Organometallic Reactivity of Metal-Centered Radicals

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This project, funded by the Inorganic, Bioinorganic and Organometallic Chemistry program, will support Professor Bradford Wayland of the University of Pennsylvania in his continued studies of metal-centered radicals and their reactions with organic radicals, particularly in the context of olefin polymerization. Recent discoveries of new methods for establishing quasi-living polymerization in free-radical polymerization of olefins will be explored. This is accomplished through the use of metal-centered radicals to trap the growing end of the polymer. The key feature to the success of this strategy is the strength of the metal-carbon bond that is formed, which must be sufficiently weak that the reaction is rapidly reversible to allow for facile chain growth. Living radical polymerization will provide a new approach for producing low polydispersity homopolymers and block copolymers of olefins that can only be polymerized by radical initiation. The initial research will center on the use of alkyl cobalt tetramesitylporphyrin complexes to initiate and control living radical polymerization of acrylates and to provide a prototype for establishing guideline criteria for the design of metal complexes that promote similar living polymerization processes. Experiments are outlined to determine the metal-carbon bond strengths of a variety of potentially useful alkyl derivatives. The properties of polymers not only depend upon their chemical structures but also upon their molecular weight. Until recently it has been difficult to control the molecular weight and especially the dispersion of molecular weights of the individual molecules of polymers of certain types of monomers. This research explores a newly discovered way, which involves the generation of `living` polymer chains, to control the molecular weight including dispersion. The term `living` means that the individual polymer molecules are capable of adding more monomer units if they are added to the system. This allows the use of block polymerization strategies in which a second type of monomer is added to the living polymer so that a region of a new type of polymer is added to the original chain. By alternating additions of two (or more) types of monomers, polymers with new material properties that are not characteristic of those of either monomer can be prepared.