An Integrated Laboratory Approach toward the Preparation of

Laboratory Experiment pubs.acs.org/jchemeduc

An Integrated Laboratory Approach toward the Preparation of Conductive Poly(phenylenevinylene) Polymers Timm A. Knoerzer,* Gary J. Balaich, Hannah A. Miller, and Scott T. Iacono* Department of Chemistry, United States Air Force Academy, USAF Academy, Colorado 80840, United States S Supporting Information *

ABSTRACT: Poly(phenylene vinylene) (PPV) represents an important class of conjugated, conducting polymers that have been readily exploited in the preparation of organic electronic materials. In this experiment, students prepare a PPV polymer via a facile multistep synthetic sequence with robust spectroscopic evaluation of synthetic intermediates and the final product. The synthetic sequence could be applied by university instructors as a capstone project for an undergraduate organic chemistry laboratory or as a centerpiece project for either a polymer or integrated laboratory course. The initial synthetic step could be segmented and used as a modular piece in the traditional introductory organic chemistry laboratory. In either case, the resultant PPV polymer is analyzed by UV−vis and fluorescence spectroscopy to determine the quantum yield which promotes enhanced student understanding of the photophysical properties of the material. In addition, GPC analysis is completed to reveal the molecular weight and polydispersity of the polymer. Students completing this experiment gain valuable experience in organic/polymer synthesis and structural characterization utilizing GC/MS, GPC, NMR, UV−vis, and fluorescence spectroscopy, as well as in the mechanistic aspects and practical application of some of the classic transformations in organic chemistry including the SN2 reaction, nucleophilic acyl substitution, organometallic chemistry, and Wittig reaction. KEYWORDS: Upper-Division Undergraduate, Polymer Chemistry, Organic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Chromatography, NMR Spectroscopy, Synthesis, UV-Vis Spectroscopy

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INTRODUCTION The synthesis of polymeric materials has led to important applications in the modern world, especially in the development of fibers, films, adhesives, elastomers, plastics, and other consumer-oriented products. In particular, conducting polymers have been exploited for the production of organic electronics because they possess the intrinsic ability to delocalize electrons by virtue of extended conjugation. By intentional manipulation of organic polymer structure, the HOMO−LUMO band gap can be narrowed allowing access to the conduction band. The purposeful excitation by means of chemical, light, or electrical energy promotes electrical conduction in these materials, ultimately producing an emission of light. The discovery of the first electrically conductive polyacetylene1 led to a renaissance in the utilization of light emissive organic conjugated polymers for the commercial preparation of highly efficient light emitting devices, magnetic storage media, photovoltaics, electro-optics, and chemical sensors.2 Classes of reasonably conductive polymers include, but are not limited to, poly(acetylene) (PA), poly(thiophene) (PT), and poly(phenylenevinylene) (PPV), which serve in the broad field of organic electronics (Figure 1). Although polymeric materials have become more prevalent in the undergraduate curriculum, few reports have described bulk synthetic experiments that are suitable for a laboratory course. For example, this Journal3 has described only two bulk syntheses involving the production of a conductive polymer This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

Figure 1. Representative examples of conductive polymers.

with several other reports4 in this Journal focusing on thin-film electropolymerization. Moreover, a search of the literature beyond this Journal revealed only sparse representation of viable synthetic experiments focused on the bulk preparation of electrical conductive polymers.5 Therefore, a need remains for the development of hands-on experimentation that can be readily exploited in the undergraduate chemistry laboratory for investigating this unique class of conductive polymers. More importantly, this experiment demonstrates a unique strategy for achieving the PPV polymeric product (using the Wittig reaction) while simultaneously providing a multistep approach for the synthesis of conductive polymers from simple, commercially available starting materials.

