Inhibited initiator concentration

Relatively high concentrations of organic peroxide or azo initiators are needed to obtain complete polymerization. After the reaction peak exotherm, polymerization slows down. Initiator concentrations must be high enough to complete conversion. Polymerization is inhibited by oxygen and copper, lead, and sulfur compounds (11). 

Thermal Decomposition of GIO2. Chloiine dioxide decomposition in the gas phase is chaiacteiized by a slow induction period followed by a rapid autocatalytic phase that may be explosive if the initial concentration is above a partial pressure of 10.1 kPa (76 mm Hg) (27). Mechanistic investigations indicate that the intermediates formed include the unstable chlorine oxide, CI2O2. The presence of water vapor tends to extend the duration of the induction period, presumably by reaction with this intermediate. When water vapor concentration and temperature are both high, the decomposition of chlorine dioxide can proceed smoothly rather than explosively. Apparently under these conditions, all decomposition takes place in the induction period, and water vapor inhibits the autocatalytic phase altogether. The products of chlorine dioxide decomposition in the gas phase include chlorine, oxygen, HCl, HCIO, and HCIO. The ratios of products formed during decomposition depend on the concentration of water vapor and temperature (27). 

Table 3.1 shows the kinetic parameters for cell growth, rate models with or without inhibition and mass transfer coefficient calculation at various acetate concentrations in the culture media. The Monod constant value, KM, in the liquid phase depends on some parameters such as temperature, initial concentration of the carbon source, presence of trace metals, vitamin B solution, light intensity and agitation speeds. The initial acetate concentrations in the liquid phase reflected the value of the Monod constants, Kp and Kp. The average value for maximum specific growth rate (/xm) was 0.01 h. The value... 

Another important argument for the use of the organic solvent is the reverse hydrolytic reactions that become feasible [61,75]. The inhibition of the biocatalyst can be reduced, since the substrate is initially concentrated in the organic phase and inhibitory products can be removed from the aqueous phase. This transfer can shift the apparent reaction equilibrium [28,62] and facilitates the product recovery from the organic phase [20,29,33]. A wide range of organic solvents can be used in bioreactors, such as alkanes, alkenes, esters, alcohols, ethers, perfluorocarbons, etc. (Table 1). 

This study has shown that typical coating biocides can be encapsulated within modified silica frameworks. These porous frameworks offer a means to inhibit the aqueous extraction of the biocide. In such combinations the biocides retain their anti-microbial properties, while controlled delivery facilitates a dynamic equilibrium to maintain a minimum inhibitory concentration at the coating interface, over an extended time period. There is evidence that biocide housed in such frameworks has a longer effective activity for a given initial concentration, since it is to some extent protected from the usual environmental degradation processes. 

Another important characteristic of inhibitors is the time of their inhibition action. If an inhibitor is consumed only in chain termination reactions, this time is determined by the initial concentration [InH]0, stoichiometric coefficient of inhibition / and Vj. In this case, the rate of inhibitor consumption is vInH = v //. Side reactions of InH with dioxygen and hydroperoxide shorten the inhibitory period and increase the rate of inhibitor consumption. Therefore, an inhibitor is efficient when it provides a minimal chain length v and its own loss in side reactions w is low. Assuming that an efficient inhibitor has w < 0.25, we get the inequality 4k 2[InH][02] < v which can be transformed, by substituting the correlation equation from Table 14.7, into the following equation... 

Assays are set up containing enzyme and inhibitor (or water for controls) in the initial volumes shown. Inhibitor concentrations listed are initial concentrations. Enzyme and inhibitor should be preincubated together if time-dependent inhibition is suspected. The assay is started by transfer of 20 pL of the contents of each well in groups 1 and 3 to a corresponding well in groups 2 and 4, respectively. Thereafter, 100 pL of substrate at an initial concentration of 30 pM (3 x K ) is added to all wells. The reaction wells then contain components at the concentrations listed in O Table 4-4. 

