Mild Route To Convert SiH Compounds to Their Alkoxy Analogues

Abstract

Alkoxysilanes, particularly ethoxysilanes, are widely used crosslinkers for the moisture induced, room temperature vulcanization of silicones. Karstedt's platinum catalyst is shown to be effective at converting commercially available small hydrosiloxanes containing 1-8 SiH groups, and higher molecular weight telechelic and pendant hydrosiloxane polymers, into the corresponding alkoxycompounds. Kinetic studies demonstrated that relatively high concentrations of platinum -500 ppm -were required for practical reaction rates to be realized. As with other silyl ether forming processes, steric hindrance at both carbon (alcohol) and silicon center retard reaction progress.

Reliable Condensation Curing Silicone Elastomers with Tailorable Properties

“…The molecular weight of C2T was determined by SEC ( Table 1 ). Ethoxy-terminated octakis(dimethylsiloxy)-T8-silsesquioxane ((QM OEt ) 8 ) was synthetized via ethanolysis of octakis(dimethylsiloxy)-T8-silsesquioxane ((QM H ) 8 ) in the presence of Karstedt´s catalyst [ 30 ]. Hostaphan RN 190/190 µm (polyester plastic carrier) was supplied by Mitsubishi Polyester Film.…”

Kinetics of catalyzed dehydrocondensation of hydrogen functionalized siloxane

“…With MeOH being the simplest primary alcohol with the most accessible structure for possible chemical interactions, it was expected to be the most reactive among the alcohols selected. The lower activity (compared to iOctOH) can be attributed to the significantly lower solubility of polysiloxanes like PDMS and PMHS in methanol 19,45 . A similar suggestion has been made previously for ethanol by Safa et al 18 In general, the more polar the molecule, the poorer solvent it is expected to be for the nonpolar PMHS molecules.…”

“…In the initial stages, the Pt‐catalyzed dehydrocondensation reactions appear to be of zeroth order in respect to PMHS. The reaction rate for the formation of H 2 , and thus, the loss of SiH groups over time is considered to be equal, as Juraskova et al have determined from 1 H‐NMR signals in the reaction of ethanolysis of hydride terminated siloxane (H‐PDMS) 19 . Given that no side‐reactions take place, the general rate of reaction can be considered as …”

“…The question of how does the foamed structure form in such a multi‐component system has already been answered by many authors 17 . Using alcohols in the dehydrocondensation reaction with polyhydrosiloxane results in the formation of alkoxysiloxane polymer (silyl ether) and H 2 evolution 18,19 . This reaction has also been described as alkoxylation or dehydrogenative alcoholysis of PMHS, or simply, alcoholysis in the literature.…”

“…The most recent study, by Jurásková et al monitored the ethanolysis of PMHS (7%–9%, 15%–18%, and 45–55% H‐functionalized) among other hydrosiloxanes which are widely known in the silicone industry. It was found that higher catalyst concentrations are needed (>15 ppm Pt in total volume—recalculated) to reach the full conversion, the cause was attributed to the higher steric hindrance near the repetitive MeHSiO units 19 . Although the order of alcohol efficiency was shown for the reaction with H‐terminated PDMS only, it still mostly depicted the effect of the steric bulk of alcohols, but with some exceptions.…”