19. The Simple Compounds
                                   of Carbon
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      Polymers and Rubber

In this "vulcanization" process, the sulfur atoms cross-link between adjacent polyisoprene chains and hold them more nearly stationary, opposing any outside stretch or deformation.

Soft rubbers contain 1-2% sulfur; hard rubbers may have as much as 35%. Cross-linking of polymer chains is a standard method today of producing a hard, mechanically strong plastic or resin.

Some plants synthesize the all-trans isomer of polyisoprene, known as guttapercha. Guttapercha is hard and horny rather than rubbery, because the orderly trans-polyisoprene chains can pack next to one another easily in crystalline regions within the polymer.

Transpolyisoprene in guttapercha is hard and semicrystalline, but cis-polyisoprene in natural rubber is soft and amorphous. The biggest single hurdle in making usable synthetic rubbers was finding a way of putting together a pure cis polymer.

Simple polymerization of isoprene in the laboratory yields a mixture of cis and trans bonds. More subtle methods of polymerization had to be perfected before a dependable method of making a pure cis polymer was developed in 1955.

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