Molecular weights measured by vapor

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. 

Bitumens are colloid systems, as are crude oils, and consist of the two colloidal components, petroleum resins and asphaltenes, dispersed in a dispersion medium. To investigate the composition of the system, a colloid precipitation according to Neumann [4-10] is carried out. The chemical nature of the bitumen and its components were determined by element analysis, where the atomic ratio H/C includes an indicator of the aromacity. Further characterization is performed by measuring the average relative particle mass (mean of the molecular weight M) by vapor pressure osmometry. 

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.)...

Hansen, E.H. and Schnitzer, M. Molecular weight measurements of polycarboxylic acids in water by vapor pressure osmometry. Anal Chim. Acta 46, 247-254 (1969). 

Thioacetophenone polymerizes spontaneously to a white solid. Conversion to polymer is increased by irradiation with ultraviolet light Highest conversion reported is 74%. Polymerization is also brought about by such anionic initiators as AlEt2Cl, AlEtj, BF3 - OEt, and SnC. Molecular weights measured on tetra-hydrofuran or brazene solutions using a vapor-pressure osmometer were in the 1000-2000 range. These are minimum values, since the polymer decomposes readily. 

Bruceton coal was depolymerized in refluxing phenol using p-toluenesulfonic acid (HOTs) catalyst (29). The products were 91% extractable into pyridine and had a number average molecular weight of 400 in pyridine, measured by vapor pressure osmometry. Yet most of the materials in this solution have molecular weights above 2500. 

Molecular weight measurements on the polymers were made using a Knauer vapor pressure osmometer (Utopia Instrument Co.) with pyridine as the solvent at 60°C. Additional measurements with a similar osmometer were made by Galbraith Laboratories, Inc. using chloroform as the solvent at 45°C, the osmometer calibration factor having been obtained with phenacetln. 

Polymer molecular weights may be measured by vapor phase or solution osmometry, or more usually by size exclusion chromatography (SEC or GPC). If the polymer is hydrophilic but neutral, measurements will usually be made in a polar organic solvent such as 1,4 dioxane or dimethyl formamide. If insoluble in organic solvents, measurements may be performed in water, a less than ideal solvent for this type of measurement. The situation is further complicated if dealing with a polyelectrolyte polymer (especially containing weak pH sensitive functionality). SEC measurements become a function of nominal charge... 

Functionality. The number of carboxyl equivalents was determined from the potentlometrlc acid-base titration. The number molecular weight iii of the CTPnBA was determined by the Vapor Pressure Osmometer measurement. The product of the number of acid equivalents by the molecular weight divided by the weight of the titrated PnBA sample is the calculated average number of carboxyl groups per chain of the poly n-butyl acrylate. 

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