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Catalyst Development
- Structural Characterization of an ATRP Catalyst Complex
- Structure of a Catalyst Complex
- Kinetic Studies on ATRP
- Determination of Reaction Parameters in an ATRP Reaction
- Measurement of Rate Constants of Activation and Deactivation
- Determination of the Redox Potential of Copper Complex
- Solvent Effects and Selection of a Catalyst for Aqueous Media
- Other Catalyst Systems
- Some Side Reactions
Structural Characterization of an ATRP Catalyst Complex
Structure of a Catalyst Complex The structure of the catalyst selected for a given reaction affects the kinetics of an ATRP.
Mt is a transition metal with two stable oxidation states differing by 1 (m <-> m+1)
Therefore, structural characterization of a series ATRP active copper(I) and copper(II) complexes with a spectrum of ligands continues to be studied within the Matyjaszewski group using a variety of analytical tools. A recent review on the structural aspects of copper catalyzed ATRP (1) provides background on the fundamentals of transition metal catalyzed atom transfer reactions, including ATRA and ATRP. The review focuses on the structure of a catalyst complex formed with bidentate, tridentate and tetradentate nitrogen ligands. The role of the ligand in ATRP is to solubilize the Cu salts and to tune the Cu catalyst activity. The choice of ligand greatly influences the effectiveness of the catalyst in a specific polymerization reaction. Nitrogen-based ligands generally work well for Cu-mediated ATRP.
Within the review complex forming ligands discussed included:
2,2'-bipyridine (bpy),
4,4'-di(5-nonyl)-2,2'-bipyridine (dNbpy),
N,N,N',N'-tetramethylethylenediamine (TMEDA),
N-propyl(2-pyridyl)methanimine (NPrPMI),
2,2':6',2"-terpyridine (tpy),
4,4',4"-tris(5-nonyl)- 2,2':6',2"-terpyridine (tNtpy),
N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA),
N,N-bis(2-pyridylmethyl)octylamine (BPMOA),
1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA),
tris[2-(dimethylamino)ethyl]amine (Me6TREN),
tris[(2-pyridyl)methyl]amine (TPMA),
1,4,8,11-tetraaza-1,4,8,11-tetramethylcyclotetradecane (Me4CYCLAM) and
N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN)
The structures of copper(I) and copper(II) complexes with the following ligands are discussed in the context of how structure affects catalyst activity in addition to solvent and temperature.
diethylenetriamine (DETA),
triethylenetetramine (TETA),
N,N-bis(2-pyridylmethyl)amine (BPMA),
tris[2-aminoethyl]amine (TREN) and
1,4,8,11-tetraazacyclotetradecane (CYCLAM)
N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN)
The structures of the ligands most frequently employed for copper based catalysts are presented below:
Aspects of the structural studies concentrate on the stoichiometry between the complex forming ligand and the copper centers, (2-3) determination of the geometry of the formed complexes, and their solution behavior. (4-6) Techniques used include solid state X-ray crystallography, Extended X-ray Absorption Fine Structure (EXAFS) (4), Electrospray Ionization Mass Spectrometry (ESI-MS), (6) UV-Vis, Raman and Far IR spectroscopy.
A series of Cu(II) complexes were examined and they adopted either a trigonal bipyramidal structure, as in the case of the dNbpy ligand, or a distorted square pyramidal coordination, in the case of triamines and tetramines. (7)
Crystal structures of [CuII(dNbpy)2Br][CuIBr2] and [CuII(PMDETA)Br2]
Experimental (dotted line) and calculated (solid line) k3 (k) functions (a) (k range: 4.20 - 14.6 Å-1) and their Fourier transforms (b) for CuIBr/Me6TREN in toluene at room temperature.
Depending on the type of amine ligand, the complexes were either neutral (triamines) or ionic (bpy and tetramines). The counterions in the case of the ionic complexes were either a bromide anion (Me4cyclam and HMTETA) or the linear [CuIBr2]- anion (dNbpy). No direct correlation was found between the length of the CuII-Br bond and the deactivation rate constant in ATRP, which suggests that other parameters such as the entropy for the structural reorganization between the Cu(I) and Cu(II) complexes might play an important role in determining the overall activity of the catalyst in ATRP.
1. "Structural aspects of copper catalyzed atom transfer radical polymerization," Pintauer, T.; Matyjaszewski, K. Coordination Chemistry Reviews 2005, 249, 1155-1184.
2. "Effect of [bpy]/[Cu(I)] Ratio, Solvent, Counterion, and Alkyl Bromides on the Activation Rate Constants in Atom Transfer Radical Polymerization;" Nanda, A. K.; Matyjaszewski, K. Macromolecules 2003, 36, 599-604.
