James Mcvie

The acid-catalyzed condensation of methyl substituted model oligosiloxanes bearing silanol and ethoxysilane functions

Abstract Kinetics of reactions proceeding in a system containing pentamethyldisiloxane-l-ol, 1, and 1-ethoxy-heptamethyltrisiloxane, 2, catalyzed with trifluoracetic acid have been studied in methylene chloride solution. The heterofunctional condensation involving the Si-OH group and the Si-OEt group competes with the homofunctional condensation of the silanol substrate. Rates of both these two reactions are similar to each other, however, they are influenced by the medium effect which is dependent on the concentration of the reactants. Consecutive hydrolysis of the ethoxysiloxane substrate as well as ethanolysis of the silanol substrate occur extensively leading to functional group interconversion. Specific rates of these processes exceed those of the condensation reactions by about two orders of magnitude. These competing processes were also studied separately in EtOH-1 and H2O-2 systems. The competition of the condensation reactions and the functional group interconversion was also studied in a system containing methyltriethoxysilane and pentamethyldisiloxane-l-ol, 1. The substitution of the subsequent ethoxy functions with 1 in methylene chloride solution in the presence of trifluoroacetic acid was followed in separate experiments using compounds of the MeSi(OSiMe2 OSiMe3)n(OEt)3−n series, n = 0–2, as substrates.