THE ACID-CATALYZED CONDENSATION OF METHYL-SUBSTITUTED MODEL OLIGOSILOXANES BEARING SILANOL AND ETHOXYSILANE FUNCTIONS

Kinetics of reactions proceeding in a system containing pentamethyldis iloxane-1-ol, 1, and 1-ethoxy-heptamethyltrisiloxane, 2, catalyzed wit h 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 sila nol substrate. Rates of both these two reactions are similar to each o ther, however, they are influenced by the medium effect which is depen dent on the concentration of the reactants. Consecutive hydrolysis of the ethoxysiloxane substrate as well as ethanolysis of the silanol sub strate occur extensively leading to functional group interconversion. Specific rates of these processes exceed those of the condensation rea ctions by about two orders of magnitude. These competing processes wer e also studied separately in EtOH-1 and H2O-2 systems. The competition of the condensation reactions and the functional group interconversio n was also studied in a system containing methyltriethoxysilane and pe ntamethyldisiloxane-1-ol, 1. The substitution of the subsequent ethoxy functions with 1 in methylene chloride solution in the presence of tr ifluoroacetic acid was followed in separate experiments using compound s of the MeSi(OSiMe2OSiMe3)n(OEt)3-n series, n = 0-2, as substrates.