INTERCHAPTER S Synthetic Polymers The formation of nylon by a condensation polymerization reaction at the interface of water and hexane, two immiscible solvents. The lower water layer contains the compound hexanedioyl dichloride, Cl C(CH2)4C Cl O O hexanedioyl dichloride The reaction produces nylon and HCl(aq). The polymer forms at the interface between the two solutions and is drawn out as a continuous strand. University Science Books, ©2011. All rights reserved. www.uscibooks.com S. SYnthetic POLYMers S1 Some molecules contain so many atoms (up to tens ̣ ̣ HOCH2CH2 + H2C=CH2 → HOCH2CH2CH2CH2 of thousands) that understanding their structure would seem to be an impossible task. By recognizing The product of this step is also a free radical that can that many of these macromolecules exhibit recur- react with another ethylene molecule according to ring structural motifs, however, chemists have come ̣ to understand how these molecules are constructed HOCH CH CH CH + H C=CH → 2 2 2 2 2 2 ̣ and, further, how to synthesize them. These mol- HOCH2CH2CH2CH2CH2CH2 ecules, called polymers, fall into two classes: natu- ral and synthetic. Natural polymers include many The product here is a reactive chain that can grow of the biomolecules that are essential to life: pro- longer by the sequential addition of more ethylene teins, nucleic acids, and carbohydrates among them. molecules. The chain continues to grow until some Synthetic polymers—most of which were developed termination reaction, such as the combination of in just the last 60 or so years—include plastics, syn- two free radicals, occurs. The polyethylene molecules thetic rubbers, and synthetic fibers. We shall study formed in this manner typically contain thousands of synthetic polymers in this Interchapter and natural carbon atoms. polymers in the next one. The polymerization of ethylene can be written Enormous industries have been built around schematically as synthetic polymer chemistry, which has profoundly nH C=CH → —(CH CH )— changed the quality of life in the modern world. It 2 2 2 2 n monomer polymer is estimated that about half of all industrial research chemists are involved in some aspect of polymer The notation —(CH2CH2 )—n means that the group chemistry. Few of us have not heard of nylon, rayon, enclosed in the parentheses is repeated n times; it polycarbonate, polyester, polyethylene, polystyrene, also serves to identify the monomer unit. The free Teflon®, Formican®, and Saran, all of which are syn- radical that initiates the polymerization reaction is thetic polymers. The technological impact of polymer not indicated because n is large and thus the end chemistry is immense and continues to increase. group constitutes only a trivial fraction of the large polymer molecule. The precise number of monomer S-1. Polymers Are Composed of Many molecules incorporated into a polymer molecule is Molecular Subunits Joined End to End not important for typically large values of n. Polymer syntheses generally produce polymer molecules with The simplest very large molecule, or macromolecule, a range of n values. The polymer properties are is polyethylene. Polyethylene is formed by joining described in terms of the average value of n. It makes many ethylene molecules, H2C=CH2, end to end. little difference whether a polyethylene molecule con- The repeated addition of small molecules to form a sists of 5000 or 5100 monomer units, for example. long, continuous chain is called polymerization, and Polyethylene is a tough, flexible plastic that is used the resulting chain is called a polymer (poly = many; in the manufacture of packaging films and sheets, mer = unit). The small molecules or units from which wire and cable insulation, ice cube trays, refrigerator polymers are synthesized are called monomers. dishes, squeeze bottles, bags for foods and clothes, The polymerization of the monomer ethylenẹ trash bags, and many other articles. Other well-known can be initiated by a free radical, such as HO , the polymeric materials are made from other monomers. hydroxyl radical. (Recall from Section 7-7 that a free For example, polytetrafluoroethylene or Teflon® is radical is a species having one or more unpaired elec- produced from the monomer tetrafluoroethylene trons.) The first step in the polymerization of ethyl- according to ene is the reaction described by F F F F ̣ ̣ n C C C C HO + H2C=CH2 → HOCH2CH2 F F � �n F F The product is a free radical that can react with tetrafluoroethylene polytetrafluoroethylene another ethylene molecule to give (Teflon®) S2 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly Teflon® is a tough, nonflammable, and exceptionally cal insulation, plastic pipes, and cryogenic bearings. inert polymer with a slippery surface that is used for Table S.1 lists some other common polymers made nonstick surfaces in pots and pans, and in