Polymers, Synthetic

Polymers are large molecules composed of repeated chemical units. The smallest repeating unit is called a mer. The term polymer is derived from the Greek words poly and mers meaning "many parts." Linear polymers are like ropes. For a polymer chain of 10,000 units (a typical length), a standard half-inch-thick rope would be about 128 meters (140 yards) long to represent the length-to-thickness ratio. Polymers are synthesized naturally and artificially to perform a wide variety of specialized tasks.

Basic Polymer Science

A polymer is generally described in terms of a single repeat unit, such as the following example:

The number of repeat units in a chain is called the degree of polymerization (DP) or chain length. Thus, a poly(propylene) chain 5,000 units long would have a DP of 5,000 and an "n" value of 5,000. Because most polymer mixtures contain chains of varying lengths, the chain length is often referred to in terms of average chain length or average DP.

At either end of the polymer chain are end groups. (Because the chain is often thousands of units long, the end groups are usually omitted.) For (poly)propylene (shown in Figure 1) the repeating carbons (C-C-C-C-C-C-C) form the polymer backbone and represent the atoms that connect the chain together. In vinyl polymers, so called because they are generally derived from substituted vinyl reactants or monomers (Figure 2), the polymer backbone is composed of only carbon atoms. An example is poly(propylene), which has five mers represented (Figure 3).

Condensation polymer backbones include non-carbon atoms. For example, polyesters have oxygen atoms and nylons have nitrogen atoms in the backbone in addition to carbon atoms (Figure 4).

Unsymmetrical reactants, such as substituted vinyl monomers, react almost exclusively to give what are called "head-to-tail" products where the substituents occur on alternative carbon atoms:

Figure 5. –CH₂–CHX–CH₂–CHX–CH₂–CHX–CH₂–CHX–

Occasionally a "head-to-head, tail-to-tail" configuration occurs. For most vinyl polymers this structure occurs less than 1 percent of the time in a random manner throughout the chain.

Figure 6. –CH₂–CHX–CHX–CH₂–CH₂–

Even with the head-to-tail configuration, a variety of structures are possible. These include a simple linear homopolymer structure

Figure 7. –M–M–M–M–M–M–M–M–M–M–M–M–M–M–M–M–M–

and branched structures with varying amounts and lengths of branching.

Copolymers are polymers derived from two different monomers (M andN). Saran, a component of Saran Wrap, is one example (Figure 9). Other examples of copolymer structures are depicted in Figure 10.

Some linear chains have extensions (beyond the substitution) coming off the polymer backbone. These extensions are called branches and influence a polymer's properties. Branches may be long or short, frequent or infrequent. For example, so-called low density polyethylene (LDPE) has between forty and one hundred short branches for every 1,000 ethylene units, whereas high density polyethylene (HDPE) has only one to six short branches for every 1,000 ethylene units (Figure 11). Branching discourages the chains from fitting close together so that the structure will be amorphous with relatively large amounts of empty space. Regular structures with little or no branching allow the polymer chains to fit close together, forming a crystalline structure. Crystalline structures are generally stronger, more brittle, of higher density, more resistant to chemical penetration and degradation, less soluble, and have higher melting points. For example, HDPE has a density of 0.97 gram per milliliter and a melting point of about 130°C (266°F), whereas LDPE has a density of about 0.92 gram per milliliter and a melting point of about 100°C (212°F).

Polymer chains can be connected to one another chemically or physically, much like a knot can connect two pieces of string. These connections are called crosslinks and cause the connected chains to act as a single unit

(Figure 12). Some materials can have only a few crosslinks, such as permanent press materials where the fabric contour is locked into place with crosslinks. Others materials such as Bakelite and ebonite are heavily crosslinked; these are hard, brittle, non-flexible materials.

Physical Properties of Polymers

The properties of polymers are dependent on many factors including inter- and intrachain bonding, the nature of the backbone, processing events, presence/absence of additives including other polymers, chain size and geometry, and molecular weight distribution.

While most materials have melting/freezing and boiling/condensing points, polymers do not boil because the energy necessary to put a polymer into the vapor state is greater than the bond energies of the atoms that hold the polymer together, thus they degrade prior to boiling. In order for a polymer to be flexible, its various units or segments must be able to move. The glass transition temperature (T_g) is the temperature where polymer units or segments can move but the entire chain cannot. Most vinyl polymers have T_g values below room temperature so that they appear to be flexible and act as rubber and plastic materials. Most condensation polymers have T_g values above room temperature and are used as hard plastics and fibers. The temperature where entire chain movement occurs is called the melting point (T_m) and is greater than the T_g.

Many polymers are themselves brittle at room temperature. For these polymers to become more pliable, additives called plasticizers that allow segmental mobility, and consequently segmental flexibility, are added. For synthetic polymers such as poly(vinyl chloride) (PVC) and polystyrene (Figure 13), plasticizers are added that allow the polymers to be flexible.

The inflexible regions of a polymer, such as crystalline regions, are often referred to as "hard" regions. Conversely, the flexible regions of a polymer, where segmental mobility occurs, are referred to as "soft" regions. This combination of hard and soft can be illustrated with so-called segmented polyurethanes (Figure 14). The urethane portion of such polymers is involved in hydrogen bonding and is considered "hard," while the polyether portion, flexible at room temperature, is considered "soft." These segmented polyurethanes are sold under a number of trade names including Spandex.

History of Synthetic Polymers

While polymers form the basis of life, the history of synthetic polymers is relatively recent. Some of the key polymers that have been developed since the early days of polymer science include:

Vulcanized rubber. In the mid-1800s, American scientist Charles Goodyear began working with rubber to try to make it more temperature stable. After many unsuccessful attempts, he accidentally allowed a mixture of sulfur and pre-rubber to touch a hot stove. The rubber did not melt but only charred a little. By 1844 Goodyear had been given a patent for a process he called "vulcanization" after the Roman god of fire, Vulcan. Vulcanization is the crosslinking reaction between the rubber chains and the sulfur.

Bakelite. After years of work in his chemistry lab in Yonkers, New York, Leo Baekeland announced in 1907 the synthesis of the first truly synthetic polymeric material, later dubbed "Bakelite." It was generally recognized by leading organic chemists of the nineteenth century that phenol would condense with formaldehyde, but because they did not understand the principles of the reaction, they produced useless crosslinked materials. Baekeland's main project was to make hard objects from phenol and formaldehyde and then dissolve the product to reform it again in a desired shape. He circumvented the problem by placing the reactants directly in a mold of the desired shape and then allowing the reactants to form a hard, clear solid—Bakelite (Figure 15). It could be worked (i.e., cut, drilled, and sanded), was resistant to acids and organic liquids, was stable at high temperatures, and did not break down when exposed to electrical charge. By adding dyes to the starting materials the objects became available in different colors. Bakelite was used to make bowling balls, phonograph records, telephone housings, cookware, and billiard balls. Bakelite also acted as a binder for textiles, sawdust, and paper, forming a wide range of composites including Formica laminates. Many of these combinations are still in use in the twenty-first century.

Neoprene. Chemist and Catholic priest Julius A. Nieuwland did extensive work in the 1920s on acetylene. He found that acetylene could be made to add to itself forming dimers and trimers. Arnold Collins, a chemist at the Dupont Company in the lab of Wallace Carothers, continued work on the project and in 1930 ran the reaction described by Nieuwland, purifying the reaction mixture. He found a small amount of material that was not vinylacetylene or divinylacetylene. After setting the liquid aside, it solidified into a material that seemed rubbery and even bounced. This new rubber was given the name Neoprene (Figure 16). Neoprene has outstanding resistance to gasoline, ozone, and oil in contrast to natural rubber and is used in a variety of applications including electrical cable jacketing, window gaskets, shoe soles, industrial hose, and heavy-duty drive belts.

Nylon. In the early 1930s Wallace Carothers and his team of chemists at Dupont were investigating synthetic fibers in order to find a synthetic alternative to silk. One promising candidate was formed from the reaction of adipic acid with hexamethylenediamine and was called fiber 66 because each monomer-containing unit had six carbons. It formed a strong, elastic, largely insoluble fiber with a relatively high melting temperature. DuPont chose this material for production. Such polyamides were given the name "nylons"; thus was born nylon 6,6 (Figure 17).

Poly(vinyl chloride). While PVC was initially formed by German chemist Eugen Baumann in 1872, scientists at B. F. Goodrich discovered in 1926 how to make sheets and adhesives from it, starting the "vinyl age." PVC's many applications include water pipes and joints, building materials, food packaging, wire insulation, and medical components.

Polystyrene. While polystyrene was probably first formed by German apothecary Eduard Simon in 1839, it was almost 100 years later, in 1930, that the German chemical company I. G. Fraben placed polystyrene on the market. Polystyrene-molded parts became common place by 1935. Applications of polystyrene include loose-fill packaging "peanuts," shape-molded packaging, and disposable utensils.

Polyacrylonitrile. Rohm and Haas Company bought out Plexiglas (polyacrylonitrile [Figure 18]; also known as acrylic and as a fiber sold under tradenames such as Orlon) from a British firm in 1935 and began production of clear plastic parts and goods, including replacements for glass in camera lenses, aircraft windows, clock faces, and car tail lights.

Poly(vinyl butyral). The polymer poly(vinyl butyral) (PVB) was first used in automotive safety glass in 1938 to prevent flying glass resulting from automobile accidents and continues to be utilized in the twenty-first century for this purpose (Figure 19).

Other important synthetic polymers. World War II helped shape the future of polymers. Wartime demands and shortages encouraged scientists to seek substitutes and materials that exceeded currently available materials. During and after the war new materials were developed, spurred by needs in the electronics, medical, communications, food, aerospace, and other industries. The aromatic nylons (armids) Kevlar (capable of stopping a speeding bullet and used as tire cord) and Nomex (used in constructing fire-resistant garments) were developed. Polycarbonates sold under the trade names of Merlon and Lexon were developed that substituted for glass in many automotive products such as tail lights. Other key developments included polytetrafluoroethylene, a slick material also known as Teflon; polysiloxanes, also know as silicones, which have an extremely wide temperature-use range and were a component of the soles of the shoes that first touched the moon; and polyester fibers and plastics such as poly(ethylene terephthalate) (PET), used in carbonated drink bottles (Figure 20).

