In the Laboratory
Synthesis and Characterization of a Hyperbranched Copolymer
W
A. Timothy Royappa Department of Chemistry, University of West Florida, Pensacola, FL 32514;
[email protected] Hyperbranched polymers are “macromolecules possess[ing] a randomly branched structure” (1), a schematic diagram of which is shown in Figure 1a. Such polymers can carry a large number of derivatizable functional groups at the chain ends. As a result, they show promise as carrier molecules for drug delivery, gene therapy, catalysts, dyes, sensors, etc., and since their rheological properties are quite different from those of corresponding linear polymers, they have also been proposed for use in viscosity modification, as melt modifiers, compatibilizers, and lubricants (2, 3). Hyperbranched copolymers of glycidol synthesized by the method described here are also used in chromatographic coatings (4). By contrast, polymer chain branching is often considered undesirable in commercial plastics, since branching leads to deterioration of tensile properties such as strength and modulus, owing to the decreased ability of branched polymer chains to pack closely. Hyperbranched polymers are usually made in a one-pot synthesis, in contrast to dendrimers (a different class of highly branched polymers possessing a well-defined arborescent structure, and predicted to have the same applications as hyperbranched polymers), which have to be synthesized by elaborate multistep techniques. A schematic diagram of a dendrimer is depicted in Figure 1b. There are three major routes to hyperbranched polymers: by condensation of polyfunctional monomers (5), polymerization of a vinyl monomer bearing an initiating group (“self-condensing vinyl polymerization”) (6 ) and polymerization of cyclic monomers (“ring-opening multibranching polymerization”) (1). This last and most recent type of polymerization will be explored in this experiment, which is suitable for an undergraduate polymer chemistry course or an advanced organic synthesis laboratory; it is for students familiar with standard techniques such as FTIR and NMR, but not necessarily with polymer-related methods such as gel permeation chromatography or differential scanning calorimetry. Ring-Opening Polymerization of Glycidol Glycidol (2,3-epoxy-1-propanol), can be polymerized both cationically (7) and anionically (1) to yield a hyperbranched polyether carrying abundant hydroxyl groups. Polyglycidol is therefore extremely polar, being soluble only in water and polar organic solvents. However, copolymers of glycidol with less polar comonomers such as epichlorohydrin (8) and propylene oxide (9) can also be made. These are soluble in a wide range of organic solvents. Propylene oxide has the advantage of being consumed at virtually the same rate as glycidol, preventing the formation of a viscous polyglycidol precipitate during copolymerization (8), thereby obviating the use of a mechanical stirrer. This synthesis can therefore be carried out using common laboratory ware, and the various analyses use readily available solvents.
a
b
Figure 1. Structure of (a) a hyperbranched polymer and (b) a dendrimer.
The BF3-initiated cationic copolymerization of glycidol with propylene oxide entails an oxonium ion intermediate, and a reaction mechanism (7, 8) is shown in Figure 2. For simplicity, nucleophilic attack is shown at the least sterically hindered epoxide carbon, although attack can occur at the more highly substituted (more electrophilic) carbon as well (7). Propagation path “a” is the canonical route for the ringopening polymerization of epoxides in general, whereas path “b” is specific to glycidol. Both paths involve nucleophilic attack by the monomer on the terminal oxonium ion (active end) of the growing polymer, and are hence referred to as the “active chain end” (ACE) mechanism (7). Branching can only occur when a hydroxyl group on the polymer attacks a protonated monomer, which is termed the “activated monomer” (AM) mechanism (7 ). This gives rise to “dendritic” or branching carbons on the chain, marked by an asterisk in the repeating motif O–CH2–C*H(O)–CH2–O, whose spectral signature provides the sole evidence that the AM mechanism plays a role in this polymerization (Fig. 3). Active participation by hydroxyl groups is somewhat unusual in cationic polymerizations, though by no means unknown. Protic species are generally excluded from such polymerizations, since they may quench these reactions, as shown in the termination step in Figure 2. This feature of the present reaction, and the hyperbranched polymer geometry which it generates, mark the major differences from other published polymerization experiments (10). The representative structure of the hyperbranched copolymer shown in Figure 3 can be seen to include numerous branch points in the chain, which are again marked by asterisks at the dendritic carbons. Experimental Procedure A clean, dry, 250-mL three-neck round-bottom flask is fitted with a thermometer and condenser equipped with a drying tube. After this setup is mounted on a magnetic stir-
JChemEd.chem.wisc.edu • Vol. 79 No. 1 January 2002 • Journal of Chemical Education
81
In the Laboratory
HO
Initiation
+
BF3
+ – H (BF3OH)
H2O (trace)
O O
H H+
O
+
O
R R = CH2OH (glycidol) or CH3 (propylene oxide)
+
O
HO
Propagation(a)
+
O
+ O
+
OH
O x
R
R
R
R
R
Propagation(b) OH
O+
+
OH
OH
H + O
Branching OH
+
+
H O+
H OH deprotonation + O OH reprotonation
O
O
H + O
R
Figure 3. Representative structure of a hyperbranched poly(glycidolco-propylene oxide) molecule.
