Development of Controlled/Living Radical Polymerization
- Review Articles on CRP
- Factors Influencing the Evolution of CRP (Focusing on ATRP)
- Equilibria for the Three Major CRP processes
- Features of Controlled/Living Radical Polymerizations
- First-order Kinetic Behavior
- Pre-determinable Degree of Polymerization
- Narrow Molecular Weight Distribution
- Long-lived Polymer Chains
Development of Controlled/Living Radical Polymerization
The following figure shows that there have been over 9000 papers published on CRP since 1993 and more than 5000 on ATRP since 1995. There have been over 1200 published US patents and US patent applications utilizing CRP for the preparation of materials targeting specific applications in the same time frame. The reason is an interest in expanding the market for specialty functional materials prepared from the wide range readily available radically (co)polymerizable monomers. Applications include coatings, adhesives, surfactants, dispersants, lubricants, gels, additives, thermoplastic elastomers, electronics, biomaterials, in fact almost any market requiring a material with a specific set of well defined properties.
Furthermore, with the development of ATRP systems that require very low concentrations of catalyst, we now believe that it is now possible to use standard FRP industrial equipment for a controlled synthesis of materials containing one or more segments prepared by ATRP. (1)
The data displayed in the figure above is current to August 2007 and was obtained by conducting a search on SciFinder Scholar using the following terms: "controlled radical polymn" or "living radical polymn" ("SUM CRP" in the figure), "ATRP or atom transfer (radical) polymn" (i.e. SUM ATRP in the figure refers to ATRP only, this does not include terms like metal mediated or metal catalyzed radical polymerization), "NMP or SFRP or nitroxide mediated polymn or stable free polymn" ("SUM SFRP") and "RAFT or reversible addition transfer or degenerative transfer or catalytic chain transfer" ("SUM DT"). The latter two terms were refined with a term radical polymn. since they coincide with other popular names such as N-methylpyrrolidone or raft-associated proteins.
There is a lot of overlap between the terms but the figure does show that there has been a continuous increase in the interest using various controlled polymerization procedures for preparation of functional materials.
In the qualitative schematic shown above the expansion in utility seen for all CRP processes is a consequence of an increase in the range of controllably polymerized comonomers, development of new RAFT agents, removal of RAFT agents from chain ends, new nitroxides, development of new ligands for ATRP, procedures for reduction in concentration of catalyst, halogen exchange, expansion of the scope of bi-phasic processes, and development of bio-conjugate structures.
Review Articles on CRP
There have been four ACS Symposia on CRP in the past decade. Proceedings from the symposia held in 1997, 1999, 2002 and 2005 have been published (2-5) with the next symposium to be held at the ACS Meeting in Philadelphia on August 17-21, 2008. (6) The fourth ACS Symposium was held at the Fall ACS Meeting in Washington DC, 2005, and over 200 talks and posters were presented at the full five day symposium.
Several extensive reviews and book chapters on Controlled/Living Radical Polymerization (CRP) have also been published (7-16) with a recent review on all CRP processes providing a balanced perspective. (17)
(1) Jakubowski, W.; Matyjaszewski, K. Polymer Preprints 2007, 48(2),256-7
(2) "Controlled Radical Polymerization. (Proceedings of a Symposium at the 213th National Meeting of the American Chemical Society, held 13-17 April 1997, in San Francisco,California.)": ACS Symp. Ser., 1998; 685, edited by Matyjaszewski, K.
(3) "Controlled/Living Radical Polymerization. Progress in ATRP, NMP, and RAFT. (Proceedings of a Symposium on Controlled Radical Polymerization held on 22-24 August 1999, in New Orleans.)": ACS Symp. Ser., 2000; 768, edited by Matyjaszewski, K.
