In this module, we have attempted to provide an introduction to plastics in particular, polyolefin polymers. Our primary goal is to show how chemists can control the structure and properties of polyolefins by selecting the appropriate monomer, rationally modifying the catalyst, changing the reaction conditions, etc. By intelligent choice of reagents, catalysts, and conditions, chemists have the ability to synthesize plastics of their own design.
Portions of this module, particularly Sessions 2 and 4, are similar in content and depth to Chapter 10 ("The World of Plastics and Polymers") in Chemistry in Context (A. Truman Schwartz et al., Wm. C. Brown Publishers, 1994) and students should be encouraged to read this chapter as a supplement. However, since our primary goal is to illustrate how chemists can control polymerization reactions, a molecular-level understanding of the polymerization reaction mechanism is required. We introduce this topic in Session 3, using the example of ethylene polymerization on a homogeneous (molecular) metallocene-type catalyst. The cyclopentadienyl ligands on this catalyst can be simply viewed as rigid balls that occupy coordination sites around the metal center. Therefore, a prior knowledge of organometallic chemistry is definitely not required by the student (or the instructor). However, some fundamental organometallic reaction steps (olefin coordination, alkyl migration,
b-hydrideelimination) do need to be introduced in order for students to understand how a polymer chain grows on a metal center. Again, these steps can be discussed at a variety of levels, depending on the students' background and sophistication.
In Session 5, we analyze the various isomers that can result when a monosubstituted olefin, propylene, is polymerized. The orientation of the propylene units along the chain (head-to-tail vs. head-to-head, tail-to-tail vs. random) and the orientation of the pendant methyl groups along the polymer backbone (isotactic vs. syndiotactic vs. random) are considered. This discussion begs the question: "How can chemists control the way in which propylene monomers are stitched together to form polypropylene?"
In Sessions 6 and 7, we answer this question by looking closely at the mechanism of propylene polymerization on a metallocene-type catalyst. Building on the material in Session 3, we show that it is the orientational preferences of the propylene monomers that dictate the way in which the polypropylene polymer grows. Furthermore, by modifying the cyclopentadienyl ligands in rational ways, chemists can cause polymers with different tacticities (isotactic, syndiotactic, atactic) to form. Session 8 asks the students to use their newly-acquired knowledge to design a new polyolefin polymer and a catalyst to direct its synthesis.
This module assumes some background knowledge of bonding (
s-and p-bondingin organic compounds) and the three-dimensional structure of organic compounds. A previous exposure to the topics of symmetry and stereochemistry would be helpful but is not essential; these subjects can be introduced within the context of the module.
This module is appropriate for a general chemistry course directed at chemistry majors, pre-medical students, and engineering students. Since some knowledge of structure and bonding are assumed, it should probably be used toward the end of the course as a bridge to sophomore organic chemistry. It is also appropriate for organic chemistry courses and would provide an excellent introduction to the topics of polymer chemistry and organometallic catalysis. Non-major courses are probably better served by the treatment in Chapter 10 of Chemistry in Context (A. Truman Schwartz et al., Wm. C. Brown Publishers, 1994).
This module has been placed on the internet and is accessible to students with web connections. Each section of the module is accompanied by a series of questions; short answers are also provided. In order to assist student with the visualization of three-dimensional structures, many of the molecules in this module can be downloaded and viewed interactively. Interactive viewing allows the students to experiment with different atom representations (ball-and-stick, CPK, etc.), rotate around bonds, evaluate bonds distances and angles, and view molecules from various perspectives. In addition, we strongly recommend that students build models of polymers. A relatively inexpensive set of Cochrane's Molecular Models, consisting of 100 carbon sp3 centers (Z12, 726-4) and 800 bond tubes (Z18, 463-2), can be obtained from Aldrich Chemical Co.
(800-558-9160)for about $20.
Our principal goal is to get students thinking about how chemists can control the products of polymerization reactions through intelligent choice of monomers, catalysts, reaction conditions, etc. In so doing, students will develop an understanding of how molecular-level interactions result in polymeric products with particular structures and properties. The module also provides a good introduction to organometallic chemistry, particularly metal-based catalysis. Among the Chemical Principles in "The Tool Kit", "Structure and Bonding" and "Reactivity" are the two topics that are covered in the greatest detail.
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