The Polyamide Thread

Wallace Carothers and the Step-Growth Condensation of Synthetic Silk

In the late 1920s, most chemists still believed that natural materials like silk, rubber, and cellulose were not true molecules. Instead, they subscribed to the colloidal micelle theory, which held that small ring-shaped molecules clumped together through weak physical forces without covalent chemical bonds. German chemist Hermann Staudinger challenged that orthodoxy in 1920. He proposed that polymers were genuine macromolecules—single chains of thousands of atoms linked end-to-end by ordinary covalent bonds.

To test Staudinger's hypothesis through pure basic research, DuPont opened a new laboratory at its Experimental Station in Wilmington, Delaware, in 1928—informally dubbed Purity Hall—and hired organic chemist Wallace Hume Carothers from Harvard University. Carothers designed an unambiguous experimental test. Rather than analyzing mysterious natural gums, he would build giant molecules from the bottom up using textbook organic reactions whose mechanisms no chemist could dispute.

Carothers began by reacting difunctional organic acids (carrying a carboxyl $-\text{COOH}$ group at both ends) with difunctional alcohols (carrying a hydroxyl $-\text{OH}$ group at both ends). Each time an acid end met an alcohol end, they joined via an ester linkage and split out one molecule of water. Because both ends of the new dimer still carried reactive groups, the chain could keep growing step by step. For two years, however, Carothers's polyesters stalled at molecular weights below $4,000\text{ daltons}$ and cooled into crumbly waxes. The bottleneck was chemical equilibrium. As the molten polymer grew thicker, trace droplets of water produced by the esterification reaction could not escape the viscous melt. That trapped water drove the reverse hydrolysis reaction and chopped the chains back apart.

In April 1930, Carothers and his assistant Julian Hill built a molecular still that placed a vacuum cold finger millimeters above the heated reaction flask to trap escaping water molecules instantly. When Hill dipped a heated glass rod into the resulting $12,000\text{-dalton}$ molten polyester and pulled it back, long glossy filaments stretched across the room. After the filaments cooled, pulling on the brittle threads did not snap them. Instead, the fiber necked down, stretched to four times its initial length, and turned into a tough, silky thread. X-ray diffraction proved that this cold drawing pulled the tangled polymer chains out of random coils and locked them into parallel crystalline bundles along the fiber axis.

Hill's superpolyester proved that synthetic chains could form strong fibers, yet it melted in hot water below $80^\circ\text{C}$ and dissolved in dry-cleaning solvents. Carothers realized why natural silk survives boiling water. Silk is a protein built from amide linkages ($-\text{CO}-\text{NH}-$), where every nitrogen-hydrogen group forms a strong hydrogen bond to a neighboring carbonyl oxygen. Switching from alcohols to diamines, Carothers and Gerard Berchet synthesized polyamide 6,6 (Nylon 6,6) on February 28, 1935, by condensing a six-carbon diamine (hexamethylenediamine) with a six-carbon diacid (adipic acid). Because both monomers contain six carbons, their amide groups space out evenly. Adjacent cold-drawn chains zip together with full hydrogen-bond registration, raising the melting point to $265^\circ\text{C}$.

To explain why trace impurities or slight imbalances between the two monomers ruined a batch of nylon, Carothers derived the governing equation of step-growth polymerization:

Here $\bar{X}_n$ is the number-average degree of polymerization—the average number of monomer units linked into each chain—and $p$ is the fractional extent of reaction, representing the proportion of functional end-groups that have reacted. In ordinary small-molecule synthesis, reaching $p = 0.90$ ($90\text{ percent}$ completion) is considered an excellent yield. In step-growth polymerization, however, $p = 0.90$ yields an average chain length of only $\bar{X}_n = 10$ monomer units, a brittle powder with zero fiber strength.

Reaching the $\bar{X}_n \ge 100$ chain length required for cold-drawing into high-tenacity parachute cord and hosiery demands driving reaction completion to at least $p = 0.99$ ($99\text{ percent}$). To guarantee exact $1\text{-to-}1$ monomer balance before heating the autoclave, DuPont chemists reacted hexamethylenediamine and adipic acid first in methanol to precipitate a crystalline $1\text{-to-}1$ nylon salt before polymerizing the melt. When DuPont placed Nylon 6,6 stockings on sale across the United States on May 15, 1940, retail stores sold all four million pairs within four days.