Controlled Polymerization of Acrylamides - American Chemical Society

Chapter 8

Controlled Polymerization of Acrylamides

Downloaded by UNIV OF ARIZONA on October 2, 2015 | http://pubs.acs.org Publication Date: November 28, 2000 | doi: 10.1021/bk-2001-0780.ch008

Marina Baum, Mical E. Pallack, Jude T. Rademacher, and William J. Brittain Department of Polymer Science, The University of Akron Akron, OH 44325-3909

Polyacrylamides are an important class of water-soluble polymers, which are used in a variety of industries such as medicine, oil, textile, and waste management. Interest in the development of new polymerization methods for well-defined molecular architectures of polyacrylamides is growing due to the versatility of these polymers. Anionic polymerization has long been known to provide the control needed for synthesis of such molecules. However, stringent reaction conditions and high purity of reagents make this synthetic method unattractive for industrial use. Over recent years, several more robust "controlled" radical polymerization processes have been developed. Among them are atom transfer radical polymerization (ATRP), reversible addition fragmentation chain transfer process (RAFT), and nitroxide-mediated stable free radical polymerization (SFRP). All of these methods utilize reversible activation-deactivation methods to control the concentration of growing radicals in order to decrease termination. This chapter provides an overview of research done in the area of "controlled" polymerization of acrylamides.

© 2001 American Chemical Society

In Stimuli-Responsive Water Soluble and Amphiphilic Polymers; McCormick, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


141 Research in living free radical polymerization (i, 2) has become an active area of polymer synthesis. Work has been published on nitroxide-mediated processes, (3, 4, 5) atom transfer radical polymerization (ATRP), (/) and reversible addition fragmentation chain transfer (RAFT). (6) Most work on living radical polymerization has focused on styrene and (meth)acrylates. Only recently, has the living radical polymerization of acrylamide-based monomers become a topic of interest. (7) Polyacrylamides are an important class of watersoluble polymers with a wide range of applications in a variety of industries. (8) Due to the non-toxicity and biocompatibility of these polymers, polyacrylamides are used in medical applications for drug-delivery, implants, and DNA sequencing gels. As water-soluble polymers, polyacrylamides are used as viscosifiers in industries such as oil, irrigation pumping, and fire fighting. They are also widely utilized asflocculationagents in solid recovery, waste removal, and water clarification. Due to their adhesive and film-forming properties, polyacrylamides are important in textile and paper industries. (9) Methods for controlled polymerization would be very valuable for many of these applications. Conventional polymerizations suffer from uncontrolled chain transfer and termination, which lead to poor control of molecular weight distribution, end-group functionalities, and chain architecture. Controlled processes allow the synthesis of specific molecular weights and well-defined molecular architectures.

Anionic Polymerization Anionic polymerization has long been known to proceed in the absence of chain transfer and termination and result in narrow molecular weight distribution polymers. Xie and Hogen-Esch (10) have described the anionic polymerization of iV^dimethylacrylamide (DMA) and JV-acryloyl-# -methylpiperazine. These workers produced polyacrylamides with controlled molecular weights and narrow molecular weight distributions when polymerizations were conducted at low temperatures (-78 °C) in THF initiated in the presence of triphenylmethyl cesium ( Scheme 1).

ΝΜθ2

CONMefe

Scheme L Anionic polymerization ofDMA.


142 Recently, Nakahama and co-workers (11) anionically prepared stereospecific ρο1ν(ΛΓ, ΛΓ-dialkylacrylamides) with narrow polydispersity in the presence of Et Zn. These reactions were conducted in THF at 0 °C for 1 h. The disadvantages of anionic polymerization include extensive purification of reagents and stringent reaction conditions. More robust methods for controlled polymerizations are desired. 2


Radical Polymerization Radical polymerizations are known to be very robust systems and are widely used in industry. However, conventional free radical polymerization methods do not offer the degree of control that we seek. Over the past few decades, controlled radical polymerization methods have been developed and include SFRP, ATRP, and RAFT. SFRP and ATRP reactions are governed by the same general scheme (Scheme 2). In this reaction the persistent species (does not initiate polymerization) reversibly couples with the transient species (propagating species) to form the "dormant" species. To achieve control, the equilibrium lies strongly to the left reducing the number of active species and thus minimizing the number of termination reactions. Consequently this process also lowers the rate of propagation. In RAFT, while an active polymer chain end is growing, the reversible capping agent is a non-radical dithioester species. R

ws,CH2—CH—X "dormant" species

VNA.CH2—CH«

+

·χ

persistent species

Scheme 2. General scheme for SFRP and A TRP. The living free radical polymerization of acrylamides has not been widely studied.

