Ionic initiators

Hygroscopic Panicle Deposition Determined by Initial Ionic Concentration... 

The second factor is the type and concentration of chemicals in soil. Soils with low initial ionic strengths favor high EO efficiencies. A lower initial ionic strength is responsible for a higher conductivity of the specimen, which in turn results in a decrease in the resistance offered to current flow, and hence the ion flow is governed more by diffusion and migration. 

The charge screening effect of Ca2+ in increasing XCPS adsorption is more clearly seen in Figure 13 for the case of SiC where the NaCl content is lower (lg/1). Due to the low initial ionic strength, no adsorption is detected for both XCPS samples at low calcium concentration. The maximum adsorption level in this case is the same as that obtained in the presence of monovalent ions only (26). 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

In the liquid phase there are many examples of ionic polymerizations of olefinic compounds induced by high energy radiation. In some, such as propylene,77 1-hexene,78 1-octene,18 and n-hexadecene-1,75 the initiating ionic species is believed to be the parent ion radical while in others such as isoprene,79 isobutylene,80 butadiene,81 and 2-methylstyrene,82 it is thought to be the carbonium ion. 

Figure 13-3 puts everything together in a spreadsheet. Input values for FKH,P04, FNaiHPOj, pA i, pKn, pK3, and pA w are in the shaded cells. We guess a value for pH in cell H15 and write the initial ionic strength of 0 in cell Cl9. Cells A9 H10 compute activities with the Davies equation. With pi = 0, all activity coefficients are 1. Cells A13 H16 compute concentrations. [HT] in cell B13 is (10 PH)/yH = (10A-H15)/B9. Cell El 8 computes the sum of charges. 

Figure 13-3 Spreadsheet solved for the system 0.025 0 m KH2PO4 plus 0.025 0 m Na2HP04 with Initial ionic strength - 0 and activity coefficients = 1. 

A zwitterionic tetramethylene initiates ionic homopolymerization, while a diradical tetramethylene initiates free radical copolymerization. As initiating species, zwitterions are likely to remain in the coiled gauche-conformation and collapse to small molecules. Diradicals, on the other hand, are easily transferred to the trans-conformation. Accordingly, diradicals are more effective initiators and more radical copolymerizations occur than ionic homopolymerizations. Addition of solvent will also influence the reaction of polar tetramethylene. A polar non-donor solvent may permit carbenium ion polymerization, while a polar donor solvent impedes it. 

In the KI/C12 reaction, an interesting effect of high defect concentration in preventing growth of nuclei of a new solid phase was disclosed [103]. Solid samples differed widely in their initial ionic conductivity and hence in their initial concentration of cation vacancies. The ones with high vacancy concentration reacted very slowly with chlorine and the colour of iodine was seen uniformly as a pale coloration of the whole solid, which had reacted to the extent of only a few percent after several days. Samples with a low vacancy concentration reacted more rapidly and a sharp boundary developed between an outer, iodine-blackened completely reacted region and a pale-coloured inner region. In the un-reactive samples, electronic (presumably positive hole) conductivity appeared transiently for a few hours and then decayed. In the reactive... 

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

Soon afterwards, in 1957, isobutene was found to undergo an ionic chain polymerization when irradiated in the liquid state (12,13), demonstrating that radiation could also initiate ionic reactions in condensed systems in which charge separation was much smaller than in a gas. These findings resulted in a dramatic change in the fundamental approach to radiation chemistry. Some of the reactions occurring in polymers were also reinterpreted since ionic processes had to be taken into account (a discussion relevant to these problems is presented in ref. 14). 

Hall has introduced an empirical test to estimate the relative importance of diradical and zwitterionic forms in tetramethylene intermediates rrans-1,4-tetramethylene diradical intermediates may initiate alternating radical copolymerizations if they add to another alkene faster than they undergo conformational isomerization to the gauche form and give a cyclobutane product through carbon-carbon bond formation, while zwitterionic 1,4-tetramethylene intermediates may initiate ionic homopolymerizations. 

The conditions for breakthrough of a portion of the mixture flow through the column are indicated here for the case when an auxiliary ion predecessor C (the initial ionic form of resin is in the C-form) is sorbed less effectively than the ions being separated and the ion displacer, D, is sorbed more effectively than ions of the mixture undergoing separation. 

Equations (40) and (45) can be used again to disclose why the halftime of ion exchange for partial conversions depends so strongly on the initial ionic composition X, q be ion exchanger (Fig. 12). If tj calculated for successive ion-exchange processes with increasing values of X o with Dg/D = - 1 (no selectivity, isotopic exchange), it is dis-... 

The heterogeneous mixture of ion exchangers consists again of n different fractions present in amounts Qj mequiv, which differ in their rate coefficients and separation factors. The initial ionic composition of each fraction is the same as before and is characterized by the equivalent fraction 0 of the counterions A. If this mixture is initially stirred in V mL of pure water, Eqs. (42) and (44), given before for the homogeneous case, are still valid. Equation (39), however, has to be modified, because in a heterogeneous mixture all n fractions will release counterions B as a result of the uptake of ions A. This yields... 

In negative-ion mode, a similar treatment holds, with snperoxide as an important initial ionic species, which is involved in ion-molecule reactions with the mobile-phase constitnents. The proton-transfer reaction leading to the deprotonated analyte molecnle [M-H] ... 

---