The sol-gel process, with its associated mild conditions, offers a new approach to the synthesis of composite materials with domain sizes approaching the molecular level. Transparent organic-inorganic composites can be prepared by dissolving preformed polymers into sol-gel precursor solutions, and then allowing the tetraalkyi orthosilicates to hydrolyze and condense to form glassy SiO, phases of different morphological structures. Alternatively, both the organic and inorganic phases can be simultaneously formed through the synchronous polymerization of the organic monomer and the sol-gel precursors. Depending upon such factors as the structures of the organic and inorganic components, the phase morphology, the degree of interpenetration, and the presence of covalent bonds between the phases, the properties of these composites can vary greatly and range from elastomeric rubbers to high-modulus materials.

After receiving his Bachelors and Masters degrees at Cal State Northridge, Bruce Novak obtained his Ph.D. degree in Chemistry from Caltech. He then accepted a position in the chemistry department at the University of California at Berkeley. After four years he moved to the Polymer Science Department at the University of Massachusetts, Amherst. He is currently the Howard J. Schaeffer Distinguished Professor and Head of the chemistry department at North Carolina State University.Figure 1. Orbital energy diagram showing the perturbation of the olefin π-orbital energies as a function of substituents (after ref 10).

The aryl−aryl interchange reaction of ArPdL2I complex 1m was found to follow pseudo-first-order kinetics. A marked inhibition in the presence of excess phosphine and/or excess iodide was observed, suggesting that a dissociative pathway was involved, contrary to the analogous alkyl−aryl interchange reaction studied previously. Phosphine flooding experiments could not be performed due to a competing phosphonium salt formation reaction that occurred in the presence of excess phosphine. A deuterium labeling experiment indicated that the interchange reaction proceeded via the reductive elimination to form the phosphonium salt, suggesting that excess phosphine was acting as a trap for intermediate palladium(0) species preventing the generation of the interchanged palladium(II) complex. Substituent effect studies of the interchange reaction indicated that it was inhibited by electron-withdrawing groups on both the phosphine and palladium-bound aryl groups and by increased steric bulk on both the phosphine and palladium-bound aryl groups. Under catalytic conditions, the distribution of phosphines formed from the aryl−aryl interchange during palladium-mediated cross-coupling reactions could be modeled by statistics. Various strategies for eliminating the formation of byproducts caused by the interchange during cross-coupling reactions were screened and optimized.