(4) "Advances in Controlled/Living Radical Polymerization. (ACS Symposium held in Boston, Massachusetts)": ACS Symp. Ser., 2003; 854, edited by K. Matyjaszewski; Hardcover: 704 pages; and seven chapter within
(5) "Controlled/Living Radical Polymerization from Synthesis to Materials; (ACS Symposium held in Washington, DC August 27 to September 1, 2005) ACS Symp. Ser., 2006; 944, edited by K. Matyjaszewski
(6) The next ACS Symposium on CRP will be held August 17-21, 2008 in Philadelphia.
(7) "Handbook of Radical Polymerization", edited by K. Matyjaszewski, T. Davis, Hardcover: 936 pages; Publisher: Wiley, New York, 2002; and three chapters within.
(8) "Statistical, Gradient and Segmented Copolymers by Controlled/Living Radical Polymerizations" by Kelly A. Davis, Krzysztof Matyjaszewski; Hardcover: 203 pages; Publisher: Springer Verlag; Berlin 2002.
(9) "Atom Transfer Radical Polymerization"; Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921-2990.
(10) "Metal-Catalyzed Radical Polymerization"; Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689-3745.
(11) ""Green" atom transfer radical polymerization: from process design to preparation of well-defined environmentally friendly polymeric materials," Tsarevsky Nicolay, V.; Matyjaszewski, K. Chem Rev 2007, 107, 2270-2299.
(12) "New Polymer Synthesis by Nitroxide Mediated Living Radical Polymerizations"; Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661-3688.
(13) "RAFTing down under: tales of missing radicals, fancy architectures, and mysterious holes"; Barner-Kowollik, C.; Davis, T. P.; Heuts, J. P. A.; Stenzel, M. H.; Vana, P.; Whittaker, M. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 365.
(14) "Macromolecular design via reversible addition-fragmentation chain transfer (RAFT)/xanthates (MADIX) polymerization"; Perrier, S.; Takolpuckdee, P. Journal of Polymer Science, Part A: Polymer Chemistry 2005, 43, 5347-5393.
(15) "Living Radical Polymerization by the RAFT Process," Moad, G.; Rizzardo, E.; Thang, S. H. Australian Journal of Chemistry 2005, 58, 379-410.
(16) "Living Radical Polymerization by the RAFT Process-A First Update," Moad, G.; Rizzardo, E.; Thang, S. H. Australian Journal of Chemistry 2006, 59, 669-692.
(17) "Controlled/living radical polymerization: Features, developments, and perspectives"; Braunecker, W. A.; Matyjaszewski, K. Progress in Polymer Science 2007, 32, 93-146.
Factors Influencing the Evolution of CRP (Focusing on ATRP)
The development of CRP, and even more specifically Atom Transfer Radical Polymerization (ATRP), is based on understanding and integration of chemistry developed over the past 60 years in the fields of organic chemistry, coordination chemistry, conventional radical polymerization, and living ionic polymerizations augmented by computational, electrochemistry and inorganic chemistry that came together in the mid-1990"s. (Relevant references, 18-33)
The concept of CRP that arose from this integration of knowledge is that a controlled radical polymerization process requires selecting conditions that allow, or create, a dynamic equilibrium between a low concentration of active propagating chains and a large number of dormant chains, which are unable to propagate or self-terminate. In NMP and ATRP the further the activation/deactivation equilibrium is shifted towards dormant species, the greater the decrease in the concentration of active propagating chains, the less significant termination becomes compared to propagation.
