Small molecules, molecular weight determination

Mass spectrometry has been used to determine the molecular weight of small labile molecules precisely. The analysis of macromolecules became possible following the development of novel mass spectrometry technologies, especially, Matrix-assisted laser desorption ionization time of flight mass... 

This means that drag diffusivity across membranes is sensitive to molecular weight, since molecular volume is determined by a number of factors, including the molecular weight of the molecule. Therefore, in general, large molecules will diffuse at a slower rate than small molecules. 

Finally, Figure 8 illustrates the molecular-weight distributions obtained by GPC of a number of n-heptane asphaltenes from various crude sources. As can be seen, a wide range of molecular weights can be obtained for asphaltenes from different crude sources. Furthermore, the peak in the distribution occurs at different values for the different crudes. It is well known that molecular association of asphaltene molecules can be a problem in molecular weight determination either by GPC or VPO. However, the real extent of this problem is quite problematical. Small angle x-ray scattering... 

Gel permeation chromatography (GPC) is essentially a process for the separation of polymer molecules according to their size. The separation occurs as the solute molecules in a flowing liquid move through a stationary bed of porous particles. The method has been used extensively in biochemistry to separate biological polymer molecules from small molecule contaminants (with the use of Sephadex column). Application of the method to synthetic polymer chemistry in the 1970s has revolutionized the procedures for polymer characterization and molecular weight determination. 

Another process consists of building the epoxycyclohexane unit into the bridge of a bisphenol molecule (Scheme 50) [30]. The condensation of phenol with 3-cyclohexene-l-carboxaldehyde may occur in the presense of both acid and alkaline catalysts. If oxalic acid was used as the catalyst, the yield of the bisphenol XLI reached 60%. Still higher yield was reached when excess phenol in the presence of HCl as catalyst was applied. The bisphenol XLI (melting temp. 318.5 °C) was obtained by crystallization from isopropanol. The -isomer contained small amounts of the -and o,o-isomer [31]. The structure of the product was confirmed by IR spectrophotometry and molecular weight determination. 

This article tries to draw a comprehensive picture of what has been done in the field of dendrimers with pol nneric cores, putting emphasis on synthetic issues, on accurate molecular weight determination, and, finally, on the aggregation behavior of these intriguing macromolecules both in the bulk and at solid and fluid interfaces. First attempts to manipulate individuahzed dendronized polymers on surfaces are also described. The article starts with a small chapter summarizing some of the ideas why these make up a challenging project and where these unusual molecules are expected to have impact on the natural sciences. 

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