O
Results and Discussion
x
OH O
OH
(BF3OH)
–
+ H2O
O –
(BF3OH) R O HO
+
– H+(BF3OH)
R
Figure 2. Reaction mechanism for the copolymerization of glycidol and propylene oxide.
rer and a stir bar placed in the flask, the glassware is heated gently under a stream of dry nitrogen. When the flask is cool, 75 mL of dichloromethane, 3.5 mL of glycidol (0.053 mol), and 6.0 mL of propylene oxide (0.086 mol) are added to the flask and polymerization is initiated by adding 100 µL of boron trifluoride diethyl etherate with continuous vigorous stirring. The reaction reaches reflux within about 5 min after addition of initiator, and remains clear and colorless throughout. The maximum reaction temperature is recorded, and monomer consumption is monitored by GC. After the monomers are consumed (one hour total reaction time, maximum), 0.5 mL of deionized water is added to quench the reaction and the mixture is stirred for about 10 min. The solvent is removed by rotary evaporation and freeze-drying to yield a clear, colorless, 82
OH
O
O
OH
+ HO
+ O
+
O
Dichloromethane, glycidol, and propylene oxide are cancer suspect agents, and boron trifluoride is corrosive. These materials should only be handled with adequate ventilation. The entire reaction must be carried out in a fume hood. The large volume of dichloromethane solvent used in this experiment mitigates the highly exothermic nature of the polymerization, and it is not recommended that the monomer concentration be increased, owing to the risk of violently boiling the reaction mixture.
OH
OH O
OH
Termination H2O
O OH
Hazards
OH O
OH
(excess)
* O O
viscous polymer. The polymer is checked for solubility in different solvents and analyzed by various spectroscopic and other means.
H O
O
OH
H O
*
O OH
O
OH
O
OH
O O
oxonium ion
OH
O
O O
*
R
O
*
All chemicals used in this experiment are commercially available from Aldrich Chemical Co. (Milwaukee, WI). Monomers and solvent are best used out of freshly opened bottles, or dried overnight over 4A or 5A molecular sieves under nitrogen before use. The poly(glycidol-co-propylene oxide) synthesized in this experiment is stable in ordinary laboratory atmosphere and can be stored at room temperature in capped glass vials. The straightforward polymerization described above can be carried out during a single laboratory period and the analyses can be done within one to two weeks after the synthesis. It is a very useful diagnostic for the instructor to have students monitor the reaction temperature during the initial stages of the reaction, since absence of a vigorous reflux (temperature ∼40 °C) within the first few minutes is an indication that the reaction will not proceed to completion (8), which is possible if the solvent or monomers are contaminated with excessive moisture. Although glycidol and propylene oxide are capable of homopolymerizing under these reaction conditions, a true copolymer is formed rather than a physical mixture of polyglycidol and poly(propylene oxide), because no polyglycidol precipitate is ever observed. Monitoring the reaction by GC provides a key piece of structural evidence: it shows both monomers being consumed at roughly the same rate, even though propylene oxide is present in significant molar excess, ruling out the formation of a block or alternating copolymer and leading one to conclude that a random copoly-
Journal of Chemical Education • Vol. 79 No. 1 January 2002 • JChemEd.chem.wisc.edu
In the Laboratory 100
a CH, CH2
75
%T
CD2Cl2 CH3
50
25
80
60
40
20
4000
0
3000
2000 1800 1600 1400 1200 1000 800
600 400
cm᎑1
ppm b
Figure 5. FTIR spectrum of poly(glycidol-co-propylene oxide). CH3 CH, CH2 OH
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
ppm Figure 4. (a) 13C NMR spectrum of poly(glycidol-co-propylene oxide) in CD2Cl2. The arrow indicates the peak due to dendritic carbons. (b) 1H NMR spectrum of the same polymer in CDCl3.
mer is formed. The polymer is obtained in high yield (students average 86%), which when combined with GC evidence of almost complete monomer consumption indicates that the monomers have been polymerized and not, for example, simply evaporated out of the flask. NMR spectra of a student-prepared polymer are shown in Figure 4. The 13C spectrum in Figure 4a was taken in CD2Cl2 and the 1H spectrum in CDCl3. The peaks between 60 and 80 ppm in Figure 4a correspond to CH2 and CH carbons bonded to ether or hydroxyl oxygens. The small peak at 79.5 ppm indicated by the arrow confirms the presence of dendritic carbons responsible for chain branching (1), providing evidence for the AM mechanism. Both spectra confirm the presence of methyl groups, and the hydroxyl protons give rise to a variable, broad, featureless peak between 3.0 and 3.5 ppm in the 1H spectrum, indicated by the arrow in Figure 4b. The OH signal in the proton spectrum originates from the polymer, and not from adventitious water, which, if present, would have given rise to a peak at 1.56 ppm in this solvent (11). Infrared spectroscopy (Fig. 5) corroborates the information obtained by NMR. The strong O–H peak around 3400 cm᎑1 arises from the polymer’s hydroxyl groups, since the presence of water has been ruled out by 1H NMR. The asymmetric methyl C–H stretching band shows up at 2970 cm᎑1, and the broad C–O–C ether vibration is near 1100 cm᎑1.
