72 hours most of the remaining iron is absorbed over the ensuing 3 to 4 weeks. Various studies have yielded half-life values ranging from 5 hours (circulating iron dextran) to more than 20 hours (total iron, both circulating and bound). 

The activity of naltrexone is believed to be due to both parent and the 6- -naltrexol metabolite. The mean elimination half-life values for naltrexone and 6- -naltrexol are 4 and 13 hours, respectively. 

Metabolism/Excretion - About 50% is excreted in the urine as the unchanged drug and 30% as metabolites (20% mono-N-dealkyIdisopyramide [MND]). The plasma concentration of MND is approximately one-tenth that of disopyramide. The mean plasma half-life is 6.7 hours (range, 4 to 10 hours). In impaired renal function, half-life values ranged from 8 to 18 hours. Therefore, decrease the dose in renal failure to avoid drug accumulation. 

Mean serum clearance values ranged from 9.4 to 28.9 mL/min/kg and were independent of dose. Mean terminal elimination half-life values ranged from 8 minutes to 4.3 hours and mean steady-state volume of distribution values ranged from 0.25 to 2.88 L/kg. IV dosing 3 times/week for 2 weeks resulted in no accumulation of interferon beta-la or beta-lb in the serum of patients. 

As in humans, the rate of carbon tetrachloride excretion in rats appears to be biphasic, with an initial half-life value of 7-10 hours (Paustenbach et al. 1986a). The rapid phase was judged to reflect... 

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