3. "Effect of [PMDETA]/[Cu(I)] Ratio, Monomer, Solvent, Counterion, Ligand, and Alkyl Bromide on the Activation Rate Constants in Atom Transfer Radical Polymerization;" Nanda, A. K.; Matyjaszewski, K. Macromolecules 2003, 36, 1487-1493.
4. "Extended X-ray absorption fine structure study of copper(I) and copper(II) complexes in atom transfer radical polymerization;" Pintauer, T.; Reinoehl, U.; Feth, M.; Bertagnolli, H.; Matyjaszewski, K. European Journal of Inorganic Chemistry 2003, 2082-2094.
5. "Highly Active Copper-Based Catalyst for Atom Transfer Radical Polymerization," Tang, H.; Arulsamy, N.; Radosz, M.; Shen, Y.; Tsarevsky, N. V.; Braunecker, W. A.; Tang, W.; Matyjaszewski, K. Journal of the American Chemical Society 2006, 128, 16277-16285.
6. "Electrospray ionization mass spectrometric study of CuI and CuII bipyridine complexes employed in atom transfer radical polymerization;" Pintauer, T.; Jasieczek, C. B.; Matyjaszewski, K. J. Mass Spectrom. 2000, 35, 1295-1299.
7. "Structural comparison of CuII complexes in atom transfer radical polymerization;" Kickelbick, G.; Pintauer, T.; Matyjaszewski, K. New Journal of Chemistry 2002, 26, 462-468.
Kinetic Studies on ATRP
Determination of Reaction Parameters in an ATRP Reaction: Simply put, if you do not select the correct catalyst and correct conditions for ATRP, the reaction will not be controlled.
This section concentrates on the kinetic and thermodynamic parameters in the atom transfer equilibrium such as the determination of the activation (kact) and deactivation (kdeact) rate constants and their dependence on the structure of the copper(I) complex. Additionally, the equilibrium constant, KEQ or KATRP = kact/kdeact, is determined via an analytical solution of the persistent radical effect. (8-10)
Kinetic studies: Knowledge of the dimension and impact of the kinetic parameters in a controlled/"living" radical polymerization (CRP) is of critical importance for the design of well-defined macromolecules. The rate of polymerization, molecular weight distribution and in case of copolymerization, the distribution of the comonomers along a given polymer segment are a direct consequence of the kinetics. Therefore, understanding and measuring the kinetics parameters is a primary goal of the group
Mechanistically, ATRP is based on an inner sphere electron transfer process, which involves a reversible (pseudo)halogen transfer between a dormant species (R-X) and a transition metal complex (Mtm/Ln) resulting in the formation of propagating radicals (R*) and the metal complex in the higher oxidation state (e.g. X-Mtm+1/Ln). Radicals react reversibly with oxidized metal complexes, X-Mtn+1/Ligand, in a deactivation reaction to reform the dormant species and the transition metal complex (Mtm/Ln) in the lower oxidation state, i.e. the activator. This process occurs with a rate constant of activation, kact, and deactivation kdeact, respectively. Polymer chains grow by the addition of monomers to the periodically generated radicals in a manner similar to a conventional radical polymerization, with the rate constant of propagation, kp. Termination reactions (kt) also occur in ATRP, mainly through radical coupling and disproportionation. The following equation illustrates how the polydispersity index in ATRP, in the absence of chain termination and transfer, relates to the concentration of initiator (RX) and deactivator (D), the rate constants of propagation (kp) and deactivation (kdeact), and the monomer conversion (q). (11)
Thus, for the same monomer, a catalyst that deactivates the growing chains faster will produce polymers with lower polydispersities (smaller kp/kdeact). Alternatively, polydispersities decrease with an increasing concentration of deactivator, although at the cost of slower rates of polymerization. For example, the addition of a small amount of Cu(II) halides to the feedstock (5-10%) in a standard copper-based ATRP leads to better controlled polymerizations with decreased polymerization rates, as a consequence of instantaneous control and increased initiation efficiency due to increased rate of deactivation. (12, 13) This approach is useful in conducting ATRP in aqueous / protic media. However the addition of Cu(II) to less polar media also affects the rate of activation, as does the ratio of added ligand to transition metal, and choice of solvent for a given catalyst complex. (14, 15)
8. "Kinetic Analysis of Controlled/\"Living\" Radical Polymerizations by Simulations. 2. Apparent External Orders of Reactants in Atom Transfer Radical Polymerization," Shipp, D. A.; Matyjaszewski, K. Macromolecules 2000, 33, 1553-1559.
