Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX-XXX
pubs.acs.org/jchemeduc
Synthesis of Polystyrene and Molecular Weight Determination by 1H NMR End-Group Analysis Jay Wm. Wackerly* and James F. Dunne Department of Chemistry, Central College, 812 University Street, Pella, Iowa 50219, United States S Supporting Information *
ABSTRACT: A procedure for the solution polymerization of styrene using di-tert-butyl peroxide (DTBP) as the initiator is described. The use of DTBP allows for end-group analysis by 1 H NMR spectroscopy and calculation of the number-average molecular weight of the polymer. This experiment was designed as a laboratory introduction to polymer chemistry for second-year undergraduate students. The highest-order focus was to get students to derive an equation to calculate the number-average molecular weight by 1H NMR spectral analysis. KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Polymer Chemistry, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, NMR Spectroscopy, Polymerization, Free Radicals
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INTRODUCTION
This report describes a laboratory experiment that combines the features of a “standard” styrene solution polymerization reaction with end-group analysis by 1H NMR spectroscopy. Using 1H NMR spectroscopy to quantify the average length of polymer molecules requires higher-order thinking6 during data analysis than simply matching NMR signals to the appropriate protons. This allows students to perform end-group analysis and calculate the number-average molecular weight (Mn) in a discovery-based manner. This experiment was designed for a second-year undergraduate organic chemistry course sequence and was conducted following coverage of radical chemistry in lecture and one semester of organic chemistry lab techniques.
Polymerization reactions conducted as part of laboratory experiments continue to be commonly found within the undergraduate curriculum.1 Recently, attention to polymer chemistry topics has been increasing because of the curricular emphasis on the subject by the American Chemical Society’s Committee on Professional Training.2 Additionally, polymer chemistry experiments historically have permeated the undergraduate curriculum, as exemplified in this Journal, where no fewer than 10 examples concerning the polymerization of styrene have been published.3 The pervasiveness of polystyrene (PS) synthesis specifically is no doubt due to the past and continued utility of the product.4 Indeed, many of the vast applications of PS-based materials come from structural properties such as incorporated comonomers, tacticity, and molecular weight. This last property, which is regarded as a prominent component of an undergraduate course on polymer chemistry,5 can be introduced within the organic chemistry laboratory curriculum. Covering the concept of polymer molecular weight exposes students to fundamental polymer chemistry concepts without requiring additional instructional time. Students can employ synthetic and analytical techniques covered during the organic chemistry curriculum to explore this feature that is unique to macromolecules. While students learn in lecture that polymers have repeat units and high molecular weights, the laboratory synthesis of a polymer and determination of the average chain length can solidify these concepts. Unsurprisingly, eight of the 10 aforementioned PS syntheses included particular attention to the concept of polymer molecular weight.3 © XXXX American Chemical Society and Division of Chemical Education, Inc.
