Polymerization Reactions of Conjugated Dienes

The general character of alkene polymerization by radical and ionic mechanisms was discussed briefly in Section 10-8. The same principles apply to the polymerization of alkadienes, with the added feature that there are additional ways of linking the monomer units. The polymer chain may grow by either 1,2 or 1,4 addition to the monomer. With 1,3-butadiene, for example, 

For polymerization of 1,3-butadiene by 1,4 addition, there are no chiral carbons, but there is the possibility of cis-trans isomerism  

Exercise 13-12 Formulate chain initiation, propagation, and termination steps for the polymerization of 1,3-butadiene by a mixture of 1,2 and 1,4 addition using a peroxide catalyst. Consider carefully possible structures for the growing-chain radical. Show the expected structure of the polymer and calculate AH° for the polymerization reaction. 

A polymer made of identical repeating units is called a homopolymer. If the units are nonidentical, as when different monomers are polymerized, the product is called a copolymer. 

Many of the polymers formed from conjugated dienes are elastic and are used to manufacture synthetic rubbers. The raw polymers usually are tacky and of little direct use, except as adhesives and cements. They are transformed into materials with greater elasticity and strength by vulcanization, in which the polymer is heated with sulfur and various other substances called accelerators, with the result that the polymer chains become cross-linked to one another by carbon-sulfur and carbon-carbon bonds. Some of the cross-linking appears to occur by addition to the double bonds, but the amount of sulfur added generally is insufficient to saturate the polymer. With large proportions of sulfur, hard rubberis formed such as is used in storage-battery cases. 

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