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Nylon 6 or polycaprolactam is a polymer developed by Paul Schlack at IG Farben to reproduce the properties of nylon 6,6 without violating the patent on its production. Unlike most other nylons, nylon 6 is not a condensation polymer, but instead is formed by ring-opening polymerization. This makes it a special case in the comparison between condensation and addition polymers. Its competition with nylon 6,6 and the example it set have also shaped the economics of the synthetic fiber industry. It was given the trademark Perlon in the year 1952.
Nylon 6 begins as pure caprolactam. 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.