In the Laboratory
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
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Krzysztof Matyjaszewski* and Kathryn L. Beers Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213; *
[email protected] Brian Woodworth PPG Industries, Inc., Alison Park, PA 15101 Zachary Metzner Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213
Background Controlled/living radical polymerizations are revolutionizing the methods used to prepare polymeric materials in the academic laboratory (1, 2). The development of myriad industrial applications is also imminent. One method in particular, atom transfer radical polymerization (ATRP) (3–5) has been applied in an undergraduate organic synthesis laboratory as a simple method of demonstrating the ease with which these techniques can be applied (6 ). One of the most commonly misunderstood concepts from the literature, however, is the strong effect that different catalyst systems and subsequent choices of reaction conditions can have on the properties of the materials prepared using ATRP. ATRP is based on a delicate equilibrium between active and dormant polymer chain ends that is mediated by the exchange of a halogen atom between the active radical chain end and an inorganic catalyst typically consisting of a copper center complexed with multidentate nitrogen-based ligands. This equilibrium, which is directly affected by the oxidation/ reduction potential of the catalyst complex, along with the stability of the corresponding organic radical, has profound impact on the controlled character of the polymerization. For each combination of monomer and catalyst, the reaction conditions must be optimized to maintain a high enough concentration of deactivating species to quickly deactivate the active chain ends and prevent the formation of dead chains. Therefore, the conditions that are used for the polymerization of acrylates using a copper/tridentate amine (7) catalyst system
cannot be used for styrene. Similarly, if a different catalyst system is used for acrylates, new reaction conditions will have to be determined. Previous experiments were developed for the undergraduate laboratories in which block and statistical copolymers of n-butyl acrylate (nBA) and styrene (S) were prepared by ATRP using the CuBr/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) catalyst system. These polymerizations were carried out in sealed flasks that had been purged with nitrogen gas to remove oxygen from the reaction medium, preventing oxidation of the catalyst and irreversible termination of radical chain ends. In the presence of oxygen, Cu(I) species readily undergo oxidation to Cu(II) without initiating polymerization. The addition of Cu(0) to the system serves to (re)generate the Cu(I) species by reducing the deactivator Cu(II) (Scheme I) (8). Because the concentration of radicals in these systems is low, oxygen can preferentially react with the Cu(I) catalyst. The oxidized catalyst can then be reduced back to Cu(I) by reaction with Cu(0) until all the oxygen present in the system has been consumed. By limiting the headspace in a sealed reaction flask, ATRP proceeds normally after a short induction period during which inhibitors within the system are consumed (9). CuII/L
CuI/L
CuI
Cuo
Scheme I
JChemEd.chem.wisc.edu • Vol. 78 No. 4 April 2001 • Journal of Chemical Education
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In the Laboratory
This technique of adding Cu(0) to a solution containing air and other inhibitors has been applied successfully in an undergraduate synthetic organic lab to prepare poly(n-butyl acrylate) (pnBA) from a difunctional initiator and to measure the kinetics of homopolymerization by ATRP. The difunctional macroinitiator was then chain-extended with polystyrene of lower molecular weight to prepare a thermoplastic elastomer. For comparison, a statistical copolymer of S and nBA of similar composition was also prepared by ATRP. Kinetic data were obtained from gas chromatography (GC), molecular weights were measured by size exclusion chromatography (SEC), and the composition of the copolymers was determined using 1H NMR (300 MHz). This laboratory introduces upper-level undergraduates to the general principles of controlled/living radical polymerization and the effects of reaction conditions on polymeric products. Much like the original ATRP experiment, these experiments can be implemented in a number of ways. An organic synthesis course may focus on the kinetics of homopolymerization in controlled systems and the effects of reaction conditions on both kinetics and end products. A particularly interesting comparison can be drawn between the homopolymerization of nBA by ATRP under two sets of conditions: purging with an inert gas, without Cu(0); and without purging, with Cu(0). A polymer laboratory course or a course designed for engineering students may investigate the mechanical or thermal properties of the copolymers because there will be measurable differences between the block copolymers prepared under each set of conditions.
not be exposed to air before performing the chain extension, but should be stored at < 0 °C to reduce the contribution of further polymerization. For the statistical copolymerization of S and nBA, the monofunctional initiator methyl 2-bromopropionate was used to reduce costs. This reaction should also be performed in the second lab period. On the third day, the catalyst should be removed from the polymers prepared by ATRP and all samples should be isolated by precipitation and dried. The kinetic samples should be analyzed using GC and SEC. However, this can carry into the fourth day if necessary. Material characterization may require less than one class period if 1H NMR is sufficient to determine compositions of the copolymers.
