In the Laboratory
Synthesis of Chromophore-Labeled Polymers and Their W Molecular Weight Determination Using UV–Vis Spectroscopy Eric S. Tillman,* Amanda C. Roof, Steven M. Palmer, Beth Ann Zarko, Caton C. Goodman, and Alissa M. Roland Department of Chemistry, Bucknell University, Lewisburg, PA 17837; *
[email protected] Precise polymer synthesis continues to be a major area of research in both academic and industrial laboratories (1). While most chemistry undergraduates study polymer synthesis in lecture, experimental work is typically lacking owing to the difficulty of synthetic techniques and limited availability of instrumentation needed for polymer characterization, such as gel permeation chromatography (GPC) systems. Reports of controlled兾living radical polymerizations have recently gained attention as an alternative, less rigorous method leading to well-defined polymeric materials compared to more demanding ionic techniques (2–4). Atom transfer radical polymerization (ATRP), for example, has been shown capable of converting several vinyl monomers into well-defined polymers without strict experimental requirements (5–7). Publications in this Journal describing the synthesis of homopolymers and copolymers using ATRP have recently appeared, demonstrating the feasibility of this technique in undergraduate laboratories (8, 9). This experiment is designed for undergraduate students to gain experience in polymer synthesis and characterization. The goal of the laboratory exercise is to produce polystyrene quantitatively end-labeled with fluorene, an aromatic chromophore, and to determine its number average molecular weight using UV–vis spectroscopy. Atom Transfer Radical Polymerization
Molecular Weight Determination by UV–Vis Spectroscopy Typically, polymer products are run through a GPC system, calibrated with the appropriate standards, to quickly and accurately determine their molecular weights and polydispersities. Though there are several ways to express the molecular weight of a polymer sample, most common is the number average molecular weight, Mn (where each polymer chain contributes equally to the molecular weight of the sample). Another commonly used value is the weight average molecular weight, Mw (where larger molecules contribute more to the molecular weight of the sample) with the ratio of Mw兾Mn indicating the range of sizes of polymer chains in the sample (polydispersity). GPC systems, however, are often unavailable at principally undergraduate institutions and may also be unavailable for undergraduate labs at larger research institutions. In this experiment, the need for a GPC system is eliminated by introducing a chromophore group and UV–vis spectroscopy is used to determine the Mn values of the polymers. If the extinction coefficient of fluorene is known at the absorbance maxima near 305 nm (which students will determine), the concentration of the fluorene chromophore (and thus the polymer) in a prepared solution can be easily calculated using the Beer–Lambert law:
A ATRP relies on the equilibrium between a dormant alkyl c = (1) halide and an active polymer radical. The necessary compolε nents of ATRP are an alkyl halide initiator, a vinyl monowhere A is the experimentally determined absorbance of the mer, a suitable metal catalyst, and a ligand. The redox-active, ligand-bound metal catalyst, commonly a multidentate, nisample, l is the pathlength of the cell, ε is the extinction coefficient of the chromophore at a specific absorbance, and c trogen-based ligand linked to copper(I), participates in the exchange of the halogen atom located at the end of the polyis the concentration of the chromophore in mol L᎑1. This mer. This generates the active propagating radical and the necessitates that the fluorene:polymer ratio is 1:1, and the deactivating copper(II) species. The general reaction mechaobserved absorbance is due exclusively to the polymer-bound nism is shown in Scheme I. As can be deduced from the ATRP + + kact reaction mechanism, the polymer is inserted R–X + [Cu(I)ligand] R + [X–Cu(II)ligand] between the C⫺X bond of an initiating alkyl kdeact. halide species, incorporating the alkyl group of the initiator into the polymer. The polymer is quantitatively labeled using a chromophoren times kp R′ based initiator under controlled兾living conditions. In this laboratory, 9-bromofluorene is used as the alkyl halide initiator of styrene, X resulting in polystyrene quantitatively end+ + kact R + [Cu(I)ligand] R + [X–Cu(II)ligand] labeled with the aromatic chromophore fluo′ ′ ′ ′ R R R kdeact. R n−1 n−1 rene (see Scheme II, boxed). The mechanistic steps involved in the specific reaction as well as the polymer formed are shown in Scheme Scheme I. The general ATRP reaction mechanism, where RX is an alkyl chloride or II. alkyl bromide and R´ is an organic group. www.JCE.DivCHED.org
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In the Laboratory
Br
+ [Cu(I)ligand]
H
kact
+
kdeact
n times
+
H
kp
+
[BrCu(II)ligand]
Ph
Br Ph
+
n
+ [Cu(I)ligand] H
kact kdeact
Ph
Ph
+ [BrCu(II)ligand]
n−1
+
H
Scheme II. The mechanistic steps involved in the reaction between 9-bromofluorene and styrene.
