Commentary pubs.acs.org/jchemeduc
Lengthening the Chain: Polymers in General Chemistry John W. Moore*,† and Conrad L. Stanitski‡ †
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States Chemistry Department, Franklin and Marshall College, P.O. Box 3003, Lancaster, Pennsylvania 17604, United States
‡
ABSTRACT: Polymers are currently underrepresented in the undergraduate chemistry curriculum, especially in general chemistry. Nevertheless, there are good reasons and ways to include them in the two-semester (three-quarter) general chemistry course sequence. We describe both a course and a textbook in which polymers and biopolymers are integrated into the general chemistry curriculum.
KEYWORDS: First-Year Undergraduate/General, Curriculum, Polymer Chemistry, Textbooks/Reference Books, Applications of Chemistry, Materials Science, Molecular Properties/Structure “a state-of-the-art overview” of chemistry. It is organized around several major themes: the relationship of molecular structure to properties of matter, the rates of transformations of one chemical structure into another, and chemical equilibrium and thermodynamics. Once developed, these themes are applied to acid−base chemistry and electrochemistry. Although AGC is a one-semester course for well-prepared students, the same themes have been incorporated into the two-semester general chemistry sequence at UW-Madison, and they continue to be used in both settings. The first third of AGC is an extended story that consists of these parts: • Atomic electronic structure can rationalize properties of elements. • Atoms combine to form arrays of ions or to form molecules. • Properties of the substances formed depend on their structures and on forces of attraction between structural units. • Several different structures can have the same elemental composition and chemical formula. • Structural features of small molecules can be predicted and applied to the structures of larger molecules. • Molecular structures determine the sizes and types of non-covalent attractions among molecules. • Macromolecules can be formed from small molecules by addition or condensation reactions. • Macromolecules consist of small units repeated many times.
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t now seems implausible that not quite a century ago, chemists did not believe in the existence of high-molecularweight compounds, ones we know today as polymers. Starting in the 1920s, research by Hermann Staudinger led him to propose that natural rubber and other such compounds were macromolecules,1 high-molecular-weight compounds made up of repeating units. This groundbreaking insight contrasted with the then-conventional wisdom among organic chemists that such compounds were simply aggregates of small molecules. In fact, Heinrich Wieland, the recipient of the 1927 Nobel Prize in chemistry, advised Staudinger, “Dear colleague: Drop the idea of large molecules; organic molecules with a molecular weight higher than 5000 do not exist...”2 Fortunately, Staudinger persisted and was awarded the 1953 Nobel Prize in Chemistry for his characterization of natural rubber as a polymer of isoprene. Today, we would be hard-pressed to imagine living without polymers, both natural and synthetic. Envision a residence hall room or student apartment and take away the polymers. What would be left? Not much! Despite this direct applicability of chemistry to their lives, college students have little knowledge or understanding of this important category of compounds. For example, most general chemistry students would not recognize that the very paper on which their textbook is printed is cellulose, a polymer. Unfortunately, polymers are currently underrepresented in the undergraduate chemistry curriculum especially in general chemistry. Nevertheless, there are good ways to include them.
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A COURSE THAT INTEGRATES POLYMER TOPICS How polymer topics can be incorporated into a typical general chemistry sequence is demonstrated by a course one of us developed together with colleagues at the University of Wisconsin-Madison more than 15 years ago. This course, Advanced General Chemistry (AGC), was designed to provide © XXXX American Chemical Society and Division of Chemical Education, Inc.
Special Issue: Polymer Concepts across the Curriculum Received: October 24, 2016 Revised: January 20, 2017
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DOI: 10.1021/acs.jchemed.6b00811 J. Chem. Educ. XXXX, XXX, XXX−XXX
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• Properties of macromolecules depend on their structures, which often depend on non-covalent forces between different parts of the same large molecule. • Biological macromolecules epitomize the ways that molecular functions depend on molecular structures and non-covalent interactions. Why this sequence? It is a great story that illustrates the power of chemistry to make sense of the material world. It incorporates a broad range of major principles that are typically taught in general chemistry. It applies those principles to situations that students can see are important and that are relevant to their everyday lives. It introduces students to topics that are the subject of cutting-edge research and are not the same old thing they learned in high school. It helps students prepare for subsequent coursesin other departments as well as chemistrywhere principles of chemical structure and function are the basis for understanding new content. It approaches topics students have studied in high school from a different perspective, retaining their attention. It introduces important types of molecules, such as proteins, that can be referred to again later in the course, such as in the discussion of enzyme kinetics. Finally, it is fun, challenging, and eminently teachable.