There are several CRP processes based on this fundamental understanding but the systems that are presently receiving the most attention are Atom Transfer Radical Polymerization, (ATRP) (34, 35), which is based on the fundamental work on ATRA (18-20) and coordination chemistry (21-26); Stable Free Radical Polymerization (SFRP) (36) including Nitroxide Mediated Polymerization (NMP) (37, 38) and OrganoMetalic Radical Polymerization (OMRP) (39), and Degenerative Transfer (DT) (40, 41) processes including Reversible Addition Fragmentation Transfer (RAFT) (42, 43) and Macromolecular Design via the Interchange of Xanthates (MADIX). (44, 45) A review of the kinetics of CRP has recently been provided by Fukuda. (46)
Discussion of the Required Equilibrium Conditions for each of the Three Major CRP Processes
The dynamic equilibrium required for control over each of the three major CRP processes is shown below
1) reversible deactivation by atom transfer. e.g. ATRP
2) reversible deactivation by coupling. e.g. Nitroxide-mediated polymerization, Co-mediated radical polymerization (47)
3) degenerative transfer. e.g. Atom or group transfer (I, Te, Ge, Sn, Sb, Bi,"), Addition Fragmentation (Dithioesters (RAFT), Unsaturated polymethacrylates)
In the case of ATRP and NMP, which obey the persistent radical effect (PRE), control is attained by:
- extending, or fragmenting, the life of propagating chains during the reaction from <1s to >1 h
- enabling quantitative (and close to simultaneous) initiation, i.e. going from Ri << Rp as is the case for conventional FRP to Ri >> Rp for CRP processes which provides narrow PDI
- the ratio of polymerized monomer to added initiator to control the molecular weight, (DPn = ^[M]/[I])
- selection of comonomers and initiator allows control over functionality, composition, and topology
- all of these objectives are accomplished by formation of a dynamic equilibrium between a propagating radical and an excess of dormant species.
The work of the Matyjaszewski group is summarized within this web page.
As noted above, in the case of an ATRP reaction the appropriate selection of suitable catalyst complexes that allow an ATRP to be conducted in a variety of reaction media should be based not only on the fundamental work on transition metal catalyzed ATRA, or "Kharasch," reactions, but also on understanding redox properties (23, 24) and stability constants of transition metal complexes. (25) Together they provide the tools for catalyst selection summarized in the "Fundamentals of ATRP" and detailed in the "Mechanism and Catalyst Development" sections of this web page.
(18) "Reactions of atoms and free radicals in solution. XV. The addition of bromodichloromethane and dibromodichloromethane to olefins. The preparation of 2-alkenals," Kharasch, M. S., Kuderna B. M., and Urry W. H.; J. Org. Chem. 1948, 13: 895-902.
(19) "The design and application of free radical chain reactions in organic synthesis. Part 2." Curran, D. P. 1988, Synthesis 7: 489-513.
(20) "Kharasch and Metalloporphyrin Catalysis in the Functionalization of Alkanes, Alkenes, and Alkylbenzenes by t-BuOOH. Free Radical Mechanisms, Solvent Effect, and Relationship with the Gif Reaction," Minisci, F.; Fontana, F.; Araneo, S.; Recupero, F.; Banfi, S.; Quici, S. Journal of the American Chemical Society 1995, 117, 226-232.
(21) "Unusual selectivities of radical reactions by internal suppression of fast modes," Fischer, H.: J. Am. Chem. Soc. 1986 108: 3925-3927.
(22) "The Persistent Radical Effect: A Principle for Selective Radical Reactions and Living Radical Polymerizations," Fischer, H. Chem. Rev. 2001 101 3581-3610.
(23) "Interpretation of the Polarographic Waves of Complex Metal Ions," Lingane, J. J.; Chem. Rev. 1941; 1 - 35
(24) "Electron Transfer by Copper Centers," Rorabacher, D. B. Chemical Reviews 2004, 104, 651-697.
(25) "Ligand-based redox series," Vlcek, A. A.; Coordination Chemistry Reviews 1982, 43, 39-62.
(26) "Mechanisms of organic oxidation and reduction by metal complexes," Kochi, J. K. Science 1967, 155, 415-424.
(27) "Metal Ammine Formation in Aqueous Solution. Theory of the Reversible Step Reactions"; Bjerrum, J. P. Haase and Son: Copenhagen, 1957.
(28) "\"Living\" polymers." Szwarc, M. Nature 1956, 178, 1168-1169.
