Incorporating Polymer Science Lecture Topics into the Beginning

Aug 4, 2017 - chemistry, to promote good student performance, and to provide a sound basis for future career development. KEYWORDS: Organic Chemistry ...
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Incorporating Polymer Science Lecture Topics into the Beginning Organic Chemistry Course To Engage Students’ Interest in Current and Future Applications Bob A. Howell* Department of Chemistry, Central Michigan University, Mt. Pleasant, Michigan 48859, United States

ABSTRACT: The impact of polymeric materials on the well-being of citizens of the modern world is enormous. These materials enhance virtually every facet of lifefrom clothing and personal care items to housing and transportation. Yet despite this, and the fact that most chemists work in a polymer or polymer-related area, polymeric materials have traditionally gotten scant attention in the undergraduate chemistry curriculum. This is now beginning to change. The American Chemical Society now recognizes that undergraduate training in chemistry without inclusion of polymeric materials is inadequate. Inclusion of polymeric materials enhances the curriculum in several ways. It serves to generate student awareness, to create enthusiasm for chemistry, to promote good student performance, and to provide a sound basis for future career development. KEYWORDS: Organic Chemistry, Polymer Chemistry, Second-Year Undergraduate, Curriculum, Industrial Chemistry, Materials Science, Student-Centered Learning

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INTRODUCTION

Polymeric materials are produced in huge volume annually and impact virtually every aspect of life.1 They permit the safe packaging and distribution of food, the availability of comfortable and durable clothing, the construction of longlasting, energy-efficient, and attractive homes, the manufacture of lightweight, durable, and fuel-efficient automobiles and aircraft, and much, much more. In fact, the high standard of living enjoyed by people of the developed world would not be possible were it not for the availability of polymeric materials. These materials are pervasive in modern society.1,2 Chemists at any level of training (B.S., M.S., Ph.D.) will likely pursue a career in industry. Most of these will work in a polymer or polymer-related area.3−6 However, few will have had any significant training in the polymer area.7−11 As a consequence, substantial in-house training may be necessary to permit new hires to become productive. This has long been viewed as a deficiency in the training of chemists in the United States.1,7−12 For all of the aforementioned reasons, the incorporation of polymeric materials into the undergraduate chemistry curriculum is essential. This inclusion may be accomplished across the curriculum.8,9 It is particularly appropriate for the second-year undergraduate course in organic chemistry.12,13 © XXXX American Chemical Society and Division of Chemical Education, Inc.

POLYMER-RELATED DISCUSSION TOPICS

Vinyl Polymerization

For most beginning courses in organic chemistry a discussion of alkenes comes early in the first semester.13 The most important commercial/societal reaction of alkenes is vinyl polymerization.1,14−16 Consideration of this process permits the reinforcement of several concepts that the students have just learned and introduces them to several items that are part of their daily lives. Vinyl polymerization is a chain growth process. Radical polymerization is most common. It is robust (it can be carried out in the presence of air and water and is not terribly sensitive to the presence of impurities) and may be utilized for the polymerization of a wide range of monomers. It is a typical chain process with distinct initiation, propagation, and terminal phases. Students have already encountered this kind of reaction for the halogenation of alkanes and are familiar with radical stability. Thus, they readily appreciate the observation that most radical polymerizations occur with head-to-tail placement Special Issue: Polymer Concepts across the Curriculum Received: January 13, 2017 Revised: July 11, 2017

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This also nicely illustrates that all polymers contain amorphous regions. The portions of the chains that can be pulled out of the amorphous mass may be organized in a more regular way, i.e., they may form crystalline regions. Many polymers are semicrystalline, while others are fully amorphous. Amorphous polymers are hard and brittle below the glass transition temperature (Tg) but pliable above that temperature. The glass transition temperature may be simply defined as the temperature at which chains may flow past each other without breaking and may be illustrated by reference to a phenomenon that most students have already experienced, the bending of glass (an inorganic polymer) tubing. In this process, the tubing is heated in a flame until it is pliable (students fully appreciate that it does not melt) and then is reshaped. When it is allowed to cool (to below Tg) it again becomes rigid but in the new shape.

