The Chemistry of Nylon 6 Synthesis

Nylon 6 is made by through the ring-opening polymerization of caprolactam. To make the polymerization take place, three more components are included in the reaction mixture, sodium hydride, N-acetylcaprolactam, and polyoxyethylene. Here is a description of what each component does in the polymerization:

Sodium hydride (NaH):

Sodium hydride is the initiator in this reaction. Sodium hydride reacts with caprolactam by extracting the amide hydrogen, to produce the sodium salt, which is much more nucleophilic than the amide.



Polyoxyethylene

The salt is even more reactive when the sodium ion is out of the way. Polyoxyethylene complexes the sodium ion, leaving behind a very reactive unshielded amide anion.



N-acetylcaprolactam

The unshielded amide anion then reacts with the N-acetylcaprolactam:



N-acetylcaprolactam is much more susceptible to this sort of reaction than caprolactam, because it is an imide, whereas caprolactam is an amide. Also, if caprolactam underwent this first reaction step in place of N-acetylcaprolactam, the product would be a much less stable amine anion, as opposed to the more stable amide ion formed when N-acetylcaprolactam reacts.

At this point the initiation step of the polymerization is complete, and propagation can begin. In this step, the amide anion produced in the initiation step extracts the amide hydrogen from another molecule of caprolactam, just as the sodium hydride did during initiation.

This produces another caprolactam amide ion, that can react as you see below:

This generates a species that can react again in the same manner. This happens over and over to produce a long chain of nylon 6.

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