(29) "Exchange reactions in the living cationic polymerization of alkenes." Matyjaszewski, K.; Lin, C. H. Makromol. Chemie, Macromol. Symp. 1991, 47, 221-237.
(30) "General Kinetic Analysis and Comparison of Molecular Weight Distributions for Various Mechanisms of Activity Exchange in Living Polymerizations." Litvinenko, G.; Mueller, A. H. E. Macromolecules 1997, 30, 1253-1266.
(31) "Iniferter concept and living radical polymerization." Otsu T. J., Polym. Sci., Part A: Polym. Chem. 2000, 38, 2121-2136.
(32) "From telomerization to living radical polymerization." Boutevin, B. J., Polym. Sci., Part A: Polym. Chem. 2000, 38, 3235-3243.
(33) Bamford, C. H. In Vol. 3 of Comprehensive Polymer Science; Elsivier, 1992.
(34) "Polymerization of Methyl Methacrylate with the Carbon Tetrachloride/Dichlorotris- (triphenylphosphine)ruthenium(II)/Methylaluminum Bis(2,6-di-tert-butylphenoxide) Initiating System: Possibility of Living Radical Polymerization"; Kato, M. et. al.; Macromolecules 1995, 28, 1721-1723.
(35) "Controlled/\"living\" radical polymerization. Atom transfer radical polymerization in the presence of transition-metal complexes"; Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614-5615.
(36) "Narrow molecular weight resins by a free-radical polymerization process," Georges, M. K. et. al.; Macromolecules 1993, 26, 2987-2988.
(37) "New Polymer Synthesis by Nitroxide Mediated Living Radical Polymerizations," Hawker, C. J. et. al.; Chem. Rev. 2001, 101, 3661-3688.
(38) "Nitroxide-mediated radical processes," Studer, A.; Schulte, T. Chemical Record 2005, 5, 27-35.
(39) "Relationship between one-electron transition-metal reactivity and radical polymerization processes," Poli, R. Angewandte Chemie, International Edition 2006, 45, 5058-5070.
(40) "Controlled Radical Polymerizations: The Use of Alkyl Iodides in Degenerative Transfer"; Matyjaszewski, K. et. al.; Macromolecules 1995, 28, 2093-2095.
(41) "Chain Transfer Activity of -Unsaturated Methyl Methacrylate Oligomers". Moad, C. L. et. al. Macromolecules 1996, 29, 7717.
(42) "Tailored polymer architectures by reversible addition-fragmentation chain transfer." Rizzardo, E.; Chiefari, J.; Mayadunne, R.; Moad, G.; Thang, S.; Macromol. Symp. 2001, 174, 209.
(43) "Dithiocarbamates as universal reversible addition-fragmentation chain transfer agents." Destarac, M., D. Charmot, X. Franck and Zard S. Z.; Macromol. Rapid Commun. 2000, 21(15): 1035-1039.
(44) "Macromolecular design via the interchange of xanthates (MADIX): Polymerization of styrene with O-ethyl xanthates as controlling agents," Destarac, M.; Brochon, C.; Catala, J.-M.; Wilczewska, A.; Zard, S. Z. Macromolecular Chemistry and Physics 2002, 203, 2281-2289.
(45) "The degenerative radical transfer of xanthates and related derivatives: an unusually powerful tool for the creation of carbon-carbon bonds," Quiclet-Sire, B.; Zard, S. Z. Topics in Current Chemistry 2006, 264, 201-236.
(46) "Kinetics of living radical polymerization"; Goto, A., Fukuda T., Prog. Polym. Sci. 2004 29 329"385.
(47) "Degenerative Transfer and Reversible Termination Mechanisms for Living Radical Polymerizations Mediated by Cobalt Porphyrins," Wayland, B. B.; Peng, C.-H.; Fu, X.; Lu, Z.; Fryd, M. Macromolecules 2006, 39, 8219-8222.