9. "Toward structural and mechanistic understanding of transition metal-catalyzed atom transfer radical processes; Advances in Controlled/Living Radical Polymerization"; Pintauer, T.; McKenzie, B.; Matyjaszewski, K. ACS Symposium Series 2003, 854, 130-147.
10. "Determination of Rate Constants for the Activation Step in Atom Transfer Radical Polymerization Using the Stopped-Flow Technique"; Pintauer, T.; Braunecker, W.; Collange, E.; Poli, R.; Matyjaszewski, K. Macromolecules 2004, 37, 2679-2682.
11. "The importance of exchange reactions in controlled/living radical polymerization in the presence of alkoxyamines and transition metals," K. Matyjaszewski, Macromol. Symp 1996, 111, 47.
12. "Atom-transfer radical polymerization of styrene catalyzed by copper carboxylate complexes," K. Matyjaszewski, M. Wei, J. Xia, S. G. Gaynor, Macromol. Chem. Phys. 1998, 199, 2289.
13. "Effect of Cu(II) on the Kinetics of the Homogeneous Atom Transfer Radical Polymerization of Methyl Methacrylate," Zhang, H., Klumperman, B.; Ming, W.; Fischer H.; van der Linde R.; Macromolecules 2001, 34, 6169-6173.
14. "Effect of [CuII] on the Rate of Activation in ATRP," Matyjaszewski, K., Nanda, A. K., Tang, W. Macromolecules 2005, 38, 2015-2018.
15. "Effect of [pyridylmethanimine]/[CuI] ratio, ligand, solvent and temperature on the activation rate constants in atom transfer radical polymerization." Tang, W., Nanda, A. K., Matyjaszewski, K.: Macromolecular Chemistry and Physics 2005, 206: 1171-1177.
Measurement of Rate Constants of Activation and Deactivation
Understanding, and controlling the equilibrium, and hence the dynamics of the atom transfer process, are basic prerequisites for running a successful ATRP. Therefore it is very important within the catalyst group to correlate structure with reactivity for each of the involved reagents, both oxidation states of the transition metal complex, radicals and dormant species, in addition to solvent effects and reaction temperature in order to provide that fundamental understanding required for the selection of optimum conditions to conduct the desired reaction.
The ATRP equilibrium, (KATRP in the scheme above), is expressed as a combination of several contributing reversible reactions, including a combination of C-X bond homolysis of the alkyl halide (RX), (other transferable atoms or groups can participate in an ATRP (16)) but the most frequently employed radically transferable atoms are halogens and will be used to exemplify ATRP throughout most of following discussions) two redox processes, and heterolytic cleavage of the CuII-X bond. Therefore KATRP can be expressed as the product of the equilibrium constants for electron transfer between metal complexes (KET), electron affinity of the halogen (KEA), bond dissociation energy of the alkyl halide (KBD) and the equilibrium constant for the heterolytic cleavage of the Mtn+1-X bond (KX), which we have named the "halidophilicity" of the Mtn+1-X complex.
This means that for a given alkyl halide R-X, the activity of the catalyst in the ATRP process depends not only on the redox potential, but also on the halidophilicity of the transition metal complex. For complexes that have similar halidophilicity, the redox potential can be used as a measure of catalyst activity in the ATRP. This was demonstrated by the linear correlation between KATRP and E1/2 (i.e. KET) for a series of CuI complexes with nitrogen based ligands. (17)
It can also be concluded that for a given catalytic system in the same solvent (where KET, KEA, and KX are essentially constant), KATRP should depend only upon the energetics of alkyl halide bond homolysis, or KBD. Indeed, when the alkyl halide bond dissociation energies were calculated recently for a series of ATRP monomers/initiators, they were found to correlate well with measured values of KATRP. (17)
KATRP measured at 22 C with CuBr/TPMA; relative values of KBD determined from free energies calculated using DFT; BrPN = bromopropionitrile, BzBr = benzyl bromide, DEBrPA = N,N-Diethyl-a-bromopropionamide, EtBriB = ethyl 2-bromoisobutyrate, PEBr = 1-(bromoethyl)benzene, MBrP = methyl 2-bromopropionate.
It was proposed that such calculations could also be used to predict equilibrium constants for less reactive monomers. Knowing KBD (which can be calculated using DFT) as well as the rate constants of propagation for a given monomer, the rates of polymerization could be calculated for different monomers in ATRP under comparable conditions (same catalyst, constant KET, KEA, and KX). For example, if the ATRP of acrylonitrile reached 90% conversion in 1 second, methyl acrylate 4 hours while styrene would take 22 hours and vinyl acetate 30 years under the same conditions. (17) This calculation merely serves to demonstrate the necessity of choosing an appropriate catalyst for each monomer.
