Free radical initiators half life

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. 

Equation 6 would hold for a family of free radical initiators of similiar structure (for example, the frarw-symmetric bisalkyl diazenes) reacting at the same rate (at a half-life of one hour, for example) at different temperatures T. Slope M would measure the sensitivity for that particular family of reactants to changes in the pi-delocalization energies of the radicals being formed (transition state effect) at the particular constant rate of decomposition. Slope N would measure the sensitivity of that family to changes in the steric environment around the central carbon atom (reactant state effect) at the same constant rate of decomposition. 

The drug has a half-life of 6-8 hours. It is extensively metabolized in the liver, and stereoselective metabolism of its two isomers is observed. Since metabolism of ( R)-carvedilol is influenced by polymorphisms in CYP2D6 activity and by drugs that inhibit this enzyme s activity (such as quinidine and fluoxetine, see Chapter 4), drug interactions may occur. Carvedilol also appears to attenuate oxygen free radical-initiated lipid peroxidation and to inhibit vascular smooth muscle mitogenesis independently of adrenoceptor blockade. These effects may contribute to the clinical benefits of the drug in chronic heart failure (see Chapter 13). 

Manufacture Polymethacrylate VI improvers are free-radical initiated solution copolymers. A wide variety of peroxide or azo initiators may be used, and several are used commercially, choice dictated largely by half-life at reaction temperatures in the range 100-140°C. In turn, this is driven by manufacturing convenience and kettle productivity factors. Production is a simple batch or semi-batch process at a... 

One of the successful radical homopolymerizations of VPA was performed by Levin et al in DMF in the presence of AIBN as initiator, in a yield of 95%. PVPA was obtained in protic solvents both from pure VPA, crude VPA , and ester-containing crude VPA in the presence of initiator. Suitable protic solvents were water and aliphatic alcohols such as isopropanol, which keep the mixtures stirrable and workable. The free radical initiators that can be used are peroxides such as dibenzoyl peroxide, di-tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butyl pero3gr-2-ethylhexanoate, and ammonium or potassium persulfate. The amount of initiator necessary is 1.5-4% versus monomer and depends directly on the amount of diluent. The authors recommend that the calculated amount of initiator be added in equal portions during the reaction, after the reaction temperature has been reached, because the polymerization is highly exothermic at the start when the monomer concentration is high and relatively higher residual monomer content could be obtained. The reaction temperature was maintained between 80 °C and 110 °C and depends on the dissociation half-life of the initiator. A reaction time of between 5 and 12 h is approximately inversely proportional to the concentration of VPA monomer in solvent. The yield of PVPA varied from 32% to 60% when peroxide initiators were used and from 3% to 6% when ammonium or potassium persulfate were used. Pure PVPA can be obtained by precipitation. The homopolymerization of VPA in methanol did not occur. ... 

Most emulsion polymerisations are free radical processes (318). There are several steps in the free radical polymerisation mechanism initiation (324), propagation and termination (324, 377, 399). In the first step, an initiator compound generates free radicals by thermal decomposition. The initiator decomposition rate is described by an Arrhenius-type equation containing a decomposition constant ( j) that is the reciprocal of the initiator half-life (Ph). The free radicals initiate polymerisation by reaction with a proximate monomer molecule. This event is the start of a new polymer chain. Because initiator molecules constantly decompose to form radicals, new polymer chains are also constantly formed. The initiated monomeric molecules contain an active free radical end group. 

One of the nice features of free-radical polymerization is that values of the preexponential coefficients and activation energies (or alternately half-life values at various temperatures) can be obtained in the literature (such as in Odian (1991)) or from their manufacturers (such as Wako Chemical Corp.) for a variety of initiators, and these numbers do not normally change no matter what the fluid environment the initiator molecules are in. Thus, if we want to decompose more than 99% of the starting initiator material in the reactor, we just have to wait for the reaction to proceed up to five times the initiator half-life. The other attractive feature of free-radical polymerization is that free-radical reactions are well known and radical concentrations can be directly measured. Thus, we know, for example, that if we want to preserve radicals in solution, we should not allow oxygen gas (O2) in our system, because reactive radicals will combine with oxygen gas to form a stable peroxy radical. That is why reaction fluids were bubbled with N2, CO2, Ar, or any inert gas, in order to displace O2 gas that comes from the air. Finally, Iree-radical polymerization is not sensitive to atmospheric or process water, compared to other polymerization kinetic mechanisms. 

TFE and monomer (I) were also copolymerized in supercritical carbon dioxide using a free radical initiator such as bis(perfluoro-2-propoxypropionyl)peroxide (III) at 35°C (the half-life time of the initiator is 40 minutes at 35°C) [9], The decomposition of the initiator proceeds through a single-bond homolysis mechanism [10], resulting in the formation of perfluorinated end group that yields thermally stable polymers [9] (Scheme 16.1). The reaction conditions and properties of the copolymers of (I) and TFE obtained in supercritical carbon dioxide are shown in Table 16.1. PTFE is crystalline, so that when the amount of TFE increases in copolymers, the polymer has some microcrystalline regions. The polymers obtained in carbon dioxide have similar properties with the commercial polymers. 

