Diacetylene-containing polycarboranylenesiloxane copolymers were prepared by two different methods to yield either alternating or block copolymers with respect to the diacetylene distribution along the backbone. With each method, a set of copolymers was prepared with trisiloxane:carborane:diacetylene molar ratios of 10:9:1, 5:4:1, and 3:2:1. Both sets of copolymers yielded networks upon thermal curing. Even though the two methods produced materials of comparable molecular weights and overall compositions, the variation in the copolymer sequence caused noticeable differences in the calorimetric glass transitions for the copolymers in which the diacetylene group was most concentrated (i.e., the 3:2:1 and 5:4:1 copolymers). The copolymers were characterized by differential scanning calorimetry, thermogravimetric analysis, and 1 H solid-state NMR spectroscopy.

Boron carbide (B 4 C) is a ceramic with a structure composed of B 12 or B 11 C icosahedra bonded to each other and to three(C and/or B)-atom chains. Despite its excellent hardness, B 4 C fails catastrophically under shock loading, but substituting other elements into lattice sites may change and possibly improve its mechanical properties. Density functional theory calculations of elemental inclusions in the most abundant polytypes of boron carbide, B 12 -CCC, B 12 -CBC, and B 11 C p -CBC, predict that the preferential substitution site for metallic elements (Be, Mg and Al) is the center atom and that for non-metallic elements (N, P and S) it is generally the end of the three-atom chain in B 4 C's rhombohedral crystal lattice. However, Si, a semi-metal, seems to prefer the chain center in B 12 -CCC and icosahedral polar sites in both B 12 -CBC and B 11 C p -CBC. As a first step to testing the feasibility of elemental substitutions experimentally, Si atoms were incorporated into B 4 C at low temperatures (~200-400 o C) by high-energy ball-milling. High-resolution transmission electron microscopy showed that the Si atoms were uniformly dispersed in the product, and the magnitude of the lattice expansion and Rietveld analysis of the X-ray diffraction data were analyzed to determine the likely sites of Si substitution in B 4 C. Further corroborative evidence was obtained from electron spin resonance spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy characterizations of the samples. Thus, a simple, top-down approach to manipulating the chemistry of B 4 C is presented with potential for generating materials with tailored properties for a broad range of applications.

Oxidative addition reactions of halogens, hydrogen chloride, and methyl iodide with the dinuclear bridging amido complex Irz[p-NH(p-tolyl)l z(C0)4 are described. For Clz, Br2, MeI, and HC1, addition of 1 equiv produces Ir(1)-Ir(II1) dimers, whereas 1 equiv of IZ produces an Ir(I1)-Ir(I1) dimer. Except for the Me1 addition reaction, all of the oxidative addition processes proceed to give Ir(II1)-Ir(II1) dimers as final products. The characterization of the Me1 addition product was performed by an X-ray analysis, showing two iridium centers, with pseudooctahedral and pseudo-square-planar environments, that are separated by 3.209(1) A. Crystal data: CuH19-IIrzNz0~0.25CsH1~; triclinic Pi, a = 12.568 (3) A, b = 12.801(3) A, c = 17.248(3) A, a = 69.82(3)', 0 = 85.45(3)', y = 74.12(3)', V = 2504.8(9) A3, 2 = 4, R(F) = 0.046, and R(wF) = 0.054 for 5389 (F > 5a(F)) reflections.

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