Nylon 6 begins as pure caprolactam. As caprolactam has 6 carbon atoms, it got the name Nylon-6.
When caprolactam is heated at about 533 K in an inert atmosphere of nitrogen for about 4-5 hours, the ring breaks and undergoes polymerization. Then the molten mass is passed through spinnerets to form fibres of Nylon 6.
Polymerization of caprolactam to Nylon 6.
During polymerization, the peptide bond within each caprolactam molecule is broken, with the active groups on each side re-forming two new bonds as the monomer becomes part of the polymer backbone. Unlike nylon 6,6, in which the direction of the amide bond reverses at each bond, all nylon 6 amide bonds lie in the same direction (see figure). Nylon 6 therefore resembles natural polypeptides more closely; in fact, caprolactam would become an amino acid if it were hydrolyzed. This difference has little effect on the polymer's mechanical or chemical properties, but is sufficient to create a legal distinction.
Nylon 6 (above) has a structure similar to Nylon 6,6 (below).