fluorene. Also, the extinction coefficient of the fluorene molecule and the fluorene-labeled polymers must be the same in both wavelength and intensity, as the calibration method involves fluorene solutions. Because students precisely make up the solutions of their polymer in g L᎑1, the weight per volume, γ, is known. This value along with the molarity of the polymer solution, c, obtained from the absorbance data, is used to calculate Mn.
γ
1 = Mn c
(2)
Hazards The polymerizations described in this experiment should be performed in ventilated hoods. Tetrahydrofuran (THF) is a flammable solvent. Styrene is flammable, a toxic irritant, and a suspected carcinogen. Methanol is a flammable liquid and a toxic irritant. N,N,N´, N´´, N´´-pentamethyldiethylenetriamine (PMDETA) is combustible and easily absorbed through skin. 9-bromofluorene is an irritant. Syringes and needles must be handled carefully and disposed of properly. Waste should be collected as halogenated organic waste, which will also contain copper ions. Experimental Procedure The experiment can be accomplished over three lab periods, with each period lasting approximately 2–3 hours. All solvents and reagents used are commercially available from Aldrich and are used as received without further purification. A detailed experimental section is provided as Supplemental Material.W A 0.2 M solution of PMDETA in THF should be made up for students prior to the first lab period. Students should calculate the amounts (moles) of monomer (styrene), initiator (9-bromofluorene), catalyst (CuBr), and ligand (PMDETA in a 0.2 M solution) for the following three reaction conditions: nmonomer兾ninitiator = 25兾1, 50兾1, and 100兾1. In each case, base the calculations on using 3 mL of styrene 1216
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and a 1:1:1 mole ratio for the other reagents: ninitiator = ncatalyst = nligand. During the first lab period, the students should set up the ATRP reaction and begin the polymerization. They can be assigned a monomer-to-initiator ratio for their reaction, or be allowed to choose their own. Solid reagents (9bromofluorene and CuBr) are weighed and added to a 50mL two-necked round-bottom flask. Styrene and PMDETA solutions must be added by syringe to accurately determine the amounts introduced. After sealing the reaction vessel using rubber septa, the reactions are run under nitrogen atmosphere. Note that the PMDETA is added last after all other reactants have been placed in an oil bath at 80 ⬚C and allowed several minutes to reach thermal equilibrium. During the second lab section, the polymerization is stopped and the polymeric product is precipitated by the addition of excess methanol. The polymer is collected by vacuum filtration using a glass frit and washed with an additional 10–20 mL of cold methanol. The complete removal of copper is not necessary and will not interfere with UV– vis spectroscopy. The product is allowed to dry for at least 2 days at room temperature before analysis. The polymer is characterized using UV–vis spectroscopy during the third lab section. This also entails the determination of the extinction coefficient of fluorene by making up two THF solutions of fluorene at ∼1 × 10᎑4 and ∼3 × 10᎑4 M. For this portion, students may want to work in pairs and check with others to ensure their values are accurate. The extinction coefficient of fluorene is used to calculate the concentration of their fluorene-labeled polymer in a THF solution. To obtain a UV–vis spectrum of their product suitable for analysis, students should use a 50-mL volumetric flask and approximately 10–20 mg of polymer product. From these data, along with the known concentrations of the UV–vis samples in g L᎑1, students can calculate the Mn values of their samples. If desired, the students may turn in their samples and these can be analyzed by GPC to determine the accuracy of their Mn values.