electri- from substituted alkenes. TabLE S.1 Common polymers prepared from substituted alkenes Names Monomer Applications Structural unit polyethylene H2C CH2 machine parts, toys, buckets, �CH2 CH2 �n ethylene containers, shrink wrap, bags, films, pipes, electrical insulation polypropylene HC CH2 food packaging, ropes, clothing, CH CH2 carpeting, molded plastic parts, CH �CH � 3 labware, pipes, valves 3 n propylene polychloroethylene, HC CH2 building materials, pipes, tubing, CH CH2 polyvinylchloride, PVC, electrical insulation, signs, toys Cl �Cl �n or vinyl vinyl chloride polychloroprene, Neoprene H2C C C CH2 high-quality automobile and garden H2 H2 H hoses, wet suits, gaskets C C Cl C C chloroprene �Cl H �n ® ® CH Plexiglas , Lucite , acrylic glass, 3 helmet visors, aquariums, hard CH3 polymethyl methacrylate contact lenses, dentures, office CH C CH C 2 furniture, artificial fingernails 2 C C H C O O �H3C O O �n 3 methyl methacrylate ® Teflon , polytetrafluoroethylene, F2C CF2 nonstick coatings; lubricant coatings F F or PTFE on certain mechanical parts tetrafluoroethylene �C C � F F n ® polyacrylonitrile, Orlon , H2C CH acrylic fabrics, tents, awnings, sails, CH2 CH Acrilan®, or acrylic fibers carpet, carbon fibers (for bikes, CN tennis rackets, auto bodies, etc.) �CN �n acrylonitrile polystyrene or expanded H2C CH packaging, insulation, modeling, CH2 CH polystyrene (Styrofoam™) biological labware, toys � � n styrene University Science Books, ©2011. All rights reserved. www.uscibooks.com S. SYnthetic POLYMers S3 S-2. Nylon and Dacron Are Made by The two monomers of nylon are linked together by an N C Condensation Reactions amide bond, . A condensation polymeriza- The polymerization reaction of ethylene is an addi- H O tion reaction, such as the synthesis of nylon, gener- tion polymerization reaction because it involves the ally involves two different monomers that combine to direct addition of monomer molecules. Another type form the basic repeating unit of the polymer. A poly- of polymerization reaction is a condensation polymer- mer made from more than one type of monomer is ization reaction. In such a reaction, a small molecule, known as a copolymer, whereas one made from just such as water, is split out as each monomer is added one type of monomer is called a monopolymer. to the polymer chain. The polymerization product Nylon was first synthesized by W. H. Carothers is called a condensation polymer. The synthesis of of the Dupont Chemical Company in 1931. His work nylon is an example of a condensation polymeriza- was carried on by his colleague Paul J. Flory, who was tion reaction. Nylon is formed by the reaction of a awarded the 1974 Nobel Prize in Chemistry for his diamino compound, such as 1,6-diaminohexane, research on the dynamics and physical properties of polymers (Figure S.1). Their research played a major H N CH2CH2CH2CH2CH2CH2 N H role in launching the U.S. polymer chemical indus- H H try. Over one thousand metric tons of nylon fiber are 1,6-diaminohexane produced annually in the United States. It is used to make strong, long-wearing fibers that find extensive and a dicarboxylic acid, such as hexanedioic acid use in rugs as well as hosiery, sweaters, and other (adipic acid), clothing. Nylon resembles silk in many of its proper- ties but is cheaper to produce. This resemblance to OH C CH CH CH CH C OH 2 2 2 2 silk is not at all coincidental. Silk is a protein, and, O O as we shall learn in Interchapter T, proteins are poly- hexanedioic acid (adipic acid) mers in which the monomers are linked together by amide bonds as in nylon. These two molecules can be linked by the reaction Polyethylene terephthalate, commonly known as described by the equation shown in the box below. Dacron®, is a condensation polymer typically formed The product in this reaction is called a dimer from the monomers ethane-1,2-diol (commonly called (di = two; mer = unit). Observe that one end of the ethylene glycol), HOCH2CH2OH, a dialcohol, and dimer is an amino group, –NH , and the other end 2 benzene-1,4-dicarboxylic acid (or terephthalic acid), is a carboxyl group, –COOH. The dimer can grow by the reaction of its amino end with a dicarboxylic acid HO C C OH monomer or by the reaction of its carboxyl end with a diamine monomer. Repetition of this process yields O O nylon (Frontispiece), whose general formula is benzene-1,4-dicarboxylic acid (terephthalic acid) a dicarboxylic acid. The reaction proceeds by the for- N CH2CH2CH2CH2CH2CH2 N C CH2CH2CH2CH2 C mation of an ester linkage between a carboxyl group �H H O O�n on the dicarboxylic acid and an –OH group of the nylon dialcohol. The basic polymer unit in Dacron® is H N CH2CH2CH2CH2CH2CH2 N H + OH C CH2CH2CH2CH2 C OH H H O O H N CH2CH2CH2CH2CH2CH2 N C CH2CH2CH2CH2 C OH + HOH H H O O S4 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly Figure S.1 Wallace H.