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OVERVIEW OF THE PROCEDURE This procedure was constructed on the basis of previously published PS syntheses with modification of the initiator.3 It has been completed by 170 students during seven different semesters (12 laboratory periods) over 5 years and yielded some PS for every student group. The procedure was designed to be completed in two 3 h periods, with the first focusing on synthesis and the latter period focusing on analysis and calculations. Students are required to make a notebook table detailing all of the chemical reagents used in the lab, with noted attention to hazardous chemicals, prior to beginning the experiment. In the lab, students are assigned a partner and then separate the Special Issue: Polymer Concepts across the Curriculum Received: October 25, 2016 Revised: September 12, 2017
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DOI: 10.1021/acs.jchemed.6b00814 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Scheme 1. Polymerization Mechanism Worth Full Credit on Student Reports (Bracketed Notation Is Optional)
which the students were already familiar. In the 1−2 h of time remaining after the 1H NMR spectrum had been obtained, students were afforded the opportunity to ask questions as they proceeded through their analysis (Table 1). Students requested
styrene from the inhibitor by running it through a column of alumina and collect the styrene in a preweighed round-bottom flask. Toluene, 0.1 equiv of di-tert-butyl peroxide (DTBP, also known as Luperox DI), and a stir bar are added to the flask, which is then equipped with a reflux condenser. The reaction is run at reflux for 1 h, at which point the solution is cooled to room temperature. The polymer is precipitated in a solution of methanol (15× v/v), vacuum-filtered, and transferred to a preweighed watch glass. This watch glass is stored at ca. 60 °C for 1 week to evaporate remaining solvent. Alternatively, the experiment may be completed in a single week by excluding this drying period. While the presence of residual solvent does skew the percent yield calculations, it does not impact the Mn determination of the polymer (vide infra). Students typically measure the mass of their PS product and obtain 1H NMR and IR spectra, though they are allowed to use other techniques if they so choose. The 1H NMR spectra are checked by the instructor to ensure that reasonable shimming, correct peak shifts, and proper integration values have been obtained. The discovery-learning emphasis comes in the determination of Mn and is covered in more detail below. The approach utilized to write the report following the completion of this lab has been described in detail elsewhere.7 Briefly, students are required to write journal-style lab reports where the Results and Discussion section requires an analysis of the mechanism (Scheme 1), yield, IR spectrum, and 1H NMR spectrum. The 1H NMR analysis is broken down into confirmation of the PS synthesis and determination of Mn. Assessment of student learning via the lab reports indicates that the vast majority of students were able to properly calculate Mn, although this successful calculation came with a wide-ranging amount of instructor-provided assistance (vide infra).
Table 1. Process for the Determination of Mn Step
Analysis Procedure
1 2
Identify and assign the five distinct peaks/peak clusters due to the PSa Calculate the repeat unit:end group ratio (factoring the hydrogen ratios) Identify and remove peaks (m) with improper integration values Multiply repeat unit weight by the calculated ratio Optional: add the weight of the end group
3 4 5 a
CHCl3, TMS, and water are present in every sample. Other predictable impurities (e.g., acetone, toluene, styrene, and grease) are also often present.
a variety of help ranging from the correct assignment of the signals in the 1H NMR spectrum to the more challenging concepts of translating a polymer signal’s integration into the corresponding “n” value of the chain. The use of DTBP specifically as the radical initiator allows for the calculation of the polymer’s Mn. Other common initiators used in polystyrene laboratories, such as benzoyl peroxide or azobis(isobutyronitrile) (AIBN), contain peaks obscured by the styrene repeat unit in a 1H NMR spectrum. In contrast, use of DTBP provides a single, well-resolved signal at 1.2 ppm (Figure 1). After correctly assigning each signal and
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HAZARDS All of the materials in this experiment, including the heating sources and hot glassware, should be handled with care and used within a fume hood. Styrene, di-tert-butyl peroxide, toluene, and methanol are flammable and should not be used near open flames. Styrene, alumina, di-tert-butyl peroxide, toluene, methanol, and chloroform are all inhalation hazards. PS has no known hazards. Proper personal protective equipment should be worn throughout the lab procedure.
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DISCUSSION
Figure 1. 1H NMR spectrum of a representative student PS sample in CDCl3. The spectrum was collected on a 300 MHz instrument.
