Chapter 5
Downloaded by NORTH CAROLINA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch005
Factors Determining the Performance of Copper -Based Atom Transfer Radical Polymerization Catalysts and Criteria for Rational Catalyst Selection Nicolay V. Tsarevsky, Wei Tang, Samuel J. Brooks, and Krzysztof Matyjaszewski Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213
The performance of copper-based ATRP catalysts can be predicted based on the stability constants of the Cu and Cu complexes with the ligand L (β and β , respectively). Both β and β should be large in order to prevent catalyst deactivation through competitive coordination of monomer and/or polymer. A high β /β ratio is required for high catalytic activity. Catalysts for which the Cu complex is more halogenophilic are more active, and provide better polymerization control due to the less pronounced deactivator dissociation. If ATRP is carried out in aqueous media, in addition to the above requirements, the ratio β /(β ) [L] should be low to prevent disproportionation of the Cu complex. Acidic monomers may be polymerized if the ligands meet all outlined requirements and are also as weakly basic as possible. II
II
I
I
II
I
II
I
II
II
I
2
I
56
© 2006 American Chemical Society
In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
57
Downloaded by NORTH CAROLINA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch005
Introduction In the past decade, atom transfer radical polymerization (ATRP)(l-3) has surfaced as one of the most powerful synthetic techniques in polymer science. It allows for the synthesis of well-defined polymeric materials with predetermined molecular weight and composition, and narrow molecular weight distribution derived from substituted styrenes, (meth)acrylates, acrylonitrile, or acrylamides. The use of multifunctional initiators and inimers makes it possible to control the molecular architecture of the polymers and many examples of star, brush, and hyperbranched copolymers prepared by ATRP have been reported.(2,4) The polymers synthesized by ATRP are halide-terminated and can participate in numerous reactions with nucleophiles. In addition, the use of functional initiators allows for the controlled synthesis of homo- or heterotelechelic macromolecules.(5) Post-polymerization modifications of either the end group(s) or the monomer units is another strategy to prepare functional polymers. The development of ATRP catalysts that can be used in ppm quantities while maintaining a satisfactory control over polymerization and sufficiently high reaction rates is a challenge that needs to be addressed since it is of major importance for making ATRP an environmentally benign process.(6) ATRP is a metal complex-catalyzed process, and understanding the role the catalyst plays and describing the side reactions in which it can participate, are crucially important for improving the control over the polymerization of "difficult" monomers such as unsaturated acids, amides, and vinyl esters. In addition, carrying out well-controlled polymerizations in environmentallyfriendlyreaction media such as water, alcohols, or supercritical carbon dioxide is highly desirable. To accomplish this, the interaction of the metal complex with various solvents that can alter its activity should be understood. A recent extensive review deals with the structural characterization of ATRP catalysts.(7) Successful attempts to correlate the activity of various ATRP catalysts with the nature of both the central metal and the ligands have been made.(8) Herein, the basic criteria for rational selection of appropriate ligands for copper-based ATRP catalysts are outlined.
Evaluation of ATRP Catalyst Performance The ATRP Equilibrium and its Components The copper-mediated atom transfer process is presented schematically in Figure 1; the equilibrium constant K = k ^ t / k ^ is a product of four simpler equilibrium constants.(8) A T R P
In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
58 kact R-X + Cu'L
-
n
R' + XCu'În
Contributing Reactions R-X x
.
R* • X*
χ©
+
Cu'L
Cu»L • e®
n
n