Features of Controlled/Living Radical Polymerizations (CRP)
The introductory slide from the home page, reproduced below, indicates that Controlled/"Living" Radical Polymerization (CRP) procedures can be used for the preparation of copolymers incorporating a broad spectrum of radically (co)polymerizable monomers forming materials with predetermined molecular weight, and narrow molecular weight distribution, if desired. The most recent work on conducting an ATRP with low concentration of transition metals indicate that some control over the breadth of the MWD is also possible. Here, and elsewhere in the text the word "control" and/or "controlled" means that if the polymerization process conditions are selected so that the contributions of the chain breaking processes are insignificant compared to chain propagation, allowing synthesis of polymers with predetermined molecular weights, low polydispersities and site specific functionalities.
Features of Controlled Radical Polymerization Processes
It is widely accepted that a controlled polymerization process should display the following features: (This summary is predominately reproduced from reference 48.)
- First-order Kinetics Behavior
- Pre-determinable Degree of Polymerization
- Narrow Molecular Weight Distribution
- Long-lived Polymer Chains
Feature 1. First-order kinetics behavior, i.e. the polymerization rate (Rp) with respect to the monomer concentration ([M]) is a linear function of time. This is due to the lack of termination, so that the concentration of the active propagating species ([P*]) is constant.
kp is the propagation constant.
The consequence of equation CRP.2 and the effect of changes in P* are illustrated in below
Feature 1. Illustration of the dependence of ln([M]0/[M]) on time
This semilogarithmic plot is very sensitive to any change of the concentration of the active propagating species. A constant [P*] is revealed by a straight line. An upward curvature indicates an increase in [P*], which occurs in case of slow initiation. On the other hand, a downward curvature suggests a decrease in [P*], which may result from termination reactions increasing the concentration of the persistent radical, or some other side reactions such as the catalytic system being poisoned or redox processes on the radical.
It should also be noted that the semilogarithmic plot is not sensitive to chain transfer processes or slow exchange between different active species, since they do not affect the number of the active propagating species.species.
Feature 2. Predeterminable degree of polymerization (Xn), i.e. the number average molecular weight (Mn) is a linear function of monomer conversion.
This result comes from a constant number of chains throughout the polymerization, which requires the following two conditions:
- that initiation should be sufficiently fast so that nearly all chains start to grow simultaneously;
- no chain transfer occurs that increases the total number of chains
The following figure illustrates feature 2 and shows that the ideal growth of molecular weights with conversion, as well as the effects of slow initiation and chain transfer on the molecular weight evolution.
Figure illustrating feature 2: The dependency of molecular weight on conversion
It is important to recognize that the evolution of molecular weight is not very sensitive to chain termination, since the number of chains remains unchanged. The effect of termination is only observable on the plot when coupling reactions for polymers with very high molecular weights start to play a significant role.
Feature 3. Narrow molecular weight distribution although this feature is very desirable, it is not necessarily the result of a controlled polymerization, which only requires the absence of chain transfer and termination, but ignores the effect of rate of initiation, exchange and depropagation. Substantial studies (33-35) indicate that in order to obtain a polymer with a narrow molecular weight distribution, each of the following five requirements should be fulfilled.
- The rate of initiation is competitive with the rate of propagation. This condition allows the simultaneous growth of all the polymer chain.
- The exchange between species of different reactivity is faster than propagation. This condition ensures that all the active chain termini are equally susceptible to reaction with monomer for a uniform growth.
- There must be negligible chain transfer or termination.
- The rate of depropagation is substantially lower than propagation. This guarantees that the polymerization is irreversible.
- The system is homogenous and mixing is sufficiently fast. Therefore all active centers are introduced at the onset of the polymerization.
This should yield a Poison distribution, as quantified in equation CRP.4.
According to equation CRP.4, polydispersity (Mw/Mn) decreases with increasing molecular weight.
