Molecular weight from osmometry

The details of the measurements made on these samples are discussed elsewhere (4, 5). The certified values from the certificates for all the samples are shown in the Appendix. For SRM 705 the number-average molecular weight by osmometry, 170,900, and the weight-average... 

FIGURE 7.8 Determination of number average molecular weight by osmometry. [Graph reconstructed from data by Roughton and Kendrew, Haemoglobin, Butterworth, London, 1949, p. 197.]... 

There are now several types of automatic osmometers that operate with essentially zero flow and that reach equilibrium very rapidly, usually within minutes. Osmotic equilibrium depends on an equal and opposite pressure being developed. The critical part of their design relates to the method of automatic adjustment of the osmotic pressure of the solution side so that the activity of the two sides is equal. Since several concentrations usually need to be run, the time required to determine a molecular weight by osmometry has been reduced from a week to a few hours by these automatic instruments. 

It should be mentioned that direct measurements of PVOCCl molecular weights are difficult since the chloroformate groups are very polar and reactive. The quantitative modification of PVOCCl by N-methylbenzylamine leads to a polymer which is soluble and stable in toluene and in THF. Thus it is possible to measure its molecular weight by osmometry and by GPC and to deduce that of the starting PVOCCl Moreover the values of M determined from... 

The molecular weight distributions of the oligomers were determined by means of gel permeation chromatography. These were done by Dr. Julian F. Johnson who was then at the Chevron Research Company. A Waters Analytical G.P.C. Model 300 was used. A combination of one 100,000 R, one 15,000 A, one 100 R, and one 45 R column was used. The columns were calibrated with normal alkanes and monodisperse polystyrenes. Samples were dissolved in toluene to obtain 60 mg/100 ml concentration they were eluted with toluene at a flow rate of 1 ml per minute at room temperature. The results were machine-computed to obtain relative molecular weights. From these, absolute values of the molecular weights were obtained by means of scaling factors calculated from experimental viscosity or vapor pressure osmometry data. The calibration curve is shown... 

There are data on the reactions of thin films of lanthanoids with naphthalene in ether solution at room temperature [72]. Cerium and europium under these conditions easily dissolve to give colourless solutions, from which pale brown solids can be obtained after removal of the solvent and the excess of naphthalene in vacuum. The solids readily decompose on heating in vacuum. The molecular weight measurements (osmometry), the H NMR spectra and the comparison of UV spectra of these products (Ce 350, 375, 407, 435 nm, Eu 350, 375, 405, 450 nm) with the spectra of (CioHg)2Cr suggested the bis(naphthalene)lanthanoid structure for the obtained compounds ... 

The measurement techniques most frequently used are derived from Raoult s and Van t Hoff s laws applied to cryometry, ebulliometry, osmometry, etc. They are not very accurate with errors on the order of ten per cent. Consequently, the molecular weight is often replaced by correlated properties. The mean average temperature or viscosity can thus replace molecular weight in methods derived from ndM. 

In these unit conversions on H, we have used the facts that 1 atm = 760 Torr and the ratio of densities PHg/ soin - /Psoin t onverts from Torr to millimeters of solution. These numerical examples show that experiments in which Apj, ATf, or ATj, are measured are perfectly feasible for solutes of molecular weight 100, but call for unattainable sensitivity for polymeric solutes of M = 10 . By contrast, osmometry produces so much larger an effect that this method is awkward (at least for 1% concentration) for a low molecular weight solute, but is entirely feasible with the polymer. 

However, even homopolymer molecular weight aver calculated from GPC frequently do not agree with those from osmometry and li t scattering or for that matter with those from another set of GPC operating conchtions Copolymers... 

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 ... 

Of the preponderance of small ions, the colligative properties of polyelectrolytes in ionising solvents measure counterion activities rather than Molecular weight. In the presence of added salt, however, correct Molecular weights of polyelectrolytes can be measured by membrane osmometry, since the small ions can move across the membrane. The second virial coefficient differs from that previously defined, since it is determined by both ionic and non-ionic polymer-solvent interactions. 

Absolute methods provide the molecular weight and the degree of polymerization without any calibration. Their calculation from the experimental data requires only universal constants such as the gas constant and Avogadro s number, apart from readily determinable physical properties such as density, refractive index, etc. The most important methods in use today are mass spectrometry, osmometry, light scattering, and - to some extent - sedimentation and diffusion measurements. Also, some chemical and spectroscopic methods (determination of end-groups) are important because of their relative simplicity. 

Dextrans of low molecular weight have also been fractionated by gel chromatography. Bremner and coworkers,67 in preparing (by alkaline degradation) dextrans of low molecular weight suitable for incorporation in the clinically important iron-dextran complex, fractionated their product on Sephadex G-50, with (O M sodium chloride as the eluant. Separate fractions, ranging in Mn (as determined by osmometry) from 1510 to 4860, were obtained in this way. 

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