Even with this early commercial activity, little was actually known about polymers. German chemist Herman Staudinger studied the polymerization of isoprene (a five-carbon hydrocarbon containing a double bond that is obtained as a product of the degradation of natural rubber by heating) as early as 1910. Intrigued by the difference between this synthetic material and natural rubber he began to study giant molecules. Many of his fellow scientists told him there was no such thing as giant molecules and that he was wasting his time. By 1920 he published a summary of his studies and correctly proposed linear structures for polystyrene and polyoxymethylene. X-ray studies were used to support the concept of macromolecules.

Wallace Hume Carothers is considered to be the father of synthetic polymer science. In 1927 the DuPont Company began a program of fundamental research in the areas of colloid chemistry, catalysis, organic synthesis, and polymer formation. Carothers, then a Harvard instructor, was persuaded to join the DuPont group. Carothers looked at the construction of giant molecules from small molecules to form synthetic polymers. His intention was to prepare molecules of known structure through the use of known organic chemistry and to "investigate how the properties of these substances depended on constitution." Over the course of his career, Carothers filed for over fifty patents and was involved in the discovery of nylon and the synthetic rubber neoprene.

From his studies Carothers established several concepts. First, polymers could be formed by employing already known organic reactions but with reactants that had more than one reactive group per molecule. Second, the forces that bring together the individual polymer units are the same as those that hold together the starting materials: namely, primary covalent bonds. Much of the polymer chemistry names and ideas that permeate polymer science were standardized through his efforts.

Types of Synthetic Polymers

Elastomers. Elastomers are polymers possessing chemical and/or physical crosslinks (Table 1 and 2). These crosslinks allow the stretched, deformed segments to return to their original locations after the force is removed. The "use" temperature must be above the T_g to allow ready chain slippage as the rubbery material is flexed and extended. The forces between the chains should be minimal to allow easy movement of these chain segments. Finally, the chains must be present in an amorphous, disorganized fashion. As force is applied and the material distorts or elongates, the randomly oriented chains are forced to align and take more ordered positions with the chains, forming crystalline regions that resist ready movement. As the force is removed the material has a tendency to return to its original disorganized state and therefore its pre-stretched shape. The formation of the crystalline regions as the material is stretched gives the material a greater tensile strength(i.e. an increased force is necessary for further elongation) at high extensions. Crosslinked vinyl polymers are ideal materials to be used in elastomers: the attractive forces between chains is low and their T_g is below room temperature.

Thermosets and thermoplastics. Thermosets are materials that have sufficient crosslinking present so that they are prevented from being soluble and melting when heated. Such materials are therefore difficult to recycle. Thermoplastics are materials that melt on heating and generally contain little or no crosslinking. They can be recycled more easily through heating and reforming. Linear polymers are thermoplastic materials.

Fibers. Fibers require materials with a high tensile strength and high modulus (high force required for elongation). This requires polymers with strong forces between the chains and chains that are symmetrical to allow for good crystalline formation. Condensation polymers exhibit these properties and so are most utilized as fibers. Fibers are normally linear and drawn (pulled) in one direction, producing higher mechanical properties in that direction. If the fiber is to be ironed, its T_g should be above 200^oC. Branching and crosslinking are undesirable since they inhibit crystalline formation. Even so, some crosslinking may be present to maintain a given orientation, such as desired in permanent press clothing. While most fibers are made from condensation polymers, new treatments allow some fibers to be made from olefinic materials such as polypropylene (Table 3).

Plastics. Plastics require properties that are intermediate between elastomers and fibers. Engineering plastics can be readily machined, cut, and drilled. Condensation polymers are typically engineering plastics while vinyl polymers are typically plastics. Table 4 contains a listing of the most common engineering plastics and plastic materials and Table 5 the volume of engineering plastics and plastics produced in the United States.

Coatings. Coatings and adhesives are generally derived from polymers that are considered to be plastics, although there are major groups that do not. For instance, silicone rubbers are elastomers that can be used as adhesives. Coatings, or coverings, are generally highly viscous (low flowing) materials. Coatings protect surfaces from the degradative effects of oils, oxidative chemical agents, extreme temperatures, rain, snow, and ionizing radiation. Coatings must adhere to the surface they are applied to. Coatings are typically a mixture of a liquid (vehicle or binder/adhesive) and one or more colorants (pigments). Coatings often also contain a number of so-called additives that can furnish added protection against ionizing radiation, increase the rate of drying and/or curing (crosslinking), and prevent microorganism growth. Coatings are specially formulated for specific purposes and locations and can be divided into five groups:

• Oil paints consist of a suspension of pigment (colorant) in a drying oil such as linseed oil.
• Oil varnishes consist of a polymer, either natural or synthetic, dissolved in a drying oil together with the necessary additives such as catalyst that promotes crosslinking of the drying oil.
• Enamels are oil varnishes with pigment added.
• Lacquers are polymer solutions to which pigments have been added.
• Latex paints are polymer latexes, often poly(methyl methacrylate) and polyacrylonitrile, to which pigments have been added. They account for well over one half of the commercial paint used.

Hardening or drying consists of removal of solvent (evaporation) and/or crosslinking of a drying oil that contains C=C units.

Adhesives. In contrast to coatings that must adhere to only one surface, adhesives are used to join two surfaces together (Table 6). Adhesion for both adhesives and coatings can occur through a number of mechanisms including physical interlocking, chemical adhesion where primary bonding occurs between the adhesive and the surfaces being joined, secondary bonding where hydrogen bonding or polar bonding occurs, and viscosity adhesion where movement is restricted because of the viscous nature of the adhesive material. Adhesives can be divided according to the type of delivery of the adhesive or by the type of polymer:

• Solvent-based adhesives like model airplane glue contain a volatile solvent that dissolves part of the plastic and when dry forms a solvent weld.
• Pressure-sensitive adhesives like those used on Post-It-Notes often contain the same adhesive material used in more permanent adhesives like Scotch Tape except in lesser amounts.
• Reactive adhesives are short chained polymers or monomers that solidify through polymerization or crosslinking after application.
• Plywood is formed from the impregnation of thin sheets of wood with resin that dries after the sheets are pressed together. Phenolic thermosets such as those developed by Bakelite are often used as the resins for plywood.
• Adhesives made from cyanoacrylates are among the best known adhesives, sold under trade names such as Super Glue and Crazy Glue. Monomers such as butyl-alpha-cyanoacrylate (Figure 16) polymerize spontaneously in the presence of moisture. The presence of the cyano and acrylate groups, both quite polar, makes this a particularly good adhesive; it is used in surgery and for mechanical assemblies.

Sealants and caulks. Sealants and caulks provide a barrier to the passage of gases, liquids, and solids; maintain pressure differences; and moderate mechanical and thermal shock. While adhesives are used for "load transfer" and require high tensile and shear strengths, sealants act as insulators and shock attenuators and do not require high tensile and shear strengths.

Films and sheeting. Films are two-dimensional forms of plastic, thick enough to be coherent, but thin enough to be flexed, creased, or folded without cracking. Most films are produced from materials from the elastomeric and plastic categories. Sheeting is a two-dimensional form of plastic that is thicker (generally greater than 250 micrometers) than film and is generally not easily flexed, creased, or folded without cracking.

Composites. Composites are materials that contain strong fibers or reinforcement embedded in a continuous phase called a matrix. They are found in jet fighters such as stealth fighters and bombers, in the "reusable" space shuttle, in graphite golf clubs, in synthetic human body parts, and for many years in marine craft (fibrous glass).

Laminates. The combination of an adhesive and an adherent is a laminate, a type of composite. Commercial laminates are produced on a large scale with wood as the adherent and phenolic, urea, epoxy, resorcinol, or polyester resins as the adhesive. Plywood is an example of a laminate. Laminates of paper or textile include Formica and Micarta. Laminates of phenolic, nylon, or silicone resins with cotton, asbestos, paper, or glass textiles are used as mechanical, electrical, and general purpose structural materials.

Conductive polymers. Most polymers are nonconductive and polymers such as polyethylene, polypropylene and polytetrafluoroethylene (Teflon) are used as insulators. Even so, some polymers have been found to conduct electricity. An example is polyacetylene; oxidation with chlorine, bromine, or iodine vapor makes polyacetylene film 10⁹ (1,000,000,000) times more conductive than the non-treated film (Figure 17). This treatment with a halogen is called "doping." Other polymers including polyaniline, polythiophene, and polypyrrole have been found to be conducting after doping and these materials are now being used in a variety of applications (Figure 18). Doped polyaniline is employed as a conductor and as an electromagnetic shielding for electronic circuits. Polythiophene derivatives are used in field-effect transistors. Polypyrrole is used in microwave-absorbing "stealth" screen coatings and in sensing devices. Poly(phenylene vinylidene) derivatives are used in the production of electroluminescent displays.

Polymer Synthesis

The process by which polymers are formed from monomers is called polymerization. Polymerization occurs by one of two basic reactions: addition or condensation. In addition polymerization, entire monomers are linked together to form long chains. In condensation polymerization, some small molecules (such as water) are released as polymer is formed.

Polymerization reactions may be divided into two major categories: stepwise processes and chain-type processes. In the step-wise process, reactants are brought together and heated. Initially short chains are formed and only at the end of the reaction are long chains formed. Reactions generally require hours to form the polymers. It is by this process that condensation polymers are generally made.

Vinyl polymers are formed using a chain-type process that involves three steps:

• Initiation. This first step requires that the monomer's double bond is broken. This can occur by means of heat or light, or by the addition of other chemical compounds that have less stable bonds. The decomposition products of these chemical compounds add to the vinyl monomer, causing the double bond to break. These materials are called initiators because they start the polymerization process.
• Propagation. This second step involves growth of the polymer chain by the addition of monomer units. This occurs rapidly, within fractions of a second.
• Termination. Finally, the growth of the chain is stopped (terminated).

The process of initiation, growth, and termination continues until the monomer is consumed. Reactions often occur at or below room temperature.

Synthetic Routes

Starting materials are often referred to as feedstocks. Most of the starting materials (monomers) employed in the synthesis of synthetic polymers like polystyrene, polyethylene, and nylons are derived indirectly from fossil fuels. The term fossil fuels refer to materials formed from the decomposition of once-living matter.