Nitroxide-Mediated SFRP A classic example of controlled radical polymerization is SFRP, where a growing radical reversible recombines with a scavenging radical forming a


143 dormant species (Scheme 3). In this reaction, the carbon-oxygen bond of the "dormant" species undergoes thermalfragmentationto give a stable, persistent nitroxide radical and a reactive polymeric radical. Benoit, Chaplinski, Braslau, and Hawker (3) reported nitroxide-mediated polymerization of DMA by use of an alkoxyamine initiator. These reactions were carried out in bulk using 2, 2, 5trimethyl-3-(l-phenylethoxy)-4-phenyl-3-azahexane initiator with the addition of 0.05 eq of the corresponding nitroxide radical at 120 °C. Polymers with narrow molecular weight distributions and molecular weights ranging from 4,00050,000 g/mol were synthesized.


^ v w CH2

ÇH

Ο—Ν

^

*:

CH2 CH ·

+ ·0

Ν

Scheme 3. Nitroxie-mediated SFRP scheme.

Reversible Addition Fragmentation Chain Transfer Technique Another method for controlled free radical polymerization is RAFT. Controlled character is obtained by a reversible transfer of a dithio moiety between active and "dormant" species (Scheme 4). Le, Moad, Rizzardo, and Thang (12) reported the RAFT polymerization of DMA using 2-phenylprop-2-yl dithiobenzoate as chain transfer agent. These reactions were conducted in benzene at 60 °C using AIBN as the initiator to afford near-monodisperse polymers with specific molecular weights. The researchers reported low conversions (13-31%) after 1 h.

.s—p

-s—P„

n

ζ

•s. Ζ

Scheme 4. General RAFT scheme.


144 Atom Transfer Radical Polymerization ATRP is a controlledfreeradical polymerization based on a reversible redox activation-deactivation method to reduce the concentration of growing radicals (Scheme 5). Here, a metal halide complexed with a ligand reversibly donates a halogen to a growing chain-end forming a "dormant" species and thus minimizing the number of reactive species. Matyjaszewski and co-workers (/) have extensively investigated ATRP of styrenes and acrylates. They have demonstrated that this is a viable method for the synthesis of near-monodisperse polymers and copolymers with predetermined molecular weights. Downloaded by UNIV OF ARIZONA on October 2, 2015 | http://pubs.acs.org Publication Date: November 28, 2000 | doi: 10.1021/bk-2001-0780.ch008

k R

X

+

a

Mt /Ligand

^

n

"

.

R.

+ -Mt X

n + 1

/ Ligand

( ^ J - * - R-R k

p

Scheme 5. General A TRP scheme. These successful living polymerizations of styrene- and acrylate-based monomers led to the investigation of ATRP for acrylamido monomers. Recent work by Teodorescu and Matyjaszewski (13) and Brittain and co-workers (14) have shown that copper-based ATRP of DMA is not controlled. Both research groups investigated various solvents, ligands, initiators and copper halides and observed broad polydispersities and lack of agreement between experimental and theoretical M . The uncontrolled nature of ATRP of DMA was confirmed by the polymer growth resumption experiment in which only a smallfractionof the chain ends increased in molecular weight. (14) It was found that a variety of solvents could be used to obtain high DMA conversions including DMF, water and toluene. Polymerizations conducted in water proceeded most rapidly. In fact, complete monomer conversions could be obtained in minutes. This is consistent with literature reports on the conventional radical polymerization of (meth)acryiamides where water exerts an accelerating effect on polymerization. (15) Of course, these fast reaction times are undesirable for controlledTliving" radical polymerization. An important mechanistic feature of controlled radical polymerizations is the ability to maintain a low radical concentration in order to minimize spontaneous, bimolecular termination. Rapid polymerizations indicate a high concentration of radicals. Interestingly, Wirth and co-workers (16) have claimed a living polymerization of acrylamide using ATRP with surface-immobilized initiators. The evidence for living polymerization was based on a correlation between film thickness and monomer concentration; also narrow molecular weight distributions were cited by Wirth and co-workers as evidence that their n



145 polymerization was living. A truer test of living behavior in Wirth's system would have been polymer growth resumption. It is possible that aerylamide behaves differently than DMA or that surface-initiated polymerizations are mechanistically different than solution polymerizations. Teodorescu and Matyjaszewski (13) offered several reasons why there is lack of control in the copper-mediated ATRP of (meth)acrylamides. These reasons include: 1) deactivation of catalyst by polymer complexation, 2) strong bond between bromine and the terminal monomer unit in the polymer, and 3) nucleophilic displacement of terminal bromine by the penultimate amide group. They concluded that thefirsttwo reasons could not be solely responsible for lack of control in the polymerizations. Brittain and co-workers (14) cited the organic literature which supports the contention that the uncontrolled polymerization is due to slow deactivation of the chain ends. Several examples have been reported of radical generation from alkyl bromides where the reaction is promoted by complexation of Lewis acids to amide bonds. (17) A particularly relevant example is that of Sibi and Ji (18) where the alkyl bromide (Scheme 6) is more readily transformed to the corresponding radical when a Lewis acid is present (e.g., MgBr or a lanthanide). A similar example of this effect was observed by Porter and co-workers. (79) Both of these reports demonstrate that the presence of a Lewis acid can have a profound effect on C-Br bond lability in chemical structures bearing an imide group alpha to the bromine. Using the reactivityselectivity principle, it was concluded that the formation of a C-Br bondfroman amide-substituted radical would be less favorable in the presence of a Lewis acid. ATRP involves metals that can serve as Lewis acids. It is the presence of the metal and its complexation to the amide functionality that slows deactivation in ATRP of (meth)acrylamides and makes the process an uncontrolled polymerization. 2