of the monomer to generate the more stable propagating radical. Ethylene Polymerization

Historically, low-density poly(ethylene) (LDPE) was the first vinyl polymer of commercial importance. It was first prepared by Gibson and Fawcett at Imperial Chemical Industries (ICI) in Great Britain. It was discovered by accident as a consequence of attempted cycloaddition reaction in a sealed tube at high pressure.16 The expected reaction did not occur, but a white solid was formed. This was identified as poly(ethylene). It was ultimately determined that the tube contained a minute leak that allowed oxygen into the reaction mixture, which served as a radical initiator. This development came at a very opportune time, early in World War II (WW II). The British had developed radar, but at the time cable insulation was made of paper, and failure was frequent. The use of poly(ethylene) solved the insulation problems, and radar significantly enhanced the Allied war effort. During ethylene polymerization, the propagating radical undergoes intramolecular chain transfer (“backbiting”). The radical chain end abstracts a hydrogen atom from the methylene group five from the end. The radical formed propagates (adds monomers) to continue the chain and leave a butyl branch. Why does this process occur so readily? It does so via a six-membered activated complex. The students are aware from an earlier discussion of cycloalkane stability that sixmembered rings are relatively strain-free. This provides an opportunity to reinforce this concept. The presence of branches along the main chain prevents tight packing of the chains and results in lower density than that observed for linear poly(ethylene). This is the same phenomenon that causes branched alkanes to have lower boiling points than their unbranched counterparts (another opportunity to reinforce something learned earlier). Several other useful concepts may be introduced at this time. Chain termination by radical coupling (combination) or disproportionation may be related to similar processes encountered earlier in the course or in general chemistry (the decomposition of copper(I) to form copper(0) and copper(II), for example). Students readily appreciate the idea that, depending on when termination occurs, chains may be longer or shorter and that products of polymerization will have a range of molecular sizes. This leads directly to a discussion of average molecular weight and how this may be expressed. The simplest is in terms of the average number of mer units incorporated per chain, which leads to the number-average molecular weight (Mn). Another convenient way to express this quantity is in terms of the average mass of the chains, which gives the weight-average molecular weight (Mw). The physical properties are better reflected by Mw than by Mn. This may be illustrated using a number of simple analogies. One that is particularly well-liked by students is the 10,000 lb. elephant with four 1 lb. mosquitos on its back.17 The Mn for the ensemble is about 2000 lb. (10,004/5) while Mw is approximately 10,000 lb. If one is stepped on by the ensemble, the consequences are little impacted by the presence of the mosquitos (low-molecular-weight chains). Dispersity, the ratio of Mw/Mn, may then be understood. The concept of chain entanglement becomes apparent as a mixture of chains is considered. This may again be illustrated by analogy to a strainer of spaghetti noodles. In an attempt to pull a noodle from the mass, it initially pulls out smoothly but then meets resistance and stops because it is entangled with other chains.