There are four basic routes by which polymers are synthesized industrially:

Melt process. Also referred to by other names including high melt, bulk melt, bulk, or neat. The melt process is an equilibrium-controlled process in which polymer is formed by driving the reaction toward completion, usually through removal of the byproduct or condensate. Thus, in the reaction of a diacid and a diol to form a polyester, water is removed, causing the reaction to proceed towards polymer formation. The reactants are employed "neat" (without solvent); any other needed materials such as catalysts are added to the reaction vessel. Heat is applied to melt the reactants, permitting them to come into contact with one another. Additional heat can be added and the pressure reduced, but heat control is important since most of these reactions are exothermic. These reactions typically take several hours to days before the desired polymer is formed. The product yield is necessarily high.

Solution process. Solution polymerizations are also equilibrium processes, with the reaction also often driven by removal of the small byproduct. The product may be recovered from the reaction system through addition of the reaction liquid to a non-solvent, removal of the solvent, or direct precipitation of the polymer from the reaction system. Because the reaction is often run at a lower temperature, more reactive reactants are generally required.

Suspension process. Water-insoluble monomers can be polymerized as suspended droplets in a process called suspension polymerization. Coalescing of the droplets is prevented by use of small amounts of water-soluble polymers such as poly(vinyl alcohol).

Emulsion process. The emulsion process differs from suspension polymerization in the size of the suspended particles and in the mechanism.

Polymer Companies

About 10,000 companies in the United States are active in synthetic polymers. These companies can be divided into three groupings:

Manufacturers. Over 200 companies produce the "bulk" polymers that are used by the other two groupings of companies. While most of these produce the bulk polymers in large quantities, some produce what are called "specialty polymers," those used in special applications on a small scale.

Processors. While some companies produce their own polymers, most purchase the raw polymer material from a manufacturing company. Processors may specialize in the use of selected polymers, such as polypropylenes, polyethylenes, or nylons; in a particular mode of processing; or in the production of particular markets such as films, sheets, laminates, adhesives, or coatings.

Fabricators and finishers. The large majority of companies are involved in the fabrication and finishing of polymer-containing products. Fabrication can be divided into three broad areas: machining, forming, and fashioning.

see also Adhesives; Fibers; Nylons; Plastics; Polyesters; Polymers, Natural.

Charles E. Carraher Jr.

Bibliography

Carraher, Charles E., Jr. (2003). Giant Molecules: Essential Materials for Everyday Living and Problem Solving, 2nd edition. Hoboken, NJ: Wiley.

Carraher, Charles E. Jr. (2003). Polymer Chemistry. New York: Dekken.

Craver, Clara D., and Carraher, Charles E., Jr. (2000). Applied Polymer Science: 21st Century. New York: Elsevier.

Morawetz, Herbert (1995). Polymers: The Origins and Growth of a Science. New York: Dover.

Salamone, Joseph C., ed. (1999). Concise Polymeric Materials Encyclopedia.M Boca Raton, FL: CRC Press.

POLYMERS

CONCEPT

Formed from hydrocarbons, hydrocarbon derivatives, or sometimes from silicon, polymers are the basis not only for numerous natural materials, but also for most of the synthetic plastics that one encounters every day. Polymers consist of extremely large, chain-like molecules that are, in turn, made up of numerous smaller, repeating units called monomers. Chains of polymers can be compared to paper clips linked together in long strands, and sometimes cross-linked to form even more durable chains. Polymers can be composed of more than one type of monomer, and they can be altered in other ways. Likewise they are created by two different chemical processes, and thus are divided into addition and condensation polymers. Among the natural polymers are wool, hair, silk, rubber, and sand, while the many synthetic polymers include nylon, synthetic rubber, Teflon, Formica, Dacron, and so forth. It is very difficult to spend a day without encountering a natural polymer—even if hair is removed from the list—but in the twenty-first century, it is probably even harder to avoid synthetic polymers, which have collectively revolutionized human existence.

HOW IT WORKS

Polymers of Silicon and Carbon

Polymers can be defined as large, typically chain-like molecules composed of numerous smaller, repeating units known as monomers. There are numerous varieties of monomers, and since these can be combined in different ways to form polymers, there are even more of the latter.

The name "polymer" does not, in itself, define the materials that polymers contain. A handful of polymers, such as natural sand or synthetic silicone oils and rubbers, are built around silicon. However, the vast majority of polymers center around an element that occupies a position just above silicon on the periodic table: carbon.

The similarities between these two are so great, in fact, that some chemists speak of Group 4 (Group 14 in the IUPAC system) on the periodic table as the "carbon family." Both carbon and silicon have the ability to form long chains of atoms that include bonds with other elements. The heavier elements of this "family," however (most notably lead), are made of atoms too big to form the vast array of chains and compounds for which silicon and carbon are noted.

Indeed, not even silicon—though it is at the center of an enormous range of inorganic compounds—can compete with carbon in its ability to form arrangements of atoms in various shapes and sizes, and hence to participate in an almost limitless array of compounds. The reason, in large part, is that carbon atoms are much smaller than those of silicon, and thus can bond to one another and still leave room for other bonds.

Carbon is such an important element that an entire essay in this book is devoted to it, while a second essay discusses organic chemistry, the study of compounds containing carbon. In the present context, there will be occasional references to non-carbon (that is, silicon) polymers, but the majority of our attention will be devoted to hydrocarbon and hydrocarbon-derivative polymers, which most of us know simply as "plastics."

Organic Chemistry

As explained in the essay on Organic Chemistry, chemists once defined the term "organic" as relating only to living organisms; the materials that make them up; materials derived from them; and substances that come from formerly living organisms. This definition, which more or less represents the everyday meaning of "organic," includes a huge array of life forms and materials: humans, all other animals, insects, plants, microorganisms, and viruses; all substances that make up their structures (for example, blood, DNA, and proteins); all products that come from them (a list diverse enough to encompass everything from urine to honey); and all materials derived from the bodies of organisms that were once alive (paper, for instance, or fossil fuels).

As broad as this definition is, it is not broad enough to represent all the substances addressed by organic chemistry—the study of carbon, its compounds, and their properties. All living or once-living things do contain carbon; however, organic chemistry is also concerned with carbon-containing materials—for instance, the synthetic plastics we will discuss in this essay—that have never been part of a living organism.

It should be noted that while organic chemistry involves only materials that contain carbon, carbon itself is found in other compounds not considered organic: oxides such as carbon dioxide and monoxide, as well as carbonates, most notably calcium carbonate or limestone. In other words, as broad as the meaning of "organic" is, it still does not encompass all substances containing carbon.

Hydrocarbons

As for hydrocarbons, these are chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. Every molecule in a hydrocarbon is built upon a "skeleton" of carbon atoms, either in closed rings or in long chains, which are sometimes straight and sometimes branched.

Theoretically, there is no limit to the number of possible hydrocarbons: not only does carbon form itself into seemingly limitless molecular shapes, but hydrogen is a particularly good partner. It is the smallest atom of any element on the periodic table, and therefore it can bond to one of carbon's four valence electrons without getting in the way of the other three.

There are many, many varieties of hydrocarbon, classified generally as aliphatic hydrocarbons (alkanes, alkenes, and alkynes) and aromatic hydrocarbons, the latter being those that contain a benzene ring. By means of a basic alteration in the shape or structure of a hydrocarbon, it is possible to create new varieties. Thus, as noted above, the number of possible hydrocarbons is essentially unlimited.

Certain hydrocarbons are particularly useful, one example being petroleum, a term that refers to a wide array of hydrocarbons. Among these is an alkane that goes by the name of octane (C₈H₁₈), a preferred ingredient in gasoline. Hydrocarbons can be combined with various functional groups (an atom or group of atoms whose presence identifies a specific family of compounds) to form hydrocarbon derivatives such as alcohols and esters.

REAL-LIFE APPLICATIONS

Types of Polymers and Polymerization

Many polymers exist in nature. Among these are silk, cotton, starch, sand, and asbestos, as well as the incredibly complex polymers known as RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), which hold genetic codes. The polymers discussed in this essay, however, are primarily of the synthetic kind. Artificial polymers include such plastics (defined below) as polyethylene, styrofoam, and Saran wrap; fibers such as nylon, Dacron (polyester), and rayon; and other materials such as Formica, Teflon, and PVC pipe.

As noted earlier, most polymers are formed from monomers either of hydrocarbon or hydro-carbon derivatives. The most basic synthetic monomer is ethylene (C₂H₄), a name whose-ene ending identifies it as an alkene, a hydrocarbon formed by double bonds between carbon atoms. Another alkene hydrocarbon monomer is butadiene, whose formula is C₄H₆. This is an example of the fact that the formula of a compound does not tell the whole story: on paper, the difference between these two appears to be merely a matter of two extra atoms each of carbon and hydrogen. In fact, butadiene's structure is much more complex.

Still more complex is styrene, which includes a benzene ring. Several other monomers involve other elements: chloride, in vinyl chloride; nitrogen, in acrylonitrile; and fluorine, in tetrafluoroethylene. It is not necessary, in the present context, to keep track of all of these substances, which in any case represent just some of the more prominent among a wide variety of synthetic monomers. A good high-school or college chemistry textbook (either general chemistry or organic chemistry) should provide structural representations of these common monomers. Such representations will show, for instance, the vast differences between purely hydrocarbon monomers such as ethylene, propylene, styrene, and butadiene.

When combined into polymers, the monomers above form the basis for a variety of useful and familiar products. Once the carbon double bonds in tetrafluoroethylene (C₂F₄) are broken, they form the polymer known as Teflon, used in the coatings of cooking utensils, as well as in electrical insulation and bearings. Vinyl chloride breaks its double bonds to form polyvinyl chloride, better known as PVC, a material used for everything from plumbing pipe to toys to Saran wrap. Styrene, after breaking its double bonds, forms polystyrene, used in containers and thermal insulation.

POLYMERIZATION.

Note that several times in the preceding paragraph, there was a reference to the breaking of carbon double bonds. This is often part of one variety of polymerization, the process whereby monomers join to form polymers. If monomers of a single type join, the resulting polymer is called a homopolymer, but if the polymer consists of more than one type of monomer, it is known as a copolymer. This joining may take place by one of two processes. The first of these, addition polymerization, is fairly simple: monomers add themselves to one another, usually breaking double bonds in the process. This results in the creation of a polymer and no other products.