Experimental Procedure
Results
Depending on the objectives of the course and the allotted time for the experiment, there are several ways in which the work can be accomplished. Students can work independently or in teams to accomplish separate stages of the experiments. Separate stages might include measuring kinetics of free radical and controlled/living radical polymerizations for comparison, chain extension and statistical copolymerization to prepare the two material samples, and material characterization. Several parts of the whole can be effective in demonstrating synthetic polymer chemistry in three or four 3-hour lab periods. All reagents used in the experiment are commercially available. Inhibitors were not removed from the monomers and all reagents were used as received from Aldrich or ACROS chemicals. It is particularly important for successful ATRP in air that the headspace in the reaction flasks be kept to a minimum so that the volume of air is low relative to the catalyst concentration. It is also critical to maintain continuous stirring because the reactions are heterogeneous and it is important to expose the maximum surface area of the Cu(0) to solution to facilitate the reduction of Cu(II). Students should calculate the molar concentrations and ratios of all of the reagents used in ATRP to obtain theoretical molecular weights of the polymer at various conversions (measured by GC). A commercially available difunctional initiator is used for the ATRP of nBA so that an ABA triblock copolymer, poly(styrene-b-n-butyl acrylate-b-styrene), can be prepared on the second day by chain-extending from both ends of the polymer. The homopolymerization reaction mixture should
The use of Cu(0) to remove residual oxygen from the medium and regenerate the catalyst in ATRP has been reported previously (8, 10). The conditions have been optimized here to suit another monomer (acrylate-based) and catalyst (CuBr/ PMDETA) that have been studied extensively under anaerobic conditions (7, 11, 12). Conditions were also modified to enable a yield of moderately high conversion polymers (~80%) in a 3-hour class period while retaining a high degree of functionality for chain extension with styrene. Using specified amounts of reagents and limiting the headspace in all reactions is necessary to limit the induction period during which oxygen is consumed and to obtain reasonable conversion and yields. One major consequence of the reaction of Cu(II)X2 with Cu(0) is that the concentration of the deactivator ([Cu(II)X2]), which is necessary to keep the concentration of radicals low (13), is reduced relative to “normal” conditions. This may result in a larger contribution of irreversible termination reactions if the experiment is not designed to favor the dormant halogen chain ends as much as possible. This is why the reactions are carried out at 60 °C instead of the 80 °C used in the absence of Cu(0). Conversely, one could argue that the temperature is raised in the experiment to which Cu(0) is not added in order to raise ka and overcome the inhibiting influence of the steady-state concentration of Cu(II)X2 present as the result of using unpurified CuBr as well as from the reaction of CuBr with inhibitors present in the monomers. Because increasing the temperature will also raise the equilibrium constant, Keq, this leads to a higher loss of the
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Hazards All reactions in this experiment should be carried out in well-ventilated fume hoods. All chemicals should be handled in fume hoods with protective clothing including lab coats and protective gloves and eyewear. Both of the monomers used in this experiment, styrene and n-butyl acrylate, are flammable, toxic irritants. Styrene is a suspected carcinogen and n-butyl acrylate is a suspected teratogen. The ligand used, pentamethyldiethylenetriamine, is combustible, flammable, and toxic and is readily absorbed through the skin and lungs. Liquid waste should be collected and labeled as halogenated organic waste containing copper salts. The use of needles and syringes was closely monitored to ensure their return and proper disposal.
Journal of Chemical Education • Vol. 78 No. 4 April 2001 • JChemEd.chem.wisc.edu
In the Laboratory 2.5 × 104
Table 1. Typical Data for the Isolated Macroinitiator and Copolymers of Styrene and n-Butyl Acr ylate
1.5
theory
2.0 × 104
Sample
1.4
Styrene (mol %)
Mn
M w/M n
0
21,900
1.17
p(S-b-n BA - b - S)
24
52,700
1.55
p(S-co-n BA)
43
12,400
1.21
pn BA
Mn
1.5 × 104
1.3
1.0 × 10
4
1.2
5 × 103
1.1
0 0.0
0.2
0.4
0.6
Mw/Mn
a
1.0 1.0
0.8
Fractional Monomer Conversion
1.4
0.8
1.2
0.7 0.6
1.0
Ln
[M]o [M]
0.5 0.8 0.4 0.6 0.3 0.4
0.2
0.2
0.1
0.0 0
20
40
60
80
100
120
140
Fractional Monomer Conversion
Figure 1. Molecular weight data for ATRP of nBA in air. [M]0:[I]0: [CuBr(PMDETA)]0:[Cu(0)] 0 = 200:1:1:3; ca. 20% acetone (v/v); T = 60 °C. (䊉) Mn; (䊊) Mw/Mn.
