Isoprene and Rubber. Part 26. Hemicolloidal Hydrorubbers

The reduction of rubber without solvent takes place only at 270°, according to experiments by Fritschi. Below this temperature rubber is reduced only with difficulty and incompletely. In the dissolved eondition, on the contrary, Pummerer and Burkard showed that rubber can be reduced at ordinary temperature. In an earlier communication it was accordingly assumed that on heating a cleavage of secondary colloid particles into primary ones (into macromolecules) takes place, and that heating, therefore, has about the same effect as strong dilution. A distinction was made between primary and secondary particles, and it was assumed that the strong diminution in viscosity which is observed in dilute rubber solutions depends upon a cleavage of the secondary colloid particles, just as do the decreases in viscosity which rubber undergoes upon heating, and which have been generally described as depolymerization. These earlier ideas of ours are, however, incorrect. An extensive cracking of the rubber molecules takes place upon heating, and this was more carefully investigated by Geiger and later by Bondy. In this way reduction is facilitated. With strong dilution, on the other hand, the rubber solution passes from the gel state to the sol state, the rubber passes into normal solution, and therefore reduction can take place more easily in a dilute solution than in a concentrated one. With reduction by heat, therefore, a derivative of unchanged rubber cannot be expected; instead the hydrorubbers obtained, for example, those first prepared by Fritschi, are not reduction products of rubber itself, but are hemicolloidal decomposition products of the latter. Only by careful reduction in cold solution can one succeed in obtaining a hydrorubber in which the rubber molecules remain unchanged.