Addition cationic

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

Many of the chemical reactions used to modify lignosulfonates are also used to modify kraft lignins. These include ozonation, alkaline—air oxidation, condensation with formaldehyde and carboxylation with chloroacetic acid (100), and epoxysuccinate (101). In addition, cationic kraft lignins can be prepared by reaction with glycidjiamine (102). 

Tsikaeva et al. [290,291] investigated the Raman and IR absorption spectra of hydrofluoric acid solutions containing a wide range of concentrations of niobium, but no other additional cations. 

Suppose we have a solution that contains lead(II), mercury(I), silver, copper(II), and zinc ions. The method is outlined in Fig. 11.20, which includes additional cations, and is illustrated in Fig. 11.21. Most chlorides are soluble so, when hydrochloric acid is added to a mixture of salts, only certain chlorides precipitate (see Table 11.4). Silver and mercury(I) chlorides have such small values of Ksp that, even with low concentrations of Cl ions, the chlorides precipitate. Lead(II) chloride, which is slightly soluble, will precipitate if the chloride ion concentration is... 

Consider that the test solution 1 contains an additional cation B2+ that is identically or less hydrophilic than cation B. Then the exchange reaction... 

X-ray powder patterns showed that the product of this reaction is indeed isomorphous with Zn2Mo308 and hence is the desired 7-electron cluster derivative. Unfortunately single crystals for a complete structure determination have not been obtained. Subsequent work (17) however showed that additional cations could... 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

In this section we are concerned with the properties of intrinsic Schottky and Frenkel disorder in pure ionic conducting crystals and with the same systems doped with aliovalent cations. As already remarked in Section I, the properties of uni-univalent crystals, e.g. sodium choride and silver bromide which contain Schottky and cationic Frenkel disorder respectively, doped with divalent cation impurities are of particular interest. At low concentrations the impurity is incorporated substitutionally together with an additional cation vacancy to preserve electrical neutrality. At sufficiently low temperatures the concentration of intrinsic defects in a doped crystal is negligible compared with the concentration of added defects. We shall first mention briefly the theoretical methods used for such systems and then review the use of the cluster formalism. 

These bimolecular reactions have provided accurate proton affinities (PAs) for many amines165,166. In addition, cation affinities are accessible, usually by combining the enthalpy of formation (AH[) of cationic species derived from PA measurements with similar data for the bare cation. Thus, the knowledge that the PA of CH3NH2 is 896166 kJmol-1 sets A//f(CH3NH3 + ) = 611 kJmor1. Since A//f(CH3+) = 1092 kJmol-1 and A//f(NH3) = —46 kJmol-1 9, the methyl cation affinity of NH3 may be deduced to be 1092 — 46 — 611 = 435 kJ mol-1. 

Additional cation stabilization by another heteroatom is also available in 2-acylmethylene azolines [255]. The protonated forms... 

It is readily seen from the intermediate addition cations and their resonance structures that attack at... 

C-2 or C-A will be unfavourable, in that one of the resonance forms features an unstable electron-deficient nitrogen cation. Attack at C-3 is the more likely, simply based on an inspection of resonance structures for the addition cation. However, electrophilic attack still tends to be unfavourable, because many electrophilic reagents, e.g. HNO3-H2SO4, are strongly acidic, and the first effect is protonation on nitrogen. Attack of E+ on to a positively... 

This may reflect that there is more charge dispersion in the addition cation from attack at C-2 than there is from attack at C-3. This is, of course, exactly the same argument as used above for C-protonation protonation (pyrrole acting as a base) also occurs at... 

This may be rationalized by considering the stability of intermediate addition cations. When the electrophile attacks at C-5 or C-8, the intermediate cation is stabilized by resonance, each having two favourable forms that do not perturb the aromaticity of the pyridinium system. In contrast, for attack at C-6 or C-7 there is only one such resonance form. We used similar reasoning to explain why naphthalene... 

Now the electrophilic attack of the aromatic system onto the acylium cation. The product is para substituted, so go for that site of attack. We then need removal of a proton, and chloride is the obvious base, but since this is complexed as AlCU, we need the latter to decompose to chloride and AICI3. We should also show resonance stabilization of the addition cation, delocalizing the charge around the ring this helps with the alternative product question. 

