Aminopropyldisiloxane

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. 

Capillary gas chromatography (GC) was used to determine the aminopropyldisiloxane concentration. An 11-m column (0.2-mm internal diameter) coated with a dime thy Isiloxane stationary phase was used. Temperature-programming techniques and a flame ionization detector were used. Tetradecane was used as an internal standard. Details of the chromatographic conditions have been reported earlier (8). A Varian Vista 402 data station simplified the calibration and analysis for both the HPLC and GC methods. 

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. 

With Tetrabutylphosphonium Siloxanolate, The final catalyst investigated in this study was tetrabutylphosphonium siloxanolate. Again, the reaction temperature was held at 80 °C because of the transience of the catalyst. The disappearance curves for D4 and aminopropyldisiloxane are shown in Figures 5 and 6, respectively. 

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... 

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.

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 ... 

Similar relative rates for the disappearance of D4 in the presence of these catalysts have been observed in the past for nonfunctionalized systems. The order of reactivity follows the expected order of increasing dissociation of the ion pair. As the ion pair dissociates, the concentration of the more active species increases. The slower reaction rate of the aminopropyldisi-loxane relative to D4 was expected to some extent on the basis of electronegativity differences, but the dramatic differences in the efficiency of incorporation of the aminopropyldisiloxane have not been reported previously. Additional factors may be contributing to the observed results, such as the expected increased solubility of the more organic (CH3)4N and (C4H9)4P ions relative to the ion. The better solubility may also lead to an increased dissociation of the siloxanolate species. 

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. 

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