Vapor number average molecular weights

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 number average molecular weights, Mq, of the polymers obtained in Table II ranged from 1300 to 3900 as determined by vapor nressure osmometry (VPO), and they were further characterized by - -H- and C-NMR spectrometry at ambient temperature and at 90°C, as well as by IR spectroscopy. The homopolymers of isopropenylferro-cene were found to have the expected structure, P, shown below, obtained by polymerization through the isopropenyl units, as indicated by spectroscopic characterization ... 

Number-average molecular weight was measured in 1,2-di-chloroethane at 40°C using a Wescan vapor pressure osmometer. 

Determination of Molecular Weight. The number average molecular weight (TEi) was measured by the vapor pressure equilibrium method with a Hitachi Molecular Weight apparatus (Model 117). 

Raoult s law The vapor pressure of a solvent in equilibrium with a solution is equal to the product of the mole fraction of the solvent and the vapor pressure of the pure solvent this relationship is used in obtaining number-average molecular weights. 

The objective of this present work was to investigate the feasibility of using GPC/DV for absolute molecular weight determination of hydroxypropylated lignins. In order to verify the validity of the universal calibration method, vapor phase osmometry (VPO) was used to provide reference number average molecular weight values. Comparisons with LALLS results have also been made and will be reported in another publication. 

Vapor Phase Osmometry. A Wescan Model 233 vapor phase osmometer was used to obtain number average molecular weights. The lignin solutions were made up with HPLC grade tetrahydrofuran (THF) and shaken manually until the solutions were clear. The experiments were conducted at 30°C. Number average molecular weights were determined by multistandard calibration (41), a procedure found to greatly enhance reproducibility and accuracy of the results. Experiments were conducted immediately after sample preparation and three days later. 

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

Number average molecular weight was determined in benzene solution by Hitachi 117 vapor pressure osmometer(VPO) at 42°C. 

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.)...

NUMBER AVERAGE MOLECULAR WEIGHT (MN) 260 BY VAPOR PHASE OSOMETRY... 

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. 

The number average molecular weight was measured in benzene solution by Mechrolab membrane osmometer 502 and also by Hitachi vapor pressure osmometer 115. 

Several vapor pressure osmometers are now commercially available. Although they are mainly used for determining number-average molecular weights in aqueous and organic solvents, they can also be employed to evaluate the total osmolality of biological solutions or dissociation and activity coefficients. Each model has its own technical characteristics. However, all are comparable in terms of general measurement procedure and sensitivity. 

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

The enthalpy change for this polymerization is AWp = —6.5 Real mor. The polymerization reaction in this problem is finished at a fixed steam pressure (1 atm). The equilibrium concentration of H2O in the polymer melt varies with temperature and steam pressure in this case. Tlte enthalpy of vaporization of H2O is about 8 Real mol . Compare the limiting values of number average molecular weight of the polyamide produced at 280 and 250°C final polymerization temperatures. Hint Recall that the variation of an equilibrium constant K with temperature is given by r/(ln K)/d /T) = —AH/R, where AH is the enthalpy change of the particular process and R is the universal gas constant. Calculate Ki and the equilibrium concentration of H2O in the melt at 250°C and use Eq.(10-8).]... 

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