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(1996–2000)

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Mechanisms and Catalyst Development

Summary

Understanding the Mechanistic Parameters of Atom Transfer Radical Polymerization (ATRP): Research conducted in the Matyjaszewski group in the area of developing an understanding of mechanistic parameters of transition metal catalyzed ATRP covers several topics:

  • Developing a comprehensive structure-reactivity relationship for each component of the reaction including:
    • structure of complexes and active centers
    • optimization of conditions for various degrees of polymerization & topologies
    • understanding/minimization of side reactions
    • improved cross-propagation kinetics for preparation of blocks
    • conducting reactions in various media
  • Catalyst optimization and catalyst removal/recycling
    • expanding range of monomers
    • reducing amount of catalyst
    • immobilization, removal, recycling the catalyst; e.g. the hybrid catalyst system

On this page we provide a summary of the mechanism of ATRP and in other pages we discuss in greater detail the chemistry involved in some Controlled Radical Polymerization (CRP) processes, in particular ATRP, and the spectrum of new materials that can now be prepared by CRP processes.

The development of ATRP is discussed in much greater detail in the following pages on this site:

The ultimate aim of these projects is to establish a structure-property correlation for the catalytic system and explore the potential of utilizing ATRP for the polymerization of an expanding range of monomers thereby allowing the preparation of materials with properties that closely match those required for specifically targeted applications.

Mechanism (Summary)

The group has historically focused on copper based catalysts, and this work forms the foundation for most of the observations/discussions/conclusions on these pages, although other transition metals have been examined and continue to be examined both within the group and by many other researchers throughout the world.

Mechanistic studies are crucial to any future developments of ATRP. Correlating reaction parameters, including measurement of the rate constants for initiation, activation, deactivation, and hence overall reaction rate; and evolution of molecular weight distribution with initiator, monomer and catalyst structure, solvent composition, and temperature should ultimately lead to the development of more active catalysts that can be specifically selected for optimum synthesis of targeted materials.

This means that one catalyst does not work for every monomer: selection of an appropriate catalyst system, reaction medium, and reaction conditions are required.

For instance if conditions are defined employing a lower activity catalyst complex, such as a copper/N-(n-alkyl)-2-pyridylmethanimine, they will not provide a controlled polymerization with a catalyst complex that is a million times more active, such as a complex formed with Me6TREN. This difference does not mean a different mechanism is operating only that process conditions have to take into account the reaction parameters.

See Starting Points

Most ATRP reactions require the addition, or in situ formation, of four essential components for an ATRP reaction:

  • a molecule, which we have called a (macro)initiator, with at least one transferable atom or group, frequently a halogen, R-X, where X = Cl of Br;
  • a transition metal (compound),
  • a ligand that forms a complex with the transition metal (compound) to modify solubility and catalyst activity,
  • one or more radically (co)polymerizable monomers.

Other language can, and has been used to describe this process, such a transition metal mediated living radical polymerization or OSET-LRP, but the basic principles underlying all processes using the direct addition or in situ formation of these four components are the same.

One or more of these functions can be combined in a single molecule, e.g. an initiator and monomer, which directly forms a (hyper)branched structure when (co)polymerized.

The general mechanism of ATRP is shown below.

The existence of a radical, (Pn*), here we are using IUPAC nomenclature, has been proposed in copper-mediated ATRP and is based on several experimental observations. (1)

Mechanistically, ATRP is based on an inner sphere electron transfer process, (3) which involves a reversible homolytic (pseudo)halogen transfer between a dormant species, an added initiator or the dormant propagating chain end, (Pn-X) and a transition metal complex in the lower oxidation state (Mtm/Ln) resulting in the formation of propagating radicals (R*) (4) and the metal complex in the higher oxidation state with a coordinated halide ligand (e.g. X-Mtm+1/Ln).

The active radicals form at a rate constant of activation (kact), subsequently propagate with a rate constant (kp) and reversibly deactivate (kdeact), but also terminate (kt). As the reaction progresses, radical termination is diminished as a result of the persistent radical effect, (PRE), (5) and the equilibrium is strongly shifted towards the dormant species (kact << kdeact).

Addition of the persistent radical, (X-Mtm+1/Ln), to the initial reaction medium increases the efficiency of initiation by avoiding the need to form the persistent radical by early stage termination reactions. This results in better polymerization control and higher initiation efficiency.

There are presently several ways to set up the ATRP equilibrium and they will be addressed in greater detail on various pages throughout the site.

