Reducing end-group analysis

Reducing end-group analysis (Smith and Montgomery, 1956) is implicitly the most direct way to determine molecular weights of linear polysaccharides, because a chemical reaction establishes one-to-one correspondence between 

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Notwithstanding the problems associated with the Donnan distribution, a pectin Mn was obtained from e = 10-2 g mL-1 dispersion in 0.05-M sodium chloride and reported to have approximated Mn by reducing end-group analysis (Fishman et al., 1986). 

Gel filtration of the hemicellulose was done on a G-50 Sephadex column with water as eluent by the National Food Research Institute of the CSIR. Neutralized samples were applied to the column and fractions (6 mL) collected, of which 1-mL aliquots were assayed by the phenol-sulfuric acid test ( ). Approximate calibration (D.P. versus elution volume on the G-50 column) was done by total carbohydrate and reducing sugar end group analysis (0) using enzymatically debranched starch as reference material. 

Number-average molecular weights, M, were obtained from colorimetric data from end-group analysis (reducing sugar) according to procedure described earlier (19) ... 

An implication of the kinetic analysis presented in Sec. IV.A is that the rate of chain scission of polyesters can be retarded by endcapping to reduce the initial carboxylic acid end-group concentration. Alternatively, the rate may be increased by acidic additives that supplement the effect of the carboxy end groups. The first expectation was confirmed by partial ethanolysis of high molecular weight... 

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. 

Starnes and Bovey (1) pioneered the method of I3C NMR analysis of reduced poly(vinyl chloride) (PVC) to study the microstructure of PVC. Tri-n-butyltin hydride ((n-Bu)3SnH) was found to completely dechlorinate PVC resulting in polyethylene (PE) whose microstructure (branching, end-groups, etc.) could be sensitively studied by 13C NMR. 

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