Secondary exclusion

The phenomenon of secondary exclusion is caused by the higher diffusion rate of the smaller polymer species compared with the larger ones. The smaller macromolecules may timely occupy the packing pores, which are otherwise accessible also for larger polymer species. Thus the elution rate of the large macromolecules is further accelerated. The term secondary exclusion was coined by Altgelt [102] and the theory of process was elaborated by Janca [103]. 

The concentration dependences of Vr in SEC are likely affected also by the viscosity effects and by the secondary exclusion [103]. 

In some cases, several secondary processes may manifest themselves simultaneously. A typical combined secondary process is the so-called concentration effect in gel chromatography, i.e., the often observed rise of the retention volumes of macromolecular substances with the increased sample concentration. The concentration effect is caused by a decrease in the macromolecular coils dimensions in solution with growing concentration, further by viscosity and osmotic effects as well as by secondary exclusion — and in the case of soft gels also by altering the pore geometry due to deswelling of gel particles in the zone of the sample. 

The phenomenon of secondary exclusion originates from the higher diffusion rate of the smaller polymer species compared with the larger ones. The smaller macromolecules may timely occupy the packing pores,... 

Parasitic processes - which include mixing, both radial and axial diffusion, viscosity effects, broadening and deformation of sample zone, as well as the secondary exclusion. 

Combined occurences - for example concentration effects that result from crowding plus viscosity plus secondary exclusion and that cause increase of retention volumes with injected polymer concentration... 

Secondary exclusion effects, i.e. size exclusion of solutes from the volume surrounding the contact points of the support particles, are of little ijtportanoe in ordinary SBC and may be included in the separation characteristics of the support. The major concern will in this case be the selection of a proper solute for the determination of the void volume (105). However, in the determination of the pore size distribution this effect must be eliminated or else corrected for (106). As shorn by Schou and Larsen (106) this effect is very small for solute sizes of vp to 3 % of the particle size. Thus for a support of 5 ym particle size a 1 % error in may be expected for solutes with a radius of 800 A. 

Pauli exclusion principle (Section 1 1) No two electrons can have the same set of four quantum numbers An equivalent expression is that only two electrons can occupy the same orbital and then only when they have opposite spins PCC (Section 15 10) Abbreviation for pyndimum chlorochro mate C5H5NH" ClCr03 When used in an anhydrous medium PCC oxidizes pnmary alcohols to aldehydes and secondary alcohols to ketones... 

Diester/Ether Diol of Tetrabromophthalic Anhydride. This material [77098-07-8] is prepared from TBPA in a two-step reaction. First TBPA reacts with diethylene glycol to produce an acid ester. The acid ester and propylene oxide then react to give a diester. The final product, a triol having two primary and one secondary hydroxyl group, is used exclusively as a flame retardant for rigid polyurethane foam (53,54). 

Secondary alcohols, such as isopropyl alcohol, j -butyl alcohol, 2-pentanol, 3-pentanol, cyclopentanol, and cyclohexanol, have been autoxidized to hydroxyaLkyl hydroperoxides (1, X = OH R = H) (10,44). These autoxidations usually are carried out at ca 20°C with uv radiation in the presence of a photosensitizer, eg, benzophenone. a-Oxygen-substituted dialkyl peroxides (2, X = Y = OH and X = Y = OOH), also are formed and sometimes they are the exclusive products (10). 

It is generally accepted that, all other things being equal, the lower the secondary creep, the better the fiber is in terms of wear, shape retention, and crease resistance. This does not mean that glass, which has no secondary creep, is better in abrasion resistance than high tenacity viscose rayon, which has secondary creep, because the respective energy absorption capacities of these two materials, exclusive of secondary creep, are not equal. Nor does it mean that fibers that exhibit secondary creep are of no value. For fabrics to meet the requirements of wear, crease resistance, and shape retention, the load and extension yield points should not be exceeded during use. 

Polypeptide chains are folded into one or several discrete units, domains, which are the fundamental functional and three-dimensional structural units. The cores of domains are built up from combinations of small motifs of secondary structure, such as a-loop-a, P-loop-p, or p-a-p motifs. Domains are classified into three main structural groups a structures, where the core is built up exclusively from a helices p structures, which comprise antiparallel p sheets and a/p structures, where combinations of p-a-P motifs form a predominantly parallel p sheet surrounded by a helices. 

Axial alcohols e.g. 60) are formed predominantly when platinum is used in moderately acidic media. The use of acetic acid alone as solvent affords the equatorial alcohol as the main product from both and 7-keto compounds. Addition of 2-10% of a strong acid e.g. hydrochloric acid) to this solvent leads to the axial alcohol as the primary, if not the exclusive, product. Primary and secondary alcohols may be converted in part to the corresponding acetates under these conditions. 

