The best steric stabilizers

The best steric stabilizers are found empirically to be amphipathic block or graft copolymers. One of the comonomers should generate a homopolymer that is nominally insoluble in the dispersion medium whereas the other should polymerize to form a polymer that is soluble in the dispersion medium. 

The idea that the best steric stabilizers are amphipa ic in character has long been recognized by manufacturers of commercial nonionic surfactants. These are classified by the empirical HLB (an acronym for hydrophilic-lipophilic balance) scheme which endeavours to scale the relative solubilities of the two contrasting components in aqueous and nonpolar dispersion media. 

The effectiveness of amphipathic polymer molecules in imparting steric stabilization can be understo if a second identical particle is imagined to approach the one portrayed in Fig. 2.2. The stabilizing moieties must be mutually repulsive if the polymer is to impart stability. In these circumstances, the Brownian collision stresses the stabUizing molecules, which endeavour to escape from the stress zone. This escape can be effected either by desorption from the particle surface or by laterd movement over the particle surface 

Virtually any polymer that is appropriately soluble in the dispersion medium is effective as a stabilizing moiety and any polymer that is insoluble in the dispersion medium is effective as an anchor polymer. Some typical stabilizing moieties and anchor polymers for aqueous and nonaqueous dispersion media are shown in Table 2.3. 

Random copolymers are usually less effective as steric stabilizers than block or graft copolymers, although it should not be overlooked that under certain conditions they are able to confer enhanced steric stabilization (see Chapter 9). 

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The best steric stabilizers are amphipathic block or graft copolymers such as poly(oxyethylene lauryl ether) (Mr 1200). Commercial nonionic surfactants are classified according to the hydrophilic-lipophilic balance (HLB), which scales the relative solubilities of the two components in aqueous and nonaqueous media. The need for the anchor part of the stabilizing molecule can be eliminated if the stabilizing moieties can be covalently bonded to the latex particles. 

The best steric stabilization will occur when the polymer covers the interface fully (sparse covering can cause build-up of attractive forces) and forms a thick unruptured film. A good solvent will dissolve the danghng chains of the polymer thoroughly. (A good solvent causes the polymer chains to stay apart in solution.)... 

The X = Y = NR2 case probably does represent the best scenario for stability for both the carbocation and the carbene. This is not only due to the strongly electron-donating character of the NR2 groups but also because the presence of four R groups provides the best steric protection. Nevertheless other heteroatoms can be considered, especially in conjunction with nitrogen, and systems with X... 

A series of well-defined A-B block copolymers of polystyrene-block-polyethylene oxide (PS-PEO) were synthesized [11] and used for emulsion polymerization of styrene. These molecules are ideal since the polystyrene block is compatible with the polystyrene formed and thus it forms the best anchor chain. The PEG chain (the stabilizing chain) is strongly hydrated with water molecules and it extends into the aqueous phase forming the steric layer necessary for stabilization. However, the PEG chain can become dehydrated at high temperature (due to the breaking of hydrogen bonds) thus reducing the effective steric stabilization. Thus the emulsion polymerization should be carried out at temperatures well below the theta (0) temperature of PEG. 

Since equatorial attack is roughly antiperiplanar to two C-C bonds of the cyclic ketone, an extended hypothesis of antiperiplanar attack was proposed39. Since the incipient bond is intrinsically electron deficient, the attack of a nucleophile occurs anti to the best electron-donor bond, with the electron-donor order C—S > C —H > C —C > C—N > C—O. The transition state-stabilizing donor- acceptor interactions are assumed to be more important for the stereochemical outcome of nucleophilic addition reactions than the torsional and steric effects suggested by Felkin. 

Finally we learned that if we analyze the first factor (substrate), we will find two effects at play electroiucs and sterics. We saw that Sn2 reactions require primary or secondary substrates because of sterics—it is too crowded for the nucleophile to attack a tertiary substrate. On the other hand, SnI reactions did not have a problem with sterics, but electronics was a bigger issue. Tertiary was the best, because the alkyl groups were needed to stabilize the carbocation. 

Figure 2.22. (a) Disjoining pressure vs. thickness isotherm for an emulsion film stabilized by 0.1% BSA, ionic strength of 10 mol/1 NaCl, oil phase = hexadecane. The dots are the experimental data, dashed line is the double-layer contribution to the total disjoining pressure, and the solid line is the best fit done supposing additivity of the double-layer and steric forces, (b) Force vs. distance profiles for ferrofluid emulsions stabilized with mixed BSA-Tween-20 adsorption layers. The total concentration of the Tween-20 is kept constant = 5CMC, pH = 5.8. (Adopted from [78].)... 

Propagation, or chain growth, takes place in a head-to-tail configuration as a result of resonance stabilization and steric factors by carbocation (M+) addition to another monomer molecule. The head stabilizes the cation best so it is the growing site while the least sterically... 

Stability, activity and chemo- and enantioselectivity increased with increasing steric demand of the ortho substituent R. Introduction of the trimethylsilyl group at this position (ligand 38) therefore resulted in an excellent enantioselective system which belongs among the best Pd catalysts described so far for asymmetric hydrovinylation. Almost 70% conversion was observed within 15 min. The product was obtained in 78.5% ee and only a small amount of the isomerization products was detected in the reaction mixture. However, at higher conversions, isomerization of the product to the internal achiral olefin took place. Therefore,... 

Structures. The methyl radical is planar and has D symmetry. Probably all other carbon-centerd free radicals with alkyl or heteroatom substituents are best described as shallow pyramids, driven by the necessity to stabilize the SOMO by hybridization or to align the SOMO for more efficient pi-type overlap with adjacent bonds. The planarity of the methyl radical has been attributed to steric repulsion between the H atoms [138]. The C center may be treated as planar for the purpose of constructing orbital interaction diagrams. 

A study of the scope of the reaction has shown that mono- and disubstituted alkenes and, particularly, aryl-substituted alkenes are the best substrates. Various limitations have been noted, some not unexpected (sensitivity to steric effects), others quite surprising (complex reactivity of cyclohexenes). Nevertheless, the exceptionally high stability of the reagent should make it available from the shelf, and in appropriate cases its use is to be considered as an alternative to the Simmons-Smith reaction. 

The stability of heparin-amine complexes depends to a large extent on the structure of the constituent amine. The best results were obtained for dimethylaniline and pyridine (Table 6) 65). The amount of bound heparin is, as is seen, proportional to the amount of immobilized amine, while the molar ratio amine/heparin is predetermined by the structure of the surface. Microscopically rough surfaces were shown to prevent association between heparin and amine as the portion of the amine is sterically inaccessible for the macromolecules of heparin. 

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