Molecular weight determination vapor-phase osmometry

Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthaUc, or pyromellitic anhydride (264). Eor lower molecular weights (higher hydroxyl numbers), E- and C-nmr methods have been developed (265). Molecular weight deterrninations based on coUigative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

Table XVII. Reproducibility of Molecular Weight Determinations by Vapor-Phase Osmometry (Butarez CTL Lot 18)...

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. 

Fig. 8.1.5. Calibration of styragel columns with polystyrene standards ( ) and with lignin samples (o). The molecular weights of the latter were determined by vapor phase osmometry. (Mansson 1981)...

The sizes determined in this work are the apparent molecular sizes and not necessarily the sizes of the asphaltene and maltene molecules at process conditions. Association efforts for asphaltene molecules have been observed for both vapor-phase osmometry molecular weight and viscosity measurements (14, 15). The sizes reported here were measured at 0.1 wt % in tetrahydrofuran at room temperature. Other solvent systems (chloroform, 5% methanol-chloroform, and 10% trichlorobenzene-chloroform) gave similar size distributions. Under these conditions, association effects should be minimized but may still be present. At process conditions (650-850°F and 5-30% asphaltene concentration in a maltene solvent), the asphaltene sizes may be smaller. However, for this work the apparent sizes determined can be meaningfully correlated with catalyst pore size distributions to give reasonable explanations of the observed differences in asphaltene and maltene process-abilities (vide infra). In addition, the relative size distributions of the six residua are useful in explaining the different processing severities required for the various stocks. Therefore, the apparent sizes determined here have some physical significance and will be referred to just as sizes. 

Molecular weight determinations by a technique related to vapor-phase osmometry indicated that the Ti(tartrate) is dimeric [490], so n = 2. The kinetic rate expression is as follows [489] ... 

The functionality / of low-molecular-weight functionalized prepolymers (e.g., polyether polyols used in the preparation of polyurethanes) is often determined from Eq. (4.31) by combining functional group analysis with another suitable method of molecular weight determination such as vapor phase osmometry. Note that in this respect the functional groups do not have to be end groups. 

The polymer molecular weights were determined by vapor-phase osmometry in chloroform using polystyrene standards of molecular weight 6 X 102, 2.9 X 103, and 3.6 X 103. The results are also shown in Table II and range from 2 X 103 to 8 X 103, which is the general range for this type of cyclopolymerization (2,17) unless multifunctional monomers are present. 

The molecular weights were determined at Galbraith Labs in Knoxville, Tenn using vapor-phase osmometry in DMF. 

Since cannot be derived theoretically, Ke is usually determined by calibration with substances of known molecular weight. As with all the other molecular-weight-determination methods, only apparent molecular weights Mapp are obtained for finite concentrations (see Sections 6.4 and 6.5) because of the effects of the virial coefficients and/or association, Mapp must therefore be extrapolated to the concentration C2 -> 0. In vapor-phase osmometry, low amounts of nonvolatile impurities interfere with the result, but volatile impurities do not, since they pass into the vapor phase. 

Average molecular weight determination, e.g., by vapor phase osmometry [199]... 

The pol3nners were white or pale yellow powders and were thermally decomposed above 240 C without melting. The molecular weight of the polymers was determined by vapor phase osmometry (VPO), Table 2. All polymers were Insoluble in water, slightly soluble in methanol and n-butanol, soluble in chloroform and 2,2,2-trifluoroethanol (TFE), and very soluble in trifluoro-acetic acid (TFA). 

Molecular weights of oligomer fractions are determined by sulfur analysis, making use of the assumption that each oligomer molecule contains one octyl mercaptan residue This assumption is adopted as valid on the basis of comparison with a few results from vapor phase osmometry, shown in Table I ... 

Polychelates of chlorendic acid [86] are prepared by bubbling oxygen through a mixture of the acid and metal in acetone (130). Copper-, manganese-, and lead-containing polymers were reported. Molecular weights of 1700 to 1900 were determined by vapor-phase osmometry on the polymers. Elemental analyses supported a cyclic structure for the oligomer. 

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