The equilibrium can be approached from both sides:

  • a standard or "normal" ATRP starting with RX/Mtn (an ATRP initiator and a catalyst in a lower oxidation state) (1) and,
  • a "reverse" ATRP which starts with radicals generated from a standard free radical initiator and the added X-Mtn+1 species. Successful polymerizations have been carried out starting with conventional free radical initiators, such as AIBN (6) and BPO (7) and higher oxidation state transition metal complexes. The higher oxidation state catalyst complex for an ATRP can also be activated by adding Mt0, or many other reducing agents, which then reduces the higher oxidation state transition metal complex, X-Mtm+1/Ln to form the X-Mtm/Ln activator in situ. (8) {See Initiation of ATRP}

There is abundant evidence that ATRP operates via a radical mechanism, Such as:

  • Chemoselectivity, cross propagation kinetics (9) and kinetic isotope effects (4) are similar to that for conventional radical polymerizations. However, the nature of a CRP with intermittent activation and deactivation can result in some deviation in cross propagation kinetics from those seen in a standard radical polymerization. (10)
  • The polymerization is initially retarded by the presence of a small amount of oxygen as does the presence of true radical inhibitors, such as galvinoxyl and TEMPO that directly react with a growing radical and inhibit chain growth by terminating the reaction.
  • On the other hand antioxidants, such as phenols, used to prevent in situ formation of peroxides (11) which can initiate polymerization of (meth)acrylates, actually act to increase the rate of an ATRP reaction by reducing the concentration of the deactivator, X -Mtm+1. (12) Indeed, back in 1946, Walling (who studied with Kharasch) warns against the fact that misuse of the term antioxidants, calling them inhibitors, can lead to confusion about their mechanism of operation. (11)
  • Furthermore, ATRP is converted into a system which displays conventional radical polymerization characteristics upon the addition of octanethiol as a chain transfer reagent. (13)
  • Chain transfer in BA polymerization also resembles the conventional radical process. (14)
  • ATRP can be carried out in the presence of water (15, 16) or other protogenic reagents, and is tolerant of a variety of functionalities on the comonomers, just as in the case of standard radical polymerization reactions. (17)
  • Moreover, the reactivity ratios, which are very sensitive to the nature of the active centers, are nearly identical to those reported for the conventional radical copolymerization of the same monomers but are very different from those for anionic, group transfer, and cationic systems. (18-23)
  • Regioselectivity and stereoselectivity are similar to, and do not exceed, that for a conventional radical polymerization. All the polymers formed by ATRP have regular head-to-tail structures with the dormant species of the typical secondary/tertiary alkyl halide structures, as evidenced by NMR. In addition, polymers have the similar tacticity as those prepared by a conventional free radical process. (10, 21)
  • A racemization study using optically active alkyl halides also supports the intermediacy of a radical. (24)
  • EPR studies have revealed the presence of X-Mtn+1 species resulting from the persistent radical effect in normal ATRP reactions. (25-30)
  • If a "classic" ATRP reaction is driven to high levels of monomer conversion a doubling of the molecular weight, or cross-linking in multi-functional initiator systems, has been observed as a result of radical-radical termination reactions. (31)
  • Cross-exchange between different halogens (32) and different polymerization systems (thermal and ATRP or nitroxide mediated and ATRP) demonstrates they have the same intermediates and supports a common radical mechanism. An equimolar mixture of initiators for nitroxide-mediated polymerization and ATRP leads to the preparation of polystyrene with a unimodal molecular weight distribution (MWD). (33)
  • Propagating free radicals have been observed directly by EPR in the polymerization of dimethacrylates. (34)

Activation rate constants (kact), for a specific ATRP, are typically determined from model studies in which the transition metal complex is reacted with a model alkyl halide in the presence of a radical trapping agent, such as TEMPO. (30) The rates are typically determined by monitoring the rate of disappearance of the alkyl halide in the presence of excess activator (CuIX/Ln) and excess TEMPO, which traps radicals faster than CuIIX2/Ln. Under such conditions, the activation rate constant can be kinetically isolated from the deactivation rate constant and is given by ln([RX]0/[RX]t) = -kact[CuIX/Ln]0t .

The values were also determined for some polymeric systems using GPC techniques and similar kinetic expressions. However, both methods are limited in measuring fast rate constants with the maximum upper limit of approximately 2 M-1s-1. This prompted us to consider the stopped-flow technique for measuring very fast activation rate constants for model systems in copper-mediated ATRP, (31) since the mixing time (ca. 1 ms) and speed of data collection (one full spectrum ca. every 1 ms for diode-array UV-visible spectrophotometers) allow the measurement of pseudo-first order rate constants up to ca. 1.5*102 s-1 (t1/2 = ca. 5 ms). The stopped-flow technique allowed determination of activation rate constants which were previously not accessible due to the limitations in measuring fast reaction rates with current GC and NMR techniques and demonstrated the effectiveness of the stopped-flow technique to measure fast reaction rates opening up a new way to systematically determine activation rate constants and activation parameters for other highly active ATRP complexes. Unpublished results have expanded the range of both techniques and it is possible to push the upper limit of GPC techniques to ~10 M-1s-1 and stop flow measurements to 7.5 *103 s-1.
The structures of some complex forming ligands used in conjunction with CuIBr or CuICl are shown below.

