What happened? Why did acetic acid, the same acetic acid found in vinegar, bring the
molecular weight under control? Imagine a growing chain of nylon polymer. A growing chain
of nylon polymer has either an amine or an acid group on its end. Amine end groups react with
acid groups, and vice versa. As you can see in the picture above, when an amine end group
reacts with adipic acid, the result is a chain that has a reactive acid group on the end.
The chain can keep growing. The acid group on the end can react with either a hexamethylene
diamine molecule or a growing nylon chain with an amine end group. Either way, the chain
continues to grow.
But if acetic acid reacts with the amine end group, the result is a chain with an unreactive
group at the end. The chain can't grow any more. This is how acetic acid limits molecular
weight.
In 1934 Wesley R. Peterson was facing the kind of
problem that most
polymer
chemists would love to
have. He was making nylon polymer which had a
molecular weight
that was too high. If the weight is
too high, the nylon polymer cannot be spun into yarn. Not to worry, Peterson added a
calculated amount of acetic
acid to his recipe, and the problem was solved. Then the nylon polymer he made had
the desired molecular weight.
References
1. Hermes, Matthew. Enough for One Lifetime: Wallace Carothers, Inventor of Nylon. Washington, D.C.: American Chemical Society; Philadelphia: Chemical Heritage Foundation, 1996.2. Labovsky, Joseph. Oral history by John K. Smith, 24 July 1996. Philadelphia: Chemical Heritage Foundation.