Specific Influences in the Polymerization of Dienes. Chemical Factors Influencing the Homogeneity of Synthetic Rubbers during Production

At the present time, when so much synthetic rubber is being produced, the question of the polymerization of dienes is most important. Many thousands of tons of GR-S and related rubbers are being made by this means, and yet the process is only very imperfectly understood. Polymerization is still an uncontrolled reaction, although much work has been done in an attempt to overcome the deficiencies. The troubles are caused by the fact that the dienes do not polymerize in a regular way, because of the formation of branch chains and cross-linkages, which are a direct result of these branch chains. These irregularities must be controlled before satisfactory products can be obtained, and the present paper is written to correlate isolated evidence and present fresh material so that the processes involved during polymerization may be more perfectly understood and possibly controlled. It is natural in the first instance to examine natural rubber to see if any indication of the lines to follow can be obtained. Unfortunately, however, even natural rubber is not perfect, and from various sources of evidence it has been shown that branching does occur in the isoprene chains. This branching is, however, different from that in synthetics, and is quite regular. As will be seen shortly, it is difficult on chemical or physico-chemical grounds to see why this should be so, as the only directive group in the isoprene unit is the methyl side-group. It seems logical to conclude that natural rubber is not formed by a polymerization process, but that carbon and hydrogen atoms combine in such a way that they form directly a natural rubber-molecule.