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William F. Coleman Wellesley College Wellesley, MA 02481
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Edward W. Fedosky University of Wisconsin–Madison Madison, WI 53715
We offer two new items from the JCE WebWare peerreviewed collection. Find these new additions to the entire collection Only@JCE Online at http://jchemed.chem.wisc.edu/ JCEWWW/Features/WebWare/collection/reviewed/index.html. Copoly: A Tool for Understanding Copolymerization and Monomer Sequence Distribution of Copolymers
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Massoud Miri*, Department of Chemistry, Rochester Institute of Technology, Rochester, NY 14623;
[email protected]. Juan A. Morales-Tirado, Xerox Co., Webster, NY 14580
The study of the composition and monomer sequence distribution of binary copolymers is often complicated because of the many definitions of representative properties for the sequence distribution, the numerous calculations required, and occasionally the abstract treatment of the statistical processes describing the copolymer formation. Copoly resolves these issues with a user-friendly, highly visual interface to perform all calculations. Using Microsoft Excel and Word, Copoly is compatible with Windows and Mac OS. In Copoly the students enter up to five independent data parameters using the Data Input Window, and immediately see the results. To obtain diagrams for a copolymerization obeying a second-order Markovian process, the fraction of one monomer, fA, and the reactivity ratios, rA, rB, rA´ and rB´ need to be entered; for a first-order Markovian process only the first three of these are required. For a Bernoullian- or zeroth-order Markovian process only fA and rA are required. The results are displayed on separate sheets labeled: 1. Copolymerization Diagrams, 2. Dyads and Triads, 3. Sequence Length Distribution, 4. Simulated Copolymer Design, and 5. Summary. Sheets 1–4 show side-by-side views of three diagrams for potentially all three of the Markovian process orders. In the Summary sheet, the four types of diagrams presented in sheets 1–4 are shown together, being calculated for a secondorder process, which depending on the input data, may simplify to a first- or zeroth-order Markovian process as subsets. In addition, there are three supplements to the Excel worksheet integrated as Word files. In Supplement 1 all applied equations are given in their conventional notation, which are easier to comprehend than the Excel formula bar. Supplement 2 presents 20 selected cases with input data and comments. Supplement 3 provides further instructions. Students can learn to interpret the distributions of nads (for example, triads) and sequence lengths in combination with a simulated copolymer design, and ascertain how these key copolymer properties are related. They find that copolymerizations with tendencies to alternation or block formation do not necessarily lead to copolymers with easily
Figure 1. Copoly, showing an excerpt of sheet 2. Dyads and Triads, and the Data Input Window.
recognizable patterns. Furthermore they can determine how to obtain a desired copolymer composition and monomer sequence distribution based on adjustments of the feed composition and the reactivity ratios. This tool can be used as a homework assignment or for self-tutoring. Accompanying documentation describes the theoretical background and provides information on how to effectively use this program. How Accurate Is the Steady State Approximation?
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Lars Ole Haustedt and Jonathan M. Goodman, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, UK.
In a common two-step reaction, A reacts reversibly to form B, which reacts irreversibly to form C. Although this is one of the simplest two-step mechanisms, it is impossible to find analytical solutions to the rate equations that describe the changes in concentration of the three components over time. The problem of integrating the rate equations is often overcome by the use of approximations. The steady-state approximation assumes that the concentration of B remains constant; this assumption leads to analytical solutions to the rate equations. How good this approximation is can be analyzed by a variety of programs (1–7). In general, however, these programs have to be downloaded, sometimes compiled, and then installed, a process that is inconvenient for a large class of students and impractical for students who want to study on their own computers. An alternative approach to solving the rate equations is to use the pre-equilibrium approximation, that is, A and B are always present in their equilibrium ratios, equivalent to assuming that k2 is much slower than both k1 and k᎑1.
JChemEd.chem.wisc.edu • Vol. 80 No. 7 July 2003 • Journal of Chemical Education
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A Java applet is presented here that allows students to make interactive analyses of reactions of this type and to assess the reliability of the steady-state and equilibrium approximations for themselves. This applet allows students to compare the results of these approximations, and also to investigate the results of applying thermodynamic and kinetic controls. The Java applet has these advantages: it needs no installation (provided a Web browser is available), it is very compact (less than 7 KB) and thus downloads quickly, and the Java applet runs on a variety of different computers. Literature Cited 1. 2. 3. 4. 5.
Pavlis, R. R. J. Chem. Educ. 1997, 74, 1139. Viossat, V.; Ben-Aim, R. I. J. Chem. Educ. 1993, 70, 732. Miller. S. I. J. Chem. Educ. 1985, 62, 490. Tardy, D. C.; Cater, E. D. J. Chem. Educ. 1983, 60, 109. Volk, L.; Richardson, W.; Lau, K. H.; Hall, M.; Lin, S. H. J. Chem. Educ. 1977, 54, 95. 6. Pyun, C. W. J. Chem. Educ. 1971, 48, 194. 7. DeTar, D. F. J. Chem. Educ. 1967, 44, 193.
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Figure 2. A view of the applet with the settings k1 = 1.1, k-1 = 1.0, and k2 = 0.1; [A0] = 1.0. [B0] = [C0] = 0.
Journal of Chemical Education • Vol. 80 No. 7 July 2003 • JChemEd.chem.wisc.edu
