List of Faculty
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Cassandra
L. FraserPolymeric Metal Complexes. Metallo-biomaterials for Medicine and Biotechnology. Bio-inspired and Sustainable Design. Responsive Nanoscale Assemblies.
Our research program is concerned with the synthesis, properties and applications of metal complexes with polymeric ligands. These multifunctional targets are prepared by combining coordination chemistry with living polymerization methodologies. Like metalloproteins, polymeric metal complexes feature site-isolated metal centers with responsive properties, and offer many opportunities for modification; they can form higher order assemblies, and function as soluble agents, films and coatings, or bulk materials. Along with fundamental studies in polymer synthesis and properties, we are also exploring uses for these materials in medicine, biotechnology, and photonics. Of particular interest are biocompatible and degradable polymers for drug delivery, biomaterials, molecular imaging, and sustainable design.
We approach this bio-inspired design project in a systematic way, working from the core of polymeric metal complexes outward. First, we develop efficient approaches to functionalized ligands and their metal complexes. Initially, our efforts concentrated on bipyridine systems; however, more recently, the polymeric metal complex platform has also been expanded to include beta-diketonates such as dibenzoylmethane. This common ligand for metals also serves as a UV absorber in sunscreens and has exhibited cancer preventative and therapeutic properties in model systems. Next, we test the compatibility of these reagents as initiators with various living polymerizations. The scope of cationic, radical, anionic, and other controlled methods are probed through investigations of reaction kinetics. Divergent metalloinitiation, and convergent chelation and coupling methods are explored. With an ever-expanding "toolkit" of macroligands available, diverse transition metal and lanthanide materials are easily accessed using the modular template approach. Suitable derivatization of chain ends, side chains or ancillary ligands allows network formation, attachment to surfaces, chemical and biological recognition, catalytic activity, and other modes of use.
The extension of coordination chemistry to "well-defined" polymeric ligands and embedding chromophores in discrete macromolecular environments are areas that are rich for exploration. In addition to the new synthetic methodologies that are emerging, some fascinating and even unexpected materials properties have also been observed when metals are embedded in well-defined macromolecular environments. Polymeric metal complexes exhibit interesting responses to heat, light, reagents and other stimuli in solution and as films, which makes them attractive for sensing and imaging applications. Certain iron polymers are reversibly thermochromic, and akin to biomineralization processes in nature, hierarchically structured block copolymers function as both iron delivery systems and nanoscale templates for controlled metal cluster formation upon annealing. Coupling triggered changes at the metal center with structural transformations in ordered films in this way introduces unprecedented dynamic features into nanoscale materials. Exciting recent findings include the discovery that multifunctional iron dibenzoylmethane initiators also serve as catalysts for lactide ring opening polymerization, and entirely unexpected air sensitivity is noted for iron tris(bipyridine) complexes in poly(ethylene glycol) environments with radical processes implicated. The biological activity of these materials is explored with collaborators in the UVA Cancer Center, whereas block copolymer assemblies are investigated through collaboration with materials scientists at MIT. Because the properties and stabilities of metal complexes are highly tunable through variation of the metal ion, the ligand set, and other molecular parameters, this makes the coordinate bond a remarkably versatile platform for macromolecular assembly.
Representative Publications:
“Synthesis and Unexpected Reactivity of Iron Tris(bipyridine) Complexes with Poly(ethylene glycol) Macroligands” Pfister, A.; Fraser, C. L. Biomacromolecules 2005, submitted.
“Iron Tris(dibenzoylmethane)-centered Polylactide Stars: Multiple Roles for the Metal Complex in Lactide Ring Opening Polymerization” Gorczynski, J. L.; Chen, J.; Fraser, C. L. J. Am. Chem. Soc. 2005, submitted.
“Poly(epsilon-caprolactone) Macroligands with Beta-diketonate Binding Sites: Synthesis and Coordination Chemistry” Bender, J. L.; Shen, Q.-D.; Fraser, C. L. Tetrahedron 2004, 60, 7277-85.
“Iron Tris(bipyridine)-Centered Star Block Copolymers: Chelation of Triblock Macroligands Generated by ROP and ATRP” Johnson, R. M.; Fraser, C. L. Macromolecules 2004, 37, 2718-27.
“Metalloinitiation Routes to Biocompatible Poly(lactic acid) and Poly(acrylic acid) Stars with Luminescent Ruthenium Tris(bipyridine) Cores” Johnson, R. M.; Fraser, C. L. Biomacromolecules 2004, 5, 580-8.
"Bipyridines" Fraser, C. L.; Smith, A. P; Comprehensive Coordination Chemistry II; Meyer, T. J.; McCleverty, J. A., Eds; Elsevier Ltd.: Oxford, UK, 2004; Vol. 1, Ch. 1, pp. 1-23.
“Synthesis of Bipyridine-Centered Diblock Copolymers” Smith, A. P.; Fraser, C. L. Macromolecules 2003, 36, 2654-60.
“Ruthenium-Centered Heteroarm Stars by a Modular Coordination Approach: The Effect of Polymer Composition on Rates of Chelation” Smith, A. P.; Fraser, C. L. Macromolecules 2003, 36, 5520-5.
“Site-Isolated Luminescent Europium Complexes with Polyester Macroligands: Metal-Centered Heteroarm Stars and Nanoscale Assembles with Labile Block Junctions” Bender, J. L.; Corbin, P. S.; Fraser, C. L.; Metcalf, D. H.; Richardson, F. S.; Thomas, E. L.; Urbas, A. M. J. Am. Chem. Soc. 2002, 124, 8526-7.
"Iron Cluster and Microstructure Formation in Metal-Centered Star Block Copolymers: Metal-Centered Iron Tris(bipyridine)-Centered Polyoxazolines" Park, C.; McAlvin, J. E.; Fraser, C. L.; Thomas, E. L. Chem. Mater. 2002, 14, 1225-30.
“Biocompatible Polyester Macroligands: New Subunits for the Assembly of Star-Shaped Polymers with Luminescent and Cleavable Metal Cores” Corbin, P. S.; Webb, M. P.; McAlvin, J. E.; Fraser, C. L. Biomacromolecules 2001, 2, 223-32.
“Metal Complexes with Polymeric Ligands: Chelation and Metalloinitiation Approaches to Metal Tris(bipyridine)-Containing Materials” Fraser, C. L.; Smith, A. P. J. Polym. Sci. Part A Polym. Chem. 2000, 38, 4704-16. (Review)
“Metal Template-Assisted Block Copolymer Synthesis: Use of Solvent Polarity to Control Chain Conformation and Reactivity at the Metal Core” Fraser, C. L.; Smith, A. P.; Wu, X. J. Am. Chem. Soc. 2000, 122, 9026-7.
“Synthesis of Thermochromic Iron(II) Tris(bipyridine)-Centered Star-Shaped Polyoxazolines and Their Bipyridine-centered Macroligand Counterparts” McAlvin, J. E.; Scott, S. B.; Fraser, C. L. Macromolecules 2000, 33, 6953-64.
“Architectural Diversity Via Metal Template-Assisted Polymer Synthesis: A Macroligand Chelation Approach to Linear and Star-Shaped Polymeric Ruthenium Tris(bipyridine) Complexes” Wu, X.; Fraser, C. L. Macromolecules 2000, 33, 4053-60.