Systems with slow exchange do not follow this perfect distribution but PDI's are defined by the following equation. (52)
A polymerization that satisfies all five prerequisites listed above is expected to form a final polymer with a polydispersity less than 1.1 for Xn greater than 10.
Feature 4. Long-lived polymer chains. This is a consequence of negligible chain transfer and termination. Hence, all the chains retain their active centers after the full consumption of the monomer. Propagation resumes upon introduction of additional monomer. This unique feature enables the preparation of block copolymers by sequential monomer addition.
The significance of controlled polymerization as a synthetic tool is widely recognized and polymers having uniform predictable chain length are readily available. Controlled polymerization provides the best opportunity to control the bulk properties of a target material through control of the multitude of possible variations in composition, functionality and topology now attainable at a molecular level.
Through appropriate selection of the functional (macro)initiator, copolymers formed in a "living"/controlled polymerization process can have any desired topology. Further, as noted at the foot of the figure showing what CRP can do, we highlight that mechanistic transformations permit the use of macroinitiators or macromonomers prepared by other polymerization procedures in any CRP process which allows incorporation of a spectrum of functionalities and polymer segments prepared by any other controlled polymerization process into segments of copolymers prepared by CRP.
Indeed a plethora of previously unattainable polymeric materials have been prepared. Numerous examples of gradient, (53) block (54) and graft (55) copolymers have been reported, as well as polymers with complex architectures, including comb shaped polymer brushes, (56) stars, (57) and hyperbranched (58) copolymers. Progress has been made in the synthesis of each of these materials and the procedures are discussed in other sections of the web page.
(48) In "Atom transfer radical polymerization in aqueous dispersed media"; Qiu, J.; PhD Thesis, 2000, Carnegie-Mellon Univ., Pittsburgh, PA, USA, p 327 pp.
(49) "The importance of exchange reactions in controlled/living radical polymerization in the presence of alkoxyamines and transition metals", Matyjaszewski, K.; Macromol. Symp. 1996 111 47-61.
(50) "A multistate mechanism for homogeneous ionic polymerization. II. The molecular weight distribution." Coleman, B. D., Fox, T. G.; J. Am. Chem. Soc. 1963 85: 1241-1244.
(51) "Cationic Polymerizations: Mechanisms, Synthesis, and Applications." Matyjaszewski, K., Editor: Publisher: (Dekker, New York, N. Y.), 1996, 768 pp.
(52) "The importance of exchange reactions in controlled/living radical polymerization in the presence of alkoxyamines and transition metals," Matyjaszewski, K. Macromol. Symp. 1996, 111, 47-61.
(53) "Gradient Copolymers by ATRP"; Matyjaszewski, K.; Ziegler, M. J.; Arehart, S. V.; Greszta, D.; Pakula, T. J. Phys. Org. Chem. 2000, 13, 775.
(54) "Statistical, gradient, block, and graft copolymers by controlled/living radical polymerizations"; Davis, K. A.; Matyjaszewski, K. Adv. Polym. Sci. 2002, 159, 2.
(55) "How to make polymer chains of various shapes, compositions, and functionalities by ATRP"; Gaynor, S. G.; Matyjaszewski, K. ACS Symp. Ser. 1998, 685, 396.
(56) "The Synthesis of Densely Grafted Copolymers by ATRP"; Beers, K. L.; Gaynor, S. G.; Matyjaszewski, K.; Sheiko, S. S.; Moeller, M. Macromolecules 1998, 31, 9413.
(57) "The synthesis of functional star copolymers as an illustration of the importance of controlling polymer structures in the design of new materials"; Matyjaszewski, K. Polym. Int. 2003, 52, 1559.
(58) "Preparation of Hyperbranched Polyacrylates" Matyjaszewski, K.; Gaynor, S. G. Macromolecules 1997, 30, 5192-5194; 7034-7041; & 7042-7049.
Click to go to section 03: Fundamentals of an ATRP Reaction