Figure 6. Differential scanning calorimetry (DSC) thermogram of poly(glycidol-co-propylene oxide). The glass transition temperature Tg is approximately ᎑50 °C.
Differential scanning calorimetry (DSC) of a student polymer sample in Figure 6 shows a well-defined glass transition Tg in the vicinity of ᎑50 °C, at the inflection point in the curve. This low Tg is characteristic of the highly flexible polyethers (12). Gel permeation chromatography (GPC) is used to assess polymer molecular weight, and this scan is shown in Figure 7 for a student sample. The small peaks at ca. 19.8 min and 20.2 min indicate the presence of small ring species formed as by-products, and the negative solventrelated peaks beyond 20.5 min can be ignored. Only a relative measure of the molecular weight of such hyperbranched polymers can be obtained by GPC using linear polymer standards. The average molecular weight of polyethers synthesized by Lewis acid-catalyzed polymerizations is known to be low, owing to extensive chain transfer during polymerization (12). Also, because GPC is a measure of polymer size and hyperbranched polymers have a more
JChemEd.chem.wisc.edu • Vol. 79 No. 1 January 2002 • Journal of Chemical Education
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In the Laboratory
in the polymerization are commercially available and elaborate synthetic apparatus is not needed.
Detector Respone / mV
150
100
Acknowledgments
50
0
-50
-100
-150 0
5
10
15
20
25
Retention Time / min Figure 7. GPC trace of the copolymer.
compact structure than their linear counterparts, reported – – molecular weights will be low: Mn ≈ 1280 g/mol and Mw ≈ 2150 g/mol (average student values). Students unfamiliar with polymer-related terminology may be referred to Chapter 1 in ref 12 for a thorough explanation of the terms used above. As expected from its structure, the copolymer dissolves rapidly in water and most organic solvents except for very nonpolar ones such as hexane. Conclusions In this experiment, students are introduced to a nonlinear polymer architecture of current relevance, ionic and ringopening polymerization reactions, and the synthesis of copolymers. The involvement of a hydroxyl group in an otherwise standard cationic ring-opening polymerization lends pedagogical interest to the experiment by helping students differentiate between the “active chain end” and “activated monomer” propagation mechanisms. GC monitoring of the reaction, NMR, FTIR and solubility studies shed light on the structure of the polymer and the mechanism of polymerization. DSC and GPC provide training for students in these important polymer characterization techniques. Students who performed this experiment in a polymer chemistry course and in an advanced synthesis course at the University of West Florida uniformly appreciated learning how to synthesize information from such varied analytical methods to form a coherent picture of polymer structure. The chemicals involved
84
Partial support for this work was provided by the National Science Foundation’s Division of Undergraduate Education through grants DUE-9851416 (GPC) and USE-9250257 (FTIR). I acknowledge financial support from the University of West Florida and Florida State University and express my special gratitude to the Department of Chemistry at Florida State University for assistance with NMR and DSC measurements. Students Tracy Marks, Philan Nguyen, Terry Odom, Roy Schleicher, and Kwok Wong are thanked for their valuable input. W
Supplemental Material
A handout for students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Sunder, A.; Hanselmann, R.; Frey, H.; Mülhaupt, R. Macromolecules 1999, 32, 4240. 2. Schmaljohann, D.; Häußler, Pötschke, P.; Voit, B. I.; Loontjens, T. J. A. Macromol. Chem. Phys. 2000, 201, 49. 3. Freemantle, M. Chem. Eng. News 1999, 77 (Sep 6), 37. 4. Varady, L.; Yang, Y. B.; Cook, S. E.; Regnier, F. E. Coated Media for Chromatography; U.S. Patent 5,030,352, Jul 9, 1991. 5. See, for example, Kim, Y. H. J. Polym. Sci. Polym. Chem. Educ. 1998, 36, 1685. 6. Fréchet, J. M. J.; Henmi, M.; Gitsov, I.; Aoshima, S.; Leduc, M. R.; Grubbs, R. B. Science 1995, 269, 1080. 7. Tokar, R.; Kubisa, P.; Penczek, S.; Dworak, A. Macromolecules 1994, 27, 320. 8. Royappa, A. T. J. Appl. Polym. Sci. 1997, 65, 1897. 9. Sunder, A.; Mülhaupt, R.; Frey, H. Macromolecules 2000, 33, 309. 10. Prior ring-opening polymerization experiments, old and new, have described syntheses of linear polymers, for example: France, M. B.; Uffelman, E. S. J. Chem. Educ. 1999, 76, 661. Mathias, L. J.; Viswanathan, T. J. Chem. Educ. 1987, 64, 639. 11. Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512. 12. Odian, G. Principles of Polymerization, 3rd ed.; Wiley: New York, 1991.
Journal of Chemical Education • Vol. 79 No. 1 January 2002 • JChemEd.chem.wisc.edu