Much more complex is the process known as condensation polymerization, in which a small molecule called a dimer is formed as monomers join. The specifics are too complicated to discuss in any detail, but a few things can be said here about condensation polymerization. The monomers in condensation polymerization must be bifunctional, meaning that they have a functional group at each end. When characteristic structures at the ends of the monomers react to one another by forming a bond, they create a dimer, which splits off from the polymer. The products of condensation polymerization are thus not only the polymer itself, but also a dimer, which may be water, hydrochloric acid (HCl), or some other substance.

A Plastic World

A DAY IN THE LIFE.

Though "plastic" has a number of meanings in everyday life, and in society at large (as we shall see), the scientific definition is much more specific. Plastics are materials, usually organic, that can be caused to flow under certain conditions of heat and pressure, and thus to assume a desired shape when the pressure and temperature conditions are withdrawn. Most plastics are made of polymers.

Every day, a person comes into contact with dozens, if not hundreds, of plastics and polymers. Consider a day in the life of a hypothetical teenage girl. She gets up in the morning, brushes her teeth with a toothbrush made of nylon, then opens a shower door—which is likely to be plastic rather than glass—and steps into a molded plastic shower or bathtub. When she gets out of the shower, she dries off with a towel containing a polymer such as rayon, perhaps while standing on tile that contains plastics, or polymers.

She puts on makeup (containing polymers) that comes in plastic containers, and later blow-dries her hair with a handheld hair dryer made of insulated plastic. Her clothes, too, are likely to contain synthetic materials made of polymers. When she goes to the kitchen for breakfast, she will almost certainly walk on flooring with a plastic coating. The countertops may be of formica, a condensation polymer, while it is likely that virtually every appliance in the room will contain plastic. If she opens the refrigerator to get out a milk container, it too will be made of plastic, or of paper with a thin plastic coating. Much of the packaging on the food she eats, as well as sandwich bags and containers for storing food, is also made of plastic.

And so it goes throughout the day. The phone she uses to call a friend, the computer she sits at to check her e-mail, and the stereo in her room all contain electrical components housed in plastic. If she goes to the gym, she may work out in Gore-tex, a fabric containing a very thin layer of plastic with billions of tiny pores, so that it lets through water vapor (that is, perspiration) without allowing the passage of liquid water. On the way to the health club, she will ride in a car that contains numerous plastic molds in the steering wheel and dashboard. If she plays a compact disc—itself a thin wafer of plastic coated with metal—she will pull it out of a plastic jewel case. Finally, at night, chances are she will sleep in sheets, and with a pillow, containing synthetic polymers.

A SILENT REVOLUTION.

The scenario described above—a world surrounded by polymers, plastics, and synthetic materials—represents a very recent phenomenon. "Before the 1930s," wrote John Steele Gordon in an article about plastics for American Heritage, "almost everything people saw or handled was made of materials that had been around since ancient times: wood, stone, metal, and animal and plant fibers." All of that changed in the era just before World War II, thanks in large part to a brilliant young American chemist named Wallace Carothers (1896-1937).

By developing nylon for E. I. du Pont de Nemours and Company (known simply as "DuPont" or "du Pont"), Carothers and his colleagues virtually laid the foundation for modern polymer chemistry—a field that employs more chemists than any other. These men created what Gordon called a "materials revolution" by introducing the world to polymers and plastics, which are typically made of polymers.

Yet as Gordon went on to note, "It has been a curiously silent revolution…. When we think of the scientific triumphs of [the twentieth century], we think of nuclear physics, medicine, space exploration, and the computer. But all these developments would have been much impeded, in some cases impossible, without… plastics. And yet 'plastic' remains, as often as not, a term of opprobrium."

AMBIVALENCE TOWARD PLASTICS.

Gordon was alluding to a cultural attitude discussed in the essay on Organic Chemistry: the association of plastics, a physical material developed by chemical processes, with the condition—spiritual, moral, and intellectual—of being "plastic" or inauthentic. This was symbolized in a famous piece of dialogue about plastics from the 1967 movie The Graduate, in which a nonplussed Ben Braddock (Dustin Hoffman) listens as one of his parents' friends advises him to invest his future in plastics. As Gordon noted, "however intergenerationally challenged that half-drunk friend of Dustin Hoffman's parents may have been… he was right about the importance of the materials revolution in the twentieth century."

One aspect of society's ambivalence over plastics relates to very genuine concerns about the environment. Most synthetic polymers are made from petroleum, a nonrenewable resource; but this is not the greatest environmental danger that plastics present. Most plastics are not biodegradable: though made of organic materials, they do not contain materials that will decompose and eventually return to the ground. Nor is there anything in plastics to attract microorganisms, which, by assisting in the decomposition of organic materials, help to facilitate the balance of decay and regeneration necessary for life on Earth.

Efforts are underway among organic chemists in the research laboratories of corporations and other institutions to develop biodegradable plastics that will speed up the decomposition of materials in the polymers—a process that normally takes decades. Until such replacement polymers are developed, however, the most environmentally friendly solution to the problem of plastics is recycling. Today only about 1% of plastics are recycled, while the rest goes into waste dumps, where they account for 30% of the volume of trash.

Long before environmental concerns came to the forefront, however, people had begun almost to fear plastics as a depersonalizing aspect of modern life. It seemed that in a given day, a person touched fewer and fewer things that came directly from the natural environment: the "wood, stone, metal, and animal and plant fibers" to which Gordon alluded. Plastics seemed to have made human life emptier; yet the truth of the matter—including the fact that plastics add more than they take away from the landscape of our world—is much more complex.

The Plastics Revolution

Though the introduction of plastics is typically associated with the twentieth century, in fact the "materials revolution" surrounding plastics began in 1865. That was the year when English chemist Alexander Parkes (1813-1890) produced the first plastic material, celluloid. Parkes could have become a rich man from his invention, but he was not a successful marketer. Instead, the man who enjoyed the first commercial success in plastics was—not surprisingly—an American, inventor John Wesley Hyatt (1837-1920).

Responding to a contest in which a billiard-ball manufacturer offered $10,000 to anyone who could create a substitute for ivory, which was extremely costly, Hyatt turned to Parkes's celluloid. Actually, Parkes had given his creation—developed from cellulose, a substance found in the cell walls of plants—a much less appealing name, "Parkesine." Hyatt, who used celluloid to make smooth, hard, round billiard balls (thereby winning the contest) took out a patent for the process involved in making the material he had dubbed "Celluloid," with a capital C.

Though the Celluloid made by Hyatt's process was flammable (as was Parkesine), it proved highly successful as a product when he introduced it in 1869. He marketed it successfully for use in items such as combs and baby rattles, and Celluloid sales received a powerful boost after photography pioneer George Eastman (1854-1932) chose the material for use in the development of film. Eventually, Celluloid would be applied in motion-picture film, and even today, the adjective "celluloid" is sometimes used in relation to the movies. Actually, Celluloid (which can be explosive in large quantities) was phased out in favor of "safety film," or cellulose acetate, beginning in 1924.

Two important developments in the creation of synthetic polymers occurred at the turn of the century. One was the development of Galalith, an ivory-like substance made from formaldehyde and milk, by German chemist Adolf Spitteler. An even more important innovation happened in 1907, when Belgian-American chemist Leo Baekeland (1863-1944) introduced Bakelite. The latter, created in a reaction between phenol and formaldehyde, was a hard, black plastic that proved an excellent insulator. It soon found application in making telephones and household appliances, and by the 1920s, chemists had figured out how to add pigments to Bakelite, thus introducing the public to colored plastics.

SYNTHETIC RUBBER.

Throughout these developments, chemists had only a vague understanding of polymers, but by the 1930s, they had come to accept the model of polymers as large, flexible, chain-like molecules. One of the most promising figures in the emerging field of polymer chemistry was Carothers, who in 1926 left a teaching post at Harvard University to accept a position as director of the polymer research laboratory at DuPont.

Among the first problems Carothers tackled was the development of synthetic rubber. Natural rubber had been known for many centuries when English chemist Joseph Priestley (1733-1804) gave it its name because he used it to rub out pencil marks. In 1839, American inventor Charles Goodyear (1800-1860) accidentally discovered a method for making rubber more durable, after he spilled a mixture of rubber and sulfur onto a hot stove. Rather than melting, the rubber bonded with the sulfur to form a much stronger but still elastic product, and Goodyear soon patented this process under the name vulcanization.

Natural rubber, nonetheless, had many undesirable properties, and hence DuPont put Carothers to the task of developing a substitute. The result was neoprene, which he created by adding a chlorine atom to an acetylene derivative. Neoprene was stronger, more durable, and less likely to become brittle in cold weather than natural rubber. It would later prove an enormous boost to the Allied war effort, after the Japanese seized the rubber plantations of Southeast Asia in 1941.

NYLON.

Had neoprene, which Carothers developed in 1931, been the extent of his achievements, he would still be remembered by science historians. However, his greatest creation still lay ahead of him. Studying the properties of silk, he became convinced that he could develop a more durable compound that could replicate the properties of silk at a much lower cost.

Carothers was not alone in his efforts, as Gordon showed in his account of events at the DuPont laboratories:

One day, an assistant, Julian Hill, noticed that when he stuck a glass stirring rod into a gooey mass at the bottom of a beaker the researchers had been investigating, he could draw out threads from it, the polymers forming spontaneously as he pulled. When Carothers was absent one day, Hill and his colleagues decided to see how far they could go with pulling threads out of goo by having one man hold the beaker while another ran down the hall with the glass rod. A very long, silk-like thread was produced.

Realizing what they had on their hands, DuPont devoted $27 million to the research efforts of Carothers and his associates at the lab, and in 1937, Carothers presented his boss with the results, saying "Here is your synthetic textile fabric." DuPont introduced the material, nylon, to the American public the following year with one of the most famous advertising campaigns of all time: "Better Things for Better Living Through Chemistry."

The product got an additional boost through exposure at the 1939 World's Fair. When DuPont put 4,000 pairs of nylon stockings on the market, they sold in a matter of hours. A few months later, four million pairs sold in New York City in a single day. Women stood in line to buy stockings of nylon, a much better (and less expensive) material for that purpose than silk—but they did not have long to enjoy it. During World War II, all nylon went into making war materials such as parachutes, and nylon did not become commercially available again until 1946.

As Gordon noted, Carothers would surely have won the Nobel Prize in chemistry for his work—"but Nobel prizes go only to living recipients…." Carothers had married in 1936, and by early 1937, his wife Helen was pregnant. (Presumably, he was unaware of the fact that he was about to become a father.) Though highly enthusiastic about his work, Carothers was always shy and withdrawn, and in Gordon's words, "he had few outlets other than work." He was, however, a talented singer, as was his closest sibling, Isobel, a radio celebrity. Her death in January 1937 sent him into a bout of depression, and on April 29, he killed himself with a dose of cyanide. Seven months later, on November 27, Helen gave birth to a daughter, Jane.

