On-line GPC

Gel permeation ehromatography (GPC)/normal-phase HPLC was used by Brown-Thomas et al. (35) to determine fat-soluble vitamins in standard referenee material (SRM) samples of a fortified eoeonut oil (SRM 1563) and a eod liver oil (SRM 1588). The on-line GPC/normal-phase proeedure eliminated the long and laborious extraetion proeedure of isolating vitamins from the oil matrix. In faet, the GPC step permits the elimination of the lipid materials prior to the HPLC analysis. The HPLC eolumns used for the vitamin determinations were a 10 p.m polystyrene/divinylbenzene gel eolumn and a semipreparative aminoeyano eolumn, with hexane, methylene ehloride and methyl tert-butyl ether being employed as solvent. 

On-line GPC technique for measurement of peak molecular weight of polystyrene... 

Perhaps the most revolutionary development has been the application of on-line mass spectroscopic detection for compositional analysis. Polymer composition can be inferred from column retention time or from viscometric and other indirect detection methods, but mass spectroscopy has reduced much of the ambiguity associated with that process. Quantitation of end groups and of co-polymer composition can now be accomplished directly through mass spectroscopy. Mass spectroscopy is particularly well suited as an on-line GPC technique, since common GPC solvents interfere with other on-line detectors, including UV-VIS absorbance, nuclear magnetic resonance and infrared spectroscopic detectors. By contrast, common GPC solvents are readily adaptable to mass spectroscopic interfaces. No detection technique offers a combination of universality of analyte detection, specificity of information, and ease of use comparable to that of mass spectroscopy. 

The static laser light scattering apparatus used as an on-line GPC detector has been popular for a while. Here, we illustrate another but less known method of combining the results from (gel permeation chromatography) and DLS. The basic principle is as follows There is a similarity between these two tools in that the translational diffusion coefficient D obtained by DLS and the elution volume V in GPC are related to the hydrodynamic size of a given macromolecule. In a first approximation, if the hydrodynamic size is proportional to the molar mass, we have... 

The possibilities for analysis of oligomeric systems can be demonstrated by considering chlorinated materials [13]. On-line GPC-NMR has been applied to mixtures of the following chlorinated oligomers ... 

Figure 7.1.13 On-line GPC-NMR data obtained for a mixture of chlorinated oligomers...

In conclusion, on-line GPC-NMR coupling provides a fast and efficient method for the characterization of polymers in general and, in particular, for copolymers of different chemical compositions and stereoregularity. 

Hatada, K. Ute, K. Yamamoto, M. Nishimura, T. Kashiyama, M. On-line GPC/NMR analyses of block and random copolymers of methyl and butyl methacrylates prepared with tert-butylmagnesium bromide. Polym. Bull. 1989,21, 489 95. 

Recent developments in the sensitivity and resolution of NMR spectrometers permitted us to use one as a real-time detector for high-performance liquid chromatography as demonstrated in recent review articles.332,333 Recently we developed an on-line GPC/NMR system in which a 500-MHz 1H NMR spectrometer was used as a detector.334-339 By using on-line GPC/NMR the molecular weight of a polymer can be determined without a calibration curve if the polymer sample contains a known amount of end-group per polymer molecule. 

The molecular weight dependence of polymer structures such as tacticity or copolymer composition can also be easily evaluated by on-line GPC/NMR. Data on the molecular weight dependence of polymer structures are very useful for understanding the mechanism of polymerization or the properties of polymers, and are usually collected by the fractionation of the polymer and the subsequent structural analysis of each fraction. However, this method requires a lot of time and rather a large polymer sample. The on-line GPC/NMR method is very useful for this purpose it needs only a few hours and a very small sample (0.5 1.0mg). 

Our on-line GPC/NMR system consists of a JASCO TRI ROTAR-V chromatograph and a JEOL JNM-GX500 spectrometer (a 16-bit A/D converter was installed). A 2- or 3-mm (i.d.) glass tubing with tapered structure at both ends is employed as an NMR observation flow cell (Fig. 30) the detection volumes are 0.060 and 0.140 ml, respectively. A similar type of flow cell was used in the on-line HPLC/NMR experiments.340,341 The tapered structures at both ends are needed to prevent turbulent flow in the inlet path of the cell, which would cause broadening of the elution band. When a cell without tapering was used, some broadening of the band was observed.334... 

When the log(Mn) determined from the H NMR spectrum in each file is plotted against the elution time, the plots for all three PMMA samples fall on a single straight line. The linear relation of the log(Mn) versus elution time obtained by the on-line GPC/NMR experiment should be the most accurate calibration curve for highly isotactic PMMA. The Mw/Mn values calculated from the GPC/NMR method agreed well with those obtained from the normal GPC/RI method using standard polystyrenes. 

The on-line GPC/NMR technique is also useful for studying the molecular weight dependence of polymer properties such as tacticity and copolymer composition. This type of information is very important for an understanding of the mechanism of polymerization. The polymerization of MMA by anionic initiators often involves multiple active species with different reactivities and stereospecificities. For example, Grignard reagent exists in the Schlenk equilibrium ... 

Fig. 34. A contour plot (a) and cross-sections (b-d) of the on-line GPC/NMR data for PMMA prepared with t-C4H9Li—(n-C4H9)3Al Al/Li = 1.0) in toluene at —78°C.

Poly(vinyl alcohol) is usually prepared by the hydrolysis of poly(vinyl acetate) and often contains a small number of unreacted vinyl acetate units. The on-line GPC/NMR analysis of several commercial poly(vinyl alcohol)s revealed that the number of vinyl acetate units depended on the molecular weight of the polymer, lower molecular weight fractions containing larger numbers of acetate units than the higher molecular weight fractions.338... 

Bosch JV, Ortiri A, Momabal B (1998) Development of a highly stable multiple wavelength IR detector fon on-line GPC, CRYSTAF and TREF analysis. In Proceedings International GPC Symposium, Arizona, OctobCT 1998, 633-640... 

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