The structure of a poly(dimethylsiloxane) (PDMS) layer at the surface of hydrophilic silica has been studied by means of DSC, proton NMR T2 relaxation experiments, and 1 H and 29 Si NMR spectroscopy. The samples were prepared using (1) the adsorption of PDMS from a PDMS solution onto the silica surface followed by a thermal treatment and (2) mechanical mixing of PDMS with the silica followed by the separation of bound rubber. It was shown that these procedures caused the formation of chain loops chemically attached to the silica surface at both chain ends. The average length of the grafted chains varied from about four to eight Si-O bonds. The experiments provided information on the mobility of the grafted chains, which is related to the structure of the grafted layer. The grafted PDMS layer was found to consist of immobilized chain segments at the PDMS-silica interface and mobile chain portions outside the interface. The chain immobilization at the interface caused a substantial decrease in the heat capacity at Tg and suppressed crystallinity of the grafted PDMS. The interface fraction increased proportionally with a decreasing chain length. About four dimethylsiloxane pendant chain units next to the grafting site were immobilized due to chain anchoring to the silica surface. A small fraction of -SiO(CH3)2-chain units was immobilized as a result of physical adsorption at the silica surface. The fraction of physically adsorbed chain units appears to be proportional to the number of residual silanol groups at the silica surface. The mobility of the chain portions outside the interface was found to differ significantly in the various samples studied and to increase with an increasing average length of the grafted chains. The NMR method allowed us to make a distinction between a dense "brushlike" structure of the grafted layer containing grafted chains of a fairly uniform length and a layer containing a significant fraction of long chain loops outside the densely grafted layer. It was found that the structure of the grafted layer is to a great extent dependent on the grafting procedure employed.

Increasing the mechanical stability of artificial polymer materials is an important task in materials science, and for this a profound knowledge of the critical mechanoelastic properties of its constituents is vital. Here, we use AFM-based single-molecule force spectroscopy measurements to characterize the rupture of a single silicon-oxygen bond in the backbone of polydimethylsiloxane as well as the force-extension behavior of this polymer. PDMS is not only a polymer used in a large variety of products but also an important model system for highly flexible polymers. In our experiments, we probe the entire relevant force range from low forces dominated by entropy up to the rupture of the covalent Si-O bonds in the polymer backbone at high forces. The resulting rupture-force histograms are investigated with microscopic models of bond rupture under load and are compared to density functional theory calculations to characterize the free-energy landscape of the Si-O bond in the polymer backbone.

Density functional theory (DFT) was used to explore the different mechanistic possibilities for the hydrosilylation reaction between methyldimethoxysilane and methylvinyldimethoxysilane catalyzed by the Ru(II) complex dicarbonyldichlorobis(triphenylphosphine)ruthenium(II) (A1). Reaction enthalpy profiles of the Chalk-Harrod, modified Chalk-Harrod, and σ-bond metathesis mechanisms were computed for several different active forms of A1. A total of 10 different pathways with different catalytic cycles and different induction steps were compared. We predict that a σ-bond metathesis mechanism involving the formation of a hydride analogue of A1 is most favored, in contrast to the commonly accepted Chalk-Harrod mechanism of hydrosilylation. The B3LYP-calculated activation energy within the catalytic cycle (∆H act ) 21.8 kcal/mol) is small enough to make A1 a reasonable catalyst for this reaction under the normally applied experimental conditions.

We present first principles molecular dynamics simulations of stretched siloxane oligomers in an environment representative of that present in single molecule atomic force microscopy experiments. We determine that the solvent used (hexamethyldisiloxane) does not influence the stretching of the siloxane in the high force regime or the rupture process, but trace amounts of water can induce rupture before the maximum siloxane extension has been attained. This would result in a significantly lower rupture force. The simulations show that the rupture of a covalent bond through a reaction with a molecule from the environment, which would not normally occur between the species when the polymer is not stressed, is possible, opening a route to mechanically induced chemical reactions. The attack of the normally hydrophobic siloxane by water when it is stretched has wider implications for the material failure under tensile stress, where trace amounts of water could induce tearing of the material.

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DMF (20 mL). To the solution thus obtained was added dropwise over 10 min sulfur monochloride (0.040 mL, 0.50 mmol) in dry DMF (5.0 mL) under nitrogen at ±50 C. While being stirred overnight the reaction mixture was allowed to reach room temperature, and the resultant brown precipitate was filtered and washed with methanol. The crude product was purified by column chromatography (silica gel, carbon disulfide) and recrystallized from carbon disulfide/hexane (1:2, v/v) to afford 64.0 mg (0.141 mmol) of airstable black plate-like crystals in 27 % yield. mp (decomp.) 183.0±183.6 C. IR (KBr) [cm ±1 ] n = 2921, 2853, 1508, 1456, 959, 915, 768, 511, 468. 1 H NMR (CS 2 /CDCl 3 ) (5:1, v/v) [ppm] d = 3.29 (m, 4H). Anal. Calcd. for C 8 H 4 S 11 : C 21.25, H 0.92, S 78.19; found, C 21.22, H 0.89, S 77.89.Crystallographic Data Collection and Structure Determination: X-ray crystal structure analyses were performed for 4 and its ClO 4 ± salt. Their crystal and experimental data are shown in Table 1. Data collection and refinement conditions: Rigaku AFC-5R diffractometer, 296 K, o±2y mode up to 2y = 60, o scan speed 8.0 deg min ±1 , graphite monochromated Mo Ka radiation (l = 0.71069 ). No decay correction was applied. The structure was solved by a direct method (SHELXS86) [12]. The atomic scattering factors were taken from the International Tables for X-ray Crystallography [13]. Full-matrix least-squares refinements with all non-hydrogen atoms anisotropic were carried out for 4 and 4×ClO 4 ×(chlorobenzene) 0.5 except for the solvent molecule of the perchlorate salt, which was refined isotropically. Hydrogen atoms were included, but their positions were not refined.The potential application of monodisperse metals and semiconductor colloids in such fields as nonlinear optics, photovoltaic cells, catalysis and new laser technologies has led to research into the preparation and stabilization of nanosized particles. [1±8] The classical routes comprise

