When the Carothers team began trying to make nylon, no one really thought much about this. But one scientist named Wesley R. Peterson noticed that this reaction could be useful to control molecular weight.

Good nylon polymer must have a molecular weight of at least 10,000. To get a high molecular weight, exactly the same number of molecules of hexamethylene diamine as adipic acid must be reacted together. This balance is tricky to accomplish, but Peterson used the nylon salt to overcome the problem, and developed a method which is still used to make nylon today. In this method, adipic acid is mixed with hexamethylene diamine at room temperature. The two react, of course. The acid and the base neutralize each other, and we're left with a salt which is usually refered to as nylon salt.

The nylon salt is then purified. This nylon salt is a crystalline solid. When the time to make nylon comes, one just has to heat the nylon salt to over 285 ^oC, and it reacts to become nylon polymer.

This sounds more complicated than simply mixing adipic and hexamethylene diamine and heating them. But remember, to make high molecular weight nylon, there must be exactly the same number of adipic acid molecules as hexamethylene diamine molecules. Because nylon salt is made from an exact 1:1 ratio of adipic acid and hexamethylene diamine, using nylon salt ensures that there is an exact 1:1 mixture of the two when nylon polymer is being made.

This method was so successful that Peterson made batches of nylon polymer with molecular weights that were TOO high! If nylon has molecular weight much higher than 20,000 it becomes impossible to spin into yarn. So Peterson had to invent a way to bring the molecular weight down, oddly enough. His trick was to add acetic acid to the reaction mixture. This trick helped produce a spinable nylon polymer.