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dx.doi.org/10.1021/ed500227m | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Scheme 1. Monomer Synthesis and Polymerization to Conjugated PPV Polymer

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EXPERIMENTAL OVERVIEW

HAZARDS All organic solvents in this experiment include flammable liquids that are toxic by inhalation. Prudent use of personal protective equipment is recommended and all manipulations described here were carried out in a standard operating fume hood. Always consult the materials safety data sheet (MSDS) for chemicals before executing this laboratory with referenced CAS numbers. Specific emphasis is placed on the preparations of compounds 3 to 5, which employ metal halogen exchange via use of n-butyl lithium. Use of protective gloves and lowered hood sash are highly recommended during the addition step. It is recommended that n-butyl lithium should be transferred using NEW disposable plastic syringes with a luer lock (e.g., NORMJECT) and not glass syringes. This reagent is a strong base and very hydroscopic, thus reacting violently with water. Therefore, scrupulous drying of glassware and use of anhydrous reaction conditions are recommended. Disposal of the needle and syringe requires flushing with hexanes and dispensing contents into tert-butanol. Proper handling of organolithium reagents should be followed (see the links provided in the Notes for Students in the Supporting Information). Reaction products have unknown or ill-defined toxicities. As a result, prudent practice dictates that the student should handle the compounds as if they are toxic using all standard personal protective equipment. Additional hazards are described in the Instructions for Students in the Supporting Information.

The goal of this integrated project is to prepare a PPVconjugated polymer, poly[2,5-di(n-hexyloxy)-1,4-phenylene(vinylene)-co-1,4-phenylene] 5, utilizing step-growth Wittig polymerization from monomers 3 and 4 (Scheme 1). The specific experimental goals include the following: (1) analysis of primary literature to identify optimal laboratory procedures, (2) reinforcement of venerable reactions in organic synthesis, (3) expansive structural characterization of synthetic products, and (4) evaluation of conductive properties via quantum yield of the resultant conductive polymer. This project can be readily segregated into two separate experiments: (1) the one-step synthesis of intermediate 2 and (2) the multistep preparation of monomeric intermediates 3 and 4 as well as polymer 5 for a polymer (or materials) chemistry course or advanced synthesis course. Alternatively, the entire synthetic process can be undertaken as a project for an appropriately themed advanced or integrated laboratory course. A description and table are included in the Instructor Notes (Supporting Information) that show the time commitment necessary to complete tasks associated with this experiment. The synthesis of intermediate 2 has been extensively tested over three semesters in the introductory organic chemistry laboratory course with 208 students with excellent reproducibility. In addition, the syntheses of monomer 3, monomer 4, and polymer 5 as well as fluorescence quantum yield determination have been effectively evaluated for three semesters in the department’s polymer synthesis course with a total of 36 students.

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RESULTS

Monomer Synthesis

A key approach in this experiment was for students to be provided with actual experimental methods sections from the primary literature that students must be able to read and analyze to extract the relevant strategies required to execute experiments (Experimental Goal 1). This practice strongly reinforced a student’s ability to read, analyze, and write in a manner consistent with the expectations of professional chemists. For example, the first synthetic transformation that leads to the preparation of monomer 2 required students to evaluate two alternative synthetic approaches found within the primary literature for which they must decide which set of reaction conditions might lead optimally to the desired product. Students acquired the experimental sections readily via online access to experimental supplementary information6 without need to access the full article. A discussion of the decisionmaking process was led during a 15 min prelaboratory session in order to identify that the optimal procedure for promoting the desired SN2 reaction employed K2CO3 in DMF7 as the

EXPERIMENT

All laboratory manipulations, consumables, equipment, and glassware are typical of a standard synthesis laboratory. Products are characterized by NMR spectroscopy, infrared (IR) spectroscopy, gas chromatography/mass spectrometry (GC/MS), gel permeation chromatography (GPC), UV−vis spectroscopy, and fluorescence spectroscopy. Each synthetic step produces products with acceptable student yields and reasonable purity with no major side products. This experiment requires 6 × 3 h laboratory periods with students working in pairs to effectively complete the project inclusive of purification and characterization of reaction products. The procedural details for all synthetic steps, structural characterization details, and pedagogical guidance for instructors are provided in the Supporting Information. B




