Bulkiness effect

Knowing the deleterious effects of ionising radiations on PTFE, much attention was paid on their effects on PVDF although the latter had been told to behave well under irradiation. As an example, PVDF multifilament yarns can be y irradiated up to the absorption of 80 kGy without any effect on their Young s modulus [38], However, the structure of the polymer was somewhat modified as its energy to break kept constant and even increased by about 50% while the adsorbed dose was less than 8 kGy but decreased for higher adsorbed doses, down to 1/3 of its maximum value (1/2 of its initial value) when the adsorbed dose reached 81 kGy. To avoid these bulky effects, the interest of... 

Ionic Nature Cationic Uses Surfactant softener for textiles Features Exc. elastic and bulky effect Properties Liq. 14% act. 

With a standard image intensifier, characterized by a gain of more than x10,000, the quantum well effect is clearly avoided. Nevertheless this very high gain reduces the image dynamics unless strong attenuation is introduced at the tube output (iris or neutral filter). Also a standard intensifier is bulky, affected by pincushion distortion and magnetic fields which can be a serious limitation in some applications. 

The effect of ligands on the endo-exo selectivity of Lewis-acid catalysed Diels-Alder reactions has received little attention. Interestingly, Yamamoto et al." reported an aluminium catalyst that produces mainly exo Diels-Alder adduct. The endo-approach of the diene, which is normally preferred, is blocked by a bulky group in the ligand. 

The reaction corresponds to a proton transfer and not to a net formation of ions, and thus the AS is of minor importance in the whole series, especially for the two t-Bu derivatives. This last effect is believed to be due to a structure-promoting effect of the bulky alkyl groups in the disordered region outside the primary hydration sphere of the thiazolium ion (322). 

In the foregoing discussions of theoretical models and experimental results, we have focused on linear polymers. We have seen the effect of chain substituents on viscosity. All other things being equal, bulky substituents tend to decrease f and thereby lower 17. The effect is primarily due to the opening up of the liquid because of the steric interference with efficient packing arising from the substituents. With side chains of truly polymeric character, the picture is quite different. 

The bulkiness of the substituent groups increases moving down Table 4.2. Also moving down the table, the melting points decrease, pass through a minimum, and then increase again. As is often the case with reversals of trends such as this, there are two different effects working in opposition in these data ... 

The effect of a bulky substituent like a phenyl group on the hydrocarbon chain apparently decreases chain flexibiUty sufficiently to allow more intimate alignment between molecules, less free volume, and therefore a high value for Tg. 

In the methacrylate homologous series, the effect of side-chain bulkiness is just the opposite. In this case, however, the pendant groups are flexible and offer less of an obstacle to free rotation than the phenyl group in polystyrene. As chain bulk increases, molecules are wedged apart by these substituents, free volume increases, and Tg decreases. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

Synthesis. Iminoboranes, thermodynamically unstable with respect to oligomerization can be isolated under laboratory conditions by making the oligomerization kineticaHy unfavorable. This is faciUtated by bulky substituents, high dilution, and low temperatures. The vacuum gas-phase pyrolysis of (trimethylsilylarnino)(aLkyl)haloboranes has been utilized as an effective method of generating iminoboranes RB=NR as shown in equation 19 for X = F,... 

Hydrolysis reactions involving tetrahedral intermediates are subject to steric and electronic effects. Electron-withdrawing substituents faciUtate, but electron-donating and bulky substituents retard basic hydrolysis. Steric effects in acid-cataly2ed hydrolysis are similar to those in base-cataly2ed hydrolysis, but electronic effects are much less important in acid-cataly2ed reactions. Higher temperatures also accelerate the reaction. 

AT-Oxidation is very sensitive to steric effects, since 1-substituted lumazines and pterins give only 5-oxides and the presence of bulky substituents at position 7 also directs oxidation to N-5. The pteridine 5-oxide (52) and 8-oxide (53) and the 5,8-dioxide (55) contain the AT-oxide groups as such, even when the possibility of AT-hydroxy tautomers exists, as in (53) i(54). 

The pK values are approximately additive and a linear relationship of the type pKT " -pK° + Y. pKm holds for the whole set (pK° is the pK. of pyrazole itself and is the effect of a substituent m at position n). Deviation from the additivity is found when two bulky substitu ts are in contiguous positions. Instead of discussing pK, values, the authors consider ApisTm which are mean values and thus more significant since they correspond to several pairs of compounds. 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

Polycarbonates have also been prepared from diphenyl compounds where the benzene rings are separated by more than one carbon atom. In the absence of bulky side groups such polymer molecules are more flexible and crystallise very rapidly. As is to be expected, the more the separating carbon atoms the lower the melting range. This effect is shown in data supplied by Sehnell" Table 20.11). 

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