Siloxanolate equilibration catalyst

Siloxanolate Catalysts. The initial step for the study of the kinetics of base-catalyzed siloxane equilibration reactions was the preparation of a number of well-defined siloxanolate catalysts. The catalysts were prepared separately, prior to the equilibration reactions, so that a homogeneous moisture-free system with a known concentration of active centers might be obtained. The catalysts studied included potassium, tetramethylammonium, and tetrabutylphosphonium siloxanolate. 

The reaction scheme for the preparation of aminopropyl-terminated di-fimctionalized oligomers is illustrated in Scheme II. The reaction proceeds by the anionic equilibration of the cyclic siloxane tetramer, D4, in the presence of l,3-bis(3-aminopropyl)tetramethyldisiloxane. The equilibration process begins immediately upon addition of the siloxanolate catalyst, and samples were removed as a function of time for the purpose of the kinetic study. 

With Tetramethylammonium Siloxanolate, The second system studied was the tetramethylammonium-siloxanolate-catalyzed equilibration process. The reaction temperature was held at 80 °C to avoid decomposition of the siloxanolate end groups. The effect of catalyst concentration on the disappearance of D4 is shown in Figure 3. The reaction of D4 proceeded fairly... 

Mechanism of Equilibration. The generally accepted mechanism for the base-catalyzed ring-opening polymerization of cyclosiloxanes involves attack of the basic catalyst at the silicon atom (15). It has been proposed, and generally accepted, that the active species is a partially dissociated siloxanolate anion (13). In the results presented in this chapter, significant differences in reaction rates were observed as the corresponding cation of the siloxanolate species was varied. The more rapid disappearance of D4 and aminopropyldisiloxane in the presence of these catalysts increased in the following order ... 

Significant differences were observed in the rate of incorporation of D4 and l,3-bis(3-aminopropyl)disiloxane for similar concentrations of potassium, tet-ramethylammonium, and tetrabutylphosphonium siloxanolate catalysts. The rate differences affected the reaction times that were required to obtain a completely equilibrated reaction mixture with the desired molecular weight. The potassium catalyst required excessively long reaction times or high concentrations before sufficient incorporation of the aminopropyldisiloxane was realized. The tetramethylammonium and tetrabutylphosphonium catalysts were much more efficient for the preparation of controlled-molecular-weight aminopropyl-terminated polysiloxane oligomers. 

Transient siloxanolate anionic catalysts prepared by reacting four moles of D-4 with one of tetramethyl ammonium hydroxide at 80 C are effective for equilibrating "neutral" systems such as the epoxy ( ), "basic" dimethyl-amino (64) or aminopropyl (59,67) end-blockers and D-4. With "acidic" functionality on the end-blocker, we have successfully utilized trifluoroacetic acid for the equilibrations. Further details of the oligomer synthesis and their utilization in segmented copol)nners will be described in future publications. 

The piperazine capped disiloxane as discussed in 6 above could easily be equilibrated under exactly identical conditions to those discussed for the primary amine system earlier, that is, the disiloxane, cyclic tetramer, and 0.5 weight percent of the siloxanolate catalyst were heated to 80°C for 44 hours. The work-up procedure was identical, that is, the catalyst was deactivated at 150°C for 3 hours. The excess cyclics were then stripped under vacuum (0.5 torr, lOO C) and the material was further characterized by amine endgroup titration. 

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