THE RUBBER INDUSTRY — GIBBONS 211 



mercial interest on account of high cost. Although other sources 

 of isoprene were proposed and patented, no commercial use is being 

 made of this synthesis. 



Butadiene and DimethyTbutadiene. — These two homologs of iso- 

 prene have also been investigated. Lebedev in 1910 (13) reported 

 that a rubberlike polymer is produced by heating a butadiene. 

 Kondakov in 1900 (10) reported that when 2,3-butadiene was heated 

 with caustic potash he obtained a rubberlike plastic material. 



Apparently no attempt was made to commercialize these two dis- 

 coveries until the shortage of rubber in Germany during the World 

 War induced German chemists to investigate possible substitutes for 

 natural rubber. They concentrated on the synthesis from dimethylbu- 

 tadiene, probably because this hydrocarbon was more easily made than 

 butadiene. The rubber produced by the polymerization of dimethyl- 

 butadiene was known as methyl rubber. It was used particularly for 

 hard rubber, and by the spring of 1918 over 30 tons per month were 

 utilized for this purpose. 



Soft products produced from methyl rubber had low elasticity, and 

 the tires and tubes made from it were short-lived. 



After the war, interest in synthetic rubber lapsed for several years 

 but was renewed about 1926, probably on account of the rapid increase 

 in the price of natural rubber about that time. Since 1926 most of 

 the activity appears to have been on the butadiene type of rubber. 

 The polymers of butadiene and vulcanizates made from them are much 

 closer to natural rubber in properties than those of dimethylbutadiene. 

 In addition, better products were made by cross-polymerizing bu- 

 tadiene with other compounds. At present, substantial quantities 

 of these synthetic rubbers, known as Buna S and Buna N, are produced 

 and used in Germany in place of natural rubber. 



Chlorobutadiene. — Polymerized chlorobutadiene, known as neo- 

 prene, was the first synthetic rubber used commercially in this country. 

 Unlike the earlier members of this group, neoprene is not a polymerized 

 hydrocarbon but is the polymer of /^-chlorobutadiene. For pui'poses 

 of rough comparison, /3-chlorobutadiene may be regarded as isoprene 

 with a chlorine atom in place of the methyl group. The polymerized 

 product is characterized by a much higher resistance to oils and other 

 organic solvents than natural rubber and a much higher resistance 

 to cracking from light and ozone; it is used for the manufacture of 

 products where these enhanced properties are sufficiently important to 

 offset the increased cost. 



The vulcanized products of butadiene and chlorobutadiene are es- 

 sentially rubbery in character, and possess good strength, good stretch, 

 and the ability to recover their original form when the stress is 

 removed. 



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