The initiation step

This will obviously take place on the necessarily existing coordination vacancies at the surface of the crystal, by -coordination of the A1 derivative and a metathetic exchange implanting R groups which foreshadow the future growing chain (Fig. 5). That has been semiquantitatively confirmed by analytical determination with labeled groups. 

Polymerization of styrene and dienes in hydrocarbon solvents 3.1. THE INITIATION STEP 

There is a large difference in initiation rates between the two initiators, but in both cases the reaction order in lithium alkyl is fractional, whereas the dependence on monomer concentration is, as expected, of the first order. The lines drawn have slopes of 1/4 (sec.-BuLi) and 1/6 (n-BuLi). There seems to be a clear relationship between association number (n) and reaction order and simple mechanisms can be suggested [17] (although not, of course, proved) of the type. 

To predict the required orders, the equilibrium must be rapidly established and the concentration of free butyllithium very small and its reactivity with monomer not large enough to disturb the equilibrium. The dissociation process can proceed via intermediate degrees of association, provided that the major species present is the n-fold associate and that intermediate species are also unreactive. Similar results have been found for isoprene [33, 36], diphenylethylene and fluorene [43—45] in benzene. The results are summarized in Table 1. 

Kinetic parameters for the reaction of lithium alkyls with styrene, isoprene and 1,1-diphenylethylene in benzene at 30°C 

Experiments in cyclohexane and hexane show that benzene cannot be regarded as an inert solvent and that the initiation mechanism differs between aliphatic and aromatic solvents. Studies on intermolecular exchange [38, 48—50], which presumably reflects the primary dissociation process, also indicate differences between the two types of solvent as well as between different lithium edkyls. Recent experiments with tert.-butyl-lithium [46], which has the lowest intermolecular exchange rate of the alkyls, indicate a more complex behaviour in the initiation reaction in benzene. This may be connected with a lower rate of dissociation of this alkyl. 

Once the radicals have been generated, the next step is for them to initiate polymerization. Some of them will he successful in this regard, while others will not. If the fraction of radicals that succeed in initiating polymerization is given by ft, the quantum yield for each initiation reaction 9, quantum yield of the chain initiation is given by 9 - 9,- -. 

The initiation step competes with the quenching (scavenging) of primary radicals by various additives (Ad) (such as stabilizers and antioxidants) and by oxygen, as 

Oxygen readily reacts with most initiating radicals by forming peroxy radicals that do not effectively initiate polymerization, so much so that in the early stages of irradiation this reaction channel removes the photogenerated primary radicals  

It is only when most of the oxygen has been consumed that the primary radicals become available for initiation.  

Given that the rate of generation of primary radicals, at a given light flux, is a function of the concentration of photoinitiator, the induction period depends also on initiator concentration. In order to eliminate this induction period of polymerization in industrial scale applications, many photocuring processes are carried 

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The initial step in the pathway is the condensation of erythrose-4-phosphale with phosphoenolpyruvate, yielding dehydroquinic acid, which by elimination of the elements of water affords dehydroshikimic acid reduction of the 3-keto group to hydroxyl gives shikimic acid. 

The initial step in alkane hydrogenolysis is the dissoeiative adsorption, or reaetive stieking of the alkane. One might suspeet that this first step may be the key to the stnieture sensitivity of this reaetion over Ni surfaees. Indeed, the reaetive stieking of alkanes has been shown to depend markedly on surfaee stnieture... 

Breton J, Martin J-L, Fleming G R and Lambry J-C 1988 Low-temperature femtosecond spectroscopy of the initial step of electron transfer in reaction centers from photosynthetic purple bacteria Biochemistry 27 8276... 

The electron density, pj, of the embedded cluster/adsorbate atoms is calculated using quantum chemistry methods (HF, PT, multireference SCF, or Cl). The initial step in this iterative procedure sets to zero,... 

The initial step is to identify which database, from a few thousands worldwide (about 10 000 in 2002), provides the requested information. The next step is to determine which subsection of the topic is of interest, and to identify typical search terms or keywords (synonyms, homonyms, different languages, or abbreviations) (Table 5-1). During the search in a database, this strategy is then executed (money is charged for spending time on some chemical databases). The resulting hits may be further refined by combining keywords or database fields, respectively, with Boolean operators (Table 5-2). The final results should be saved in electronic or printed form. 

Fig. 6.9 Variation in mean squared displacement during the initial steps of a molecular dynamics simulation of argon.

Each chlorine atom formed m the initiation step has seven valence electrons and IS very reactive Once formed a chlorine atom abstracts a hydrogen atom from methane as shown m step 2 m Figure 4 21 Hydrogen chloride one of the isolated products from... 

Can you write an equation for the initiation step that precedes these propagation steps ... 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

In this section we discuss the initiation step of free-radical polymerization. This discussion is centered around initiators and their decomposition behavior. The first requirement for an initiator is that it be a source of free radicals. In addition, the radicals must be produced at an acceptable rate at convenient temperatures have the required solubility behavior transfer their activity to... 

Under identical conditions, OX was approximately ten times as reactive as its isomers. It was proposed that the initial steps in the mechanisms of formation of each isomer ate the same. 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

The capture of solar energy as fixed carbon in biomass via photosynthesis is the initial step in the growth of biomass. It is depicted by the equation... 

