JUNE 1, 1916.] 



THE INDIA RUBBER WORLD 



467 



What the Rubber Chemists Are Doing. 



SYNTHESIS .\> 



[KlXTl'RE OF CAOUTCHOUCS. 



THE researches of I. 1. Ostromyslenski and F. 1'. Kosche- 

 lev on the chemistry of caoutchouc are here condensed 

 from the abstracts published in the Journal of the 

 Society of Chemical Industry (March 31, 1916). The authors 

 lind that much of the work published since 1911 on the poly- 

 merization of the dioletines is inaccurate and many of the 

 patents are useless. These papers describe peculiar processes of 

 iiomerization of two different forms of isoprene-caoutchouc 

 and methods for the catalytic polymerization of dioletines, 

 both to normal and abnormal caoutchoucs. These methods 

 give practically quantitative results. 



Caouprene bromide is identical with the symmetrical bro- 

 mide of erythrene-caoutchouc. Removal of hydrogen bromide 

 from either yields dehydrocaouprene and a homologous 

 compound is similarly obtained from the bromide of natural 

 Para caoutchouc. 



Reduction of caouprene bromide by zinc dust yields eryth- 

 rene-caoutchouc and more or less isomeric caouprene. Caou- 

 prene is transparent and forms either a viscous liquid or an 

 amorphous, elastic mass. Benzoyl peroxide at 140 to 158 

 degrees F. converts caouprene into normal erythrene-caout- 

 chouc. 

 SYNTHESIS OF NATURAL CAOUTCHOUC IJY WA\ OF B MYKCENE. 



Isoprene heated cautiously at 176 to 194 degrees F. yields 

 B-myrcene. This hydrocarbon is a colorless, mobile liquid, 

 soluble in all the organic solvents. When heated with 

 sodium and barium peroxide at 140 to 158 degrees F. it is 

 converted quantitatively into normal isoprene caoutchouc, 

 whereas under similar conditions isoprene gives an abnormal 

 caoutchouc. 



STRUCTURE OF CAOUTCHOUCS. 



The bromide of natural caoutchouc is homologous with 

 caouprene bromide and possesses unicyclic structure. Under 

 the action of zinc dust it is converted readily and quanti- 

 tatively into free caoutchouc; therefore the latter possesses 

 a unicyclic structure. Willstatter has shown that multi- 

 membered unicyclic groupings of unsaturated hydrocarbons 

 are readily transformed into polycyclic groupings, and 

 Harries has proposed for natural caoutchouc a structure in 

 which such isomerism is assumed. That free caoutchouc 

 and its halogen compounds possess unicyclic structures has 

 been demonstrated experimentally. 



The conversion of unsaturated compounds into cyclic 

 compounds may be regarded as a process of intramolecular 

 polymerization. For example, in the polymerization of vinyl 

 bromide, at the moment when the trimeride appears, the 

 action proceeds in two directions: (1) A small part of the 

 trimeride undergoes intramolecular polymerization with 

 formation of the stable six-membered ring compound, 1.3.5- 

 tribromocyclohexane: (2) The remainder of the trimeride 

 unites with unchanged monomeride until a 32-menibered 

 chain of carbon atoms is formed. This then undergoes ring 

 formation to form the symmetrical bromide of erythrene- 

 caoutchouc. 



CONVERSK 



The discovery of the formation of B-myrccnc in the 

 process of conversion of isoprene into caoutchouc throws 

 light on the mechanism of this process. In the first phase, 

 the hydrogen atom and the residual radicle from a molecule 

 of isoprene combme at the ethylenic linking of a second 



molecule, gi\ing the dimcride, B-myrcenc. The latter is 

 then converted into the trimeride, and so on, until the octa- 

 meride is formed. This then undergoes "intramolecular 

 polymerization" to isoprene-caoutchouc. This polymeriza- 

 tion of chemically pure B-myrcene may possibly represent 

 the only synthesis of natural caoutchouc, that is, of a sub- 

 stance perfectly identical with natural Para caoutchouc, both 

 in the general structure of its nucleus and also in the posi- 

 tions of the methyl groups and double linkings of the 

 molecule. There is reason to believe that tropical plants 

 synthesize natural caoutchouc by way of B-myrcene or myr- 

 cene-Iike hydrocarbons or their dimerides and not by the 

 polymerization of isoprene. The sap of plants frequently 

 contains compounds with an atom grouping like that of 

 myrcene, such as geranoil, linalool, nerol. etc., and dehydra- 

 tion of these unsaturated alcohols would lead immediately 

 to the corresponding myrcenes. 



COMP.VRISOX OF l)RAZn.I.\N AND PLANTATION METHODS OF 

 PREPARINC. PARA RUBBER. 



G. Stafford Whitby recently reported the results of his 

 researches on this topic before the London Section of the 

 Society of Chemical Industry. 



In order to obtain strictly comparable results, the two 

 methods mentioned were applied to separate portions of the 

 same lot of freshly tapped latex. The Brazilian method 

 employed for the preparation of the fine hard Para rubber 

 was followed as closely as possible, the latex being coagu- 

 lated on a paddle in smoke from burning wood and 

 palm nuts. The plantation method adopted as typical was 

 the preparation of smoked sheet by diluting the latex, coagu- 

 lating it with acetic acid (0.5 per cent of the weight of dry 

 rubber), holding out the coagulum, drying the sheets, and 

 linally, hanging them in a smoke house for 22 days. The 

 products obtained by these two methods were subjected to 

 precisely similar vulcanization tests, the results of which 

 were expressed in accordance with a scheme worked out by 

 P. Schidrowitz to indicate the duration of heating required 

 for a "perfect" cure and the mechanical properties of the 

 vulcanized product. 



The results indicate that the Brazilian method is not 

 superior to the plantation method, and the rubber coagulated 

 by the Brazilian method required longer curing in the vul- 

 canization tests. Such differences as were observed the 

 author attributed in part to a specific and deleterious action 

 of phenolic substances derived from the smoke, and the 

 jiresence of formaldehyde may also be responsilile to some 

 extent. 



From vulcanization tests on a very large number of 

 samples, the author concluded that oxidation and the dis- 

 coloration which it produces have no effect on the vulcan- 

 izing properties of the rubber. The outer and more dis- 

 colored portions of balls of fine hard Para rubber were 

 found to be equal in quality to the inner portions. 



Separate investigation of rubber from young and from 

 old trees gave no support to the widely accepted opinion 

 that tlie latex from young trees is inferior. 



In further experiments it was ascertained that air-dried 

 plantation sheet rubber is quite equal, if not superior, in 

 quality to smoked sheet rubber. 



In the discussion following the paper Professor Henry 

 E. Armstrong said that the impression he had formed when 

 recently in Ceylon was that they understood very little of 

 what they were doing, and that more research work was 