Coordination Polymerization

In the early 1950s, coordination polymerization was developed by Karl Ziegler in Germany. This permitted the development of linear poly(ethylene) or high-density poly(ethylene) (HDPE). The properties of this material are superior to those of LDPE for many applications. Several years later, both Dow Chemical and Exxon developed metallocene catalysts (soluble Ziegler catalysts) for the polymerization of ethylene and other 1alkenes (α-olefins). This opened the door for the preparation of a range of new materials, principally copolymers of ethylene and an l-alkene (often l-hexene or l-octene). These materials are branched, but the branches are uniform in size (butyl or hexyl branches, respectively) and can be incorporated at any desired level. These materials, termed linear low-density poly(ethylene) (LLDPE), have physical and mechanical properties that are better than those of LDPE but inferior to those of HDPE. This has allowed them to displace HDPE in some applications for which LDPE is unsuitable but for which the better properties (and cost) of HDPE are not required. Giulio Natta had early realized that Ziegler catalysts could be utilized for the polymerization of α-olefins and was largely responsible for the development of poly(propylene). Because of chain transfer to a monomer to form a conjugatively stabilized allyl radical (which provides an opportunity to revisit concepts of bonding and stabilization that students have learned earlier), propylene cannot be polymerized to high molecular weight using radical techniques, and poly(propylene) was not available until the advent of coordination polymerization. Both poly(ethylene) and poly(propylene) have had enormous impacts on the development of modern society. Ziegler and Natta shared the Nobel Prize in 1963.18 Poly(styrene) and Copolymers

Poly(styrene) is another polymer that students encounter regularly in their daily lives. It is lightweight, transparent, and inexpensive. Its major deficiency is brittleness, which limits its utility. It is used for the production of light coverings, inexpensive toys, wine tumblers, disposable cutlery, blister packaging for pharmaceuticals, transparent packaging for pastries, etc. Most students have encountered it in one or more of these applications. All students are familiar with foamed poly(styrene) (Styrofoam). Most will know it as the material for disposable coffee cups. However, its much more important use is in home insulation. In both cases, it is the insulating property of the material that is important. It is immensely useful in modern home construction. It is the material (2 to 4 in.) between the B

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exterior (often vinyl siding) and the sheeting (usually plywoodthin sheets of wood veneer held together with lots of phenolic resin). The mechanical properties of general purpose poly(styrene) may be dramatically improved by the incorporation of an elastomer, usually poly(butadiene). The resulting material, high-impact poly(styrene) (HIPS), is much more resistant to crack propagation than is general-purpose poly(styrene). Students are usually aware of the difference from exposure to the use of inexpensive cutlery for summer picnics. A fork made from general-purpose poly(styrene) tends to break when plunged into a piece of barbecued chicken, while one made from HIPS simply bends and permits the completion of the meal. It is copolymers of styrene, styrene−acrylonitrile (SAN) and acrylonitrile−butadiene−styrene (ABS), that have risen to greatest prominence. In particular, ABS is an outstanding material with great utility. It is tough and durable. Almost all of the appliances that students encounter in their homes (refrigerator, washer/dryer, dishwasher, etc.) have housings constructed of ABS. Some will have had the experience of backing a chair into the refrigerator door. The door is not ruined as a consequence (does not dent or crack) but simply pops back to the original shape when the chair is removed. This same property makes ABS a preferred material for automobile side panels. Because of its toughness and durability, the surface of much modern furniture is made from ABS. In fact, the desktops at which students are working may be ABS. It may be given the grain appearance of any wood but is much more longlasting and will last through several generations of students.

and windows, and the other half is used for water-borne coatings. Most students will have had some experience in painting a room in their home, a garage, or an outbuilding and will appreciate the easy cleanup associated with use of modern paints. Some will have experienced the consequences of leaving an uncleaned brush overnight. This leads naturally to a discussion of the curing of the paint to form an impervious protective layer on the surface to which it has been applied. This protective layer is essential for everything from home exteriors and outdoor furniture to automobiles. Poly(acrylate)s are also used in coatings and were a major product of the Rohm and Haas Company. In fact, Rohm and Haas became the world’s premier coatings company. This led to its acquisition by the Dow Chemical Company. Sweater Fabric and Carbon Fiber: Poly(acrylonitrile)