protein structures can be tailored to a specific molecular task. Students can readily see that natural selection might result in highly “tuned” structures that are very efficient. The role of the smaller number of monomers from which DNA is formed and the ability of that structure to code for polymer synthesis and to replicate also call upon students to apply what they have learned earlier. Introducing polymers immediately after the discussion of intermolecular forces provides a great way to revisit that topic and emphasize its importance. The structure of DNA and both α-helix and β-sheet secondary protein structure provide a good review and significant motivation for learning about the importance of hydrogen bonding. The broad range of noncovalent forces that maintain tertiary and quaternary protein structure is used to review all non-covalent forces and to reemphasize the importance of hydrophobic interactions. It is pointed out that how a protein chain folds to achieve its native state is not yet known in detail and that students might contribute to learning more about it. Demonstrations include hydrolysis of wool in strong base and following unfolding and refolding of a protein using fluorescence.6,7 Having introduced polymers and biopolymers, we make certain to return to them later in the course. In the section on chemical kinetics we discuss enzyme catalysis, emphasizing that enzymes are highly active and very specific because of the complementarity of enzyme and substrate structures and the non-covalent forces between them. Enzyme catalysis is related to an energy versus reaction coordinate diagram, enabling review of that topic. Students do a laboratory exercise in which they measure rates of an enzyme-catalyzed reaction. The ability of an inhibitor to fit into an active site, competing with or excluding substrate, also reviews principles of molecular structure. In the section on acid−base chemistry, we discuss the zwitterion structures for amino acids and demonstrate that the solubility of protein molecules depends on pH. The section on Gibbs energy includes photosynthesis and carbohydrates as animals’ source of Gibbs energy. We describe partitioning of that energy into small quantities available via ATP hydrolysis to drive other metabolic processes.
Specific Polymer Topics in General Chemistry
We introduce macromolecules by describing initiation, propagation, and chain termination for free-radical addition polymerization of ethylene. Students can readily extend their knowledge of a double bond and recognize that its π bond enhances the reactivity. To demonstrate that polymers are huge molecules, we show the shear thickening of poly(acrylamide), Mw > 5 × 106, which when stirred forms a cone instead of a vortex.3 Properties of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are demonstrated using typical containers made from each material, and the branched structure of LDPE is shown using a figure. We point out the broad range of monomers that can form addition polymers. Examples of plastics described by all of the recycling codes are shown so that students are aware of the importance of polymers in their lives. We introduce the idea of a copolymer, illustrate it with the structures of styrene−butadiene rubber and natural rubber, and allude to the importance of synthetic rubber in World War II. We also describe vulcanization and crosslinking. Formation of a polyurethane foam is demonstrated.4 Poly(ethylene terephthalate), with which students will be familiar as the material in disposable plastic water bottles, is used to introduce condensation polymerization. Students are already familiar with condensation of alcohols with carboxylic acids to form esters and with amide formation, so assimilating condensation polymerization is not difficult. The nylon rope trick demonstration provides another example of condensation polymerization.5 We point out the need for two functional groups either in the same molecule or in two different molecules as well as the fact that a small molecule is a byproduct of the condensation reaction. Hydrogen-bond crosslinks in nylon 6-6 are shown on space-filling drawings of several polymer chains, and their effect on polymer properties is emphasized. Condensation polymerization is a logical lead-in to a discussion of biopolymers: proteins and DNA. The synthesis of proteins extends the idea of copolymerization to include many different monomers, and we point out that with the astronomical number of different possible polymers, specific
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WHY INCLUDE POLYMERS? There are many good reasons to include polymers and biopolymers in general chemistry: • If we are to provide an overview of chemistry, we should include aspects that are the focus of current research in the field and that are crucial for most businesses that are chemistry-based. • For many students, a general chemistry course will be their only chemistry course, and such students should be aware of the great importance of polymers in their lives. • Polymers provide a context within which many general chemistry topics can be taught, exemplified, and reviewed: bonding, functional groups, molecular structure and reactivity, structure−function relationships, noncovalent forces between and within molecules, and catalysis. • Polymers nicely illustrate how desired macroscopic properties can be attained by changes in molecular structures, i.e., how molecules can be “tuned” to meet specific needs. • Most students in general chemistry plan to major in fields where fundamental knowledge of polymers is B
