3 STRUCTURE OF GELS 59 



high-polymer compounds are formed, such as rubber in the case con- 

 sidered here. 



Apart from this type of chain formation, high molecular weight sub- 

 stances may also be formed by etherification of alcohohc groups (Fig. 50) 

 or by a process of esterification between carboxylic and hydroxylic groups 

 with elimination of water. This way of interlinking is distinguished as 

 condensation from the polymerization of unsaturated compounds. It leads to 

 equally long molecules; the chains are then, however, no longer all-carbon 

 chains like those in polystyrene or rubber, but always contain oxygen atoms 

 as interconnecting links. When polyvalent alcohols react with each other, 

 no chain-like, but net-hke or even spatial giant molecules are formed, such 

 as probably occur in the insoluble huminic acids and in the insoluble 

 cutins (see p. 295). By way of introduction, however, we shall confine the 

 discussion to the somewhat simpler conditions in the high-polymer carbo- 

 hydrates with linear chain molecules. 



The high-polymer molecules may become so large as to assume the 

 properties of colloid particles. Staudinger (1936a) designates these giant 

 molecules as macromolecules and the branch of science dealing with their 

 constitution and chemical behaviour as niacromokcidar chemistry. 



Polysaccharides. The same principles by which disaccharides are 

 formed (see Fig. 35/37, p. 39), govern the formation of polysaccharides, 

 which are of outstanding importance in plant physiology. Here too, 

 the monoses are interlinked by 1-4 oxygen bridges with elimination 

 of water, and this polycondensation may embrace a large number of 

 monomer molecules. In cellulose the successive links of /3-glucose are 

 rotated through 180°. In starch, however, the a-glucose residues can 

 interact without being rotated (Fig. 50). The cellulose chains have a 

 digonal screw axis as an element of symmetry, contrary to the starch 

 chains, which have not. Consequently, the cellulose molecules are 

 more stable and straightened out, whereas the starch molecules tend 

 to become more convolute because they are less symmetrical. This 

 morphological difference is doubtless one of the reasons for the 

 difference in behaviour between starch and cellulose. Possibly it is 

 also responsible for the tendency of the starch molecule towards 

 branching (see Fig. 1 5 2b, p. 3 1 1). The mannans occurring in corozo nut 

 and in the rhizomes of Amorphophalhis konjak (see Fig. 160) can be 

 derived in a similar way from mannose as starch and cellulose from glu- 

 cose. The two monoses differ only in the different position of the H- 

 and OH-groups at the second C-atom. For the chain in mannan from 

 corozo nut, locahzed in the cell waU, Meyer and Mark (1930, p. 168) 