chloride monomer 4 (Scheme 1). This exemplar synthesis complimented a similar iterative preparation of oligomeric 5 as a subunit using olefin metathesis and provided an excellent opportunity to compare/contrast polymerization strategies.8 The choice of base was important to promote effective polymerization with n-BuLi (2.1 equiv), providing the most efficient production of the desired phosphonium ylide; students must be adequately briefed on the hazards associated with use of n-BuLi prior to use. Upon addition of monomer 3, students observed the formation of a visible precipitate in solution (Scheme 2), suggesting the collapse of the oxaphosphetane

solvent. Best practice showed that the prelaboratory discussion should occur during the lesson immediately prior to commencement of the experiment to allow time for student preparation (e.g., prelaboratory research, notebook preparation, and calculations). Once identified, the optimal strategy was used to synthesize monomer 2 via the Williamson ether synthesis (Experimental Goal 2) of 2,5-dibromohydroquinone (1) with 1-bromohexane in the presence of K2CO3 in DMF (Scheme 1). The original procedure prescribed an overnight reflux, but this reaction was accomplished with a shorter 1 h reflux with acceptable outcomes. The shorter reaction generally resulted in lower yields (30−40% compared with 60−80%), but the truncated time did not appear to compromise the ability to isolate a reasonable amount of pure product for further synthetic manipulation. The product was ultimately purified by either recrystallization or column chromatography, affording a solid (off-white to pale purple) that was of high purity according to GC/MS and NMR analyses (Supporting Information Figures S1−S5). In either case, ample product was yielded to allow full characterization and for continuation of the polymer synthesis. Monomer 2 was readily converted to the dialdehyde 3 (Scheme 1) by iterative lithium-halogen exchange and nucleophilic substitution with DMF (Experimental Goal 2). This particular transformation from 2 to 3 was attractive because it reinforced fundamental organometallic chemistry as well as nucleophilic substitution reactions involving carbonylcontaining molecules. In this case, the organolithium nucleophile facilitates expulsion of the (CH3)2NH group from DMF leading to a useful discussion point especially as it pertained to the application of acid−base principles and their effect on chemical equilibrium. Although the (CH3)2NH group is a strongly basic leaving group, its relative basicity is less than that of the incoming carbanionic nucleophile, thus the reaction proceeded conclusively toward the aldehyde substitution product. Upon synthesis of dialdehyde 3, the phosphonium chloride monomer 4 was prepared from commercially available starting materials using a primary literature strategy from Organic Syntheses (Experimental Goals 1 and 2) and as described in the Supporting Information. This protocol was optimized for undergraduates by shortening the original reaction time to 1.5 h, making this step feasible for a conventional chemistry laboratory course. This reaction produced generous quantities of the product (quantitative yields were common), which was used by students without purification in the subsequent polymerization strategy. Each monomeric product provided the opportunity to complete spectroscopic analyses to elucidate the reaction product (Experimental Goal 3). Several additional experimental options are available, if desired. For example, this product could be further characterized by IR, 13C NMR, 13C DEPT NMR, and 2D NMR techniques, depending upon the instrumental infrastructure of the institution and the goals of the course. Further details regarding the use of instrumentation for the elucidation of product structures are described in the Supporting Information.

Scheme 2. Step-Growth Polymerization (Wittig Coupling) of the Aldehyde Monomer with in Situ Generated Phosphonium Ylide