When asked to identify their favorite sweater material, most students will respond “wool”. When asked for the source of wool, someone will ultimately say “sheep”. When asked where sheep are raised, some prodding is sometimes required to identify New Zealand. Despite the fact that New Zealand has more sheep than people, it soon becomes apparent that not enough sheep exist to provide everyone a sweater. So, what to do? Students will quickly come to the conclusion that it or a suitable substitute must be produced by another means. The substitute is poly(acrylonitrile) (PAN). As a fiber in the United States it is known as Orlon.21 Sweaters produced from this material have the look and feel of wool but have other properties that are superior to those of wool. It does not shrink in high humidity or have an “itchy” feel. Poly(acrylonitrile) fiber is widely used in the production of carpeting. More importantly, it is the precursor for carbon fiber. The utilization of carbon fiber polymer composites was essential for the development of the aerospace and modern aircraft industries. Most students are aware that the Boeing 787 Dreamliner is the first commercial “all-composite aircraft”. Students will also be aware of some lesser uses of carbon fibersome will have used “graphite” tennis rackets, golf clubs, or fishing rods.

Poly(vinyl chloride) and Poly(methyl methacrylate) and Applications

Poly(vinyl chloride) (PVC) is the third-largest-volume commercial polymer. Interestingly, it was not the discovery or synthesis of PVC that established it as an important material. The defining event was the discovery of methods for plasticization of PVC by Waldo Semon at B.F. Goodrich that made it an important commercial product.19 PVC requires the presence of large amounts of effective plasticizers, usually esters, to be processed. The two largest uses of PVC are in the formation of vinyl siding for home construction and pipe extrusion. PVC pipe is used in most plumbing applications PVC pipes are much more durable than pipes made from metal and free of the corrosion and contamination problems often associated with these items. PVC is used in many other applications, including the construction of sewers and culverts for highways. Lesser uses include the production of roofing shingles, floor tile, shower curtains, sheet used in packaging, raincoats and hats, and many others. Poly(methyl methacrylate) (PMMA) was another polymer that had a major impact on the conduct of WW II. As a plastic, it is known as Plexiglas.20 It is transparent and durable, but its most useful attribute is that it does not shatter when impacted by a projectile. This makes it much more desirable than standard glass (a silicon−oxygen polymer that is very brittle at room temperature) for the construction of aircraft canopies. The windows of today’s commercial aircraft are made from this same material. This material was developed at the Rohm and Haas Company (the history and background of these two men, both named Otto, is an interesting side story that will be of interest to students). Currently, about half of the production of PMMA is used as Plexiglas for the construction of storm doors

Polymers in the Kitchen: Poly(vinylidene chloride) and Poly(tetrafluoroethylene)

Vinylidene chloride polymers form the base for important barrier plastic packaging.22 The homopolymer, poly(vinylidene chloride), is not commercially viable. At its melt temperature, it undergoes catastrophic thermal degradation to release large volumes of hydrogen chloride and form a highly porous carbon. However, copolymers containing a low level (10−15%) of an alkyl acrylate, often methyl acrylate, are processable and are widely used in food packaging. These materials display properties of a good barrier to the transport of small molecules (oxygen) to prevent spoilage and flavor and aroma compounds to prevent flavor scalping on the supermarket shelf. As flexible wraps, these polymers are used for packaging of meats, cheeses, etc. They are also used as the central barrier layer in rigid containers of various kinds. The common household wrap called Saran Wrap is made from the copolymer with vinyl chloride. Poly(tetrafluoroethylene) (PTFE, Teflon) was a serendipitous discovery of Roy Plunkett at DuPont. For students, this discovery emphasizes the importance of observation and attention to detail. Plunkett was taking tetrafluoroethylene from a small cylinder for use in cycloaddition reactions. A standard procedure was to place the cylinder on a balance, open the valve, and release the required mass of tetrafluoroethylene. C