An alternative starting network is one without stream spHts. The networks from the TI method maximize energy recovery and may introduce heat-load loops. Stream spHts ate not made in the initial steps of network invention. The ED method is proposed to be one in which heuristic rules and strategies would be used to improve the networks developed by the TI method. The importance of a thermodynamic base for evolutionary rules is stressed in this proposal, but there is no expHcit guidance for the evolutionary process. 

The initial step of production is carried out in a titanium reactor (34) because of the high corrosivity of maleic acid to most metals under the drastic reaction conditions used. The other steps are performed in stainless steel equipment. Improved purification processes for malic acid have been patented (37,38). 

Concentrate. An action to intensify in strength or purity by the removal of valueless or unneeded constituents, ie separation of ore or metal from its containing rock or earth. The concentration of ores always proceeds by steps or stages. Liberation of mineral values is often the initial step. Concentrate also means a product of concentration, ie, enriched ore after removal of waste in a beneficiation mill. 

Decomposition of Austenite. In heat-treating steels, the initial step is usually to heat the steel into the austenite region (>723° C) and then control the cooling process to produce the desired stmeture. The phase diagram (Fig. 2) shows that austenite decomposes into the two phases d and Fe C... 

The two main termination steps for neutral solutions are HO + HO — H2O2 + 2 O3 and HO + HO3 — H2O2 + O3 + O2. An alternative mechanism has been proposed that does not involve HO and HO but has a different initiation step (26). Three ozone molecules are destroyed for each primary event. In the presence of excess HO radical scavengers, ie, bicarbonate, the pseudo-first-order rate constant at 20°C for the initiation step is 175 X. This yields an ozone half-hfe of 66 min at pH 8. In distilled water = 50 mmol/L), the half-hfe is significantly lower, ie, 7 min. 

There are three commercial routes to ADN in use. The first method, direct hydrocyanation of 1,3-butadiene [106-99-0] has replaced an older process, cyanation via reaction of sodium cyanide with 1,4-dichlorobutane [110-56-5] owing to the lower cost and fewer waste products of the new process. During the initial steps of the direct hydrocyanation process, a mixture of two isomers is generated, but the branched isomer is readily converted to the linear 3-pentenenitrile [4635-87-4]. 

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

The initial step in the recycle of metals is the physical segregation of the metals from other materials. Eor new scrap this process is straightforward for old scrap it can be expensive. This classification and segregation of scrap is of importance to the producers of the metals from secondary materials. Historically, much of the classification has rehed on hand sorting which can be rehable, but it is labor intensive. The recycling of automotive scrap is illustrative of the... 

For some apphcations, eg, foam mbber, high soHds (>60%) latices are requited. In the direct process, the polymerization conditions are adjusted to favor the production of relatively large average particle-size latices by lowering the initial emulsifier and electrolyte concentration and the water level ia the recipe, and by controlling the initiation step to produce fewer particles. Emulsifier and electrolyte are added ia increments as the polymerization progresses to control latex stabiUty. A latex of wt% soHds is obtained and concentrated by evaporation to 60—65 wt % soHds. 

Ether Cle va.ge, This is commonly observed as the initial step in the metaboHsm of the phenoxy herbicides 2,4-D (1), (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), and mecoprop (17). A wide variety of bacteria have been isolated which are able to catalyze this reaction (eq. 

The initiating step in these reactions is the attachment of a group to the sulfoxide oxygen to produce an activated intermediate (5). Suitable groups are proton, acyl, alkyl, or almost any of the groups that also initiate the oxidations of alcohols with DMSO (40,48). In a reaction, eg, the one between DMSO and acetic anhydride, the second step is removal of a proton from an a-carbon to give an yUde (6). Release of an acetate ion generates the sulfur-stabilized carbonium ion (7), and the addition of acetate ion to the carbonium ion (7) results in the product (eq. 15) ... 

V-methyl aniline formed in the initial step to give the leuco base of methyl violet. Treatment with aqueous acid produces the dye. Because Michler s hydrol may also react with dimethyl aniline instead of the /V-methyl aniline to give crystal violet, commercial-grade methyl violet is usually a mixture. A cobalt complex has converted 4,T-dimethylaminodipheny1methane and dimethyl aniline in the presence of atmospheric oxygen to crystal violet in one step (50). 

Biotin is produced by a multistep pathway in a variety of fungi, bacteria, and plants (50—56). The estabUshed pathway (50,56) in E. coli is shown in Figure 6. However, Htde is known about the initial steps that lead to pimelyl-Co A or of the mechanism of the transformation of desthiobiotin to biotin. Pimelic acid is beheved to be the natural precursor of biotin for some microorganisms (51). 

Isolation. Isolation procedures rely primarily on solubiHty, adsorption, and ionic characteristics of the P-lactam antibiotic to separate it from the large number of other components present in the fermentation mixture. The penicillins ate monobasic catboxyHc acids which lend themselves to solvent extraction techniques (154). Pencillin V, because of its improved acid stabiHty over other penicillins, can be precipitated dkecdy from broth filtrates by addition of dilute sulfuric acid (154,156). The separation process for cephalosporin C is more complex because the amphoteric nature of cephalosporin C precludes dkect extraction into organic solvents. This antibiotic is isolated through the use of a combination of ion-exchange and precipitation procedures (157). The use of neutral, macroporous resins such as XAD-2 or XAD-4, allows for a more rapid elimination of impurities in the initial steps of the isolation (158). The isolation procedure for cephamycin C also involves a series of ion exchange treatments (103). 

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