Tetramethylammonium siloxanolate catalyst

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. 

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. 

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. 

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... 

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-... 

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. 

Figure 9. Effect of tetramethylammonium (0.095 mol %, ) and tetrabutyl-phosphonium (0.066 mol %, Q) siloxanolate catalysts on the rate of reaction of aminopropyldisiloxane (DSX). The targeted M was 1500, and the reaction was carried out at 80 °C.

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 rate of disappearance of the starting materials was followed as one approach to determine the effect of catalyst type and concentration on the rate of the ring-opening polymerization. Results are presented in this chapter for the potassium-siloxanolate-catalyzed system, as well as for the analogous tetramethylammonium- and tetrabutylphosphonium-siloxanolate-catalyzed systems. 

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... 

---