These temperature properties determine the polymerization initiation temperature and the length of time the free-radical initiation process is active. For example, low-temperature peroxides possess a relatively short free-radical half-life and offer a low-temperature initiation but also a lower peak temperature, while a high-temperature peroxide with a relatively long... 

Under appropriate conditions and in the presence of free-radical initiators, cyclopentadiene and MA will also react to form a saturated 1 2 copolymer.Copolymerizations are most effectively carried out at 80-205°C. In general, the highest yield of 1 2 copolymer is obtained when the initiator is used at a temperature where it has a short half-life. At a given temperature, the higher the initiator concentration or more rapidly the initiator decomposes, the higher the yield, independent of total reaction time. Copolymerizations may be run in bulk or solvents such as dioxane or chlorobenzene. The amount of solvent is critical, i.e., the yield of copolymer decreases with increasing solvent concentration unless the initiator concentration is increased. 

In general, free radicals initiators are used under conditions of half-life times of about 10 hours. The decomposition of peroxides can be single-step or multistep for example, dicumyl peroxide (DICUP) decomposes as follows ... 

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. 

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). 

A novel cross-linked polystyrene-divinylbenzene copolymer has been produced from suspension polymerization with toluene as a diluent, having an average particle size of 2 to 50 /rm, with an exclusive molecular weight for the polystyrene standard from about 500 to 20,000 in gel-permeation chromatography. A process for preparing the PS-DVB copolymer by suspension polymerization in the presence of at least one free-radical polymerization initiator, such as 2,2 -azo-bis (2,4-dimethylvaleronitrile) with a half-life of about 2 to 60 min at 70°C, has been disclosed (78). 

In the comparison of organic peroxides as free-radical polymerization initiators, one of the measures used is the temperature (T) required for the half-life (tm) to be 10 h. If it is desired to have a lower T, would ty2 be greater or smaller than 10 h Explain briefly. 

A list of some peroxo compounds that generate free radicals is given in Table 3.3, extensive information can be found in the literature. The initiators are selected according to their thermal half-lives to ensure that at the polymerization temperature they provide a source of free radicals. The rate equation for the thermal half-life is given by tj/2 = 0.693 kj, where kj is the rate constant for the thermal decomposition. In technical applications one often uses the tempera-... 

Peroxides decompose when heated to produce active free radicals which in turn react with the mbber to produce cross-links. The rate of peroxide cure is controlled by temperature and selection of the specific peroxide, based on half-life considerations (see Initiators, free-radical Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triallyl isocyanurate, and diallyl phthalate, act as coagents in peroxide cures, they are not vulcanization accelerators. Instead they act to improve cross-link efficiency (cross-linking vs scission), but not rate of cross-link formation. 

Thermal decomposition of peroxides initially forms oxygen-centered free radicals from the oxygen—oxygen bond homolysis. These radicals are reactive intermediates generally having very short lifetimes, i.e., half-life... 

Effect of Peroxides. In addition to benzoyl peroxide, lauryl-, acetyl-, 2,4-dichlorobenzoyl-, and methyl ethyl peroxide, and tert-hvXy hydroperoxide were studied and gave satisfactory results. The effectiveness of the peroxide is relatively independent of the half-life of the peroxide (Table X). By contrast, the catalyst AIBN is much less satisfactory, as found for methylvinylpyridine and acrylonitrile. The difference between these peroxides and AIBN suggests that the AFR polymer is not formed by a simple uncatalyzed free radical system which would give a graft polymer or a simple mixture of polypropylene and polyacrylate. It is well known that for the polymerization of acrylates AIBN is at least as good if not better than peroxide in initiating the free radical reaction (2). 

During an initial lag period, DMM undergoes conversion to the monomethylated form, and distribution from blood to tissues (half-life of uptake into hair, 6 days). This biotransformation may occur at sites rich in metabolic enzymes such as the skin, intestinal flora, the liver, and macrophages. Free-radical mechanisms are proposed as another possible means of dealkylation. 

The metabolic clearance rate of PSA follows a two-compartment model with initial half-lives of 1.2 and 0.75 hours for free PSA and total PSA and subsequent half-lives of 22 and 33 hours. Because of this relatively long half-life, 2 to 3 weeks may be necessary for the serum PSA to return to baseline levels after certain procedures, including transrectal biopsy, transrectal ultrasonography, transurethral resection of the prostate, and radical prostatectomy. Prostatitis and acute urinary retention can also elevate PSA concentration. Although the digital rectal examination has no clinically important effects on serum PSA levels in most patients, in some it may lead to a twofold elevation. 

A survey of the literature reveals that there are several commercially available radical initiators used in free radical transformations which are conducted in both aqueous and ionic liquid media (see Figure 4.1 for structures). The most popular thermal radical initiator is 2, 2 -azobisisobutyronitrile (AIBN), which has a half-life (t /2) in toluene of 1 h at 81 °C and 10 h at 65 °C.7,10... 

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