Determination of the equilibrium rate constant is crucial in order to understand the kinetics of an ATRP. Assuming steady-state kinetics, the rate of polymerization is given by:
This equation means that the rate of polymerization is controlled by the ratio of CuI to CuII and not the absolute amount of catalyst present in the reaction medium. Understanding the implications of this equation contributed to the development of ARGET ATRP once the concept of AGET ATRP was developed. Experimentally, the values of KATRP can be determined by direct analysis of the polymerization mixture (by EPR, NMR, GC, GPC, IR...) or by the study of low molecular weight model compounds. Furthermore, while some side reactions (thermal-initiation of monomer, elimination reactions, transfer reactions, degradation of the catalyst..) and some physical parameters (viscosity, inhomogeneity...) may have an important effect on the kinetics of CRP the influence of these parameters may also be investigated by model studies or by computer simulation. (18-20)
Historically, (21) we considered that the persistent radical effect (PRE) was the primary contributor to the degree of control seen in ATRP and operated with some differences between lower activity catalysts and high activity complexes. For example when Klumperman and Fischer (22) examined polymerization of MMA in the presence of a copper complex formed with N-(n-hexyl)pyridylmethaniminethe results showed that initially added Cu(II) can have strong effects on the kinetics of the ATRP depending on the [Cu(II)]0/[Cu(I)]0 ratio. When 10% Cu(II) relative to Cu(I) was added at the beginning of the polymerization, the kinetics were described by Fischer's equation (ln([M]0/[M]) varies with t2/3) with the reaction orders for initiator, Cu(I) and Cu(II), close to or the same as those in Fischer's equation, verifying the applicability of Fischer's equation in lower activity ATRP systems. On the other hand, when [Cu(II)]0/[Cu(I)]0 >= 0.1, the kinetics can be interpreted by Matyjaszewski's equation (ln([M]0/[M]) varies with t). The polymerization rate shows almost first order with respect to the concentration of the initiator and Cu(I) and inverse first order with respect to the concentration of Cu(II), suggesting that the "self-regulation" and radical termination becomes less important for the ATRP process when enough Cu(II) is added at the beginning of the reaction. This result has potential for better control of ATRP systems.
Recently new equations were derived for the evolution of the persistent radical over time for both non-equimolar and equimolar ATRP reactions. From these new equations, KATRP values were obtained for several ATRP systems using UV-vis spectrometric or GC measurements. Fischer's original equations for the PRE could be used only for systems that formed low amounts of persistent radical and consequently operated for systems with relatively low KATRP values. For systems that were driven to higher conversion and more Cu(II) was formed and for more reactive systems, significant deviations from linearity in t1/3 plots are observed and are presently being examined. (23)
kact: The initial report on determination of the rate constant for activation (kact) for different models of the polymerization system employed both small molecule and macromolecular model initiating species. The structures of the reagents shown in the following schematic describe a few of the ligands and model compounds selected to mimic the polymeric chain ends used in the kinetic studies. (24)
The activation rate constants were measured using HPLC or GC under the kinetic isolation conditions achieved by trapping the generated radical with a ten fold excess of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) with respect to the alkyl halide as shown below. (25)
Typical values of the activation rate constants measured at 35 C indicated that 2-bromoisobutyrate is approximately 10 times more reactive than the other alkyl halides and 1-phenylethyl bromide is ~20 times more reactive than the corresponding chloride. (17) This difference dramatically decreases at higher temperatures due to the higher activation energy for the latter.
PMDETA forms more reactive Cu(I) complexes than dNbpy. Me6TREN is ~104 times more active than the dNbpy-based complex. Indeed we have measured differences in activity of copper based catalyst complexes with different ligands that cover a range of 106.
kdeact: Deactivation rate constants were determined by trapping 1-phenylethyl radicals using TEMPO in a competitive clock reaction. The 1-phenylethyl radical was generated by the thermal decomposition of the 1-(N,N-(2-methylpropyl-1)-(1-diethylphosphono-2,2-dimethyl-propyl-1-)-N-oxyl)-1-phenylethane (PESG1). (25) The alkoxyamine can be obtained from Arkema under the trade name BlocBuilderTM and is one of the most versatile control agents available for NMP.
[Arkema website: http://www.arkema-inc.com/index.cfm, then select products and under product selection choice you will see the information.]