Ph

Ph

Scheme 6. Radical formation in the presence of a Lewis acid. Sawamoto and co-workers (20) investigated ATRP of DMA using RuCl (PPh ) in conjunction with an alkyl halide as an initiator in the presence of Al(Oz-Pr) in toluene at 60-80 °C. They observed faster polymerization rates 2

3

3

3



146 than those of methacrylates and obtained polymers with controlled molecular weights and polydispersities of -1.6. Polymerization proceeded via the activation of the terminal carbon-halide bond by the ruthenium complex. Living character was determined by polymer growth resumption experiments in which most of the polymer chains resumed growth upon addition of the second charge of monomer. Success of these polymerizations suggests that the metal catalyst has a profound effect on the nature of these reactions. It seems that Ru metal does not have the same complexing characteristic as Cu with the amide group. In summary, the most promising methods for controlled radical polymerization of acrylamides are SFRP and RAFT. More in-depth research into these areas is necessary for better understanding of the controlled nature of these reactions. Copper-based ATRP is not a viable system for controlling acrylamide polymerization. However, new results in ruthenium-based ATRP show promise.

References 1. Matyjaszewski, K. Controlled Radical Polymerization; ACS Symposium Series 685; American Chemical Society: Washington, DC 1998. 2. Hawker, C. J. Acc. Chem. Res. 1997, 30, 373. 3. Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, C. J. J. Am. Chem. Soc. 1999, 121, 3904. 4. Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer, G. K. Trends Polym. Sci. 1994, 2, 66. 5. Odell, P. G.; Veregin, R. P. N.; Michalak, L. M.; Georges, M . K. Macromolecules 1997, 30, 2232; and references contained therein. 6. Chong, Y. K.; Le, T. P. T.; Moad, G.; Rizzardo, Ε.; Thang, S. H. Macromolecules 1999, 32, 2071. 7. Li, D.; Brittain, W. J. Macromolecules 1998, 31, 3952. 8. Shalaby, S. W.; McCormick, C. L.; Butler, G. B. Water-Soluble Polymers: Synthesis, Solution Properties and Applications, ACS Symposium Series 467, American Chemical Society: Washington, DC 1991. 9. Polymeric MaterialsEncyclopedia;Salamone, J. C., Ed.; CRC Press, Inc.: Boca Raton, 1996, 47. 10. Xie, X.; Hogen-Esch, T. E. Macromolecules 1996, 29, 1746. 11. Kobayashi, M.; Okuyama, S.; Ishizone, T.; Nakahama, S. Macromolecules, 1999, 32, 6466. 12. Le, T.; Moad, G.; Rizzardo, Ε.; Thang, S. H. Intern. Pat. WO 98/01478, 1998. 13. Teodorescu, M.; Matyjaszewski, K. Macromolecules 1999, 32, 4826.


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14. Rademacher, J. T.; Baum, M.; Pallack, Μ. Ε.; Brittain, W. J.; Simonsick, W. J. Macromolecules 2000, 33 15. Hamilton, C. J.; Tighe, B. J. In Comprehensive Polymer Science; Eastmond, G. C.; Ledwith, Α.; Russo, S.; Sigwalt, P., Ed.; Pergamon Press: New York, 1989; Vol. 3, Chapter 20. 16. Wirth, M. J.; Huang, X. Macromolecules 1999, 32, 1694. 17. Sibi, M.; Porter, N. A. Acc. Chem. Res. 1999, 32, 163; Mero, C. L.; Porter, N. A. J. Am. Chem. Soc. 1999, 121, 5155. 18. Sibi, M. P.; Ji, J. Angew. Chem., Int. Ed. Engl. 1996, 35, 190. 19. Wu, J. H.; Radinov, R.; Porter, N. A. J. Am. Chem. Soc. 1995, 117, 11029. 20. Senoo, M.; Kotani, Y.; Kamigaito, M.; Sawamoto, M.; Macromolecules, 1999, 32, 8005.


Controlled Polymerization of Acrylamides - American Chemical Society

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