DOI: 10.1021/acs.jchemed.6b00811 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Table 1. Chemical Concepts and Polymer Context Concept Hydrogen bonding Non-covalent forces Molecular weight Bonding using C, H, and O Bonding using C, H, O, and N Chemical structure and function Functional group Reaction mechanism Activation energy and catalysis Active site and enzyme catalysis Enzyme inhibition Gibbs energy
Polymer Context
Chapter
DNA structure; α-helix and β-pleated sheet structures of proteins Hydrogen bonding in DNA; Protein tertiary structure Chain length and degree of polymerization; osmotic pressure Polysaccharides, triglycerides DNA; 1°, 2°, 3° protein structure
7, 10 7, 10 7, 10, 13 10 7, 10
Natural polymersbiomolecules; Synthetic polymersdesigned properties by control of monomer type, chain length, branching, and cross-linking Polyesters, polyamides Addition and condensation polymerization Enzyme action
7, 10
Polysaccharide hydrolysis
11
Folic acid synthesis and sulfa drug action Metabolism of carbohydrates, fats, and proteins
11 16
6, 10 10 11
at least one textbook is available that integrates polymer topics with general chemistry concepts; that textbook has been used in the course described above for 15 years.12 Both the course sequence and the textbook take advantage of the fact that polymers offer a convenient context in which to directly teach, exemplify, and review many major general chemistry concepts. The major chemical concepts in the textbook that are developed, expanded, and illustrated within a polymer context are summarized in Table 1. Other sources of information about polymers that may be useful to those who want to adopt our approach are available.13 The basic concepts of molecular formulas and covalent bonding in alkanes are introduced early, followed by multiple covalent bonds and bond properties. The discussion of molecular shapes, molecular polarity, and non-covalent forces leads directly to the introduction of polymers with the presentation of nucleotides and DNA. Because most general chemistry students are in biology-related majors, interest is raised among them through this early application of polymers and a detailed chemical description of nucleotide sequences in this, arguably, the most significant biomolecule. This discussion is also a context to describe the profound structural effects of directed hydrogen bonding among complementary base pairs in contrast to the covalent bonding within the nitrogen bases. In addition, polymers afford a rich context in which to review polymer-related organic functional groups: carboxylic acids, esters, amines, amides, and amino acids. Modern polymers, some of which are crafted to mimic natural macromolecules, have specific properties created by controlling the monomer types, chain length, chain branching, and cross-linking. The specific properties dictate the polymer’s use, as in the case of LDPE and HDPE discussed earlier. In this approach, the use of polymers as a context for teaching major chemical concepts in general chemistry predates and is in line with the new curriculum requirement in the most recent CPT Guidelines, which address the current underrepresentation of polymers in most undergraduate chemistry curricula.8 In addition, the Spring 2016 CPT Supplement Macromolecular, Supramolecular, and Nanoscale (MSN) Systems in the Curriculum goes on to note that “Much of the traditional undergraduate curriculum in chemistry focuses on the synthesis and characterization of small discrete molecules. But many types of materials are not well-described from this perspective. These include macromolecules (whether synthetic or bio-
valuable: biomedical sciences, engineering, and many others. • Macromolecules, especially biopolymers, illustrate how atoms of only a few elementscarbon, hydrogen, oxygen, and nitrogencan bond with each other to form compounds with a broad range of useful properties. • Polymers can be presented as part of a logical sequence (rather than a separate silo) that shows students how molecular-level thinking can guide the understanding and design of really important substances. • The ACS Committee on Professional Training (CPT) Guidelines (2015) state that “the principles that govern [macromolecular, supramolecular, mesoscale, and nanoscale] systems must be part of the curriculum required for certified graduates. This instruction must cover the preparation, characterization, and physical properties of such systems.”8 A basic introduction to such systems at the general chemistry level can contribute to students’ ability to better understand more details in subsequent courses. The reasons for not including polymers in general chemistry are much less convincing. Other topics are said to be more fundamental, which leaves no time to discuss polymers, but most of the general chemistry concepts we need to teach can be readily applied to and exemplified by polymers, so little extra time is needed.9 It is argued by some that students will learn about polymers in more advanced courses. If polymers are included in such courses, that is great, but not all students take those courses, and polymers are sufficiently important that all students should learn some basic facts and principles pertaining to them. It is more difficult to find appropriate textbook and other support materials for teaching polymers, but that has not prevented others from incorporating polymers and biopolymers at the general chemistry level, even in the laboratory.10,11
