Siloxanolate catalysts preparation

Materials. Octamethylcyclotetrasiloxane, D4, was generously supplied hy General Electric Company. l,3-Bis(3-aminopropyl)tetramethyldisiloxane (to be referred to subsequently as aminopropyldisiloxane) was obtained from Petrarch Systems, Inc. These materials were dried over calcium hydride and vacuum distilled prior to use. Potassium hydroxide, tetramethylammonium hydroxide pentahydrate, and tetrabutylphosphonium bromide used in the preparation of the siloxanolate catalysts were used as received from Aldrich. 

Catalyst Preparation. The potassium siloxanolate catalyst was prepared by charging finely crushed potassium hydroxide, D4, and toluene to a flask equipped with an overhead stirrer and an attached Dean-Stark trap with condenser. Argon was bubbled through the solution from below the level of the liquid to promote the elimination of water via a toluene azeotrope as the reaction proceeded. Typically, a D4/KOH molar ratio of 3 1 was used with enough toluene to form an approximately 50% (wt/vol) solution. The catalyst was allowed to form at 120 °C for 24 h, during which time the toluene-water mixture was eliminated and collected in the Dean-Stark trap. The clear catalyst was then diluted to an —35% (wt/vol) solution with dry toluene and stored in a desiccator until use. 

The tetramethylammonium siloxanolate catalyst was prepared similarly by charging tetramethylammonium hydroxide, D4, and an azeotropic agent to the flask and heating the reaction at 80 °C for 24 h. The lower reaction temperature was necessary to avoid decomposition of the ammonium catalyst. Under most conditions, this procedure produces an active catalyst that is not completely homogeneous. Although not precisely defined, some carbonate is known to be present in addition to the siloxanolate. 

The tetrabutylphosphonium siloxanolate catalyst was prepared by reacting the potassium siloxanolate catalyst with a solution of tetrabutylphosphonium bromide in toluene. The reaction resulted in a precipitate of KBr and the formation of homo-... 

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. 

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 major difference in catalytic eflSciency among these three siloxanolate systems was in the rate of incorporation of the aminopropyldisiloxane. This area has not been examined in detail in the past, and the results presented in the previous tables and figures provide valuable information on the effectiveness of the various catalysts for the preparation of aminopropyl-fimctionalized oligomers. The data presented show that the tetramethylammonium catalyst reacts with the aminopropyldisiloxane somewhat more slowly than does the tetrabutylphosphonium catalyst. The difference is clearly observed in Figure 9, which shows that the aminopropyldisiloxane... 

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