The number‐average molecular weight of native hardwood xylans

A 4‐O‐methylglucuronoxylan has been isolated in a yield of 90% by direct alkaline extraction of trembling aspen (Populus tremuloides) wood. The number‐average molecular weights of the acetate and valerate derivatives were determined in chloroform‐ethanol and of the xylan itself in dilute cupriethylenediamine. There was no evidence for the occurrence of molecular association of the acetate in chloroform‐ethanol. Tetra‐chloroethane, dimethyl sulfoxide, and cadoxen were unsuitable solvents. The results showed that the polysaccharide had a number‐average molecular weight of 31,500, corresponding to a degree of polymerization of 200. The constants K and a in the Staudinger‐Mark‐Houwink equation were determined for cupriethylenediamine solutions of a series of celluloses and xylans, the values obtained being 2.2 × 10−2 for K in both cases, and 0.78 and 0.72 for a, respectively. It was concluded that cellulose and xylan probably have the same conformation in aqueous alkali, intermediate between a compact and a free‐draining coil. Since the xylan framework is known to contain two branches per average molecule, the results would probably also imply that the branches are relatively short.