The response of a silicone polymer fragment to external stresses is considered in terms of a mechanochemical reaction. The quantum chemical realization of the approach is based on a coordinate-of-reaction concept for the purpose of introducing a mechanochemical internal coordinate (MIC) that specifies a deformational mode. The related force of response is calculated as the energy gradient along the MIC, while the atomic configuration is optimized over all of the other coordinates under the MIC constant-pitch elongation. The approach is applied to a set of linear silicone oligomers Si n with n = 4, 5, and 10 subjected to uniaxial tension, followed by the molecule breaking and a postfracture relaxation. Three stages of deformation, differing by structural transformation, have been detected. The observed peculiarities of the oligomer mechanical behavior are well attributed to the characteristic modes of vibrational spectra. The oligomer strength and the related Young's moduli are obtained. A cooperative radical-driven mechanism of silicone polymer fracture is suggested.

Nitrile addieren sich an die Azidliganden der planaren Komplexe cis-(R,P),Pt(N,), und trans-(Ph,P),Ir(CO)N, unter Bildung von 5-R-Tetrazolato-Verbindungen 1 -3. Kinetische Untersuchungen zeigen, daR die Geschwindigkeit der [2 + 31-Cycloadditionen rnit groRerem Donorvermogen der Phosphan-Liganden und mit elektronenarmen Nitrilen zunimmt. Der Mechanismus dieser unter milden Bedingungen verlaufenden Reaktionen wird diskutiert. Aus 3 und (Ph,P),Pd-(5-R-tetra~olat)~ werden mit HCI bzw. Acylchloriden die freien 5-R-Tetrazole bzw. disubstituierte Tetrazole erhalten. (Ph,P),Pd(N,), setzt sich mit Acetylendicarbonsaure-dimethylester zu den Triazolato-Komplexen 5 und 6 um. Mit organischen Isothiocyanaten und Thiocyanaten bilden sich Tetrazolinthionato (7)-bzw. 5-(Methylthio)tetrazolato-Komplexe (8). 'H-NMR-spektroskopisch lassen sich Bindungsisomere mit den ambivalenten heterocyclischen Liganden nachweisen. Cycloaddition Reactions of Azide Ligands in Phosphane Complexes of Palladium(II), Platinum(II), and Iridium(1)Nitriles are added to the azide ligands of the planar compounds cis-(R,P),Pt(N,), and trans-(Ph,P),Ir(CO)N, to give the 5-R-tetrazolato complexes 1 -3. Kinetic studies show that the rate of the [2 + 31-cycloaddition increases with donor strength of the phosphane ligands and with electron-poor nitriles. The mechanism of these reactions is discussed. Using HCI or acyl chlorides, from 3 and (Ph,P),Pd(S-R-tetra~olate)~ the free 5-R-tetrazoles or disubstituted tetrazoles are obtained under mild conditions. (Ph,P),Pd(N,), reacts with MeO,CC= CC0,Me to give the triazolato complexes 5, 6. Organic isothiocyanates and thiocyanates yield tetrazolinethionato (7) and 5-(methylthio)tetrazolato complexes (8). Using 'H NMR spectroscopy isomeric complexes with these ambidentate heterocyclic ligands can be detected.Wir konnten vor einigen Jahren zeigen, daR die Azidgruppe speziell in planaren Palladium(I1)-Komplexen 2, zahlreiche Reaktionen unter erstaunlich milden Bedingungen eingeht. So lassen sich Mehrfachbindungssysteme (z. B. Nitrile3) und Isocyanide4)) -in einer 1,3-dipolaren Cycloaddition nach Huisgen -an den Azidliganden addieren. Cycloadditions-Reaktionen wurden inzwischen mit verschiedenen Azido-Komplexen beschrieben 6 ) . In Fortfuhrung unserer Arbeiten iiber Reaktionen an der koordinierten Azidgruppe wird im folgenden iiber Umsetzungen von planaren phosphanhaltigen Azidopalladium(II), -platin(II)-und -iridium(I)-Verbindungen mit Alkinen, Nitrilen und verschiedenen Heterocumulenen berichtet'). Die Palladium(I1)-Komplexe des Typs (R,P),Pd(N3), besitzen im festen Zustand in Abhangigkeit vom Phosphan-Liganden trans-*) oder cis-Struktur9); in Losung liegen die cis-und trans-Isomeren im Gleichgewicht@ vor, das von Ne/-0 Verlag Chemie GmbH, D