Initiators half lives

Polymerisation can only proceed efficiently and economically if sufficient free radicals are present throughout the polymerisation. However, the presence of too many free radicals can have a deleterious effect upon the polymer, resulting in too low a molecular weight, or excessive chain grafting etc. Thus, it is desirable to know how the number of free radicals relates to the initiator concentration, initiator type, and reaction conditions, including reaction temperature. This relationship is expressed as the initiator half-life, and is defined as the time taken for half of a given mass of initiator to decompose. 

This is the standard Arhenius equation. If a maximum velocity constant (k , ) is incorporated, then  

The term ln(ln(2)/km ) is considered to be constant, and a plot of the Naperian logarithm of ti against 1/T will be linear, with a gradient of E/2.3R. 

Organic peroxides have activation energies in the region 100-150 KJ/molecule. 

Peroxy compounds with low activation energies have decomposition rates which are moderately linear with respect to temperature, and hence, are particularly suited to reaction conditions where the reaction temperature is not controlled between narrow limits. 

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The initial half-life of a second-order reaction corresponds to a decrease from Oq to UqI2 and is given by... 

The second half-life, corresponding to a decrease from Uo/2 to ao/4, is twice the initial half-life. 

Table II. Initiators Half-life Data Comparisons...

In this paper we present a meaningful analysis of the operation of a batch polymerization reactor in its final stages (i.e. high conversion levels) where MWD broadening is relatively unimportant. The ultimate objective is to minimize the residual monomer concentration as fast as possible, using the time-optimal problem formulation. Isothermal as well as nonisothermal policies are derived based on a mathematical model that also takes depropagation into account. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and time is studied. 

A series of simulations were performed to study the effect of variables such as initiator concentration, initiator half-life and activation energy on the optimum temperature and optimum time. It was assumed that initially the polymerization mixture contained S volume percent monomer, the rest of the mixture being solvent and polymer formed earlier. It was required to reduce the monomer concentration from S volume percent to 0.S volume percent in the minimum possible time. The kinetic and tbeimodyamnic parameters used are similar to those of free radical polymerization of MMA. The parameter values are given in Appendix B. 

In Figme 4 is shown the effect of initiator half-life for an initiation activation energy of 120 KJ/mol on the optimum temperature and optimum time. It can be seen that the optimum temperature is almost independent of the half-life. As expected, the optimum time increases with an increase in half-life. Closer study of the results reveals that an almost constant optimal temperature is due to high NL, Values. A much higher temperature would cause to be higher than the desired Mf. 

In this paper we formulated and solved the time optimal problem for a batch reactor in its final stage for isothermal and nonisothermal policies. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and optimum time was studied. It was shown that the optimum isothermal policy was influenced by two factors the equilibrium monomer concentration, and the dead end polymerization caused by the depletion of the initiator. When values determine optimum temperature, a faster initiator or higher initiator concentration should be used to reduce reaction time. 

From 10 to 20% of an oral dose of miconazole is excreted in the urine, mainly as metabolites, within 6 days. About 50% of an oral dose may be excreted mainly unchanged in the feces. The elimination pharmacokinetics of miconazole have been described as triphasic, with a biological half-life of about 24 h. With an initial half-life of about 0.4 h, and intermediate half-life of about 2.5 h and elimination half-life of 24 h. Very little miconazole is removed by haemodialysis [3]. 

The pharmacokinetic parameters of octane are more like those of dieldrin than DDT while the clearance of all three of these compounds is somewhat greater than that of PAH. While the initial half life of DEHP is longer (36 min.) than any other compound in Table VI, its second phase t is rapid (3 hrs.) while the is equal to that of the plasma volume. The clearance of DEHP isu about one-half that of PAH. 

Surface Water. In pond water, adsorption to suspended sediments was an important process. The initial half-life was reported to be no more than 68 d. After 120 d, 20% of the applied amount remained (Grzenda et al., 1966). The biodegradation half-life of amitrole in water is approximately 40 d (Reinert and Rodgers, 1987). 

Absorption/Distribution - Following IV administration, distribution is essentially confined to extracellular fluids and is characterized by an initial half-life of about 10 minutes. Elimination follows a first-order process and is characterized by a terminal half-life of about 1.3 hours in young healthy volunteers. As the IV dose is increased over the range of 0.1 to 0.4 mg/kg, the maximum plasma... 

Metabolism/Excretion - Carbamazepine is metabolized in the liver to the 10,11-epoxide, which also has anticonvulsant activity. It may induce its own metabolism. Initial half-life ranges from 25 to 65 hours and decreases to 12 to 17 hours with repeated doses. 

The differences in the decomposition rates of various initiators can be conveniently expressed in terms of the initiator half-life t /i defined as the time for the concentration of I to decrease to one half its original value. The rate of initiator disappearance by Eq. 3-13 is... 