In some cases products of rearrangement are obtained either partially or exclusively on treatment of Grignard reagents with epoxides. Thus, reaction of the 2/ ,3/ -epoxide (14) with methyl Grignard reagent affords a mixture of two epimeric secondary A-nor alcohols (15) in 80% yield and the tertiary hydroxy compound, 2a-methyl-5a-cholestan-2/f-ol (16) in 15 % yield. ... 

In SEC, universal calibration is often utilized to characterize a molecular weight distribution. For a universal calibration curve, one must determine the product of log(intrinsic viscosity molecular weight), or log([7j] M). The universal calibration method originally described by Benoit et al. (9) employs the hydro-dynamic radius or volume, the product of [tj] M as the separation parameter. The calibration curves for a variety of polymers will converge toward a single curve when plotted as log([7j] M) versus elution volume (VJ, rather than plotted the conventional way as log(M) versus V, (5). Universal calibration behavior is highly dependent on the absence of any secondary separation effects. Most failures of universal calibration are normally due to the absence of a pure size exclusion mechanism. 

Secondary enamino ketones such as - -octahydro-7-quinolone (64) furnish a mixture of N- and C-alkylated bases, 120 and 121, respectively, on treatment with ethyliodide (168). Alkylation of tertiary enamino ketones as, for example, 101 proceeds exclusively on the oxygen-atom, forming product 122 (206). 

Apparently, cyclization involves the formation of open-chain intermediates 342, 343, further closing up to imidazolidines 344 and oxazolidines 345 which eliminate the secondary amine, thus leading to imidazolines 346 and oxazolines 347. The latter exist in the solution exclusively in the enolic forms 348, 349 which are stabilized by conjugation and intramolecular hydrogen bonds. 

With secondary and tertiary alkyl halides an Ea-elimination is often observed as a side-reaction. As the alkyl halide reactant an iodide is most often employed, since alkyl iodides are more reactive than the corresponding bromides or chlorides. With phenoxides as nucleophiles a C-alkylation can take place as a competing reaction. The ratio of 0-alkylation versus C-alkylation strongly depends on the solvent used. For example reaction of benzylbromide 4 with /3-naphth-oxide 5 in yV,A-dimethylformamide (DMF) as solvent yields almost exclusively the /3-naphthyl benzylether 6, while the reaction in water as solvent leads via intermediate 7 to formation of the C-benzylated product—l-benzyl-2-naphthol 8—as the major product ... 

Finally the aminoquinoline bearing a primary amine at the terminal carbon atom of the side chain is itself an effective antimalarial drug. Ring opening of 2-methyltetrahydrofuran by bromine gives the dibromide, 99. The primary halide is sufficiently less hindered so that reaction with potassium phthalimide affords exclusively the product of displacement of that halogen (100). Alkylation of the aminoquinoline with lOO affords the secondary amine, 101. Removal of the phthalimide group by means of hydrazine yields primaquine (102). ... 

At room temperature, NiAl deforms almost exclusively by (100) dislocations [4, 9, 10] and the availability of only 3 independent slip systems is thought to be responsible for the limited ductility of polycrystalline NiAl. Only when single crystals are compressed along the (100) direction ( hard orientation), secondary (111) dislocations can be activated [3, 5]. Their mobility appears to be limited by the screw orientation [5] and yield stresses as high as 2 GPa are reported below 50K [5]. However, (110) dislocations are responsible for the increased plasticity in hard oriented crystals above 600K [3, 7]. The competition between (111) and (110) dislocations as secondary slip systems therefore appears to be one of the key issues to explain the observed deformation behaviour of NiAl. 

The majority of heat-resisting alloys contain at least one element which is selectively oxidised (e.g. Cr, Si, Al). The resulting scale is highly protective, but depletion of the secondary element may occur if the diffusion rate of this element in the alloy is low. Then repetitious loss of the scale can be profoundly detrimental to long-term performance, since depletion of the alloying element near the surface may occur to such an extent that its concentration falls below the critical level for the exclusive reformation of the highly protective scale... 

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

Both primary and secondary amines add to a /S-unsaturated aldehydes and ketones to yield /3-amino aldehydes and ketones rather than the alternative imines. Under typical reaction conditions, both modes of addition occur rapidly. But because the reactions are reversible, they generally proceed with thermodynamic control rather than kinetic control (Section 14.3), so the more stable conjugate addition product is often obtained to the complete exclusion of the less stable direct addition product. 

Until about 1880 the lead-acid battery was exclusively then subject of scientific study. Possible commercial utilization lacked suitable charging processes secondary cells had to be charged by means of the primary cells already known at that time. 

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