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 percent of plastics are currently recycled and the rest goes into municipal waste, making up about 30 percent 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.
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 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.