0.0 160
Time / min
Normalized Refractive Index Response
Figure 2. Semilogarithmic plot of monomer conversion vs time for ATRP of nBA in air. [M]0:[I]0:[CuBr(PMDETA)]0:[Cu(0)]0 = 200:1:1:3; ca. 20% acetone (v/v); T = 60 °C. (䊉) ln([M]o/[M]); (䊊) conversion.
a Block
copolymer contained high molecular weight peak from coupling chains, but little or no residual macroinitiator.
halogen end groups, which will reduce the functionality of the macroinitiator. An increased contamination of the homopolymer, pnBA, in the block copolymer is still a potential problem in these reactions if the reaction conditions are not monitored carefully. In fact, because under classic conditions for the ATRP of styrene the steady-state concentration of Cu(II)X2 is higher than that in the ATRP of nBA (14, 15), and because the favored mode of termination in radical polymerization of polystyrene is coupling, the effects of the lowered deactivator concentration are most clearly observed in the chain extension with polystyrene. Typical kinetic results for the ATRP of nBA in the presence of a limited amount of air and added Cu(0) are plotted in Figures 1 and 2. The linear progression of molecular weights (after the induction period) indicates the constant concentration of growing chains characteristic of a controlled process. By plotting the theoretical molecular weights based on the conversion measurements, ∆[M]/[I]0, along with molecular weights obtained from SEC, it is clear that molecular weights can be predicted by the ratio [M]0:[I]0. There is some deviation of molecular weight at low conversion, which is most likely due to a small contribution of poorly controlled chain growth at the beginning of the reaction as a result of insufficient deactivation. Polydispersities are also a clear indication of the differences in the two methods; the typical conventional radical polymerization yields polymers with polydispersities (Mw/Mn) >1.5, whereas in the case of ATRP, Mw/Mn < 1.2. The typical molecular weight data for a set of isolated (co)polymers are listed in Table 1. Comparison of the SEC traces for the pnBA macroinitiator and the ABA-triblock copolymer provides a qualitative indication of the end-group functionality of the macroinitiator (Fig. 3). As discussed above, the contribution of radical–radical coupling reactions in the styrene polymerization is increased, leading to a highmolecular-weight shoulder in the SEC trace for the block copolymer. However, what is typically shown is the weight distribution of chains in the SEC traces as a function of the response in a refractive index detector. As a result, a larger response is recorded for larger polymer chains, leading to a trace that is skewed toward high molecular weight. This is discussed further in the supplemental materials.W Acknowledgments
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104
105
106
107
Molecular Weight Figure 3. SEC traces of isolated polymer samples. Molecular weights were obtained using calibration with polystyrene standards. Solid line: difunctional poly(nBA); Dashed line: poly(S-b-nBA-b-S); Dotted line: poly(nBA-co-S).
We would like to thank Richard McCullough and Paul Ewbanks for allowing the introduction of this experiment into the advanced organic curriculum at Carnegie Mellon University. The assistance and cooperation of Karen Stump, the director of the undergraduate laboratories, are also greatly appreciated.
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Supplemental Material
A complete description of this experiment and supplemental materials are available in this issue of JCE Online. Literature Cited 1. Controlled Radical Polymerization; Matyjaszewski, K., Ed.; ACS Symposium Series 685; American Chemical Society: Washington, DC, 1998. 2. Controlled/Living Radical Polymerization; Matyjaszewski, K., Ed.; ACS Symposium Series 768; American Chemical Society: Washington, DC, 2000. 3. Matyjaszewski, K. Chem. Eur. J. 1999, 5, 3095. 4. Patten, T. E.; Matyjaszewski, K. Adv. Mater. 1998, 10, 901. 5. Patten, T. E.; Matyjaszewski, K. Acc. Chem. Res. 1999, 32, 895. 6. Beers, K. L.; Woodworth, B. W.; Matyjaszewski, K.; Metzner,
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Z. J. Chem. Educ. 2001, 78, 544–547. 7. Xia, J.; Matyjaszewski, K. Macromolecules 1997, 30, 7697. 8. Matyjaszewski, K.; Coca, S.; Gaynor, S. G.; Wei, M.; Woodworth, B. E. Macromolecules 1997, 30, 7348. 9. Matyjaszewski, K.; Coca, S.; Gaynor, S. G.; Wei, M.; Woodworth, B. E. Macromolecules 1998, 31, 5967. 10. Percec, V.; Barboiu, B.; Sluis, M. v. d. Macromolecules 1998, 31, 4053. 11. Davis, K.; Paik, H.-j.; Matyjaszewski, K. Macromolecules 1999, 32, 1767. 12. Davis, K. A.; Matyjaszewski, K. Macromolecules 2000, 33, 4039. 13. Fischer, H. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1885. 14. Kajiwara, A.; Matyjaszewski, K.; Kamachi, M. Macromolecules 1998, 31, 5695. 15. Kajiwara, A.; Matyjaszewski, K. Macromol. Rapid Commun. 1998, 19, 319.
Journal of Chemical Education • Vol. 78 No. 4 April 2001 • JChemEd.chem.wisc.edu