Molecular Weight Analysis
Prior to this experiment, students had only been exposed to the concept of polymer molecular weight as a generic “n” value when free-radical polymerizations were covered in lecture. Since the idea of polymer molecular weight was a new concept for students, they were prompted to work to determine the Mn prior to leaving the laboratory during the second week of this experiment.8 The calculation of Mn was chosen because it can be calculated using 1H NMR spectroscopy, a technique with
impurity in the 1H NMR spectrum (step 1), students calculate the average chain length according to the following equation (step 2): m
nPS =
∑i = 1 I
m · pE
E
B
Ii pi
(1) DOI: 10.1021/acs.jchemed.6b00814 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Laboratory Experiment
where Ii and pi are the integration and the number of protons associated with the ith PS signal, respectively, m is the number of PS signals used, and IE and pE are the integration and number of protons associated with the end group, respectively. Calibrating the integration of the tert-butoxy end group to 9 simplifies the equation:9 m
nPS =
∑i = 1
experiment. Therefore, it was also possible that this experiment merely determined the kinetic chain length and not in fact the true Mn value. Another mechanistic detail worth consideration is the fact that thermal initiation could have occurred under these conditions. However, the fact that pedagogically useful data are obtained on the back end of this procedure demonstrates this simplified approach to be an effective “hands-on” experience that combines organic synthesis with an introduction to polymer molecular weight concepts. Future work will include a more in-depth investigation into the mechanism of this synthesis, specifically the exact termination step, and the development of techniques to more objectively assess student learning of concepts such as analysis of 1H NMR integration data, comparisons of experimental versus theoretical spectra, and determination of polymer molecular weight. With the introductory focus of this experiment, numerous higher-order polymer chemistry concepts were not broached. However, we have found that additional concepts such as green chemistry and sustainability issues surrounding both the synthesis and applications of PS, the mechanism of termination, weight-average molecular weight (Mw) and polydispersity (PDI), the foaming of PS, the existence of a glass transition temperature (Tg), and sample morphology were of interest to some students. While these, along with other in-depth topics, could be added to this experiment, we have chosen to keep the focus on just the determination of Mn. Other laboratory procedures have been successfully utilized to allow students to approximate polymer molecular weights. However, in our laboratory thin-layer chromatography (TLC) was less useful in Mn determination because the narrow range of Mn values from student samples did not separate to an appreciable degree when subjected to the TLC analysis reported in the literature.3e When viscometry was attempted with one class, it required an extra lab week, and students were unable to achieve reproducible results.3b,c While gel-permeation chromatography (GPC) is often the standard for molecular weight analysis,3g−j our institution does not possess a GPC instrument, and therefore, the PS samples could not be analyzed by this method in a timely fashion. Finally, an excellent example of the use of 1H NMR spectroscopy for endgroup analysis has been reported.11 However, that approach was targeted toward advanced students to learn about more complex polymer systems, whereas this experiment was designed to introduce polymer molecular weight concepts by building off the unit on free radical chemistry in the organic sequence. However, these other analytical approaches as well as additional investigation into the mechanism of termination could be combined into an experiment in a course covering more advanced polymer chemistry topics.
Ii pi
(2)
m
Using the integral values from Figure 1 in eq 2 provides nPS =
34.9 3
+
21.5 2
+
9.9 1
+
22.0 2
4
Importantly, students were not given this equation; rather, it was derived, often with guidance from the instructor, on the basis of their 1H NMR spectral assignments and knowledge of the repeating structure of the polymer. However, not all peak regions provided useful integration values, as is the case with experimental data (see the Supporting Information for six representative 1H NMR spectra). In Figure 1 it can be seen that peak a overlaps with CHCl3 and peak d overlaps with H2O, causing artificially high integration values. Peak c has a slightly lower integration value that is postulated to be due to an upfield shift of some of the repeat units near the polymer terminus.10 In this experiment, students must differentiate “clean” integration data from integration values that should be excluded. If peaks a, c, and d are removed from eq 2 (step 3), it becomes nPS =
21.5 2
1
= 10.8
indicating that there are 10.8 styrene monomer units. Often students will get the same or a similar n value if they skip step 3, provided they have taken good care to purify their sample. However, if large amounts of impurities are present (e.g., toluene) the calculated n value will be significantly inflated. Once n is calculated, students determine Mn as follows (step 4): M n = n·(styrene molar mass) g = 10.8·104.15 mol g = 1119.6 mol
(3)
Students sometimes add the weight of the end group as well (step 5). However, since the weights of the end groups in typical polymers are regarded as insignificant, no points were allocated to this step in the grading rubric.
