HISTORICAL SKETCH OF CHEMISTRY OF RUBBER 9 



forms a number of addition compounds with the halogens, 

 sulphur, etc. The simplest and most definite of these is 

 the tetrabromide, Ci H 16 Br 4 , in which two atoms of bromine 

 have attached themselves at each double bond. The sulphur 

 derivatives are obtained in the process of vulcanisation. Weber 

 concluded that there is a series of sulphur compounds between 

 the limits (Ci Hi6)ioS 2 and Ci H 16 S 2 , the latter corresponding to 

 ebonite, though later work has thrown considerable doubt on 

 the accuracy of his deductions. 



The determination of the molecular weight of caoutchouc 

 has not so far been accomplished. In the liquids usually- 

 employed to dissolve it, rubber is present in the colloidal state, 

 and each particle probably consists of a large aggregate of 

 molecules. So that osmotic pressure, freezing point and 

 boiling point methods are inapplicable. As, however, in 191 2 

 Walden discovered that starch formed a true solution in 

 formamide, with the aid of which he found the molecular 

 weight to correspond to the comparatively simple formula 

 (C 6 Hi O 5 )i, a true solution of rubber may eventually be 

 obtained. In the meantime, as its simpler soluble derivatives 

 have only ten carbon atoms in the molecule, it may be 

 assumed that rubber contains no more than this number. That 

 the molecules of caoutchouc and those of many of its deriva- 

 tives are associated with each other to a high degree is in accord- 

 ance with their colloidal condition. 



From 1907 onwards the attention of a number of chemists 

 began to be directed to the problem of the artificial production 

 of rubber. Activity in this country was centred in the S}m- 

 thetic Products Company, while in Germany the question 

 was taken up by the Bayer Company and the Badische Anilin 

 und Soda Fabrik. Previous observations had shown that the 

 homologues of isoprene polymerise to substances related to caout- 

 chouc possessing properties varying from sticky resin to brittle 

 solids. So far as the consumer is concerned, the exact consti- 

 tution of a body is unimportant, provided its physical properties 

 are suitable. So that the product required is not necessarily 

 true rubber, but a substance having similar physical properties. 

 The more important members of the isoprene series that 

 have been considered are : erythrene, or buta-i : 3-diene, 

 CH 2 : CH . CH : CH 3 ; piperylene, or i-methyl-buta-i : 3-diene, 

 CH(CH 3 ) : CH .CH :CH 2 ; and isoprene, or 2-methyl~buta~i : 



