Unsymmetrical,Cyclic Diborenes and Thermal Rearrangement to a Borylborylene

Cyclic diboranes(4) based on a chelating monoanionic benzylphosphine linker were prepared through boron–silicon exchange between arylsilanes and B₂Br₄. Coordination of Lewis bases to the remaining sp² boron atom yielded unsymmetrical sp³‐sp³ diboranes, which were reduced with KC₈ to their corresponding trans‐diborenes. These compounds were studied with a combination of spectroscopic methods, X‐ray diffraction, and DFT calculations. PMe₃‐stabilized diborene 6 was found to undergo thermal rearrangement to gem‐diborene 8 . DFT calculations on 8 reveal a polar boron–boron bond, and indicate that the compound is best described as a borylborylene.     

Crosslinking Index,Molecular Weight Distribution and Rubber Equilibrium Shear Modulus During Polyfunctional Crosslinking of Existing Polymer, 6. Primary Polymer with a Schulz–Zimm Distribution of Molecular Weights

A network model for the crosslinking of already existing polymer molecules with a so‐called Schulz–Zimm distribution of their molecular weights is presented. It is an extension of previously developed statistical network models applied to the crosslinking of primary polymers with several other molecular weight distributions and with crosslinks of any functionality. The model results in the possibility to obtain more insight into the structure of polymers, especially those with narrow distributions of the molecular weight. In more detail, the model can give a perspective on structural network parameters such as the weight fractions of ideal network, of dangling polymer ends, and of those molecules not connected to the network, i. e., the sol fraction, the number of crosslinks in which a polymer molecule is bound, the functionality of the crosslinks, or the average molar mass of the polymer molecules in between the crosslinks M̄_c. Results of calculations are shown for a hypothetical crosslinking process of polymers with various molecular weight distributions. Moreover, the dependency of the network parameters on the polydispersity index and the type of molecular weight distribution is shown. Finally the increase of the functionality of the crosslinks during the ageing process of a 9.9% poly(vinyl chloride) gel as a function of the polydispersity index of the molecular weight distribution is presented.     

The enumeration of connected graphs and linked diagrams

Constructive combinatorial proofs are given for recurrence formulas which count, respectively, labeled connected graphs and linked diagrams. Previous proofs were analytical in nature. Our combinatorial proofs give rise to algorithms for selecting at random a labeled connected graph or a linked diagram.     

Use of porous biodegradable polymer implants in meniscus reconstruction. 2) Biological evaluation of porous biodegradable polymer implants in menisci

Several series of porous, biodegradable PU/PLLA foams were used for meniscus reconstruction in dogs. PLLA-fiber reinforced PU/PLLA composites, PU/PLLA, and PU foams were implanted in severe meniscus lesions. The healing process was initiated as a result of blood vessels and other cells ingrowth into these porous materials. To accomplish good ingrowth of fibrocartilaginous tissue, the materials should have interconnected macropores of 200–300n with a high total pore volume. PLLA fibers were used to reinforce the materials, but seem to retard the degradation process and the ingrowth of fibrocartilaginous tissue.     

Novel 17 substituted pregnadiene derivatives as 5 alpha-reductase inhibitors and their binding affinity for the androgen receptor

The in vitro antiandrogenic activity of four new progesterone derivatives: 4, 5, 6 and 7 (8 is a known compound) was determined. These compounds were evaluated as 5alpha-reductase inhibitors as well as by their capacity to bind to the androgen receptor in gonadectomized hamster prostate. The IC(50) value was determined using increasing concentrations of 4, 5, 6, 7 and 8 in the presence of [(3)H]T and the microsomal fraction of the hamster prostate containing the 5alpha-reductase enzyme. In this paper we also demonstrated the effect of increasing concentrations of the novel steroids upon [(3)H]DHT binding to the androgen receptors from hamster prostate which produces competition for the androgen receptor sites. The in vitro studies showed that steroids 4, 5, 6, 7 and 8 had an inhibitory activity for the 5alpha-reductase with IC(50) of: 4 (0.17 microM), 5 (0.19 microM), 6 (1 microM), 7 (4.2 microM), and 8 (2.7 microM). On the other hand, the IC(50) value for compounds 4, 5, 6, 7, 8 and DHT showed the following order of affinity for the androgen receptor: 6>7>5>DHT. Surprisingly compounds 4 and 8 did not bind to the androgen receptor. The overall data indicate that all synthesized compounds are inhibitors for the enzyme 5alpha-reductase present in the hamster prostate. In contrast, compounds 5, 6 and 7, which have a cyclohexyl group in the side chain showed a high affinity for the androgen receptor.     

In1.08Gd0.92Ge2O7: a new member of the thortveitite family

Indium gadolinium digermanium heptaoxide, In_1.08Gd_0.92Ge₂­O₇, with a thortveitite‐type structure, has been prepared as a polycrystalline powder material by a high‐temperature solid‐state reaction. As in the mineral thortveitite, the crystal structure belongs to the monoclinic system, with space group C2/m (No. 12). The precise structural parameters were obtained by applying the Rietveld method of refinement to the X‐ray powder diffraction data. This layered structure presents, on one side, a honeycomb‐like arrangement of the unique octahedral site, which is occupied randomly by In and Gd atoms, and, on the other side, sheets of isolated Ge₂O₇ diortho‐groups made up of double tetrahedra sharing a common vertex and displaying C_2h point symmetry. This compound showed a remarkable photoluminescence effect when it was irradiated with the X‐ray beam during the X‐ray diffraction measurements, and with the α beam during the Rutherford back‐scattering spectrometry experiments employed to analyze the chemical stoichiometry.