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Pedagogical Considerations
CONCLUSIONS The inclusion of polymer experiments in the organic laboratory serves as a versatile tool for addressing the American Chemical Society’s emphasis on macromolecular topics in the undergraduate curriculum. The use of standard organic laboratory techniques is successful in introducing undergraduates to the core polymer concepts. The molecular weight calculations are conducted in a discovery-based method requiring students to be rigorous in their data analysis. The introductory nature of this experiment can be used as a jumping-off point for discussion of more in-depth polymer topics.
Because of the pedagogical focus of the procedure, some of the mechanistic details have been intentionally simplified (Scheme 1). For example, it is important to note that the ambiguity of the predominant termination step of the reaction prevented assignment of one of the end groups of the polymer chain. The tert-butoxy group was taken to be one of the end groups from its role as the initiator, but the identity of the second end group varies according to the specific termination step. Termination via either disproportionation, with alkene end groups at concentrations too low to identify, or chain transfer, likely to water impurities or the toluene solvent, was likely in this C
DOI: 10.1021/acs.jchemed.6b00814 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Laboratory Experiment
2001, 78 (4), 544−546. (i) Matyjaszewski, K.; Beers, K. L.; Metzner, Z.; Woodworth, B. Controlled/Living Radical Polymerization in the Undergraduate Laboratories. 2. Using ATRP in Limited Amounts of Air to Prepare Block and Statistical Copolymers of N-Butyl Acrylate and Styrene. J. Chem. Educ. 2001, 78 (4), 547−550. (j) Tillman, E. S.; Contrella, N. D.; Leasure, J. G. Monitoring the Nitroxide-Mediated Polymerization of Styrene Using Gel Permeation Chromatography and Proton NMR. J. Chem. Educ. 2009, 86 (12), 1424−1426. (4) Maul, J.; Frushour, B. G.; Kontoff, J. R.; Eichenauer, H.; Ott, K.H.; Schade, C. Polystyrene and Styrene Copolymers. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 2007; pp 475−522. (5) The importance of understanding molecular weight in introductory polymer chemistry curricula is exemplified by (a) early coverage in polymer chemistry textbooks, (b) the fact that it was the highest-weighted topic in a survey used to formulate the 1978 ACS Polymer Chemistry exam, (c) and suggested guidelines for an introductory polymer chemistry laboratory course: (a) Stevens, M. P. Polymer Chemistry: An Introduction, 3rd ed.; Oxford University Press: New York, 1999. (b) Carraher, C. E.; Deanin, R. D. Core Curriculum in Introductory Courses of Polymer Chemistry. J. Chem. Educ. 1980, 57 (6), 436. (c) Mathias, L. J. The Laboratory for Introductory Polymer Courses. J. Chem. Educ. 1983, 60 (11), 990. (6) Rosenthal, L. C. Writing Across the Curriculum: Chemistry Lab Reports. J. Chem. Educ. 1987, 64 (12), 996−998. (7) Wackerly, J. W. A Stepwise Approach to Writing Journal-Style Lab Reports in the Organic Chemistry Course Sequence. J. Chem. Educ. 2017, DOI: 10.1021/acs.jchemed.6b00630. (8) Anecdotally, before requiring students to calculate Mn before leaving the lab, the few students who did not visit with the instructor to check their work or receive guidance on their calculations nearly always presented incorrect or unattempted molecular weight calculations in their reports. (9) In efforts to facilitate and provide students a starting point for analysis of their 1H NMR spectra, we found it pedagogically useful to set the integration value of the t-Bu peak for the students. However, other instructors may find it useful to have students set the values on their own. (10) Proper baseline correction and integration of the entire region can lead to effective integration of this region. (11) Izunobi, J. U.; Higginbotham, C. L. Polymer Molecular Weight Analysis by 1H NMR Spectroscopy. J. Chem. Educ. 2011, 88 (8), 1098−1104.