Thermal degradation of poly(methyl-n-hexylsilylene)

Thermal degradation of poly(methyl-n-hexylsilane) in the solid state in absence of oxygen reveals formation of a cyclic pentamer between 150 and 250°C. Polymer is gradually degraded to an intermediate molecular weight distribution. The weight average of this new distribution is not only temperature-dependent, but is also a function of viscosity of the polymer and nature of chain ends. As no insolubles or Si? H groups are formed, the degradation mechanism is most likely a back-biting mechanism induced by active chain ends such as silyl anions or Si? Cl rather than a homolytic cleavage of the main chain. A concurrent intramolecular rearrangement reaction is also proposed. Moreover, this study proposes an explanation to the trimodal molecular weight distribution obtained by the Wurtz coupling of dichlorosilanes with molten sodium in refluxing toluene. © 1995 John Wiley & Sons, Inc.     

ORGANISCHE PHOTOCHEMIE XXXII1,2

Abstract

Es werden drei neue Beispiele zu Photoreaktionen von Aroyl-diphenylphosphinen beschrieben, die sowohl unter α-Spaltung als auch 1.2-Sauerstoff-Transfer aus der Carbonylgruppe auf den Phosphor verlaufen. Beide Reaktionen finden auch im Massenspektrometer statt.

Three new examples to photoreactions of aroyl diphenyl phosphines which undergo α-cleavage as well as 1.2-oxygen transfer from the carbonyl group onto the phosphorous atom are described. Both reactions also occur in the mass spectrometer.     

Heteroatom Polysilylenes

Heteroatom polysilylenes with Cl, OR and NR₂ substituents attached to the silicon chain through the heteroatom are described. The relationship between the single crystal X-ray structures of octachlorocyclotetrasilane Si₄Cl₈ and perchloropolysilylene, (SiCl₂)_n, is discussed with reference to possible mechanisms for its phototopochemical synthesis from Si₄Cl₈. Substitution with alcohols yields the bis(alkoxy)polysilylenes, and the origin of the red-shifted UV spectra are discussed in terms of the electronic interaction between oxygen n and silicon orbitals. Substitution with secondary amines yields soluble, yellow polymers with even longer wavelength UV absorption maxima. Effects of the alkyl chain length on hydrolytic stability are considered.     

KINETIC STUDIES ON THE ADDITION OF p-TOLUENESULFINIC ACID TO BENZYLIDENEACETOPHENONES

Abstract

Kinetics of the addition of p-toluenesulfinic acid to p-substituted benzylideneacetophenones has been investigated. The reaction was second order, first order each in the concentration of sulfinic acid and chalcone. Effects of the p-substituents on the reaction rate were small (rho = 0.4). The reaction was strongly acid catalyzed.     

Synthesis of siloxane‐crosslinked polysilylenemethylenes by chlorodephenylation

Thermally stable polysilylenemethylenes (PSMs) with siloxane crosslinking moieties were successfully synthesized by chlorodephenylation of preformed poly(methylphenylsilylenemethylene) (PMPSM) and subsequent in situ alcoholysis/hydrolysis/condensation reactions. The simplified process and mild reaction conditions are quite advantageous. The crosslink density of these materials can be adjusted by the degree of chlorodephenylation, although an alkoxysilyl group remains to some extent. The resulting crosslinked PSMs have well defined structures in which the backbone is composed of MePhSiCH₂ and Me(MeO)SiCH₂ as well as Me(O_1/2)SiCH₂ as a crosslinking moiety. The resulting crosslinked PSMs exhibited glass‐transition temperatures ranging from 15 to 20 °C, whereas that of linear PMPSM was 22 °C. The crosslinked PSMs remained unchanged in weight below 300 °C, suggesting that they are thermally stable up to that temperature. The good solvent resistance caused by crosslinking as well as high thermal stability of these materials allow us to design new PSM‐based polymer blends and preceramic polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 416–422, 2002     

Efficient ecofriendly synthesis of pyrazole acryloyl analogs by amino acid catalysis

An easy and effective green approach of Knoevenagel condensation and arylidene formation with the substrates involves pyrazole-3-carbaldehydes and active methylenes under catalytic action of glycine (the simplest amino acid) in dimethylsulfoxide. These reactions were successfully carried out at room temperature.     