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Having done this one day, Plunkett came back the following day to carry out another reaction. He was surprised to find that, although the mass of the cylinder was the same as when it had last been used, no gas came from it. When students are asked how they might respond to this situation, they invariably respond that they would discard the cylinder and go to the stockroom for another (as might many practicing chemists). However, Plunkett was curious about what might have happenedthe mass of the cylinder was unchanged from that recorded the previous day, but it contained no gas. He took the cylinder to the machine shop and asked that it be sawed into pieces. He found that the inside walls of the cylinder were coated with a white solid; thus, was born Teflon. Students are familiar with the thermal stability and nonstick properties of Teflon from their experience with cookware containing a surface coating of the material. They are less aware of its use as insulation for wiring in space craft or as supporting pads for construction in earthquake-prone areas. A discussion of Teflon provides an opportunity to review the impact of bonding and structure on the properties of materials.

the guayule plant is that it grows in arid areas of the U.S. Southwest and its production would not displace farmland. The treatment of alkenes occurs early in the first semester of the beginning organic chemistry sequence. Inclusion of a discussion of vinyl polymers provides an opportunity to review concepts of reactivity, bonding and structure just learned from a treatment of alkanes. More importantly, it engages the students, making them aware of the importance of organic chemistry to their everyday well-being and enhancing their interest and enthusiasm for the course. In many cases, it “hooks” them on organic chemistry. Polymers To Cushion, To Wear, To Protect: Poly(urethane), Poly(glucose), Poly(ester), Poly(amide)

Opportunities to incorporate polymeric materials in a way that enhances student learning also abound in the second semester of the beginning organic course. The first usually comes with ether formation, particularly the opening of oxiranes to form polyethers. Poly(ethylene oxide) (PEG) is of particular interest because of its biocompatibility and widespread use in the medical field. Both poly(ethylene oxide) and poly(propylene oxide) of various molecular weights are widely used for reaction with isocyanates to form poly(urethane)s. Students are generally impressed to find that an automobile seat sturdy enough to support the weight of a human adult yet soft enough to provide a comfortable seat can be produced simply by varying the density of the poly(urethane) foam, bottom to top, used to make the seat. The common blowing reaction for the generation of poly(urethane) foam is also of interest and permits a discussion of the stability of β-substituted carboxylic acids. (This can be reiterated later when discussing the Hoffmann, Curtius, and Schmidt reactions for the generation of amines.) Acetals (ketals) are common protecting groups for aldehydes (ketones) and are usually introduced for this purpose. This provides an opportunity to discuss poly(acetal)s, with DuPont Delrin [poly(oxymethylene); poly(formaldehyde)] being the most prominent commercial material. More importantly, the isomeric poly(glucose)s, starch and cellulose, may be introduced. These materials are poly(acetal)s differing only in the stereochemistry of the linkage of the mer units. This difference in structure has a dramatic impact on the properties. Starch is an important human food source, while cellulose is not. Cellulose is, however, an important polymer for fiber and paper production. As a fiber, primarily from the cotton plant, it has long been used for the production of clothing, bedding, curtains, carpeting, and other household items. As a prelude to the discussion of the formation and properties of the glucose polymers, it is helpful to review the equilibrium between the two cyclic hemiacetal forms of glucose in solution. At equilibrium near room temperature, the concentration of the β isomer (equatorial hydroxyl at the anomeric carbon atom) is approximately double that of the α isomer (axial hydroxyl at the anomeric carbon atom). This can be related to the stability of substituted cyclohexanes discussed near the beginning of the course. A discussion of carboxylic acid derivatives allows the introduction of the two giants of the step-growth polymer area, poly(ester) and poly(amide). An early reaction encountered here is Fisher esterification. Reaction of a monofunctional acid with a monofunctional alcohol in the presence of an acid catalyst generates a simple ester. The question of what products may be formed from reaction of a difunctional acid with a

Natural Rubber Replacements: cis-Poly(isoprene), Poly(isobutylene), Styrene−Butadiene