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In the Laboratory
Results and Discussion
Acknowledgments This work was supported by financial contributions from ACS-Petroleum Research Fund (grant 39855-GB7), the Bucknell Undergraduate Research Program, and the Bucknell University Chemistry Department. W
Table 1. Comparison of the Molecular Weights of Fluorene End-Labeled Polystyrene, as Determined by UV–Vis Spectroscopy and GPC b
c
d
Mn (UV–vis)
Mn (GPC)
PDI
Correlation (%)
025
02800
02700
1.06
04
050
04000
04500
1.08
11
3
050
03100
03000
1.13
03
4
050
05100
04700
1.15
09
5
050
06800
06600
1.03
03
6
100
11500
11700
1.05
02
7
100
12500
11800
1.04
06
Run
[M]/[I]
1 2
a
a
b
Monomer-to-initiator ratio in the initial solution. Molecular weight obtained by UV–vis spectroscopy, using the Beer–Lambert law with ε = ᎑1 ᎑1 c 7800 M cm . Molecular weight by GPC using polystyrene standards. d Percent correlation was determined by dividing the difference of the Mn values by the Mn value determined by GPC and multiplying by 100.
RI trace UV trace (305 nm)
12
13
14
15
16
17
18
19
20
Volume Efluent / mL Figure 1. GPC traces of polystyrene using 9-bromofluorene as an initiator. Solid line: RI trace; dashed line UV trace (305 nm). (Table 1, run 6). 2.0
Absorbance
As shown in Table 1, the ATRP of styrene using 9bromofluorene as an initiator was surprisingly well controlled considering the lack of strict experimental procedures and use of unpurified reagents and solvents. Various polymer samples were also analyzed by a calibrated GPC and their Mn and PDI values were determined using refractive incex (RI) detection. The resulting polymers possessed a polydispersity index (PDI) of less than 1.15, consistent with controlled兾living systems. The correlation percentages were within a reasonable range. A typical GPC trace (Figure 1) of the polymer showed a fairly symmetrical peak on both the RI trace and the UV trace (set at 305 nm, qualitatively verifying fluorene labeling). All polymers were analyzed using UV–vis spectroscopy and their molecular weights were determined using the Beer– Lambert law in combination with their known concentrations in prepared THF solutions (in g L᎑1). The UV–vis spectra of the fluorene-labeled polystyrene (Figure 2) also illustrated a red shift from a fluorene standard, consistent with a polymer-bound aromatic chromophore (10). Our group has previously found that a competing termination reaction in this ATRP system is the radical–radical coupling of the initiating fluorenyl radical, leading to a fluorene dimer (11). The absorption of this side product occurs in the same region as the fluorene-labeled polystyrene, thus it is necessary to remove this from the polymer. Successive precipitations were sometimes needed to improve the percent correlations, presumably due to the fluorene dimer sticking to the polymeric product. However, if the polymer was washed with methanol while being collected on the filter, residual non-polymeric species were adequately removed. In typical cases, molecular weight values determined by UV–vis spectroscopy were within 10% of the actual molecular weight (as determined by GPC), proving this to be a useful technique in determining molecular weights.
1.5
fluorene
1.0
fluorene-labeled polystyrene
0.5
0.0 275
285
Supplemental Material
295
305
315
325
Wavelength / nm
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
Figure 2. UV–vis absorbance spectra of fluorene and fluorene-labeled polystyrene: data from Table 1, run 6.
Literature Cited 1. Stevens, M. P. Polymer Chemistry, An Introduction; Oxford Press: New York, 1999. 2. Controlled兾Living Radical Polymerization; Matyjaszewski, K., Ed.; American Chemical Society: Washington, DC, 1998. 3. Controlled兾Living Radical Polymerization: Progress in ATRP, NMP, and RAFT; Matyjaszewski, K., Ed.; American Chemical Society: Washington, DC, 2000. 4. Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661–3688. 5. Matyjaszewski, K.; Xia. J. Chem. Rev. 2001, 101, 2921–2990. 6. Matyjaszewski, K.; Wang, J.-L.; Grimaud, T.; Shipp, D. A.
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Macromolecules 1998, 31, 1527–1534. 7. Patten, T. E.; Matyjaszewski, K. Acc. Chem. Res. 1999, 32, 895–903. 8. Beers, K. L.; Woodworth, B.; Matyjaszewski; K. J. Chem. Educ. 2001, 78, 544–547. 9. Matyjaszewski, K.; Beers, K. L.; Woodworth, B.; Metzner, Z. J. Chem. Educ. 2001, 78, 547–550. 10. Tillman, E. S.; Hogen-Esch, T. E. Macromolecules 2001, 31, 6616–6622. 11. Goodman, C. C.; Chon, D.; Tillman, E. S. ACS Div. Polym. Sci, Polym. Prepr. 2004, 45, 1012–1013.
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