The activity of the catalyst complexes formed with this spectrum of ligands covers a wide spectrum of activity as detailed in the following pages and summarized in the interactive kinetic data section and in a recent paper (37) and a review on catalyst performance (38) which describes catalyst complexes with different ligands displaying a range of activity exceeding 106.

A similar series of activation rate constants (kact) have been determined for a variety of initiators for Cu-mediated ATRP under the same conditions. (39) The ratio of the activation rate constants for the studied alkyl (pseudo)halides also exceeds 106 times

1. "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.

2. "Radical Nature of Cu-Catalyzed Controlled Radical Polymerizations (Atom Transfer Radical Polymerization)." Matyjaszewski, K., Macromolecules 1998, 31, 4710.

3. "Inner sphere and outer sphere electron transfer reactions in atom transfer radical polymerization," Matyjaszewski, K. Macromolecular Symposia 1998, 134, 105-118.

4. "Isotope Effects and the Mechanism of Atom Transfer Radical Polymerization," Singleton, D. A.; Nowlan, D. T., III; Jahed, N.; Matyjaszewski, K., Macromolecules 2003, 36, 8609-8616.

5. "The Persistent Radical Effect: A Principle for Selective Radical Reactions and Living Radical Polymerizations," Fischer, H., Chem. Rev. 2001, 101, 3581-3610.

6. "Controlled/\"Living\" Radical Polymerization. Homogeneous Reverse Atom Transfer Radical Polymerization Using AIBN as the Initiator." Xia, J., Matyjaszewski, K.: Macromolecules, 1997, 30: 7692-7696.

7. "Homogeneous Reverse Atom Transfer Radical Polymerization of Styrene Initiated by Peroxides." Xia, J., Matyjaszewski, K.: Macromolecules, 1999, 32: 5199-5202.

8. "Zerovalent Metals in Controlled/\"Living\" Radical Polymerization" Matyjaszewski, K.; Coca, S.; Gaynor, S. G.; Wei, M.; Woodworth, B. E. Macromolecules 1997, 30, 7348-7350.

9. "Identifying the Nature of the Active Species in the Polymerization of Methacrylates: Inhibition of Methyl Methacrylate Homopolymerizations and Reactivity Ratios for Copolymerization of Methyl Methacrylate/n-Butyl Methacrylate in Classical Anionic, Alkyllithium/Trialkylaluminum-Initiated, Group Transfer Polymerization, Atom Transfer Radical Polymerization, Catalytic Chain Transfer, and Classical Free Radical Polymerization." Haddleton, D. M., Crossman, M. C., Hunt, K. H., Topping, C., Waterson, C., Suddaby, K. G.: Macromolecules, 1997, 30: 3992-3998.

10. "Factors Affecting Rates of Comonomer Consumption in Copolymerization Processes with Intermittent Activation," Matyjaszewski, K. Macromolecules 2002, 35, 6773-6781.

11. "The thermal polymerization of methyl methacrylate," Walling, C., Briggs, E. R.: J. Am. Chem. Soc. 1946 68: 1141-1145.

12. "Effect of phenol and derivatives on atom transfer radical polymerization in the presence of air" Gnanou, Y., Hizal, G.: J. Polym. Sci., Part A: Polym. Chem. 2003 42: 351

13. "Atom-transfer radical polymerization in the presence of a thiol. More evidence supporting radical intermediates." Heuts, J. P. A., Mallesch, R. and Davis T. P., Macromol. Chem. Phys. 1999 200(6): 1380-1385.

14. "Evidence for chain transfer in the atom transfer radical polymerization of butyl acrylate." Roos, S. G. and A. H. E. Muller; Macromol. Rapid Commun. 2000, 21 864-867.

15. "Controlled/\"Living\" Radical Polymerization Applied to Water-Borne Systems." Gaynor, S. G., J. Qiu and K. Matyjaszewski; Macromolecules, 1998, 31(17): 5951-5954.

16. "Emulsion Polymerization of n-Butyl Methacrylate by Reverse Atom Transfer Radical Polymerization." Qiu, J., S. G. Gaynor and K. Matyjaszewski; Macromolecules, 1999, 32 2872-2875.

17. "Functional polymers by atom transfer radical polymerization." Matyjaszewski, K., Coca, S. Nakagawa Y. and Xia J.; Polym. Mater. Sci. Eng. 1997, 76: 147-148.