Prior to WW II, rubber required by the United States was natural poly(isoprene) largely supplied by Dutch colonies in Indonesia. With the occupation of these areas by the Japanese, this source was cut off, presenting a crisis in the United States. Rubber was desperately needed for the production of truck and aircraft tires. Without these, the war effort could not go forward. The response was the formation of the Rubber Reserve Company comprising representatives of the rubber and chemical industries, government laboratories, and several academic institutions to find a way to produce rubber by synthesis. The natural material is cis-poly(isoprene) generated under the influence of enzyme catalysis. Of course, attempts to polymerize isoprene lead to the formation of the thermodynamically more stable but unuseful trans-poly(isoprene). This affords an opportunity to re-emphasize the importance of structural features on stability and propertieswhat causes the cis structure to be elastomeric? If vulcanization is included in the discussion, the importance of conjugative stabilization in the formation of allylic radicals can be reinforced. While the effort to generate a domestic supply of rubber occurred under difficult circumstances, it was ultimately successful, although some of the most successful outcomes came after the end of the war. The production of cis-poly(isoprene) was achieved in 1954 at B.F. Goodrich (using a Ziegler catalyst), at Firestone in 1955, and somewhat later at Shell. In the meantime, styrene− butadiene rubber (SBR) was developed at the Dow Chemical Company. Poly(isobutylene) was developed to be used for the production of tire inner tubes. These materials were extremely useful to the war effort and are still used today (in huge quantities). Most “natural” rubber used today is made by synthesis. The current concern about climate change and the utilization of fossil fuel sources has generated a renewed interest in biosources of rubber. Various plants, including goldenrod (Solidago spp.), guayule (Parthenium argentatum), and dandelion (Taraxacum kok-saghyz), generate a cis-poly(isoprene) latex. The levels of polymer produced are generally lower than that produced by the hevea plant (Hevea brasiliensis) grown predominantly in Southeast Asia, but it is certainly conceivable that any of these plants could be modified to make them more efficient in this production. A potential advantage of D

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been written about both the man and the material. There is an abundance of information from this period that will be of interest to students of organic chemistry. Carothers was a brilliant scientist of immense professional accomplishment but a rather tragic personal history.24 He was able to demonstrate, using simple organic reactions, that polymers are macromolecules made up of units of repeating structure. This put on a sound footing the ideas about macromolecules expressed by Hermann Staudinger somewhat earlier. Among all of his accomplishments, the one for which he is most remembered is the development of Nylon-6,6, a poly(amide) generated from the salt of hexamethylenediamine and adipic acid. The choice of which nylon to commercialize was a practical one. Both monomers could be obtained from benzene, which was available in relatively pure form at the time. Further, the salt could be crystallized to generate a pure precursor to the polymer with exactly the 1:1 stoichiometry needed for successful step-growth polymerization. The DuPont company had been founded as a munitions (powder) company and had prospered.33 Powder was in demand for clearing land and building railroads of an expanding nation. During World War I, DuPont supplied most of the powder required by the U.S. military. After the war, it was deemed to be monopolistic and had to be broken up. Three entities were formed: a division of DuPont, Hercules Powder, and Atlas Powder. At this time, DuPont recognized the need to diversifythe country was largely settled, roads had been built, and the need for powder was diminished. At that time, textile fibers were from natural origins. The most prized was silk, a poly(amide). However, silk is produced by the growing larvae of a particular moth. To be productive, the temperature and humidity must be carefully controlled and the organisms fed a diet of mulberry leaves. Even small variations in conditions can be disastrous for a silkworm colony. As a consequence, the supply of silk was limited. Although it was prized for the production of ladies’ scarves and stockings, these items were expensive and available only to the affluent. DuPont recognized that this presented an opportunity to respond to an obvious societal need. A material with the properties of silk that could be mass-produced at manageable cost was needed. A problem faced by DuPont in addressing this need was that it employed no organic chemists (and at that time was without a research arm). Through rigorous efforts, the company was able to hire Wallace Carothers away from Harvard University to head this undertaking. Ultimately, Carothers was wonderfully successful not only in generating “synthetic silk” but also in developing a number of other new materials. Nylon stockings were unveiled at the 1939 World’s Fair in New York and were a sensation. Popular magazines of the time contain pictures of women lined up for blocks to purchase their single pair of nylons (now these are essentially throw-away items). Within a short time, Nylon stockings were no longer available. The entire production of Nylon was required to support the war effort. The Normandy invasion would not have been possible without the availability of Nylon-6,6 for the production of parachutes. The story of the development of Nylon and the achievements of Wallace Carothers has many facets (far too many to recount here) that are of interest to young students and can be used to good effect in the beginning organic chemistry course.24−32 The development of poly(amide)s at DuPont did not end with Nylon-6,6. Some years later, Stephanie Kwolek, the first female scientist at DuPont, developed Kevlar, the poly(amide) from p-phenylenediamine and terephthalic acid, and Nomex,

