Siloxanolate catalysts

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. 

With Potassium Siloxanolate. The first set of kinetic data refers to the disappearance of D 4 as a function of time for the commonly used potassium siloxanolate catalyst. The reaction temperature was heM constant at 80 °C, and the targeted molecular weight was 1500 g/mol. Several concentrations of potassium siloxanolate were studied, including 0.087, 0.134, and 0.207 mol %, on the basis of the total moles of starting material. The rate of reaction of D4 increased, but even with 0.207 mol % potassium siloxanolate catalyst. 

The lack of molecular weight control in this system was more evident at the lower concentrations of catalyst studied. For example, a potassium siloxanolate concentration of 0.087 mol % produced a 4200-g/mol oligomer after 24 h of reaction time. This value is almost 3 times the desired molecular weight. These results and previous studies (8) indicate clearly that the potassium siloxanolate catalyst did not readily incorporate the aminopropyldisiloxane at moderate catalyst concentrations. 

Comparison of Catalysts. The concentration of siloxanolate catalyst in all three systems studied ranged from 0.03 to 0.20 mol %, on the basis of the total moles of starting materials. At these concentrations of catalyst, the reactions of D4 with the tetramethylammonium and tetrabutylphosphonium siloxanolate catalysts were fairly comparable, whereas the reaction with po-... 

Figure 7. Effect of potassium (0.134 mol %, and tetrabutylphosphonium (0.066 mol %, s) siloxanolate catalysts on the rate of reaction of D4- The targeted M was 1500, and the reaction was carried out at 80 °C.

The tetramethylammonium and tetrabutylphosphonium siloxanolate catalysts are compared in Figure 8. The greater eflSciency of tetrabutylphosphonium siloxanolate relative to the potassium and tetramethylammonium siloxanolate for incorporation of D4 has been described in the past (2) for nonfunctionalized siloxane systems. Despite the rate differences, all three systems were effective in reducing the D4 concentration to the same equilibrium level in a reasonable amount of time. 

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. 

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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