intermediate to form the desired alkene product and readily separable Ph3PO, which was methanol-soluble. The resulting polymeric material was characterized by an intensely orangecolored solid that was readily precipitated from methanol at room temperature. This solid was filtered and dried in a vacuum oven to provide ample product (typical student yields were 30−50%) with acceptable purity for structural characterization. Structural characterization of the polymer involved GPC and 1 H NMR analyses (Experimental Goal 3). Typical student GPC analysis revealed a number-average molecular weight (Mn) of 4000 (versus polystyrene standards in tetrahydrofuran (THF)) and a polydispersity index (PDI) of 1.9. Given the repeat unit molecular weight (434 g/mol), this result indicated approximately 9 repeat units, presumably comprised of discretely sized oligomers. The resulting bright yellow solution was capable of forming clear films using either spin or drop-casting onto microscope slides from a concentrated solution in THF. This methodology was also elegantly demonstrated by Mako and Levine in their recent publication in this Journal.3a 1H NMR results of the polymer showed clear evidence of the aliphatic ether side chain in addition to aromatic and vinylic functionalities with predominance of the E configuration for the vinyl groups as evidenced by 13.0 Hz coupling constants (Supporting Information Figure S9) in the vicinity of 6.5−6.75 ppm, which was expected for a Wittig reaction employing a stabilized ylide. In this case, the adjacent aromatic ring affords

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POLYMERIZATION AND EVALUATION OF OPTICAL PROPERTIES Polymer 5 was prepared via a step-growth chain extension through Wittig coupling (Experimental Goal 2) of the bifunctional aldehyde monomer 3 with the phosphonium C




real-world applications. The project engaged students in a multistep preparation of the polymer that can be utilized in its entirety for an undergraduate organic laboratory course or compliment an advanced course, such as polymer chemistry or advanced techniques. The methods described in this account have been successfully implemented in both organic and polymer chemistry courses encompassing over 200 students. Topics for discussion included mechanistic organic chemistry, step-growth polymerization, spectroscopic interpretation, or photophysical properties of the polymer. Strategies for student engagement are suggested in the Notes for Instructors. Notable avenues for engagement include student preparation of a journal quality methods section, Supporting Information document, and laboratory report.

an electron donating resonance effect which leads to the stabilization of the ylide. The optical properties of the PPV polymer 5 (Experimental Goal 4) were qualitatively observed from a dilute polymer solution in THF (ca. 5 mg/mL) and a solvent-cast film that revealed a high degree of luminescence upon exposure to a UV lamp (350 nm). Typical student-generated UV−vis and fluorescence (emission) spectra for polymer 5 in solution (THF) and thin films are shown in Figure 2. A single π−π absorption characteristic of phenylene−vinylene conjugation functionality was observed at λmax excitation of 460 nm in THF.

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ASSOCIATED CONTENT

S Supporting Information *

Instructions for students, spectral data, notes for the instructor, and example student work. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the undergraduates from the organic chemistry laboratory and chemistry majors from polymer chemistry courses for judiciously conducting these experiments. We would also like to thank our undergraduate research assistants, Kiah I. A. Hicks and Kirsten M. Farquhar, for their work in optimizing the reaction conditions for these experiments.

Figure 2. Solution and film absorption and emission spectroscopy of PPV polymer 5.

The thin film of PPV polymer 5 showed emission broadening compared with the dilute solution. Emission spectra from films displayed an approximately 33 nm red shift in λem from 513 to 546 nm, in addition to broadening of emission onset to near 600 nm. This result led to a discussion with students regarding the impact of π−π stacking for the independent, rigid phenylene−vinlylene chains that facilitated intermolecular transitions, ultimately lowering emission energy. The optical properties of the PPV polymer 5 were further demonstrated by performing a quantum yield determination versus an appropriate fluorescence standard. Coumarin 153 is an ideal standard due to the fact that its absorption and emission profiles correspond nicely with spectroscopic characteristics of the PPV polymer. As shown in Supporting Information Table S1, the absorbance and emission spectra for a dilution series was performed for both the coumarin 153 standard as well as the PPV polymer 5. A graph of integrated fluorescence intensity versus absorbance produced a good linear relationship used to calculate the solution quantum yield (Φ) (Supporting Information Figure S10). A typical student quantum yield for the PPV polymer 5 was 0.42, in accordance with published literature values.9

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REFERENCES

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SUMMARY The protocol for the preparation and characterization of a PPVderived polymer provided undergraduates with the experience in the preparation of a functional light-emissive polymer with D




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An Integrated Laboratory Approach toward the Preparation of

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