In natural rubber the chain has thousands of repeat units, not just five like you see in the picture above.
Polyisoprene can be made synthetically by polymeriztion of a small molecule called isoprene, with the help of special metal compounds called Ziegler-Natta catalysts.
As it comes from the tree, natural rubber latex is not good for very many things. The
resourceful people of the Americas in pre-Columbian times used it to make waterproof boots,
and balls for playing games like basketball. But the boots could only be used in warm
climates. In cold weather they would become brittle and crack. So people in cold places
could only use natural rubber for pencil erasers. That's where the word rubber
comes from.
Wouldn't you know sulfur was just the extra ingedient he needed to make rubber work in cold
weather. After mixing hot gooey rubber latex and sulfur and letting the mixture cool, he took
the rubbery solid that resulted and tacked it to the outside of his door. The cold
Massachusetts winter didn't make it brittle. What's more, it didn't become gooey when heated
anymore, either.
Goodyear was onto something here. This process for making rubber more useable became known as
vulcanization.
What Goodyear had done was this: he crosslinked the rubber. Let me explain. The sulfur
molecules each contain eight sulfur atoms, arranged in a ring, like you see on your right.
When these sulfur molecules are heated with polyisoprene molecules,
something nifty happens. The sulfur rings open, and fall apart. Fragments of the sulfur rings
will join with the polyisoprene, joining the chains together, as you see below:
Goodyear Has a Spill
Then there comes onto the scene a tinkerer named Charles Goodyear. In the winter of 1839
Goodyear was in Massachusetts trying to figure out how to make natural rubber more useful so
that he could finally make a living at his until-now fruitless tinkering. He had mixed rubber
latex and sulfur together when he had a little accident. He spilled the mixture on a hot
stovetop. When the mixture was through frying, Goodyear couldn't believe what had happend.
One Big Happy Molecule
< Back to Goodyear's Accident
This crosslinking makes the rubber stronger. It also allows the rubber to keep its shape better
when it is stretched over and over again. It keeps the rubber from getting gooey in hot
climates because, think about it, a single molecule can't flow like a substance made up of
many molecule. Think of the way you can pour a bucket full of gravel, but you can't really
pour a boulder, and you'll get the idea.
The Drawbacks of Crosslinking
Now there are some drawbacks to this crosslinking which makes natural rubber so useful. First
of all, because it doesn't get gooey and flow when it gets hot, one has to mold it into whatever
shape one wants before crosslinking. But that isn't a really big problem, just something
for an engineer to keep in mind when making things out of natural rubber. But it's related to
a bigger problem. Because rubber doesn't flow when hot after it has been crosslinked, it is
very difficult to recycle. This is a big problem. Just think of how many tires are used up each
year by all the cars in the world. That's a lot of waste to dispose. Several experimental
processes are being investigated for recycling crosslinked rubber. Another answer is to use
certain kinds of rubber which aren't crosslinked called thermoplastic elastomers.
Polymer Science Learning Center and
the Chemical Heritage Foundation