divinyl rubbers, a group of synthetic rubbers; products of the polymerization of butadiene. Upon polymerization, butadiene molecules (I) may combine with the participation of either of two or both double bonds, forming polymers with different configurations of the chemical monomer units in the macromolecule:
Two isomers—cis (II) and trans (III)—are possible for the 1, 4 configuration:
Depending on the polymerization conditions and the nature of the catalyst, butadiene rubbers that differ in content of molecules of the monomer units of the 1, 4 configuration (both cis- and trans-structure) and monomer units of the 1, 2 configuration are obtained.
Catalysts used to obtain butadiene rubbers include alkaline metals (chiefly sodium) and complex compounds of the type CoCl2 + Al(C2H5)2Cl, TiCl4 +Al (C2H5)3, and others (the so-called Ziegler-Natta coordination-ionic catalysts), and organometallic compounds (for example, lithium n-butylate). Nonstereoregular butadiene rubbers containing 60-66 percent 1, 2 monomer units and 34-40 percent 1, 4 monomer units (mainly of trans-structure) are obtained in the presence of alkali metals. The reaction of stereospecific polymerization takes place in the presence of the other catalysts listed above. This reaction results in the formation of stereoregular polymers containing about 98 percent 1, 4 monomer units, including 32-98 percent units of cis-structure. The content of the latter is determined by the type of catalyst. Butadiene rubber with high (93-98 percent), average (87-95 percent), and low (35-40 percent) content of 1, 4 -cis monomer units are obtained in the presence of cobalt, titanium, and organic-lithium catalysts respectively.
The most common nonstereoregular sodium-butadiene rubber (SKB) was the world’s first industrial synthetic rubber. Its manufacture according to Academician S. V. Lebedev’s method was organized in the USSR in 1932. Stereoregular butadiene rubber was first produced by G. Natta in Italy. The production of SKD stereoregular rubber was organized in the USSR in 1965. Stereoregular butadiene rubbers, which can be used to obtain cured rubber, are of the greatest importance.
Butadiene rubbers of various structure have the same density (900-920 kg/m3 or 0.90-0.92 g/cm3), similar dielectric properties (specific volume electric resistance 1-10 tera ohms. m, or 1014-1015 ohms. cm; dielectric pemeability 2.4-2.8) and are soluble in gasoline, benzene, chloroform, and other solvents. The brittleness temperature of stereoregular butadiene rubbers varies from -95° C (for butadiene rubber with a low content of 1, 4-cis monomer units) to -122° C(for butadiene rubber with a high content of these monomer units); for nonstereoregular SKB it is about -52° C. When exposed to heat, oxygen, ozone, ultraviolet rays, or ionizing radiation, butadiene rubbers undergo structural changes accompanied by a decrease in their solubility. The rate of these changes slows with increasing content of monomer units of the 1, 2 configuration in the rubber.
|Table 1. Properties of cured rubbers made from dometic butadiene rubbers|
|1 The rubber contains 90-93 percent monomer units of 1, 4-cis structure|
|Tensile strength [meganewtons per sq m(kg-force/cm2)].............||22(220)||16(160)|
|Rebound resilience (percent).............||58||34|
|Coefficient of frost resitance at -45° C.............||0.75||0.11|
|Durability [cm3 / (kW-hr)].............||140||350|
Sulfur, organic disulfides, peroxides, and alkyl-phenolformaldehyde resins are used to vulcanize stereoregular butadiene rubbers; chiefly sulfur is used to vulcanize nonstereoregular butadiene rubbers. Active carbon blacks are mainly used as fillers. Black-filled rubbers made from stereoregular butadiene rubbers are much stronger and more elastic, durable, and frostproof than those made from nonstereoregular butadiene rubbers (see Table 1).
stereoregular butadiene rubbers differing in content of 1, 4-cis monomer units are produced in several foreign counries. The following brands are available: Ameripol SV, cis-4, Budene, and Diene (USA); Polysar Taktine (Canada); Buna SV (Federal Republic of Germany); Intene (great Britain); and Nipol BR and Asadene (Japan).
Butadiene rubbers are general-purpose rubbers. Stereoregular butadiene rubbers are used mainly in the manufacture of tires, conveyor belts, and a variety of durable and frostproof articles. SKB rubber is used to make a variety of industrial and household rubber goods. The output of stereoregular butadiene rubbers in the capitalist countries in 1971 was about 800, 000 tons.