Oxidation of benzylic methylenes to ketones with Oxone-KBr in aqueous acetonitrile under transition metal free conditions

A green and highly efficient protocol for the oxidation of benzylic methylenes to their corresponding ketones with a combination of Oxone and KBr in aqueous acetonitrile is developed. The H₂¹⁸O labeling experiment demonstrated that the oxygen introduced into ketone originated from water. A plausible mechanism was also suggested.     

Synthesis of novel polysilylenes bearing phenol groups directly bonded to Si?Si main chains for preparing Langmuir–blodgett thin films

Polysilylenes with a phenol group directly bonded to the main chain Si atom were prepared by polymerizing dichlorosilane monomers bearing phenol groups protected by t-butyldimethylsilyl ether, followed by deprotecting the silyl ether. These amphiphilic polysilanes were applied to provide oriented thin films by the Langmuir-Blodgett technique. Stable monolayers were not obtained for polysilylenes having only alkyl and aryl substituents. However, all the polysilylenes with phenol moieties provided monolayers on a water surface. These polysilylene monolayers were transferred to hydrophobic substrates by applying the Langmuir-Blodgett technique. Among these polysilylenes, only poly(n-butyl-3-hydroxyphenylsilylene) provided multilayers in which the Si? Si main chains oriented in the dipping direction. The orientation was determined by polarized UV absorption. The orientation ratio reached 0.45. © 1993 John Wiley & Sons, Inc.     

Degradation of poly(methylphenylsilylene) and poly(di-n-hexylsilylene)

Degradation of poly(methylphenylsiylene) and poly(di-n-hexylsilylene) was studied by chemical and mechanical methods at ambient and higher temperatures. Purely thermal degradation in solid state starts as a slow process at 150°C and provides soluble and insoluble products which include cyclosilanes as well as various siloxanes. Sonication at ambient temperatures leads to the mechanical degradation of high molecular weight polymers by homolytic cleavage induced by shear forces. No cyclics are formed under these conditions. Polysilanes in the presence of strong nucleophiles degrade exclusively to cyclic oligomers. Rate of this back-biting chain reaction depends on substituents at silicon atom, alkali metal, solvents, and temperature. Electrophiles degrade polysilanes to various α,ω-difunctional oligosilanes. © 1993 John Wiley & Sons, Inc.     

A theoretical study of electronic effects in alkyne‐linked reactive intermediates: (nitrenoethynyl)methylenes, (nitrenoethynyl)silylenes,and (nitrenoethynyl)germylenes

Acetylene‐linked reactive intermediates of (nitrenoethynyl)‐X‐methylenes, (nitrenoethynyl)‐X‐silylenes, and (nitrenoethynyl)‐X‐germylenes are almost experimentally unreachable (X–M–C≡C–N; X=H ( 1 ), CN ( 2 ), OH ( 3 ), NH₂ ( 4 ), NO₂ ( 5 ), and CHO ( 6 ); M=C, Si, and Ge). The effects of the electron‐donating and electron withdrawing groups were compared and contrasted at seven levels of theory. All singlet species as ground states with one local open‐shell singlet carbene subunit (π¹π¹) and another local open‐shell singlet nitrene subunit (π¹π¹) were found to be more stable than their corresponding triplets including one local open‐shell singlet carbene (δ¹π¹) (or one local closed‐shell singlet carbene [δ²π⁰]) and another local triplet nitrene subunit (π¹π¹) with 45.94–77.996 kcal/mol singlet–triplet energy gap (ΔE_s‐t). Their relative silylenes and germylenes made reduction of ΔE_s‐t, so the triplet ground states were found for species 3 _Si , 4 _Si , 5 _Si , 2 _Ge , 3 _Ge , 4 _{Ge ,} and 5 _Ge . All the singlet silylenes/germylenes formed by one local closed‐shell singlet silylenes/germylenes (δ²π⁰) and one local closed‐shell singlet nitrene subunit (π²π⁰). Also, one local closed‐shell singlet silylene/germylene subunit (δ²π⁰) and one local triplet nitrene subunit (π¹π¹) were observed for triplet silylenes/germylenes. The singlet and triplet species 3 _Si , 4 _Si , 3 _Ge , and 4 _Ge , due to their electrophilic (Si₄/Ge₄) and nucleophilic (X₅) centers, could be identified as intermediates in chemical reactions.