Another method for the estimation of deactivation rate constants in ATRP using copper catalysts is based on the principle of reverse ATRP and is conducted under an air atmosphere, using molecular oxygen as an oxidant for the Cu(I) species. (26) The deactivation rate constants for several different reactive copper complexes were estimated by this procedure, and the results correlated with those obtained from the analysis of the evolution of molecular weight distribution with conversion and the results agreed well with competitive trapping reactions using nitroxides.
16. "Effect of (Pseudo)halide Initiators and Copper Complexes with Non-halogen Anions on the ATRP," Davis, K. A.; Matyjaszewski, K. Journal of Macromolecular Science, Pure and Applied Chemistry 2004, 41, 449-465.
17. "Effects of Initiator Structure on Activation Rate Constants in ATRP," Tang, W.; Matyjaszewski, K. Macromolecules 2007, 40, 1858-1863.
18. "Effect of [CuII] on the Rate of Activation in ATRP"; Matyjaszewski, K.; Nanda, A. K.; Tang, W. Macromolecules 2005, 38, 2015-2018.
19. "Characterization of Cu(II) Bipyridine Complexes in Halogen Atom Transfer Reactions by Electron Spin Resonance," Knuehl, B.; Pintauer, T.; Kajiwara, A.; Fischer, H.; Matyjaszewski, K. Macromolecules 2003, 36, 8291-8296.
20. "Determination of Rate Constants for the Activation Step in Atom Transfer Radical Polymerization Using the Stopped-Flow Technique," Pintauer, T.; Braunecker, W.; Collange, E.; Poli, R.; Matyjaszewski, K. Macromolecules 2004, 37, 2679-2682.
21. Controlled/\"Living\" Radical Polymerization. Kinetics of the Homogeneous Atom Transfer Radical Polymerization of Styrene, Matyjaszewski, K.; Patten, T. E.; Xia, J.; J. Am. Chem. Soc. 1997, 119, 674-680.
22. "Effect of Cu(II) on the Kinetics of the Homogeneous Atom Transfer Radical Polymerization of Methyl Methacrylate," Zhang, H.; Klumperman, B.; Ming, W.; Fischer, H.; van der Linde, R. Macromolecules 2001, 34, 6169-6173.
23. "Determination of Equilibrium Constants for Atom Transfer Radical Polymerization," Tang, W.; Tsarevsky, N. V.; Matyjaszewski, K. Journal of the American Chemical Society 2006, 128, 1598-1604.
24. "Tridentate Nitrogen-Based Ligands in Cu-Based ATRP: A Structure-Activity Study;" Matyjaszewski, K.; Goebelt, B.; Paik, H.-j.; Horwitz, C. P. Macromolecules 2001, 34, 430-440.
25. "Determination of Activation and Deactivation Rate Constants of Model Compounds in Atom Transfer Radical Polymerization," K. Matyjaszewski, H.-j. Paik, P. Zhou, S. J. Diamanti, Macromolecules 2001, 34, 5125.
26. "Measurement of Initial Degree of Polymerization without Reactivation as a New Method To Estimate Rate Constants of Deactivation in ATRP," Gromada, J.; Matyjaszewski, K. Macromolecules 2002, 35, 6167-6173.
Determination of the Redox Potential of Copper Complexes
Here we are investigating of the role of coordinating ligand in the catalyst complex and its effect on the catalyst activity. We are interested in the redox behavior of the copper complexes that could be used in an ATRP, and how the redox potential is dependent on the complex forming ligand and on the reaction media. Cyclic voltametry of copper(I) and copper(II) complexes are used to measure the potentials. For Cu(I) complexes with nitrogen based ligands KATRP is proportional to E1/2. More reducing catalysts increase KATRP. The redox potential and halidophilicity of the catalyst are affected by both the transition metal and ligand. Generally the more reducing a complex is the more active the catalyst. The order of activity is shown below for a selection of N-based ligands.
The reduction potentials can be correlated with kact and kdeact for activation 1-PhEtBr and deactivation of PhEt*, the deactivation rate constants scale reciprocally with activation rate constants.
The catalyst complex formed with Me6TREN has a very high kact AND a sufficiently high kdeact making Me6TREN a VERY efficient ligand, especially for less reactive monomers.
One reason for the high activity of the TREN based catalyst complexes is that entropy favors a high kdeact for Me6TREN since the Cu(II) atom moves just 0.13 &Mac197; inside the cavity of the ligand. (27, 28) While most work on (TREN) based ligands has been conducted using Me6TREN as the ligand the substituents on the N-atoms of the commercially available H6TREN can be readily modified using a Michael addition to attach different species to H6TREN to prepare ligands suitable for catalysts that will be soluble in the polymerization environment, control the rate of polymerization and be easy to remove from the product. The figure also shows that substituents on the ligand can significantly affect activity; c.f Me6TREN with Et6TREN. (29)
H6TREN is available in commercial quantities from Pressure Chemicals.
The structure of some of the other ligands examined in this study (30) and the relative activity of their Cu complexes are shown below:
The most active ATRP catalyst reported to date, the Cu complex dimethyl cross-bridged cyclam, (31) was identified solely based on its very low reduction potential (> 75 mV lower than that of the CuBr / Me6TREN complex).
The range of activity of catalyst complexes with different ligands therefore exceeds one million. This figure clearly indicates that conditions optimized for one catalyst complex, e.g. a pyridine-imine, NPPMI based catalyst complex, can not be used for complexes employing Me6TREN as ligand or indeed even bpy, since the activity between the complexes differ by orders of magnitude.
The general order of activities of Cu complexes is related to their structure and follows the following order: tetradentate (cyclic-bridged) > tetradentate (branched) > tetradentate (cyclic) > tridentate > tetradentate (linear) > bidentate ligands. The nature of the N atom is also important and follows the order: pyridine >= aliphatic amine > imine.
As seen from the above figure the activity of a given Cu complex strongly depends on the ligand structure and even small structural changes may lead to large differences in their activity. Note effect of different substituents on the activity of TREN-based ligands, in addition to changing the phylicity of the resulting complex the different substituents dramatically change the activity, resulting in complexes with activity spanning 5 orders of magnitude from the lower activity of AN6TREN to high activity with Me6TREN.
Recently a new, versatile and highly active copper-based complex CuBr/N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) was reported for acrylic, methacrylic, and styrenic monomers. (32) The catalyst mediated ATRP at a catalyst/initiator molar ratio of 0.005 and produced polymers with well-controlled molecular weight and low polydispersity. ATRP occurred even at a catalyst/initiator molar ratio as low as 0.001 with copper concentration in the produced polymers as low as 6-8 ppm (catalyst/monomer molar ratio = 105). The catalyst structures were studied by X-ray diffraction and NMR spectroscopy. The activator CuIBr/TPEN existed in solution as an equilibrium mixture of binuclear and mononuclear complexes but as a binuclear complex in single crystals. The deactivator CuIIBr2/TPEN complex was mononuclear. High stability and appropriate KATRP were found crucial for the catalyst required to work under high dilution or in coordinating solvents/monomers. This provides guidance for further design of highly active ATRP catalysts.
27. "Reversible Binding of Dioxygen by a Copper(I) Complex with Tris(2-dimethylaminoethyl)amine (Me6TREN) as a Ligand." Becker, M.; Heinemann, F. W.; Schindler, S.; Chem. Eur. J. 1999, 5, 3124.
28. "The crystal structure of tris(2-dimethylaminoethyl)amine nickel(II) and -copper(II) bromidesM. D. Vaira, P. L. Orioli, Acta Cryst. 1968, B24, 595.
29. "Synthesis and ATRP activity of new TREN-based ligands." Gromada, J.; Spanswick, J.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2004, 205, 551-566.
30. "Effect of Ligand Structure on Activation Rate Constants in ATRP," Tang, W.; Matyjaszewski, K. Macromolecules 2006, 39, 4953-4959.
31. "Copper-based ATRP catalysts of very high activity derived from dimethyl cross-bridged cyclam", Tsarevsky, N. V.; Braunecker, W. A.; Tang, W.; Brooks, S. J.; Matyjaszewski, K.; Weisman, G. R.; Wong, E. H. J. Mol. Catal. A: Chem. 2006, 257, 132.
32. "Highly Active Copper-Based Catalyst for Atom Transfer Radical Polymerization," Tang, H.; Arulsamy, N.; Radosz, M.; Shen, Y.; Tsarevsky, N. V.; Braunecker, W. A.; Tang, W.; Matyjaszewski, K. Journal of the American Chemical Society 2006, 128, 16277-16285.
Solvent effects and selection of a catalyst for aqueous media
The performance of copper-based ATRP catalysts can be predicted based on the stability constants of the CuII and CuI complexes with the ligand L (BII and BI, respectively). Both BII and BI should be large in order to prevent catalyst deactivation through competitive coordination of monomer and/or polymer. A high BII/BI ratio is required for high catalytic activity. More halogenophilic CuII-L complexes provide more active catalyst complexes and better control over polymerization due to the fact that there is sufficient concentration of deactivator present in the system. If the ATRP is to be carried in aqueous media, in addition to the above requirement, the ratio BII/(BI)2[L] should be low to prevent disproportionation of the CuI complex. Acidic monomers may be polymerized if the ligands meet all outlined requirements and are as weakly basic as possible.
