Viscoelastic properties of a silicone resin during crosslinking

Viscoelastic properties of a silicone resin crosslinked at various extents were characterised by means of rheology. The influence of temperature on the viscoelastic properties of the material as-delivered and in a state pre-crosslinked approximately to the gel point has been investigated by dynamic-mechanical measurements. While the glass transition temperature is increased by the crosslinking, no changes of the free volume fraction at T _g and its thermal expansion coefficient were observed. Taking the different glass transition temperatures into account, it could be shown that the corresponding WLF-parameters are the same. The molar mass and, hence, the viscosity of the material can be increased by a heat treatment. The dependence of the zero shear-rate viscosity on the weight average molar mass indicates that the existence of entanglements of the polymer molecules is not very probable.     

SYNTHESIS AND NMR SPECTROSCOPIC INVESTIGATION OF PHENYLPHOSPHORYL DERIVATIVES

Abstract

The syntheses of 16 phenyl- (1 and 3) and phenylthiophosphoryl (2) derivatives are described. The ¹³C and ¹⁷O NMR data sensitively reflect electron density changes in the phosphoryl substituents caused by p^π–d^π back-donation from X/Y to P. Taft constants σ₁ and σ_R ⁰ were derived for 16 phosphoryl substituents.     

Chiral Atropisomeric 2-Iodo-4,4′,6,6′-tetramethyl-2′-(diphenylphosphoryl)-1,1′-biphenyl-enantiodifferentiation by Rh2[MTPA]4-Adduct Formation and Conformational Analysis

Enantiodifferentiation of the chiral 2-iodo-4,4′,6,6′-tetramethyl-2′-(diphenylphosphoryl)-1,1′-biphenyl (2) can be accomplished easily by adding one mole equivalent of the enantiopure dirhodium complex Rh ^(II) ₂ [(R)-(+)-MTPA]₄ (Rh*). The internal competition of the two bindings sites in 2, the Ph ₂ P═O group, and the iodine atom was identified by variable-temperature ³¹ P NMR. Energy optimization (PM3) and ROESY spectroscopy of 2 in the absence and presence of Rh* reveal that 2 prefers a conformation in the adducts, which is the least stable one in the free molecule, i.e., adduct formation is accompanied by a rotation of the Ph ₂ P═O group about the C-2′─P bond.     

The Phenylphosphinidene C6H5P is Stable Under Unimolecular Conditions - A Theoretical Prediction

Abstract

As members of the family of low-coordinated phosphorus compounds, the phosphinidenes (RP) containing subvalent phosphorus (σ¹,λ¹-P) are of current interest. Until now, no Organophosphiaidene RP (R=Alkyl, Aryl) is known to be stable in the condensed phase. As the results of trapping experiments are questioned, the formation of RP as intermediates is still doubtful [1]. The mass spectrometric decay of some organophosphorus compounds yields radical cations [C₆,H₅,P]^+? m/z 108. For these species structures 1 and 2 are conceivable: