Organometallic Reactivity of Metal-Centered Radicals

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This award in the Inorganic, Bioinorganic, and Organometallic Chemistry program supports research on the mechanism of metal-promoted olefin polymerization by Dr. Bradford B. Wayland, Chemistry Department, University of Pennsylvania. Three goals will be pursued. First, organometallic species (M-R) will be formed in equilibrium with metal-centered and organic radicals to enable a direct method for evaluating thermodynamic parameters for organo-metal bond homolysis. Second, the details of chain transfer catalysis (CTC), which controls polymer molecular weight and introduces end group alkene functionality, will be studied. A series of persistent metalloradical complexes with minimum ligand steric demands and varying M-H bond enthalpies will be designed for application in CTC for different classes of olefin monomers. Factors responsible for the effectiveness of metallo-radical species in fast hydrogen radical abstraction and rapid addition of M-H to olefins will be determined. Finally, the role of organometallic complexes in initiating and controlling living radical polymerization (LRP) of olefins will be investigated. LRP is an approach to producing low polydispersity homopolymers and block copolymers of olefins that requires radical initiation. Organocobalt tetramesityl porphyrin complexes initiate and control LRP of acrylates and provide a prototype for establishing guideline criteria for the design of metal complexes that promote LRP processes. Ligand and polymeric radical steric demands will be used to prevent metal-centered radicals and polymeric radicals from achieving the transition state for beta-hydrogen abstraction and the persistent radical effect of metal-centered radicals is expected to suppress bimolecular termination of polymeric radicals. Several series of persistent metalloradicals will be tested for effectiveness in the synthesis of different classes of olefins. A series of water soluble and polymer supported catalysts will be synthesized to address environmental issues and the practical considerations of product separation and catalyst recovery.Polyolefins are important commercial products. This research will develop a rational approach to the synthesis of various polyolefins and potentially result in new structurally defined block copolymers with unexplored material properties. Students involved in the study will learn not only sophisticated methods involving inorganic and organic chemistry, but will be introduced to major issues and challenges in energy-related research, polymer chemistry, catalysis and materials science.