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FINDING A TEXTBOOK Of course, an appropriate textbook is key to teaching such a sequence. In general chemistry textbooks, polymers are commonly relegated to one of the last few chapters. Those chapters are typically not included in the syllabus because of the desire to cover topics perceived as being more important. Separate chapters as an introduction to polymers also make it difficult to integrate polymers with the many general chemistry concepts that are nicely exemplified using polymers. However, C
DOI: 10.1021/acs.jchemed.6b00811 J. Chem. Educ. XXXX, XXX, XXX−XXX
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(5) Shakhashiri, B. Z. 3.1 Nylon 6-10. In Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1; University of Wisconsin Press: Madison, WI, 1983; p 213. (6) University of Wisconsin-Madison Demonstration Lab. Hydrolysis of Wool in Strong Base. https://www.chem.wisc.edu/deptfiles/ genchem/demonstrations/Biochem/biochemdemos.htm (accessed January 2017). (7) Carlson, T.; Lam, K.; Lam, C.; He, J.; Maynard, J.; Cavagnero, S. Naked-Eye Detection of Reversible Protein Folding and Unfolding in Aqueous Solution. Accepted for publication. J. Chem. Educ. 2016, DOI: 10.1021/acs.jchemed.6b00507. (8) American Chemical Society Committee on Professional Training. ACS Guidelines and Evaluation Procedures for Bachelor’s Degree Programs, Spring 2015; American Chemical Society: Washington, DC, 2015. (9) Polymer Core Course Committee in General Chemistry. Polymer Chemistry for Introductory General Chemistry Courses. J. Chem. Educ. 1983, 60 (11), 973−977. (10) Schaller, C. P.; Graham, K. J.; Johnson, B. J.; Jakubowski, H. V.; McKenna, A. G.; McIntee, E. J.; Jones, T. N.; Fazal, M. A.; Peterson, A. A. Chemical Structure and Properties: A Modified Atoms-First, OneSemester Introductory Chemistry Course. J. Chem. Educ. 2015, 92 (2), 237−246. (11) Mc Ilrath, S. P.; Robertson, N. J.; Kuchta, R. J. Bustin’ Bunnies: An Adaptable Inquiry-Based Approach Introducing Molecular Weight and Polymer Properties. J. Chem. Educ. 2012, 89 (7), 928−932. (12) Moore, J. W.; Stanitski, C. L. Chemistry: The Molecular Science, 5th ed.; Cengage Learning: Stamford, CT, 2015. (13) For example, see: (a) The Macrogalleria. http://www.pslc.ws/ macrog/index.htm (accessed January 2017). (b) Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry; Howell, B. A., Ed.; ACS Symposium Series, Vol. 1151; American Chemical Society: Washington, DC, 2013. Teegarden, D. M. Polymer Chemistry: Introduction to an Indispensable Science; NSTA Press: Arlington, VA, 2003; available at https://books. google.com/books?isbn=0873552210). (14) American Chemical Society Committee on Professional Training. Macromolecular, Supramolecular, and Nanoscale (MSN) Systems in the Curriculum. https://www.acs.org/content/dam/ acsorg/about/governance/committees/training/acsapproved/ degreeprogram/macromolecular-supramolecular-nanoscalesupplement.pdf (accessed February 2017).
logical), supramolecular systems, and nano/mesoscale (MSN) systems.”14 Some CPT-approved chemistry programs address this new MSN requirement by offering a specialized course. Alternatively, the revised CPT Guidelines also allow the introduction of these concepts into existing foundation and in-depth courses across the curriculum. These guidelines are a welcome change that helps to refocus the curriculum to include topics related to current cutting-edge materials and research related to them. However, it should be noted that the CPT Guidelines consider general chemistry as an entry-level course rather than a foundation or in-depth course. Thus, the revised CPT Guidelines, unfortunately, are silent about MSN topics in general chemistry. In our view, the revised Guidelines miss a significant opportunity to advocate for the integration of polymer-based topics in the two-semester (three-quarter) general chemistry sequence.
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CONCLUSION Polymer topics can be introduced and integrated successfully into the general chemistry curriculum. These topics serve to illustrate and amplify the concepts and principles being taught at that level. Polymers are an extremely important aspect of chemistry that should be part of the education of any student who takes an introductory chemistry course. Many such students will find their knowledge of polymers helpful in subsequent coursesnot only in chemistry, but in biomedical sciences, engineering, and other disciplines. Any course that is designed to provide an overview of chemistry ought to include polymer topics.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
John W. Moore: 0000-0001-7652-0668 Notes
The authors declare the following competing financial interest(s): The authors of this paper are co-authors of a textbook described in this paper.
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ACKNOWLEDGMENTS J.W.M. acknowledges the support of colleagues and former colleagues who participated in the design and teaching of Advanced General Chemistry, especially Judith Burstyn, Arthur B. Ellis, and Mahesh Mahanthappa. We thank Elizabeth Moore for help with the graphical abstract.
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REFERENCES
(1) Staudinger, H.; Fritschi, J. Ü ber die Hydrierung des Kautschuks und über seine Konstitution. Helv. Chim. Acta 1922, 5 (5), 785−806. On p 788, the term “Makromolekel” is used for the first time. (2) Staudinger, H. From Organic Chemistry to Macromolecules, A Scientific Autobiography; Wiley: New York, 1961; p 79. (3) Shakhashiri, B. Z. 9.34 Rod Climbing by a Polymer Solution. In Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 3; University of Wisconsin Press: Madison, WI, 1989; p 235. (4) Dirreen, G. E.; Shakhashiri, B. Z. The Preparation of Polyurethane Foam: A Lecture Demonstration. J. Chem. Educ. 1977, 54 (7), 431. D
DOI: 10.1021/acs.jchemed.6b00811 J. Chem. Educ. XXXX, XXX, XXX−XXX
