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The Origin of Nylon

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  • The world we live in is saturated by plastics, polymers with a seemingly infinite versatility. Some can withstand the most extreme changes in temperature, others are malleable under the most contorting pressures, and others are simply lightweight and convenient to use.

    The Roaring 20's was a time of great innovation and revolution in society, business, and music, but also in science, which especially helped business roar. Though scientists and educators today throw the term "polymer" around like it is common slang, scientists of the 20's were waging heated debates over the class of compounds. Due to polymers' huge molecular weights, it was widely doubted that polymers were even compounds in their own right, but rather separate, smaller molecules held together by some mysterious covalent force. One European disbeliever commented he could not comprehend a molecule with a molecular weight of 100,000 grams per mole any more than the idea of "an elephant... 1500 feet long and 300 feet high". For a contemporary reference, ultra-high molecular weight polyethylene can have a molecular weight between 2 and 6 million grams per mole.

    Figure \(\PageIndex{1}\) Wallace Carothers.

    In order to prove that polymers could be synthesized efficiently and be made commercially available (thus perhaps rendering the European academic wars moot) Wallace Carothers, a scientist at Dupont, an American company, experimented with common functional groups and condensation reactions. He knew that carboxylic acids and alcohols participated in condensation reactions to produce esters. Carothers made the key observation that if the reagent alcohol and carboxylic acid were difunctional (each "end" of the molecule had the same functional group), the condensation reaction could theoretically occur to form one huge polymer.

    Figure \(\PageIndex{2}\) A difunctional carboxylic acid.

    Figure \(\PageIndex{3}\) A difunctional alcohol (primary diol)

    The theory did hold true, but Carothers' initial polymers had molecular weights of "only" 5000 to 6000 grams per mol; polymers of this relatively small size were not functional enough to be mass-produced in industry. The problem was due to an entirely different type of chemistry: equilibrium. Polymerization did effectively produce large polyesters, but, as a condensation reaction, it also produced one molecule of water for each individual condensation of the (-OH) and (-COOH) group.

    One molecule at a time does not seem like much, but over time, Carothers' reaction vessels were flooded. Carothers realized that, by Le Chatelier's Principle, if huge amounts of product were allowed to remain in the system, the reaction would shift in opposition to the reactant side. In this case, the preponderance of water was causing the reaction to slow and eventually favor the difunctional alcohols and carboxylic acid reagents.

    By installing a "molecular still" to continuously drain water from the reaction vessel, Carothers and his lab were able to synthesize a colorless, stringy material that stuck to a glass rod and could be pulled and wound like spaghetti. Though the Great Depression slowed Dupont's progress and resulted in workers being cut, Carothers was encouraged to continue in his work and find optimal difunctional groups for the most cost-efficient polymers.

    Carothers' group switched from polyethers to polyamides, chains created by the condensation reactions of diamines and dicarboxylic acids. Specifically, hexamethylenediamine and adipic acid reacted to produce the polymer dubbed "nylon 6,6".

    Figure \(\PageIndex{4}\) The repeated unit of nylon 6. This polymer differs from nylon 6,6 in that it consists a single monomer (caprolactam, a cyclic compound with an amine group and a carbonyl group on separate carbons), whereas nylon 6,6 utilizes two monomers.

    Nylon was an instant hit in the United States; 5 million pairs of nylon womens' stockings sold out on their first day at market. Though he had created the first commercially available synthetic fiber and was elected to the National Academy of Science, Carothers suffered from severe mental depression. In 1937, he mixed a cocktail of lemon juice and cyanide and committed suicide; due to his efforts, however, the synthetic fiber and plastic industry took off to the omnipresent powerhouse it is today.

    From ChemPRIME: 8.24: Condensation Polymers

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