18. "Atom transfer radical copolymerization of styrene and n-butyl acrylate." Arehart, S. V. and K. Matyjaszewski; Macromolecules 1999, 32 2221-2231.

19. "Gradient copolymers by atom transfer radical copolymerization"; K. Matyjaszewski, M. J. Ziegler, S. V. Arehart, D. Greszta, T. Pakula,; J. Phys. Org. Chem. 2000, 13, 775.

20. "Atom transfer radical copolymerization of styrene and butyl acrylate"; G. Chambard, B. Klumperman; ACS Symp. Ser. 2000, 768, 197.

21. "Atom transfer radical copolymerization of methyl methacrylate and n-butyl acrylate"; M. J. Ziegler, K. Matyjaszewski, Macromolecules 2001, 34, 415.

22. "Controlled/\"Living\" Radical Polymerization. Halogen Atom Transfer Radical Polymerization Promoted by a Cu(I)/Cu(II) Redox Process"; J.-S. Wang, K. Matyjaszewski, Macromolecules 1995, 28, 7901.

23. "Evidence for Living Radical Polymerization of Methyl Methacrylate with Ruthenium Complex: Effects of Protic and Radical Compounds and Reinitiation from the Recovered Polymers." T. Nishikawa, T. Ando, M. Kamigaito, M. Sawamoto, Macromolecules 1997, 30, 2244.

24. "Free-Radical Intermediates in Atom Transfer Radical Addition and Polymerization: Study of Racemization, Halogen Exchange, and Trapping Reactions." K. Matyjaszewski, H.-j. Paik, D. A. Shipp, Y. Isobe, Y. Okamoto, Macromolecules 2001, 34, 3127.

25. "Simultaneous EPR and Kinetic Study of Styrene Atom Transfer Radical Polymerization (ATRP)." A. Kajiwara, K. Matyjaszewski, M. Kamachi, Macromolecules 1998, 31, 5695.

26. "EPR study of the atom-transfer radical polymerization (ATRP) of (meth)acrylates." A. Kajiwara, K. Matyjaszewski, Macromol. Rapid Commun. 1998, 19, 319.

27. "Formation of Block Copolymers by Transformation of Cationic Ring-Opening Polymerization to Atom Transfer Radical Polymerization (ATRP)." A. Kajiwara, K. Matyjaszewski, Macromolecules 1998, 31, 3489.

28. "EPR and kinetic studies of atom transfer radical polymerization of (meth)acrylates." A. Kajiwara, K. Matyjaszewski, Polym. J. 1999, 31(1), 70.

29. "Electron paramagnetic resonance study of conventional and controlled radical polymerizations." A. Kajiwara, K. Matyjaszewski, M. Kamachi, ACS Symp. Ser. 2000, 768, 68.

30. "EPR Study of Atom Transfer Radical Polymerization (ATRP) of Styrene." K. Matyjaszewski, A. Kajiwara, Macromolecules 1998, 31, 548.

31. "End-Functional Poly(tert-butyl acrylate) Star Polymers by Controlled Radical Polymerization." X. Zhang, J. Xia, K. Matyjaszewski, Macromolecules 2000, 33, 2340.

32. "Utilizing Halide Exchange To Improve Control of Atom Transfer Radical Polymerization." K. Matyjaszewski, D. A. Shipp, J.-L. Wang, T. Grimaud, T. E. Patten, Macromolecules 1998, 31, 6836.

33. "Simultaneous atom transfer and nitroxide mediated controlled free radical polymerization of styrene." M. R. Korn, M. R. Gagne, Chem. Commun. 2000, 1711.

34. "Atom Transfer Radical Polymerization of Poly(ethylene glycol) Dimethacrylate." Q. Yu, F. Zeng, S. Zhu, Macromolecules 2001, 34, 1612.

35. "Determination of Activation and Deactivation Rate Constants of Model Compounds in Atom Transfer Radical Polymerization." Matyjaszewski, K.; Paik, H.-j.; Zhou, P.; Diamanti, S. J. Macromolecules 2001, 34, 5125-5131.

36. "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., and Matyjaszewsk, K., Macromolecules 2004, 37, 2679-2682.

37. "Effect of Ligand Structure on Activation Rate Constants in ATRP," Tang, W.; Matyjaszewski, K. Macromolecules 2006, 39, 4953-4959.

38. "\"Green\" Atom Transfer Radical Polymerization: From Process Design to Preparation of Well-Defined Environmentally Friendly Polymeric Materials," Tsarevsky, N. V.; Matyjaszewski, K. Chemical Reviews 2007, 107, 2270-2299.

39. "Effects of Initiator Structure on Activation Rate Constants in ATRP," Tang, W.; Matyjaszewski, K. Macromolecules 2007, 40, 1858-1863.

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