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.