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00814. A table containing a sampling of student data from the past four years, sample 1H NMR spectra, a copy of the student procedure, and rubric provided to the students (PDF, DOC) Labeled 1H NMR spectrum of PS (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jay Wm. Wackerly: 0000-0001-6975-4431 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Tanner Rathje for his assistance with TLC. REFERENCES
(1) A few recent examples are highlighted here: (a) Colombani, O.; Langelier, O.; Martwong, E.; Castignolles, P. Polymerization Kinetics: Monitoring Monomer Conversion Using an Internal Standard and the Key Role of Sample t0. J. Chem. Educ. 2011, 88 (1), 116−121. (b) Mako, T.; Levine, M. Synthesis of a Fluorescent Conjugated Polymer in the Undergraduate Organic Teaching Laboratory. J. Chem. Educ. 2013, 90 (10), 1376−1379. (c) Schneiderman, D. K.; Gilmer, C.; Wentzel, M. T.; Martello, M. T.; Kubo, T.; Wissinger, J. E. Sustainable Polymers in the Organic Chemistry Laboratory: Synthesis and Characterization of a Renewable Polymer from δ-Decalactone and L-Lactide. J. Chem. Educ. 2014, 91 (1), 131−135. (d) Chan, J. M. W.; Zhang, X.; Brennan, M. K.; Sardon, H.; Engler, A. C.; Fox, C. H.; Frank, C. W.; Waymouth, R. M.; Hedrick, J. L. Organocatalytic RingOpening Polymerization of Trimethylene Carbonate To Yield a Biodegradable Polycarbonate. J. Chem. Educ. 2015, 92 (4), 708−713. (e) Darensbourg, D. J. Copolymerization of Epoxides and CO2: Polymer Chemistry for Incorporation in Undergraduate Inorganic Chemistry. J. Chem. Educ. 2016, DOI: 10.1021/acs.jchemed.6b00505. (2) Wenzel, T. J.; McCoy, A. B.; Landis, C. R. An Overview of the Changes in the 2015 ACS Guidelines for Bachelor’s Degree Programs. J. Chem. Educ. 2015, 92 (6), 965−968. (3) (a) Wilen, S. H.; Kremer, C. B.; Waltcher, I. PolystyreneA Multistep Synthesis: For the Undergraduate Organic Chemistry Laboratory. J. Chem. Educ. 1961, 38 (6), 304−305. (b) Bradbury, J. H. Polymerization Kinetics and Viscometric Characterization of Polystyrene: A Physical Chemistry Experiment. J. Chem. Educ. 1963, 40 (9), 465−467. (c) Ander, P. Dependence of Molecular Weight of Polystyrene on Initiator Concentration: An Introductory Physical Chemistry Experiment. J. Chem. Educ. 1970, 47 (3), 233−234. (d) Mazza, R. J. Free Radical Polymerization of Styrene. A Radiotracer Experiment. J. Chem. Educ. 1975, 52 (7), 476−478. (e) Armstrong, D. W.; Marx, J. N.; Kyle, D.; Alak, A. Synthesis and a Simple Molecular Weight Determination of Polystyrene. J. Chem. Educ. 1985, 62 (8), 705−706. (f) Andrews-Henry, H. Polystyrene Kinetics by Infrared: An Experiment for Physical and Organic Chemistry Laboratories. J. Chem. Educ. 1994, 71 (4), 357. (g) Sanford, E. M.; Hermann, H. L. Bromination, Elimination, and Polymerization: A 3-Step Sequence for the Preparation of Polystyrene from Ethylbenzene. J. Chem. Educ. 2000, 77 (10), 1343−1344. (h) Beers, K. L.; Matyjaszewski, K.; Woodworth, B. Controlled/Living Radical Polymerization in the Undergraduate Laboratories. 1. Using ATRP to Prepare Block and Statistical Copolymers of N-Butyl Acrylate and Styrene. J. Chem. Educ. D
DOI: 10.1021/acs.jchemed.6b00814 J. Chem. Educ. XXXX, XXX, XXX−XXX