The interaction of phosphate with d and Na is shown in Figure 11.11. Upon adsorption of phosphate onto goethite, d adsorption dropped to zero, whereas adsorption of Na increased substantially (Nanzyo and Watanabe, 1981, 1982). It was suggested that adsorption of phosphate is followed by adsorption of additional cations at pH >7. 

Cations differ from ligands in that they influence the crystallization of ferrihydrite over a wider pH range than do ligands. They usually require mol ratios (M/(M + Fe)) of 0.05-0.1 to influence the kinetics and products of the reaction, whereas ligands are often effective at hundredfold lower concentrations. In addition, cations are often incorporated in the iron oxide structure (see Chap. 3). The effects of Ti, VO , Pb ", Cr , and the first row divalent transition elements have been investigated. These effects vary widely, although retardation predominates. 

Platinum-diphosphine complexes in [C4CiIm][PPg] have been studied via ip NMR [67,68]. Here, the insertion reaction of SnClj into the Pt-Cl bond of PtCljCbdpp) was observed, as well as the formation of an additional cationic complex. The structure of the catalyst could be elucidated via characteristic Jpp and Jpt p coupling constants. 

High acid ionomers are neutralized to various extents by several different types of metal cations, such as by manganese, lithium, potassium, calcium and nickel cations. Several types can be blended. It has been found that these by additional cations neutralized high acid ionomer blends produce compositions exhibiting enhanced hardness and resilience due to synergies, which occur during processing (12). 

Effect of cations other than TPA has also been studied and the results are given in Table 2 and Fig.3. The molar ratio of additional cation/CTA" was kept at 1.4. It can be seen that cations like tetramethylammonium or tetraethylammonium ions or even sodium also gives highly ordered MCM-41 structure The BET surface area of these samples was found to be >1000 m2/g. Fig.3 shows that the MCM-41 structures obtained with these additional cations have improved hydrothermal stability. The long range ordering was unaffected by 4 days of hydrothermal treatment. Moreover, the surface area and pore volume of the water-treated samples was only marginally lower than that of the calcined samples. 

Mesopore size distribution analysis showed a very narrow distribution with a peak pore diameter of about 27 A for the calcined samples. It can be seen that the pore wall thickness of the calcined samples was in the 18-19 A and did not change much by adding additional cations. The increased hydrothermal stability observed was, therefore, not due to any increase in pore wall thickness. For the water-treated samples, the adsorption isotherms and pore size distribution curves showed that after hydrothermal treatment the pore size distribution was... 

Physical properties of MCM-41 samples prepared with different additional cations... 

Si MAS NMR spectra of the uncalcined MCM-41 samples synthesized normally and with TPA+ and Na+ are shown in Fig 4. It was observed that the ratio of Q4/Q3 peaks was higher in samples synthesized with additional cations. The effect was most pronounced with TPA" as the additional cation. The higher Q4/Q3 ratio indicates that the silicate polymerization during the formation of the mesostructure was enhanced by the presence of the additional cations, Upon calcination, the free silanol groups are forced to condense to form Si-O-Si bond and 29Si MAS NMR of the samples showed predominantly Q4 peak. However, these... 

Fig.3 XRD patterns of MCM-41 samples synthesized with different additional cations (a) calcined samples, (b) after hydrothermal treatment (cation/surfactant = 1.4)...

This method of synthesizing MCM-41 with improved hydrothermal stability is applicable with surfactants of different chain length also XRD patterns of the calcined samples synthesized with surfactants Cu and Cm chain length with TPA as additional cation and without any cations are shown in Fig 5. XRD patterns of the hydrothermally treated samples are also included for comparison It can be seen that when no TPA was added to the synthesis... 

Fig.5 XRD patterns of MCM-41 samples synthesized with Cm and Cis surfactants without any additional cation (a) and (b), and with TPA+ as additional cation (c) and (d). Calcined samples (a) and (c), water-treated samples (b) and (d).

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