monofunctional alcohol or monofunctional acid with a difunctional acid with a difunctional alcohol may be raised. In this way, it is easy for students to appreciate that a polymer can be formed from a series of sequential esterification reactions, a typical step-growth process.23 Poly(ethylene terephthalate) is the common poly(ester) with which students are familiar. It was first produced by Whinfield and Dickson at the Calico Printers Association in 1941. Today it is produced from purified terephthalic acid and ethylene glycol. It has been a wonderfully successful product. It may be formulated as either a fiber (Dacron) or a plastic (PET). As a fiber, it is used in the production of a wide range of useful materialsclothing, carpeting, upholstery, curtains, etc. As a plastic, it is used to make trays, milk jugs, and, of course, the ubiquitous water bottle. The fiber is much more durable than traditional cotton fiber, and in the 1950s it replaced cotton for the production of much clothing. However, this was not successful and led to the common practice of generating clothing from blends of cotton and poly(ester) fibercotton for “feel” and poly(ester) for durability. This observation provides yet another opportunity to relate structure to properties. Why do people like to wear clothing containing cotton as opposed to that made from pure poly(ester)? Cotton is poly(glucose). Each mer unit contains hydroxyl groups. A monolayer of water is hydrogen-bonded to the surface of the fiber. Water is an insulator, and the garment feels warm. On the other hand, poly(ester) lacks hydroxyl groups and continuously conducts heat away from skin with which it is in contact. Therefore, it feels cold, and the wearer is uncomfortable. Several other properties can be illustrated using the blend of fibers. Poly(ester) is much more durable than cotton. Most students have had the experience of noting the change that occurs with a common T-shirt upon repeated wearing and laundry. The mass of the shirt decreases with time until ultimately all that is left is a mesh of poly(ester) fiber. This may be readily demonstrated for the class by simply displaying a new T-shirt beside an old (much laundered) T-shirt. Shirts made of cotton fiber tend to wrinkle readily and must be repeatedly ironed to look good (students’ grandmothers will remember this). A brief discussion of this phenomenon permits the reinforcement of two concepts learned earlierplasticization and glass transition. The glass transition temperature Tg for cotton is relatively low. When a student wearing a cotton shirt arrives in class and sits against a chair back, plasticization by water from the skin reduces Tg further. Warmth from the skin raises the temperature of the cotton above Tg. As the student moves in the seat, the shape of the fabric (now above Tg) readily changes, i.e., wrinkles form. At the end of class, the student walks into the hallway, the fabric cools below Tg and the wrinkles become permanent. They can, of course, be removed by raising the temperature of the fiber above Tg and reshaping it (ironing). Now that students understand poly(esterification) and have an appreciation of the PET structure, it is appropriate to introduce the nucleic acid polymers, which are analogous poly(ester)s. The nucleic acid main chain is a poly(ester) in which phosphoric acid functions as the difunctional acid and a pentose, either ribose or deoxyribose, functions as a difunctional alcohol. Historical Development of Poly(amide)

The defining event in the development of polymeric materials in the 20th century was the generation of poly(amide)s (Nylons) by Wallace Carothers at DuPont.24−32 Much has E

