Carothers thought about these reactions. What if one used an acid that could react
twice, and an alcohol that could react twice as well? What would happen if one tried to
carry out the reaction below?
Because this reaction would happen in many steps, each involving one acid and alcohol reaction,
it became known as step-growth
polymerization.
- Wallace Carothers1,2
And of course, the macromolecules he created were the first linear polyesters.
He was able to show that he had long linear macromolecules rather
than small cyclic groups because he could detect the presence
of acid and alcohol groups at the ends of the macromolecular chains.
Cyclic molecules would have no end groups, of course.
But even more importantly, they developed a whole new means of making macromolecules.
Polymers existed in nature, and certain synthetic polymers had been made accidentally before
Carothers and his team made their polyesters. But step-growth polymerization was the first
reaction designed and carried out with the specific purpose of creating high molecular weight
polymer molecules. It was the first polymerization reaction whose results had been predicted
beforehand by scientific theory.
Carothers developed a series of equations to describe the behavior of step-growth
polymerization systems which are still known as the Carothers equations today.
The step-growth polymerization principle set out by Carothers was first used
to make polyesters, and eventually nylon,
polyurethanes, polycarbonates, and many other types of polymers.
1. Carothers, Wallace. "Early History of Polyamide Fibers", 19 February 1936.
Hagely Museum and Library, Acc. 1784.
2. Furukawa, Yasu, Inventing Polymer Science, University of Pennsylvania Press,
Philadelphia, 1998.
3. Hermes, Matthew. Enough for One Lifetime: Wallace Carothers, Inventor of Nylon.
Washington, D.C.: American Chemical Society; Philadelphia: Chemical Heritage Foundation, 1996.
In order to prove Hermann Staudinger's theory that cotton and rubber were made of giant
chain-like
macromolecules,
Wallace Carothers wanted to create some giant molecules of his own. Many
types of coupling reactions had long been known to organic chemists. A coupling reaction is
a reaction which joins two small molecules together to form one bigger molecule, such as the
coupling of an acid and an alcohol to form an ester:
Many scientists were sure that one would
get a ring-like molecule like the one on the right. But Carothers knew
that ring-shaped molecules with more than six atoms in a circle are
generally unstable. So if he used acids and alcohols that were large
enough, rings probably wouldn't form. It seemed to him more likely that
one would get long chainlike macromolecules like this:
"The conviction was firmly
established in my own mind
that bifunctional condensations
involving unit length beyond
7 were...almost perfectly
intermolecular, and they were
therefore functionally capable
of almost indefinite structural
propagation."
To test his theory he had his team of chemists make a series of
polymers
from acids and
alcohols. Carothers chose this reaction, called esterification, because it was simple.
Esters would be the only product formed by the reaction. Thus, if no cyclic products were
obtained, there would be no doubt that he had created macromolecules.
References