An especially interesting case of inhibition is the internal or autoinhibition of allylic monomers (CH2=CH—CH2Y). Allylic monomers such as allyl acetate polymerize at abnormally low rates with the unexpected dependence of the rate on the first power of the initiator concentration. Further, the degree of polymerization, which is independent of the polymerization rate, is very low—only 14 for allyl acetate. These effects are the consequence of degradative chain transfer (case 4 in Table 3-3). The propagating radical in such a polymerization is very reactive, while the allylic C—H (the C—H bond alpha to the double bond) in the monomer is quite weak—resulting in facile chain transfer to monomer... 

Free-radical polymerization processes are used to produce virtually all commercial methaerylie polymers. Usually free-radical initiators tqv > such as a/o compounds or ieroxides are used to initiate the polymerisations. Photochemical and radiation-initiated polymerizations are also well known. At it constant temperature, the initial rate of the hulk or solution radical polymerization of methaerylie monomers is first-order with respect to monomer eoneentration. anil one-half order with respect to the initiator concentration. Methacrylate polymerizations are markedly inhibited by-oxygen therefore considerable care is taken to exclude air during the polymerization stages of manufacturing. 

If complete adsorptive molecular layers inhibit further dissolution of silicic acid, all experimental phenomena are explicable. In this case the final experimental concentration is not a saturation concentration which can be obtained also from high initial concentrations of silicic acid. Instead, the concentration will decrease only to the extent that the adsorption equilibrium is established. For sufficiently high initial concentrations of silicic acid and comparatively small surface areas, this will involve a multimolecular layer, whose equilibrium concentration is higher than the final experimental concentration of the self-inhibiting dissolution process. 

The kinetic description of chainwise polymerizations requires a set of rate equations accounting for inhibition, initiation, propagation, termination, and transfer steps. The polymerization rate is usually given by the propagation steps (one or more), because they consume functional groups much more frequently than any other step involving them e.g. initiation or transfer steps. However, it is always necessary to consider the overall set of kinetic equations to determine the evolution of the concentration of active species (free radicals, ions, etc.) that participate in the propagation step. 

In most media formulations for mammalian cell lines, glucose is included at an initial concentration of 10-25 mM, which decreases to around half the concentration level within the period of a batch culture. The production of lactic acid may lower the pH of the culture and this may decrease the cellular growth rate. However, if the culture is appropriately buffered then the accumulated lactate levels typically found in a batch culture should not cause any growth inhibition (Hassell et al., 1991). 

Aromatically substituted enols are easily autoxidized to the keto-hydroperoxide form (4, 5, 6, 10, 11, 12, 16), which, we postulate, would then initiate a radical polymerization. A radical mechanism is proved by the inhibiting effect of quinone and a,polymer yield is directly proportional to the square root of the initiator concentration (7). 

Stabilize new particles, thereby increasing the total number of particles. Since the nucleation period is lengthened, the polydispersity increases. Figure 14 shows that the dependence of the inhibitor concentration on the number of particles is 0.176 0.010. Conversion time curves indicate that an induction period results from the presence of the inhibitor. Since polymer-stabilized miniemulsion polymerization occurs via droplet nucleation, it should be less sensitive to oil-phase inhibition. Initiator radicals will enter the droplet one after the other until all of the inhibitor is used up, and the monomer polymerizes. This does not affect the number of droplets or particles. As seen in Fig. 15, the number of particles is proportional to the DPPH concentration raised to the power of 0.0031 0.0001. Therefore, the number of particles is essentially independent of the presence of inhibitor. 

Some reagents react with the initiating radical to give unreactive substances, a process known as inhibition. A common inhibitor for vinyl polymerisations is hydroquinone, which reacts by the transfer of two hydrogen radicals to the initiator radicals (Fig. 2.4). This gives quinone and unreactive initiator and has the net effect of causing a lag time in the polymerisation and a decrease in the initiator concentration. Monomers are often stored in the presence of inhibitor in order to prevent polymerisation. The amount and type of inhibitor may vary depending on the monomer batch and the manufacturer. For inter-laboratory comparisons of materials to be possible, it is therefore important to remove the inhibitor and purify the monomers prior to use [13]. 

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