The concentration of carbon tetrachloride appeared to decline exponentially in a biphasic manner, with an initial half-life of less than 1 hour, and a second-phase half-life of about 40 hours. Roughly similar results were observed in several volunteers who breathed carbon tetrachloride for 1-3 hours, where the half-life of carbon tetrachloride in expired air over the first several hour period after exposure was less than 1 hour (Stewart et al. 1961). 

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... 

Serum concentrations of lithium decline in a biphasic manner. An initial half-life (t a) of 0.8-1.2 hours and... 

It is a sulfated napthylamine and used as first line therapy for early hemolymphatic African trypanosomiasis (caused by T. brucei gambiense). It has very tight protein binding and having short initial half life but terminal half life is about 50 days and is excreted by kidney. It is also used for chemoprophylaxis against African trypanosomiasis. 

Pharmacokinetics Compared to alte-plase, tenecteplase has a lower plasma clearance (175mFmin vs. 570ml/min), longer initial half-life (20-24 minutes vs. 4 minutes), and may be more resistant to inactivation by plasminogen activator inhibitor-1. Its initial volume of distribution is weight related and approximates plasma volume. 

Pharmacokinetics Abciximab has an initial half-life of about 10 minutes and a second phase half-hfe of about 30 minutes, probably related to rapid binding to the platelet gp Ilb/IIIa receptors. Although abciximab remains in the circulation for up to 10 days in a platelet-bound state, platelet function generally recovers over the course of 48 hours. 

Exogenous somatostatin is rapidly cleared from the circulation, with an initial half-life of 1-3 minutes. The kidney appears to play an important role in its metabolism and excretion. 

Chloroquine is a synthetic 4-aminoquinoline (Figure 52-2) formulated as the phosphate salt for oral use. It is rapidly and almost completely absorbed from the gastrointestinal tract, reaches maximum plasma concentrations in about 3 hours, and is rapidly distributed to the tissues. It has a very large apparent volume of distribution of 100-1000 /kg and is slowly released from tissues and metabolized. Chloroquine is principally excreted in the urine with an initial half-life of 3-5 days but a much longer terminal elimination half-life of 1-2 months. 

Pentamidine is an aromatic diamidine (Figure 52-3) formulated as an isethionate salt. Pentamidine is only administered parenterally. The drug leaves the circulation rapidly, with an initial half-life of about 6 hours, but it is bound avidly by tissues. Pentamidine thus accumulates and is eliminated very slowly, with a terminal elimination half-life of about 12 days. The drug can be detected in urine 6 or more weeks after treatment. Only trace amounts of pentamidine appear in the central nervous system, so it is not effective against central nervous system African trypanosomiasis. Pentamidine can also be inhaled as a nebulized powder for the prevention of pneumocystosis. Absorption into the systemic circulation after inhalation appears to be minimal. The mechanism of action of pentamidine is unknown. 

Suramin is a sulfated naphthylamine that was introduced in the 1920s. It is the first-line therapy for early hemolymphatic East African trypanosomiasis ( brucei rhodesiense infection), but because it does not enter the central nervous system, it is not effective against advanced disease. Suramin is less effective than pentamidine for early West African trypanosomiasis. The drug s mechanism of action is unknown. It is administered intravenously and displays complex pharmacokinetics with very tight protein binding. Suramin has a short initial half-life but a terminal elimination half-life of about 50 days. The drug is slowly cleared by renal excretion. 

Half-Life. Once these activation parameters have been determined lor a initiator, half-life times tit a given temperature, i.e.. the lime required for 50 decomposition at a selected temperature, and half-life temperatures for a given period, i.e.. the temperature required for 509f decomposition of an initiator over a given time, can be calculated. Half life data arc useful for comparing the activity or one initiator with another when the half-life data arc determined in the same solvent and at the same concentration and. preferably, when the initiators are of the same class. 

Tamoxifen is a competitive partial agonist inhibitor of estradiol at the estrogen receptor and is extensively used in the palliative treatment of advanced breast cancer in postmenopausal women. It is a nonsteroidal agent (see structure below) that is given orally. Peak plasma levels are reached in a few hours. Tamoxifen has an initial half-life of 7-14 hours in the circulation and is predominantly excreted by the liver. It is used in doses of 10-20 mg twice daily. Hot flushes and nausea and vomiting occur in 25% of patients, and many other minor adverse effects are observed. Studies of patients treated with tamoxifen as adjuvant therapy for early breast cancer have shown a 35% decrease in contralateral breast cancer. However, adjuvant therapy extended beyond 5 years in patients with breast cancer has shown no further improvement in outcome. Toremifene is a structurally similar compound with very similar properties, indications, and toxicities. 

The drug s mechanism of action is unknown. It is administered intravenously and displays complex pharmacokinetics with very tight protein binding. It has a short initial half-life but a terminal elimination half-life of about 50 days. The drug is slowly cleared by renal excretion. 

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