Polymers Natural and Synthetic Rubbers

Conjugated dienes can be polymerized just as simple alkenes can (Section 8.10). Diene polymers are stmcturally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4 addition of the growing chain to a conjugated diene monomer. 

Rubber is a naturally occurring diene polymer of isoprene (2-methyl-l,3-butadiene) and is produced by more than 400 different plants. The major source is the so-called mbber tree, Hevea brasiliensis, from which the cmde material, called latex, is harvested as it drips from a slice made through the bark. The double bonds of rubber have Z stereochemistry, but gutta-percha, the E isomer of mbber, also occurs naturally. Harder and more brittle than mbber, guttapercha has a variety of minor applications, including occasional use in dentistry and as the covering on golf balls. 

A number of different synthetic mbbers are produced commercially by diene polymerization. Both cis- and fcrans-polyisoprene can be made, and the synthetic rubber thus produced is similar to the namral material. Chloroprene (2-chloro-l,3-butadiene) is polymerized to yield neoprene, an excellent, although expensive, synthetic mbber with good weather resistance. Neoprene is used in the production of industrial hoses and gloves, among other things. 

Draw a segment of the polymer that might be prepared from 2-phenyl-l,3-butadiene. Problem 14.12 

Show the mechanism of the acid-catalyzed polymerization of 1,3-butadiene. 

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Classification of Polymers Free-Radical Chain-Growth Polymerization Cationic Chain-Growth Polymerization Anionic Chain-Growth Polymerization Stereoregular Polymers Ziegler-Natta Polymerization A WORD ABOUT... Polyacetylene and Conducting Polymers Diene Polymers Natural and Synthetic Rubber Copolymers... 

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