Number vapor pressure osmometry

Among the techniques employed to estimate the average molecular weight distribution of polymers are end-group analysis, dilute solution viscosity, reduction in vapor pressure, ebuUiometry, cryoscopy, vapor pressure osmometry, fractionation, hplc, phase distribution chromatography, field flow fractionation, and gel-permeation chromatography (gpc). For routine analysis of SBR polymers, gpc is widely accepted. Table 1 lists a number of physical properties of SBR (random) compared to natural mbber, solution polybutadiene, and SB block copolymer. 

However, because of the high price, MALDI-TOF mass spectrometers have not come into wide use. Vapor pressure osmometry (VPO), an old and traditional method for estimating molecular weight, is useful in the field of CPO chemistry. The experimental error of this measurement is approximately 10% however, the obtained data are sufficiently useful to estimate the number of porphyrins in a molecule. 

Vapor Pressure Osmometry - The number average molecular weights of polymers with Mn < 20,000 were determined using "Model 232A Molecular Weight Apparatus, Wescan Instruments, Inc., 3018 Scott Blvd., Santa Clara, CA 95050". Toluene was used as solvent and the instrument was calibrated using polystyrene of Mn 9,000 and 20,400. 

The association number of Li amides, such as LrN(SiMe3)2, organolithium compounds bound at the a-position to S or Se atoms, such as LiCH2SePh, and various transition metal complexes was determined at 0°C in THF or at —35°C in Et20, by differential vapor-pressure osmometry. The method allows handling compounds sensitive to autooxidation, moisture and temperature . ... 

Vapor Pressure Osmometry. Number-average molecular weights were evaluated with a vapor pressure osmometer (Knauer) following a previously described method (18). 

Most of the work on the boric acid-diol reaction during the last twenty years has been done to determine the coordination number of the diol (number of diol molecules) in the complex and to evaluate the equilibrium constant (often called a stability constant) for a number of diol-boric acid reactions. Several techniques have been used to study these questions, including polarimetry (7), optical rotatory dispersion (8), polarography (9), conductivity (3), vapor pressure osmometry (10), and electrochemistry (II, 12, 13). The most frequently studied system has been the electrochemical (pH) titration of boric acid or borax solutions with various diols. 

To confirm the occurrence of the polymorphic transition predicted in the section 4 and to elucidate the mechanism, it is primarily necessary to clarify the enantiomeric assembly mode in the first-formed metastable crystal prior to the polymorphic transition and compare it with the stable crystal structure after the polymorphic transition with respect to a compound showing Preferential Enrichment. Since it is very possible that the stable molecular assembly structure in solution would be retained in the crystalline phase first-formed by crystallization from the same solvent,20 at first, we have investigated the enantiomeric association mode in solutions of the racemates showing Preferential Enrichment. Consequently, in our case, the variable temperature H NMR technique proved to be inapplicable to deciding which molecular association mode is more stable in solution, homochiral or heterochiral.21 Instead, the combined use of the solubility and supersolubility measurements under various conditions and the number-averaged molecular weight measurement by vapor pressure osmometry turned out to become a potent tool for this objective. 

Figure 10. Number-averaged molecular weight measurement of the racemate and nonracemic sample of ST in CHC13 at 30°C by vapor pressure osmometry. (Reprinted with permission from ref 18. Copyright 2003 American Chemical Society.)...

E> can be measured directly by membrane or vapor pressure osmometry. The application of an alternative method was described recently [64, 65]. It is based on an analysis of the sedimentation equilibrium in an analytical ul-tracentrifuge, where the solution contains the polyelectrolyte as well as a small concentration of an inert salt. In sedimentation equilibrium, the concentration gradients of both components are coupled via a Donnan-type equilibrium, which is governed by the effective charge number zeff of the polyion. Both concentration gradients can be determined in one experiment, when the polyion and the coion of the salt have sufficiently separated absorption bands in the UV or visible range. 

Infrared spectra were recorded in carbon tetrachloride in 5 mm KBr liquid cells or as a thin film between NaCl plates on a Beckman IR-20 infrared spectrometer. The number-average molecular weights were determined by vapor pressure osmometry in methylene chloride solutions (3-8 g/1) (23). 

In spite of the high reactivity of the Si -Cl bond towards nucleophiles (such as styryl carbanions) it could be feared that the double bond would be attacked by the remaining carbanions. This side reaction apparently does not take place. Thus Greber showed that the number average molecular weights obtained by double bond analysis and by vapor pressure osmometry agree which each other. 

Table 8.3.4. Number-average molecular weights of different kraft lignins determined by vapor pressure osmometry...

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