HOW PLASTICS HAVE ENHANCED LIFE.

Despite his tragic end, Carothers had brought much good to the world by sparking enormous interest in polymer research and plastics. Over the years that followed, polymer chemists developed numerous products that had applications in a wide variety of areas. Some, such as polyester—a copolymer of terephthalic acid and ethylene—seemed to fit the idea of "plastics" as ugly, inauthentic, and even dehumanizing. During the 1970s, clothes of polyester became fashionable, but by the early 1980s, there was a public backlash against synthetics, and in favor of natural materials.

Yet even as the public rejected synthetic fabrics for everyday wear, Gore-tex and other synthetics became popular for outdoor and workout clothing. At the same time, the polyester that many regarded as grotesque when used in clothing was applied in making safer beverage bottles. The American Plastics Council dramatized this in a 1990s commercial that showed a few seconds in the life of a mother. Her child takes a soft-drink bottle out of the refrigerator and drops it, and the mother cringes at what she thinks she is about to see next: glass shattering around herchild. But she is remembering the way thingswere when she was a child, when soft drinks stillcame in glass bottles: instead, the plastic bottlebounces harmlessly.

Of course, such dramatizations may seem a bit self-serving to critics of plastic, but the fact remains that plastics enhance—and in some cases even preserve—life. Kevlar, for instance, enhances life when it is used in making canoes for recreation; when used to make a bulletproof vest, it can save the life of a law-enforcement officer. Mylar, a form of polyester, enhances life when used to make a durable child's balloon—but this highly nonreactive material also saves lives when it is applied to make replacement human blood vessels, or even replacement skin for burn victims.

Recycling

As mentioned above, plastics—for all their benefits—do pose a genuine environmental threat, due to the fact that the polymers break down much more slowly than materials from living organisms. Hence the need not only to develop biodegradable plastics, but also to work on more effective means of recycling.

One of the challenges in the recycling arena is the fact that plastics come in a variety of grades. Different catalysts are used to make polymers that possess different properties, with varying sizes of molecules, and in chains that may be linear, branched, or cross-linked. Long chains of 10,000 or more monomers can be packed closely to form a hard, tough plastic known as high-density polyethylene or HDPE, used for bottles containing milk, soft drinks, liquid soap, and other products. On the other hand, shorter, branched chains of about 500 ethylene monomers each produce a much less dense plastic, low-density polyethylene or LDPE. This is used for plastic food or garment bags, spray bottles, and so forth. There are other grades of plastic as well.

In some forms of recycling, plastics of all varieties are melted down together to yield a cheap, low-grade product known as "plastic lumber," used in materials such as landscaping timbers, or in making park benches. In order to achieve higher-grade recycled plastics, the materials need to be separated, and to facilitate this, recycling codes have been developed. Many plastic materials sold today are stamped with a recycling code number between 1 and 6, identifying specific varieties of plastic. These can be melted or ground according to type at recycling centers, and reprocessed to make more plastics of the same grade.

To meet the environmental challenges posed by plastics, polymer chemists continue to research new methods of recycling, and of using recycled plastic. One impediment to recycling, however, is the fact that most state and local governments do not make it convenient, for instance by arranging trash pickup for items that have been separated into plastic, paper, and glass products. Though ideally private recycling centers would be preferable to government-operated recycling, few private companies have the financial resources to make recycling of plastics and other materials practical.

WHERE TO LEARN MORE

Bortz, Alfred B. Superstuff!: Materials That Have Changed Our Lives. New York: Franklin Watts, 1990.

Ebbing, Darrell D.; R. A. D. Wentworth; and James P.Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995.

Galas, Judith C. Plastics: Molding the Past, Shaping the Future. San Diego, CA: Lucent Books, 1995.

Gordon, John Steele. "Plastics, Chance, and the Prepared Mind." American Heritage, July-August 1998, p. 18.

Mebane, Robert C. and Thomas R. Rybolt. Plastics and Polymers. Illustrated by Anni Matsick. New York: Twenty-First Century Books, 1995.

Plastics.com (Web site). <http://www.plastics.com> (June5, 2001).

"Plastics 101." Plastics Resource (Web site). <http://www2.plasticsresource.com/plastics_101/> (June 5, 2001).

"Polymers." University of Illinois, Urbana/Champaign, Materials Science and Technology Department (Web site). <http://matse1.mse.uiuc.edu/~tw/polymers/polymers.html> (June 5, 2001).

"Polymers and Liquid Crystals." Case Western Reserve University (Web site). <http://abalone.cwru.edu/> (June 5, 2001).

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

KEY TERMS

ADDITION POLYMERIZATION:

A form of polymerization in which monomers having at least one double bond or triple bond simply add to one another, forming a polymer and no other products. Compare to condensation polymerization.

ALKENES:

Hydrocarbons that contain double bonds.

COPOLYMER:

A polymer composed of more than one type of monomer.

DIMER:

A molecule formed by the joining of two monomers.

DOUBLE BOND:

A form of bonding in which two atoms share two pairs of valence electrons. Carbon is noted for its ability to form double bonds, as for instance in many hydrocarbons.

FUNCTIONAL GROUPS:

An atom or group of atoms whose presence identifies a specific family of compounds. When combined with hydrocarbons, various functional groups form hydrocarbonderivatives.

HOMOPOLYMER:

A polymer that consists of only one type of monomer.

HYDROCARBON:

Any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms.

HYDROCARBON DERIVATIVES:

Families of compounds formed by the joining of hydrocarbons with various functional groups.

MONOMERS:

Small, individual subunits, often built of hydrocarbons, that join together to form polymers.

ORGANIC:

A term referring to any compound that contains carbon, except for oxides such as carbon dioxide, or carbonates such as calcium carbonate (i.e., limestone).

ORGANIC CHEMISTRY:

The study of carbon, its compounds, and their properties.

PLASTICS:

Materials, usually organic, that can be caused to flow under certain conditions of heat and pressure, and thus to assume a desired shape when the pressure and temperature conditions are withdrawn. Plastics are usually made up of polymers.

POLYMERIZATION:

The process whereby monomers join to form polymers.

POLYMERS:

Large, typically chain-like molecules composed of numerous smaller, repeating units known as monomers.

VALENCE ELECTRONS:

Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding.

Polymer

Condensation polymers

Resources

Polymers are made up of extremely large, chainlike molecules consisting of numerous, smaller, repeating units called monomers. Polymer chains, which could be compared to paper clips linked together to make a long strand, appear in varying lengths. They can have branches, become intertwined, and can have cross-links. In addition, polymers can be composed of one or more types of monomer units, they can be joined by various kinds of chemical bonds, and they can be oriented in different ways. Monomers can be joined together by addition, in which all the atoms in the monomer are present in the polymer, or by condensation, in which a small molecule byproduct is also formed. Addition polymers include polyethylene, polypropylene, Teflon, Lucite, and rubber. etc. Condensation polymers include nylon, Dacron, and Formica.

The importance of polymers is evident as they occur widely both in the natural world in such materials as wool, hair, silk and sand, and in the world of synthetic materials in nylon, rubber, plastics, Styrofoam, and many other materials. The usefulness of polymers depends on their specific properties. Some of the sought-after properties of the synthetic polymers over natural ones include greater strength, non-reactivity with other substances, non-stickiness, and light weight. Modern lifestyles rely heavily on qualities of the readily available synthetic polymers.

Although the 1920s became known as the “plastic age” and the plastic industry did not really boom until World War II, chemists actually began modifying very large, natural macromolecules, such as cellulose, in 1861. In the strict sense, plastic means materials that can be softened and molded by heat and pressure but the term is also sometimes used to describe other macromolecular (large-molecule) materials, whether they be structural materials, films, or fibers. The first plastic material, prepared by Alexander Parkes when he mixed nitrocellulose with wood naphtha, was patented as “Parkesine” but this material found few commercial uses. The product was improved by Daniel Spill and marketed as “Xylonite” which found a market in combs and shirt collars. In 1884, it was adopted by the Sheffield cutlery industry for producing cheaper knife handles than the traditional bone.

In 1870, in response to a contest offering $10,000 to find a substitute for the costly ivory used to make billiard balls, John Wesley Hyatt again improved on the easily deformed and flammable “Parkesine.” The new product “Celluloid,” though still flammable, could be molded into smooth, hard balls and proved to be not only a substitute for ivory billiard balls, but also replaced the expensive tortoise-shell used for mirror backings and hair or tooth brushes. It became the material of choice for George Eastman in 1889 in the development of roll film for snapshots and movies, and as such, brought in large profits.

With the success of these products, chemists began experimenting with other natural products. By the turn of the century a Bavarian chemist, Adolf Spitteler, added formaldehyde to milk and produced an ivory-like substance called “Galalith” that was used in button-making. At this time, scientists also began working with small molecules to produce large ones rather than just trying to modify large, natural molecules. Around 1910 in a reaction between phenol and formaldehyde, the Belgian photographic chemist Leo H. Baekeland produced a black, hard plastic he called Bakelite that proved to be a good insulator and a pleasing substance for use in making telephones and household appliances. It was not until the 1920s that plastics were produced that could be mixed with pigments to produce color.

It was about 1930 when scientists first began to understand and accept the evidence that polymers were giant, chain-like molecules that were flexible. American chemists were more receptive to these new ideas than were their European counterparts. In 1928 Du Pont chemical company, whose major research interest prior to this point had been gunpowder manufacture, hired Wallace H. Carothers, a chemist who chose polymer formation as his basis for research. He was able to show how the individual units of the polymer chain joined together chemically and resulted in chain growth. He soon developed a new fiber, which was marketed by Du Pont in 1938 as Nylon. It turned out to be Du Pont’s greatest money-maker and was extremely important for use in parachutes in World War II. At about the same time two other chemists, Gibson and Fawcett, who were working in England, discovered polyethylene which had an important role in World War II as radar insulators. Clearly, the “Age of Plastics” was in full swing.

Polymers are extremely large molecules composed of long chains, much like paper clips that are linked together to make a long strand. The individual subunits, which can range from as few as 50 to more than 20,000, are called monomers (from the Greek mono meaning one and meros meaning part). Because of their large size, polymers (from the Greek poly meaning many) are referred to as macromolecules.