CuI complexes are generally unstable in aqueous media and tend to disproportionate. For instance, the disproportionation of non-complexed CuI in water is characterized by an equilibrium constant as large as Kdisp =106. (33) Addition of excess ligand L can significantly affect the equilibrium and the equilibrium constant changes to a new value, determined by the stabilization of CuII relative to CuI upon coordination with L.
Therefore, for ligands forming 1:1 complexes with copper ions, the activity of the catalyst is proportional to BII/BI whereas the tendency of the CuI complex to disproportionate in aqueous solution (which should be minimized) depends on the ratio BII/(BI)2[L]. (34) Thus, a map can be constructed that can be used to select a ligand for aqueous ATRP that forms an active complex yet stable towards disproportionation.
More details on the Cu(I) disproportionation reaction, including the effect of both ligand and solvent on it are provided in a review paper, describing various redox process related to ATRP. (35)
Correlation between ATRP catalytic activity and tendency for disproportionation for several CuI complexes (Chem. Rev. 2007).
The figure shows that while the CuI/PMDETA complex is very active it is not suitable for aqueous ATRP due to very fast disproportionation. On the other hand, ligands such as bpy, HMTETA, and TPMA can all be used in aqueous media although with rather different activities. If necessary, the catalyst disproportionation in water can be suppressed by using an appropriate co-solvent or by addition of a pseudo ligand that will stabilize CuI vs. CuII, such as pyridine, which allows aqueous ATRP of ionic monomers such as sodium 4-styrenesulfonate and 2-(N,N,N-trialkylammonio)ethyl methacrylate salts, to be conducted. (35-37)
33. "Two-step electron transfer and disproportionation of simple copper(+) ion." Datta, D. Ind. J. Chem. 1987, 26A, 605.
34. "Atom transfer radical polymerization of ionic monomers in aqueous solution: Mechanistic studies and synthesis." Tsarevsky, N. V.; Pintauer, T.; Matyjaszewski, K. Polym. Prepr. 2002, 43(2), 203.
35. Electron transfer reactions relevant to atom transfer radical polymerization, Tsarevsky, N. V.; Braunecker, W. A.; Matyjaszewski, K. Journal of Organometallic Chemistry 2007, 692, 3212-3222.
36. Competitive equilibria in atom transfer radical polymerization, Tsarevsky, N. V.; Braunecker, W. A.; Vacca, A.; Gans, P.; Matyjaszewski, K. Macromolecular Symposia 2007, 248, 60-70.
37. \"Green\" Atom Transfer Radical Polymerization: From Process Design to Preparation of Well-Defined Environmentally Friendly Polymeric Materials, Tsarevsky, N. V.; Matyjaszewski, K. Chemical Reviews (Washington, DC, United States) 2007, 107, 2270-2299.
Other Catalyst Systems
Many different catalyst systems have been used in these studies. The most frequently used metals are Cu, Fe, and Ru although other transition metals that can undergo a one electron redox transition have also been examined.
The target for the continued work on the development of more active catalysts for ATRP has been the preparation of a higher quality product by reducing the catalyst loading and running under more economical conditions by lowering the reaction temperature and continually seeking to increase the range of monomers that can be controllably copolymerized.
An example of an iron based system with comparable activity was developed by Gibson. (38)
38. "Design of Highly Active Iron-Based Catalysts for Atom Transfer Radical Polymerization: Tridentate Salicylaldiminato Ligands Affording near Ideal Nernstian Behavior." O'Reilly, R. K., Gibson, V. C., White, A. J. P., Williams, D. J.: J. Am. Chem. Soc., 2003, 125: 8450-8451.
Some Side Reactions
A number of side reactions have been examined using the kinetic tools described above. These include the association/dissociation equilibrium between the vinyl monomers and the catalyst complex. (39)
The enthalpy and entropy of coordination and the relative binding constants of methyl acrylate (MA), 1-octene (Oct), styrene (Sty), and methyl methacrylate (MMA) with Cu/PMDETA were determined. (29) The affinity of the monomers to the CuI center decreased in the order MA > Oct > Sty > MMA. The consequence of monomer coordination to CuI in atom transfer radical polymerization (ATRP) was examined. The reactivity of the selected comonomers in an ATRP, and in conventional free radical copolymerization reactions, are not significantly affected by monomer coordination to the transition metal complex. However these reactions become more important as the concentration of the catalyst complex in the reaction medium is significantly reduced as in ARGET and ICAR ATRP. (40) Under such dilute conditions, catalysts with tridentate ligands such as PMDETA (to which monomers can coordinate very strongly) do not perform as well as catalysts with tetradentate ligands of otherwise comparable activity.
