End-group methods

Whelan, W. J. (1964). Determination of reducing end-groups. Methods in Carbohydrate Chemistry, Vol. IV, pp. 72-78. Academic Press, New York. 

The sample amount can be easily determined from the injection volume and the sample concentration, and no information from a concentration detector is required. With this approach, the Mn value of any polymer sample can be determined by SEC using only a viscosity detector. Other molar mass averages, however, cannot be determined. The advantage of the Goldwasser Mn method is that it can access much wider molar mass ranges than other existing methods like osmometry or end-group methods. 

Polysaccharides. Part XXVIII. The End-group Method as Applied to Starch. An Improved Method for the Estimation of Tetramethyl Glucose in Admixture with Trimethyl Glucose, E. L. Hirst and G. T. Young, J. Chem. Soc., 1247 (1938). 

Since the concentration of end groups varies inversely with molecular weight, end group methods tend to become unreliable at higher molecular weights. They can be used, where they are applicable at all, up to near 50,000. 

In a few cases, alkaline hydrolysis has proved applicable to special problems. Tryptophan is not destroyed in alkali, and analysis of alkaline hydrolyzates forms the basis of one method for quantitative determination of this amino acid (e.g., Dreze, 1960). Despite the fact that tryptophan-containing peptides should be more stable in alkali than acid, partial alkaline hydrolysis has not been employed for identification of this type of peptide. Amino acids often can be regenerated by alkaline hydrolysis from derivatives obtained by the amino-terminal end-group methods. Dinitrophenyl amino acids and phenylthiohydantoin (Fraenkel-Conrat et al., 1955) as well as hydantoin (Stark and Smyth, 1963) derivatives of amino acids can be treated in this manner. 

In the absence of good chemical end-group methods for determination of carboxyl-terminal residues or sequences, use of the carboxypeptidases remains the best means for analyses of this kind. 

Less reliable reducing end group methods include the copper number of Schwalbe, the silver number of G6tze and methods employing phenylhydrazine. The D.P. values obtained by these methods, however, are usually of about the same order of magnitude as those obtained by the method of Bergmann and Machemer. ... 

It is now interesting to speculate what structures could be classified into this middle ground. Figure 6 illustrates telechelics with polydispersities of 1.5 which are monofunctional or bifunctional with respect to end-groups. Methods are available to achieve this, e.g. using a functional dimer macro-... 

The end-group method was applied to the determination of the molecular weight of cellulose by Haworth and others. The structural formula for cellulose is discussed in Chapter 3. The two end groups have the structures... 

End-group methods produce an equivalent mass, namely the mass of polymer per mol of end-groups. For a polydisperse polymer of molecular weight Mf containing iV,. moles of polymer molecules, if the number of functional groups is /, then... 

EO fed to the autoclave was 21.546 kg together with the same quantity of PO over a time of 1.4 h and humidity content of -0.2% by weight. The total pressure was 1.7 atm and temperature was kept constant at 107°C. The product obtained was then neutralized with aqueous sulfuric acid and volatile residue was removed by vacuum distillation at 180°C at 2 mm Hg of pressure. The average molecular weight of the obtained product is 467 g/mol, as evaluated by means of the hydroxyl end group method [22-25]. 

Application of the end-group method to other polymers, such as polyesters, polyurethanes and cellulose, is reviewed in ref. 25. 

Acid Value n The measure of the free-acid content of a substance. It is expressed as the number of milligrams of potassium hydroxide (KOH) neutralized by the free acid present in 1 g of the substance. This value, also called acid number, is sometimes used in connection with the end-group method of determining molecular weights of polyesters (DeKgny P, Oldring PKT, Tuck N (eds)... 

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