Rubbers butadiene-pentadiene

Most unsaturated substances such as alkenes, alkynes, aldehydes, acrylonitrile, epoxides, isocyanates, etc., can be converted into polymeric materials of some sort—either very high polymers, or low-molecular-weight polymers, or oligomers such as linear or cyclic dimers, trimers, etc. In addition, copolymerization of several components, e.g., styrene-butadiene-dicyclo-pentadiene, is very important in the synthesis of rubbers. Not all such polymerizations, of course, require transition-metal catalysts and we consider here only a few examples that do. The most important is Ziegler-Natta polymerization of ethylene and propene. 

An important extension of Ziegler-Natta polymerization is the copolymerization of styrene, butadiene and a third component such as dicyclo-pentadiene or 1,4-hexadiene (see below) to give synthetic rubbers. Vanadyl halides rather than titanium halides are then used as the metal catalyst. 

Synthetic rubbers Butadiene, butenes, pentane, pentadiene, acrolein, acrylonitrile, dichloroethane, methyl methacrylate, methanol, phosgene, chlorobenzene, caprolactam, cyclohexane -... 

Next to isoprene, pentadienes and 2,3-dimenthyl-1,3-butadiene are produced as alkylbu-tadienes on a large scale. Poly-2,3-dimethyl-l,3-butadiene was one of the first synthetical rubbers [309, 310]. Terminally substituted 1,3-butadienes give 1,4 monomeric units each of which contains one or two asymmetric carbon atoms [R = H or alklyl group R = alkyl group (41) and (42)]. Therefore, monomers of this type can lead to different stereoregular 1,4-polymers cw-1,4 iso- or syndiotactic, trans-, A iso- or syndiotactic [311,312]. 

At this time it was becoming recognized that conjugated dienes other than isoprene could be converted into elastic substances. In 1881 von Hofmann prepared a rubber from 1,3-pentadiene and in 1892 Couturier polymerized 2,3-dimethyl-1,3-butadiene although he paid little attention to the product. 

Whilst there exists a large number of diene monomers only four are of real interest to the rubber chemist, namely 1,3-butadiene (I), isoprene (i.e. 2-methyl-1,3-butadiene) (II), 2,3-dimethyl-1,3-butadiene (III) and piperylene (i.e. 1,3-pentadiene) (IV). 

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