Robert Lehnert

Organofunctional alkoxysilanes in dilute aqueous solution: new accounts on the dynamic structural mutability

Abstract The condensation of alkyltrialkoxysilanes was studied using time-dependent 1 H- and 29 Si-NMR spectroscopy. The main goal was to correlate the results of these investigations with standard conditions in technical applications of the most commonly used commercial silane adhesion promoters. Thus, experiments with 3-aminopropyltriethoxysilane ( DYNASYLAN ® AMEO), 3-glycidyloxypropyltrimethoxysilane (DYNASYLAN® GLYMO) and 3-methacryloxypropyltrimethoxysilane (DYNASYLAN® MEMO) were carried out in aqueous solutions at a selected range of concentrations. This investigation reflects a convenient way to determine the total concentration of reactive silanol groups during the course of the reaction. Moreover, the activity of the solution in adhesion promoting processes can be monitored and measured.

NMR-spektroskopische Untersuchungen zur Hydrolyse von funktionellen Trialkoxysilanen/ NMR-Spectroscopic Investigations on the Hydrolysis of Functional Trialkoxysilanes

The hydrolysis of functional trialkoxysilanes was studied under various conditions by 1H NMR spectroscopy. The silane concentration was 1 - 2% by weight in a water/acetonesystem. Under these conditions, the reaction was found to be first order. In detail, influence of pH, grade of stirring, and size and structure of functional groups of the alkyltrialkoxysilanes were investigated in order to compare the relative reaction rates of the different compounds.

Synthesis of siloxanes. XII: Cleavage of siloxanes by hydrogen chloride

Abstract 1 H NMR spectroscopy has been used in a kinetic study of the cleavage of siloxane bonds by hydrogen chloride in dioxane. The cleavages show an induction period which is associated with the autocatalytic effect of the water formed during the reaction. The kinetic behavior can be expressed in terms of a rate law that includes rate contants for cleavage by both dioxane. HCl ( k 1 ) and H 2 O·HCl( k 2 ). The k ′ 1 ( k ′ 1  k 1 · [HCl] −4 ) and k ′ 2 ( k ′ 2  k 2 · [HCl] −3 ) values correlated with σ * values; thus ϱ * values of 1.4 and 0.7 were obtained for k ′ 1 and k ′ 2 , respectively. The σ * values of 0.35 that we previously derived for the Me 3 SiO group applies in this reaction. The reaction of 1,3-bis( p -methoxyphenyl)tetramethyldisiloxane with hydrogen chloride involves cleavage of silicon-aryl rather than SiO bonds.

Synthesis of siloxanes. XX: Stereochemical investigations of substitution reactions at cyclotrisiloxanes with new types of model compounds

Abstract Functional cyclotrisiloxanes [(RMeSiO)(R′ 2 SiO)(ClMeSiO)] (R = OSiMe 3 , OSi (OSiMe 3 ) 3 ; R′= OSiMe 3 ) and [(PhMeSiO) 2 -(XMeSiO)] (X = H, Cl), each as a mixture of two configurational isomers, have been prepared and shown to be useful as model compounds for studying the stereochemical course of substitution reactions at siloxane silicon atoms by 29 Si or 1 H NMR spectroscopy. As an example, the acetolysis of the chlorocyclotrisiloxanes was studied and found to proceed with retention of configuration.