Other concurrent reactions that may occur during ATRP and could affect its efficiency include disproportionation of the CuI catalyst in aqueous or polar media, transfer reactions associated with the complexing ligand, (35, 41) and solvent coordination. (42, 43)
Indeed with the recent development of new initiation techniques in ATRP that allow catalysts to be employed at unprecedented low concentrations (approximately 10 ppm), a thorough understanding of competitive equilibria that can affect catalyst performance is becoming increasingly important including:
- factors affecting the position of the ATRP equilibrium;
- dissociation of the ATRP catalyst at high dilution and loss of deactivator due to halide dissociation;
- conditional stability constants as related to competitive monomer, solvent, and reducing agent complexation as well as ligand selection with respect to protonation in acidic media; and
- competitive equilibria involving electron transfer reactions, including the radical oxidation to carbocations or reduction to carbanions, radical coordination to the metal catalyst, and disproportionation of the Cu-based ATRP activator.
As noted above one concern is the possibility of Outer Sphere Electron Transfer, or the reduction / oxidation of radicals by the catalysts to carbanions / carbocations (see below). (44) As noted in reference 44 the continuous development of more active and stable catalysts in ATRP has increasingly required a thorough knowledge of concurrent electron transfer reactions that can affect catalyst performance. One must select the appropriate catalyst for the targeted monomers; e.g. CuI/Me6TREN is too "reducing" for acrylonitrile and CuI/bpy is too "oxidizing" for vinyl ether. These side reactions can be viewed as arising from reactions due to formation of carbocations or carbanions and radical coordination to the metal catalyst resulting in the interplay of controlled radical polymerization mechanisms.
Outer Sphere Electron Transfer as a Side Reaction in ATRP
Attempts to directly polymerize acidic monomers (such as acrylic or methacrylic acid) by ATRP have failed due to interactions of acids with the catalyst leading to complete loss of activity. The concept of conditional stability constants (45, 46) allows the calculation of the stability of the ATRP catalyst in acidic solutions. Generally, complexes formed with very basic ligands are rather sensitive to the pH of the medium, although in the case when the ligand forms particularly stable copper complexes, the complex protonation can become negligible. A graphical representation of the dependence of the conditional stability constant B* of a complex as a function of acid concentration is shown below
39. "Quantifying Vinyl Monomer Coordination to CuI in Solution and the Effect of Coordination on Monomer Reactivity in Radical Copolymerization." Braunecker, W. A., Tsarevsky, N. V., Pintauer, T., Gil, R. R., Matyjaszewski, K. Macromolecules 2005, 38: 4081-4088.
40. "Diminishing catalyst concentration in atom transfer radical polymerization with reducing agents," Matyjaszewski, K.; Jakubowski, W.; Min, K.; Tang, W.; Huang, J.; Braunecker, W. A.; Tsarevsky, N. V. Proceedings of the National Academy of Sciences of the United States of America 2006, 103, 15309-15314.
41. "Towards understanding monomer coordination in atom transfer radical polymerization: synthesis of [CuI(PMDETA)(p-M)][BPh4] (M=methyl acrylate, styrene, 1-octene, and methyl methacrylate) and structural studies by FT-IR and 1H NMR spectroscopy and X-ray crystallography." Braunecker, W. A., Pintauer, T., Tsarevsky, N. V., Kickelbick, G., Matyjaszewski, K.: Journal of Organometallic Chemistry 2005, 690: 916-924.
42. "Effect of variation of [PMDETA]0/[Cu(I)Br]0 ratio on atom transfer radical polymerization of n-butyl acrylate." Huang, J., Pintauer, T., Matyjaszewski, K.: Journal of Polymer Science, Part A: Polymer Chemistry, 2004, 42: 3285-3292.
43. "Deactivation Efficiency and Degree of Control over Polymerization in ATRP in Protic Solvents." Tsarevsky, N. V., Pintauer, T., Matyjaszewski, K.: Macromolecules, 2004, 37: 9768-9778.
44. "Inner sphere and outer sphere electron transfer reactions in atom transfer radical polymerization." Matyaszewski, K. Macromol. Symp. 1998, 134, 105-118.
45. Schwarzenbach, G. Die Komplexometrische Titration; 2nd ed.; Enke: Stuttgart, 1956.
46. Ringbom, A. Chemical Analysis. Vol. XVI. Complexation in Analytical Chemistry; Interscience: New York, 1963.