Like strands of paper clips, polymer chains can be of varying lengths, they can have branches and they can become intertwined. Polymers can be made of one or more kinds of monomer units, they can be joined by different kinds of chemical bonds and they can be oriented differently. Each of these variations either produces a different polymer or gives the existing polymer different properties. All of these possibilities provide numerous opportunities for research and there are more chemists employed in the polymer industry than in any other branch of chemistry. Their job is to modify existing polymers so that they have more desirable properties and to synthesize new ones.

Although polymers are often associated only with man-made materials, there are many polymers that occur in nature such as wood, silk, cotton, DNA, RNA, starch, and even sand and asbestos. They can make the material soft as in goose down, strong and delicate as in a spider web, or smooth and lustrous as in silk. Examples of man-made polymers include plastics such as polyethylene, styrofoam, Saran wrap, etc.; fibers such as nylon, Dacron, rayon, Herculon, etc.; and other materials such as Formica, Teflon, PVC piping, etc. In all of these synthetic compounds, man is trying to make substitutes for materials that are in short supply or too expensive, or is trying to improve the properties of the material to make it more useful.

Most synthetic polymers are made from the non-renewable resource, petroleum, and as such, the “age of plastics” is limited unless other ways are found to make them. Since most polymers have carbon atoms as the basis of their structure, in theory at least, there are numerous materials that could be used as starting points. But the research and development process is long and costly and replacement polymers, if they ever become available, are a long way in the future. Disposing of plastics is also a serious problem, both because they contribute to the growing mounds of garbage accumulating everyday and because most are not biodegradable. Researchers are busy trying to find ways to speed-up the decomposition time which, if left to occur naturally, can take decades.

Recycling is obviously a more economical and practical solution to both the conservation and disposal of this valuable resource. Only about 1% of plastics are currently recycled and the rest goes into municipal waste, making up about 30% by volume. Because different plastics have different chemical compositions, recycling them together yields a cheap, low-grade product called “plastic lumber.” These plastics are usually ground up and the chips are bonded together for use in such things as landscaping timbers or park benches. For a higher grade material, the plastics must be separated into like kinds. To facilitate this process, many plastics today are stamped with a recycling code number between one and six that identifies the most common types. Then, depending on the kind, the plastic can be melted or ground and reprocessed. New ways of reprocessing and using this recycled plastic are constantly being sought.

In order for monomers to chemically combine with each other and form long chains, there must be a mechanism by which the individual units can join or bond to each other. One method by which this happens is called addition because no atoms are gained or lost in the process. The monomers simply “add” together and the polymer is called an addition polymer.

The simplest chemical structure by which this can happen involves monomers that contain double bonds (sharing two pairs of electrons). When the double bond breaks and changes into a single bond, each of the other two electrons are free and available to join with another monomer that has a free electron. This process can continue on and on. Polyethylene is an example of an addition polymer. The polymerization process can be started by using heat and pressure or ultraviolet light or by using another more reactive chemical such as a peroxide. Under these conditions the double bond breaks leaving extremely reactive unpaired electrons called free radicals. These free radicals react readily with other free radicals or with double bonds and the polymer chain starts to form.

Different catalysts yield polymers with different properties because the size of the molecule may vary and the chains may be linear, branched, or cross-linked. Long linear chains of 10,000 or more monomers can pack very close together and form a hard, rigid, tough plastic known as high-density polyethylene or HDPE. Bottles for milk, water, bleach, soap, etc. are usually made of HDPE. It can be recognized by the recycling code number 2 that is marked on the bottom of the bottles.

Shorter, branched chains of about 500 monomers of ethylene cannot pack as closely together and this kind of polymer is known as low-density polyethylene or LDPE. It is used for plastic food or garment bags, spray bottles, plastic lids, etc. and has a recycling code number 4. Polyethylene belongs to a group of plastics called thermoplastic polymers because it can be softened by heating and then remolded.

The ethylene monomer has two hydrogen atoms bonded to each carbon for a total of four hydrogen atoms that are not involved in the formation of the polymer. Many other polymers can be formed when one or more of these hydrogen atoms are replaced by some other atom or group of atoms. Polyvinyl chloride (PVC), with a recycling code number 3, is formed if one of the hydrogen atoms is replaced by a chlorine atom. Polypropylene (P/P), with a recycling code number 5, is formed if one hydrogen atom is replaced by a methyl (CH₃) group. Polystyrene (PS) with a recycling code number 6 is formed if one hydrogen atom is replaced by a phenyl (C₆H₅) group. Other polymers that are derivatives of ethylene include polyacrylonitrile (known by the trade name Orlon or Acrilan), when one hydrogen is replaced by a cyanide (CN) group; polymethyl methacrylate (trade name Plexiglas or Lucite), when one hydrogen is replaced by a methyl (CH₃) group and another is replaced by a CO₂CH ₃group; and polytetrafluoroethylene (Teflon), when all four hydrogen atoms are replaced by fluorine atoms.

Natural and synthetic rubbers are both addition polymers. Natural rubber is obtained from the sap that oozes from rubber trees. It was named by Joseph Priestley who used it to rub out pencil marks, hence, its name, a rubber. Natural rubber can be decomposed to yield monomers of isoprene. It was used by the early American Indians to make balls for playing games as well as for water-proofing footwear and other garments. But, useful as it was, it also had undesirable properties. It was sticky and smelly when it got too hot and it got hard and brittle in cold weather. These undesirable properties were eliminated when, in 1839, Charles Goodyear accidentally spilled a mixture of rubber and sulfur onto a hot stove and found that it did not melt but rather formed a much stronger but still elastic product. The process, called vulcanization, led to a more stable rubber product that withstood heat (without getting sticky) and cold (without getting hard) as well as being able to recover its original shape after being stretched. The sulfur makes cross-links in the long polymer chain and helps give it strength and resiliency, that is, if stretched, it will spring back to its original shape when the stress is released.

Because the supply of natural rubber was limited and because it had still other undesirable properties, chemists began experimenting to find synthetic products that would be even better than natural rubber. Today there are many monomers and mixtures of two or three different monomers, called copolymers, that can polymerize to form rubber-like substances. Neoprene, produced from 2-chlorobutadiene, was one of the first synthetic rubbers. The biggest commercial product in the United States is the copolymer, styrenebutadiene or SBR, which is composed of one styrene monomer for every three butadiene monomers.

Condensation polymers

A second method by which monomers bond together to form polymers is called condensation. The formation of condensation polymers is more complex that the formation of addition polymers. Unlike addition polymers, in which all the atoms of the monomers are present in the polymer, two products result from the formation of condensation polymers, the polymer itself and another small molecule which is often, but not always, water. These polymers can form from a single kind of monomer, or, copolymers can form if two or more different monomers are involved. Most of the natural polymers are formed by condensation.

One of the simplest of the condensation polymers is a type of nylon called nylon 6. It is formed from an amino acid, 6-aminohexanoic acid that has six carbon atoms in it, hence the name nylon 6. All amino acids molecules have an amine group (NH₂) at one end and a carboxylic acid (COOH) group at the other end. A polymer forms when a hydrogen atom from the amine end of one molecule and an oxygen-hydrogen group (OH) from the carboxylic acid end of a second molecule split off and form a water molecule. The monomers join together as a new chemical bond forms between the nitrogen and carbon atoms. This new bond is called an amide linkage. Polymers formed by this kind of condensation reaction are referred to as polyamides. The new molecule, just like each of the monomers from which it formed, also has an amine group at one end (that can add to the carboxylic acid group of another monomer) and it has a carboxylic acid group at the other end (that can add to the amine end of another monomer). The chain can continue to grow and form very large polymers. Each time a monomer is added to the chain, a small molecule byproduct of water is also formed.

All of the various types of nylons are polyamides because the condensation reaction occurs between an amine group and an acid group. The most important type of nylon is a copolymer called nylon 66, so-named because each of the monomers from which it forms has six carbon atoms. Nylon 66 is formed from adipic acid and hexamethylenediamine. Adipic acid has a carboxylic acid group at both ends of the molecule and the hexamethylenediamine molecule has an amine group at both ends of the molecule. The polymer is formed as alternating monomers of adipic acid and hexamethylenediamine bond together in a condensation reaction and a water molecule splits away.

Nylon became a commercial product for Du Pont when their research scientists were able to draw it into long, thin, symmetrical filaments. As these polymer chains line up side-by-side, weak chemical bonds called hydrogen bonds form between adjacent chains. This makes the filaments very strong. Nylon was first introduced to the public as nylon stockings (replacing the weaker natural fiber, silk) in October, 1939 in Delaware. Four thousand pairs sold in no time. A few months later, four million pairs sold in New York City in just one day. But the new found treasure was short-lived since, when the United States entered World War II in December, 1941, all the nylon went into making war materials. Women again had to rely on silk, rayon, and cotton, and some even painted their legs. Nylon hosiery did not become available again until 1946.

Another similar polymer of the polyamide type is the extremely light-weight but strong material known as Kevlar. It is used in bullet-proof vests, aircraft, and in recreational uses such as canoes. Like nylon, one of the monomers from which it is made is terephthalic acid. The other one is phenylenediamine.

Polyesters are another type of condensation polymer, so-called because the linkages formed when the monomers join together are called esters. Probably the best known polyester is known by its trade name, Dacron. It is a copolymer of terephthalic acid (which has a carboxylic acid at both ends) and ethylene glycol (which has an alcohol, OH group), at both ends. A molecule of water forms when the OH group from the acid molecule splits away and bonds with a hydrogen atom from the alcohol group. The new polymer is called polyethylene terephthalate or PET and can be recognized by its recycling code number 1.

Dacron is used primarily in fabrics and clear beverage bottles. Films of Dacron can be coated with metallic oxides, rolled into very thin sheets (only about one-thirtieth the thickness of a human hair), magnetized, and used to make audio and video tapes. When used in this way, it is extremely strong and goes by the trade name Mylar. Because it is not chemically reactive, and is not toxic, allergenic, or flammable, and because it does not promote blood-clotting, it can be used to replace human blood vessels when they are severely blocked and damaged or to replace the skin of burn victims.