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the poly(amide) from m-phenylenediamine and isophthalic acid.34 To distinguish these materials from the aliphatic poly(amide)s, they are known as aramids. Kevlar is the material from which bulletproof vests and helmets used by law enforcement and military personnel are made. Nomex is inherently fire-resistant and is the material from which protective clothing for firefighters and race car drivers is made. At this point, it is appropriate to introduce natural poly(amide)s [poly(peptide)s, proteins]. The linkage between mer units is the amide (peptide) bond, as for poly(amide)s from synthesis. However, natural poly(amide)s contain a variety of mer units, i.e., they are copolymers of several amino acids. It is instructive to compare and contrast the structures and properties of the two. In both cases, the main chains align such that hydrogen bonding is maximized. This results in the formation of β-pleated sheets and helices for proteins and accounts for the great strength of nylons. Proteins exhibit additional secondary structures that account for their unique properties as catalysts.

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CONCLUSIONS Opportunities for the incorporation of polymeric materials into the beginning organic course abound. Some of these have been detailed here, and many others are available. The intent here is not to be exhaustive but to illustrate how the introduction of polymeric materials fits naturally (all of this, after all, is organic chemistry) and may be used to reinforce concepts important to organic chemistry. The incorporation of this material enhances the courseit improves student morale and interest in the course, makes students much more aware of their surroundings and the impact of simple organic chemistry on the quality of their daily lives, makes students much more socially aware and gets them to begin to think critically about problems facing modern society, and provides an enjoyable atmosphere in which to learn the fundamentals of organic chemistry. On the basis of long experience in teaching beginning organic chemistry, the incorporation of this material takes little time but enhances the quality of the course and often improves student performance, as evidenced by responsiveness (questions, comments), performance on hour exams, and improved scores on the ACS standardized final examination.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bob A. Howell: 0000-0003-1534-4351 Notes

The author declares no competing financial interest.

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REFERENCES

(1) Wittcoff, H. A.; Rueben, B. G.; Plotkin, J. S. Industrial Organic Chemicals, 3rd ed.; John Wiley and Sons: Hoboken, NJ, 2013. (2) Seymour, R. B. Polymers are Everywhere. J. Chem. Educ. 1988, 65, 327−334. (3) United States Department of Labor, Bureau of Labor Statistics. Occupational Employment Statistics: Occupational Employment and Wages, May 2016: 19−2031 Chemists. https://www.bls.gov/oes/ current/oes192031.htm (accessed June 2017). (4) Billmeyer, F. W.; Kelley, R. N. Entering Industry: A Guide for Young Professionals; John Wiley and Sons: New York, 1975. F

DOI: 10.1021/acs.jchemed.7b00033 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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(27) Kauffman, G. B. Nylon at 50. CHEMTECH 1988, 18 (12), 725−731. (28) Patterson, G. A. A Prehistory of Polymer Science; SpringerBriefs in Molecular Science; Springer: Heidelberg, 2012; Chapter 4. (29) Naglak, G. Dress for Success; Chemical Heritage Foundation: Philadelphia, PA, 2013. (30) Morris, P. J. T. Polymer Pioneers; Center for the History of Chemistry: Philadelphia, PA, 1986; pp 58−60. (31) Wingate, P. J. Why Carothers Joined DuPont: The Hopkins Mafia. Chem. Heritage 1998, 15 (2), 44−46. (32) Kativa, H. S. Synthetic Threads. Distillations 2016, 2, 16−21. (33) Kelly, J. Explosive Growth. Invent. Technol. 1998, 13 (4), 10−20. (34) Preston, J. High-Strength/High-Modules Fibers from Aromatic Polymers. J. Chem. Educ. 1981, 58, 935−937.

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DOI: 10.1021/acs.jchemed.7b00033 J. Chem. Educ. XXXX, XXX, XXX−XXX