There are other important condensation polymers that are formed by more complex reactions. These include the formaldehyde resins the first of which was Bakelite. These plastics are thermosetting plastics; that is, once they are molded and formed, they become permanently hard and they cannot be softened and remolded. Today their major use is in plywood adhesives, Melmac for dinnerware, Formica for table and counter tops, and other molding compounds.

Polycarbonate polymers are known for their unusual toughness, yet they are so clear that they are

KEY TERMS

Addition polymer— Polymers formed when the individual units are joined together without the gain or loss of any atoms.

Condensation polymer— Polymers formed when the individual units are joined together with the splitting off of a small molecule by-product.

Copolymer— Polymers formed from two or more different monomers.

Macromolecule— A giant molecule.

Monomers— Small, individual subunits which join together to form polymers.

Plastics— A group of natural or synthetic polymers that are capable of being softened and molded by heat and pressure; also sometimes used to include other structural materials, films and fibers.

Polyamide— A polymer, such as nylon, in which the monomers are joined together by amide linkages.

Polyester— A polymer, such as Dacron, in which the monomers are joined together by ester linkages.

Polymer— Extremely large molecules made of numerous, small, repeating units.

Recycling code— Numbers between one and six that are stamped on many plastics and used for recycling purposes which identify the various kinds of polymers.

used for “bullet-proof” windows and in visors for space helmets. The tough, baked-on finishes of automobiles and major appliances are cross-linked polymers formed from an alcohol, such as glycerol, and an acid, such as phthalic acid, and are called alkyds. Silicone oils and rubbers are condensation polymers that have silicon rather than carbon as part of their structural form. These compounds are generally more stable at high temperatures and more fluid at low temperatures than the carbon compounds. They are often used for parts in space vehicles and jet planes.

See also Artificial fibers.

Resources

BOOKS

Brandup, J., et al, eds. Polymer Handbook, 4th ed. New York: John Wiley & Sons, 2003.

Dean, John A., ed. Lange’s Handbook of Chemistry, 16th ed. New York: McGraw-Hill, 2004.

Veselovskii, R.A., Vladimir N. Kestelman, Roman A Veselovsky. Adhesion of Polymers. New York: McGraw-Hill, 2001.

PERIODICALS

Amis, E.J. “Combinatorial Investigations of Polymer Adhesion.” Polymer Preprints, American Chemical Society, Division 42, no. 2 (2001): 645-646.

Leona B. Bronstein

Polymer

Polymers are made up of extremely large, chainlike molecules consisting of numerous, smaller, repeating units called monomers. Polymer chains, which could be compared to paper clips linked together to make a long strand, appear in varying lengths. They can have branches, become intertwined, and can have cross-links. In addition, polymers can be composed of one or more types of monomer units, they can be joined by various kinds of chemical bonds, and they can be oriented in different ways. Monomers can be joined together by addition, in which all the atoms in the monomer are present in the polymer, or by condensation, in which a small molecule byproduct is also formed. Addition polymers include polyethylene, polypropylene, Teflon, Lucite, and rubber. etc. Condensation polymers include nylon, Dacron, and Formica.

The importance of polymers is evident as they occur widely both in the natural world in such materials as wool, hair, silk and sand , and in the world of synthetic materials in nylon, rubber, plastics , Styrofoam, and many other materials. The usefulness of polymers depends on their specific properties. Some of the sought-after properties of the synthetic polymers over natural ones include greater strength, non-reactivity with other substances, non-stickiness, and light weight. Modern lifestyles rely heavily on qualities of the readily available synthetic polymers.

Although the 1920s became known as the "plastic age" and the plastic industry did not really boom until World War II, chemists actually began modifying very large, natural macromolecules, such as cellulose , in 1861. In the strict sense, plastic means materials that can be softened and molded by heat and pressure but the term is also sometimes used to describe other macromolecular (large-molecule) materials, whether they be structural materials, films, or fibers. The first plastic material, prepared by Alexander Parkes when he mixed nitrocellulose with wood naphtha, was patented as "Parkesine" but this material found few commercial uses. The product was improved by Daniel Spill and marketed as "Xylonite" which found a market in combs and shirt collars. In 1884, it was adopted by the Sheffield cutlery industry for producing cheaper knife handles than the traditional bone.

In 1870, in response to a contest offering $10,000 to find a substitute for the costly ivory used to make billiard balls, John Wesley Hyatt again improved on the easily deformed and flammable "Parkesine." The new product "Celluloid," though still flammable, could be molded into smooth, hard balls and proved to be not only a substitute for ivory billiard balls, but also replaced the expensive tortoise-shell used for mirror backings and hair or tooth brushes. It became the material of choice for George Eastman in 1889 in the development of roll film for snapshots and movies, and as such, brought in large profits.

With the success of these products, chemists began experimenting with other natural products. By the turn of the century a Bavarian chemist, Adolf Spitteler, added formaldehyde to milk and produced an ivory-like substance called "Galalith" that was used in button-making. At this time, scientists also began working with small molecules to produce large ones rather than just trying to modify large, natural molecules. Around 1910 in a reaction between phenol and formaldehyde, the Belgian photographic chemist Leo H. Baekeland produced a black, hard plastic he called Bakelite that proved to be a good insulator and a pleasing substance for use in making telephones and household appliances. It was not until the 1920s that plastics were produced that could be mixed with pigments to produce color .

It was about 1930 when scientists first began to understand and accept the evidence that polymers were giant, chain-like molecules that were flexible. American chemists were more receptive to these new ideas than were their European counterparts. In 1928 Du Pont chemical company, whose major research interest prior to this point had been gunpowder manufacture, hired Wallace H. Carothers, a chemist who chose polymer formation as his basis for research. He was able to show how the individual units of the polymer chain joined together chemically and resulted in chain growth. He soon developed a new fiber, which was marketed by Du Pont in 1938 as Nylon. It turned out to be Du Pont's greatest money-maker and was extremely important for use in parachutes in World War II. At about the same time two other chemists, Gibson and Fawcett, who were working in England, discovered polyethylene which had an important role in World War II as radar insulators. Clearly, the "Age of Plastics" was in full swing.

Polymers are extremely large molecules composed of long chains, much like paper clips that are linked together to make a long strand. The individual subunits, which can range from as few as 50 to more than 20,000, are called monomers (from the Greek mono meaning one and meros meaning part). Because of their large size, polymers (from the Greek poly meaning many) are referred to as macromolecules.

Like strands of paper clips, polymer chains can be of varying lengths, they can have branches and they can become intertwined. Polymers can be made of one or more kinds of monomer units, they can be joined by different kinds of chemical bonds and they can be oriented differently. Each of these variations either produces a different polymer or gives the existing polymer different properties. All of these possibilities provide numerous opportunities for research and there are more chemists employed in the polymer industry than in any other branch of chemistry . Their job is to modify existing polymers so that they have more desirable properties and to synthesize new ones.

Although polymers are often associated only with man-made materials, there are many polymers that occur in nature such as wood, silk, cotton , DNA, RNA, starch, and even sand and asbestos . They can make the material soft as in goose down, strong and delicate as in a spider web, or smooth and lustrous as in silk. Examples of man-made polymers include plastics such as polyethylene, styrofoam, Saran wrap, etc.; fibers such as nylon, Dacron, rayon, Herculon, etc.; and other materials such as Formica, Teflon, PVC piping, etc. In all of these synthetic compounds, man is trying to make substitutes for materials that are in short supply or too expensive, or is trying to improve the properties of the material to make it more useful.

Most synthetic polymers are made from the non-renewable resource, petroleum , and as such, the "age of plastics" is limited unless other ways are found to make them. Since most polymers have carbon atoms as the basis of their structure, in theory at least, there are numerous materials that could be used as starting points. But the research and development process is long and costly and replacement polymers, if they ever become available, are a long way in the future. Disposing of plastics is also a serious problem, both because they contribute to the growing mounds of garbage accumulating everyday and because most are not biodegradable. Researchers are busy trying to find ways to speed-up the decomposition time which, if left to occur naturally, can take decades.

Recycling is obviously a more economical and practical solution to both the conservation and disposal of this valuable resource. Only about 1% of plastics are currently recycled and the rest goes into municipal waste, making up about 30% by volume . Because different plastics have different chemical compositions, recycling them together yields a cheap, low-grade product called "plastic lumber." These plastics are usually ground up and the chips are bonded together for use in such things as landscaping timbers or park benches. For a higher grade material, the plastics must be separated into like kinds. To facilitate this process, many plastics today are stamped with a recycling code number between one and six that identifies the most common types. Then, depending on the kind, the plastic can be melted or ground and reprocessed. New ways of reprocessing and using this recycled plastic are constantly being sought.

In order for monomers to chemically combine with each other and form long chains, there must be a mechanism by which the individual units can join or bond to each other. One method by which this happens is called addition because no atoms are gained or lost in the process. The monomers simply "add" together and the polymer is called an addition polymer.

The simplest chemical structure by which this can happen involves monomers that contain double bonds (sharing two pairs of electrons). When the double bond breaks and changes into a single bond, each of the other two electrons are free and available to join with another monomer that has a free electron . This process can continue on and on. Polyethylene is an example of an addition polymer. The polymerization process can be started by using heat and pressure or ultraviolet light or by using another more reactive chemical such as a peroxide. Under these conditions the double bond breaks leaving extremely reactive unpaired electrons called free radicals. These free radicals react readily with other free radicals or with double bonds and the polymer chain starts to form.

Different catalysts yield polymers with different properties because the size of the molecule may vary and the chains may be linear, branched, or cross-linked. Long linear chains of 10,000 or more monomers can pack very close together and form a hard, rigid, tough plastic known as high-density polyethylene or HDPE. Bottles for milk, water , bleach , soap , etc. are usually made of HDPE. It can be recognized by the recycling code number 2 that is marked on the bottom of the bottles.

Shorter, branched chains of about 500 monomers of ethylene cannot pack as closely together and this kind of polymer is known as low-density polyethylene or LDPE. It is used for plastic food or garment bags, spray bottles, plastic lids, etc. and has a recycling code number 4. Polyethylene belongs to a group of plastics called thermoplastic polymers because it can be softened by heating and then remolded.

The ethylene monomer has two hydrogen atoms bonded to each carbon for a total of four hydrogen atoms that are not involved in the formation of the polymer. Many other polymers can be formed when one or more of these hydrogen atoms are replaced by some other atom or group of atoms. Polyvinyl chloride (PVC), with a recycling code number 3, is formed if one of the hydrogen atoms is replaced by a chlorine atom. Polypropylene (P/P), with a recycling code number 5, is formed if one hydrogen atom is replaced by a methyl (CH3) group. Polystyrene (PS) with a recycling code number 6 is formed if one hydrogen atom is replaced by a phenyl (C6H5) group. Other polymers that are derivatives of ethylene include polyacrylonitrile (known by the trade name Orlon or Acrilan), when one hydrogen is replaced by a cyanide (CN) group; polymethyl methacrylate (trade name Plexiglas or Lucite), when one hydrogen is replaced by a methyl (CH3) group and another is replaced by a CO2CH3 group; and polytetrafluoroethylene (Teflon), when all four hydrogen atoms are replaced by fluorine atoms.

Natural and synthetic rubbers are both addition polymers. Natural rubber is obtained from the sap that oozes from rubber trees. It was named by Joseph Priestley who used it to rub out pencil marks, hence, its name, a rubber. Natural rubber can be decomposed to yield monomers of isoprene. It was used by the early American Indians to make balls for playing games as well as for water-proofing footwear and other garments. But, useful as it was, it also had undesirable properties. It was sticky and smelly when it got too hot and it got hard and brittle in cold weather . These undesirable properties were eliminated when, in 1839, Charles Goodyear accidentally spilled a mixture of rubber and sulfur onto a hot stove and found that it did not melt but rather formed a much stronger but still elastic product. The process, called vulcanization , led to a more stable rubber product that withstood heat (without getting sticky) and cold (without getting hard) as well as being able to recover its original shape after being stretched. The sulfur makes cross-links in the long polymer chain and helps give it strength and resiliency, that is, if stretched, it will spring back to its original shape when the stress is released.

Because the supply of natural rubber was limited and because it had still other undesirable properties, chemists began experimenting to find synthetic products that would be even better than natural rubber. Today there are many monomers and mixtures of two or three different monomers, called copolymers, that can polymerize to form rubber-like substances. Neoprene, produced from 2-chlorobutadiene, was one of the first synthetic rubbers. The biggest commercial product in the United States is the copolymer, styrene-butadiene or SBR, which is composed of one styrene monomer for every three butadiene monomers.

Condensation polymers

A second method by which monomers bond together to form polymers is called condensation. The formation of condensation polymers is more complex that the formation of addition polymers. Unlike addition polymers, in which all the atoms of the monomers are present in the polymer, two products result from the formation of condensation polymers, the polymer itself and another small molecule which is often, but not always, water. These polymers can form from a single kind of monomer, or, copolymers can form if two or more different monomers are involved. Most of the natural polymers are formed by condensation.

One of the simplest of the condensation polymers is a type of nylon called nylon 6. It is formed from an amino acid , 6-aminohexanoic acid that has six carbon atoms in it, hence the name nylon 6. All amino acids molecules have an amine group (NH2) at one end and a carboxylic acid (COOH) group at the other end. A polymer forms when a hydrogen atom from the amine end of one molecule and an oxygen-hydrogen group (OH) from the carboxylic acid end of a second molecule split off and form a water molecule. The monomers join together as a new chemical bond forms between the nitrogen and carbon atoms. This new bond is called an amide linkage. Polymers formed by this kind of condensation reaction are referred to as polyamides. The new molecule, just like each of the monomers from which it formed, also has an amine group at one end (that can add to the carboxylic acid group of another monomer) and it has a carboxylic acid group at the other end (that can add to the amine end of another monomer). The chain can continue to grow and form very large polymers. Each time a monomer is added to the chain, a small molecule byproduct of water is also formed.

All of the various types of nylons are polyamides because the condensation reaction occurs between an amine group and an acid group. The most important type of nylon is a copolymer called nylon 66, so-named because each of the monomers from which it forms has six carbon atoms. Nylon 66 is formed from adipic acid and hexamethylenediamine. Adipic acid has a carboxylic acid group at both ends of the molecule and the hexamethylenediamine molecule has an amine group at both ends of the molecule. The polymer is formed as alternating monomers of adipic acid and hexamethylenediamine bond together in a condensation reaction and a water molecule splits away.

Nylon became a commercial product for Du Pont when their research scientists were able to draw it into long, thin, symmetrical filaments. As these polymer chains line up side-by-side, weak chemical bonds called hydrogen bonds form between adjacent chains. This makes the filaments very strong. Nylon was first introduced to the public as nylon stockings (replacing the weaker natural fiber, silk) in October, 1939 in Delaware. Four thousand pairs sold in no time. A few months later, four million pairs sold in New York City in just one day. But the new found treasure was short-lived since, when the United States entered World War II in December, 1941, all the nylon went into making war materials. Women again had to rely on silk, rayon, cotton, and some even went to painting their legs. Nylon hosiery did not become available again until 1946.

Another similar polymer of the polyamide type is the extremely light-weight but strong material known as Kevlar. It is used in bullet-proof vests, aircraft , and in recreational uses such as canoes. Like nylon, one of the monomers from which it is made is terephthalic acid. The other one is phenylenediamine.

Polyesters are another type of condensation polymer, so-called because the linkages formed when the monomers join together are called esters. Probably the best known polyester is known by its trade name, Dacron. It is a copolymer of terephthalic acid (which has a carboxylic acid at both ends) and ethylene glycol (which has an alcohol , OH group), at both ends. A molecule of water forms when the OH group from the acid molecule splits away and bonds with a hydrogen atom from the alcohol group. The new polymer is called polyethylene terephthalate or PET and can be recognized by its recycling code number 1.

Dacron is used primarily in fabrics and clear beverage bottles. Films of Dacron can be coated with metallic oxides, rolled into very thin sheets (only about one-thirtieth the thickness of a human hair), magnetized, and used to make audio and video tapes. When used in this way, it is extremely strong and goes by the trade name Mylar. Because it is not chemically reactive, and is not toxic, allergenic, or flammable, and because it does not promote blood-clotting, it can be used to replace human blood vessels when they are severely blocked and damaged or to replace the skin of burn victims.

There are other important condensation polymers that are formed by more complex reactions. These include the formaldehyde resins the first of which was Bakelite. These plastics are thermosetting plastics; that is, once they are molded and formed, they become permanently hard and they cannot be softened and remolded. Today their major use is in plywood adhesives, Melmac for dinnerware, Formica for table and counter tops, and other molding compounds.

Polycarbonate polymers are known for their unusual toughness, yet they are so clear that they are used for "bullet-proof" windows and in visors for space helmets. The tough, baked-on finishes of automobiles and major appliances are cross-linked polymers formed from an alcohol, such as glycerol , and an acid, such as phthalic acid, and are called alkyds. Silicone oils and rubbers are condensation polymers that have silicon rather than carbon as part of their structural form. These compounds are generally more stable at high temperatures and more fluid at low temperatures than the carbon compounds. They are often used for parts in space ships and jet planes.

See also Artificial fibers.

Resources

books

Brandup, J., et al., eds. Polymer Handbook. 4th ed. New York: John Wiley & Sons, 1999.

Dean, John A., ed. Lange's Handbook of Chemistry. 15th ed. New York: McGraw-Hill, 1998

Veselovskii, R.A., Vladimir N. Kestelman, and Roman A Veselovsky. Adhesion of Polymers. 1st ed. New York: Mc-Graw-Hill, 2001.

periodicals

Amis, E.J. "Combinatorial Investigations of Polymer Adhesion." Polymer Preprints, American Chemical Society, Division 42, no. 2 (2001): 645-646.

Leona B. Bronstein

KEY TERMS

Addition polymer

—Polymers formed when the individual units are joined together without the gain or loss of any atoms.

Condensation polymer

—Polymers formed when the individual units are joined together with the splitting off of a small molecule by-product.

Copolymer

—Polymers formed from two or more different monomers.

Monomers

—Small, individual subunits which join together to form polymers.

Polyamide

—A polymer, such as nylon, in which the monomers are joined together by amide linkages.

Polymer

A polymer is a very large molecule in which one or two small units is repeated over and over again. The small repeating units are known as monomers. Imagine that a monomer can be represented by the letter A. Then a polymer made of that monomer would have the structure:

-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A

In another kind of polymer, two different monomers might be involved. If the letters A and B represent those monomers, then the polymer could be represented as:

-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A

A polymer with two different monomers is known as a copolymer.

The number of monomers (As or Bs) in a polymer is very great indeed. To accurately represent the first polymer above, for example, it might be necessary to write a few hundred or a few thousand As. We would have to fill up a page or two of this book to give an accurate formula for such a polymer.

Natural polymers

Polymers are very common in nature; some of the most widespread naturally occurring substances are polymers. Starch and cellulose are examples. Green plants have the ability to take the simple sugar known as glucose and make very long chains containing many glucose units. These long chains are molecules of starch or cellulose. If we assign the symbol G to stand for a glucose molecule, then starch or cellulose can be represented as:

-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-

Again, a real molecule of starch or cellulose contains hundreds or thousands of these G units.

Words to Know

Copolymer: Polymers formed from two or more different monomers.

Monomers: Small molecules that join together to form polymers.

Plastics: A group of polymers that are capable of being softened and molded by heat and pressure.

Synthetic polymers

Scientists began to make synthetic polymers long before they really understood the structure of these giant molecules. As early as the 1860s, chemists were exploring ways in which naturally occurring polymers such as cellulose could be modified to make them more useful. These polymers eventually became known as plastics. The term comes from the fact that most early polymers could be melted, bent, and shaped.

The first truly synthetic polymer was invented around 1910 by Belgian-American chemist Leo H. Baekeland (1863–1949). Baekeland reacted phenol with formaldehyde to produce a tough, hard, material that did not dissolve in water or other solvents and that did not conduct an electric current. He named the product Bakelite. Bakelite rapidly became very popular as casing material for a number of household products, such as telephones and electrical appliances.

Credit for first recognizing the chemical nature of polymers is usually given to German chemist Hermann Staudinger (1881–1965). In 1926, Staudinger suggested that polymers are very large molecules consisting of one or two simple units (the monomers) repeated over and over again. He received the Nobel Prize in chemistry in 1953 for this discovery.

In the last half of the twentieth century, chemists invented dozens of different kinds of synthetic polymers. Most of these compounds were developed to have certain special and desirable properties, such as toughness; resistance to wear; low density; resistance to water, acids, bases, and other chemicals; and resistance to the flow of electric current.

[See also Plastics ]