Synthesis and Fractionation Studies of Functionalized Organosiloxanes

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8 Synthesis and Fractionation Studies of Functionalized Organosiloxanes Cheryl S. Elsbernd,1 Maria Spinu, Val J. Krukonis,2 Paul M. Gallagher,2 Dillip K. Mohanty,3 and James E. McGrath4 Department of Chemistry and NSF Science & Technology Center, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

The synthesis of difunctionalized aminopropyl-terminated poly— (dimethylsiloxane) via equilibrium polymerization of the cyclic tetramer in the presence of the functionalized disiloxane was demonstrated. The use of tetramethylammonium and tetrabutylphosphonium siloxanolate anionic catalysts for these reactions was studied. The reactions of the tetramethylammonium and tetrabutylphosphonium catalysts were limited to ~80 °C because of the known transience of these species at higher temperatures. The disappearance of the cyclic tetramer and disiloxane was monitored by HPLC (high-pressure liquid chromatographic) and GC (gas chromatographic) techniques, which indicated that the tetramethylammonium and tetrabutylphosphonium catalysts were much more efficient than the potassium catalyst, even when the potassium catalyst was used at much higher (e.g., 160 °C) temperatures. This behavior may be related to the higher degree of dissociation and, possibly, the enhanced solubility of the tetramethylammonium and tetrabutylphosphonium catalysts relative to the much more studied potassium catalyst. The number-average molecular weight was independent of the catalyst concentration and was influenced only by conversion and the molar ratio of the tetramer to the disiloxane. GPC (gel permeation chromatographic) characterization was also possible by derivatizing Current address: 3M Center, St. Paul, MN 55144 Current address: Phasex Corporation, 360 Merrimack Street, Lawrence, MA 01843 3 Current address: Department of Chemistry, Central Michigan University, Mount Pleasant, MI 48858 4 Author to whom correspondence should be addressed 1

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0065-2393/90/0224-0145$06.00/0 © 1990 American Chemical Society

In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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the amine to ketimine functionalities. Supercritical-fluid-fractionation studies of the functionalized siloxanes yielded samples with relatively narrow molecular weight distributions, which were compared with polysiloxane standards produced in our laboratory. The number-average molecular weights determined by titration methods and those determined by GPC experiments were in excellent agreement.

THE M I PORTANCE AND UTILITY

of multiphase copolymer systems have been well documented in the literature (1-4), with emphasis on their unique combination of properties and their potential material applications. Organosiloxane block polymers are a particularly interesting type of multiphase copolymer system because of the unusual characteristics of polysiloxanes, such as their stability to heat and UV radiation, low glass transition temperature, high gas permeability, and low surface energy (1, 2, 5). The incorporation of polysiloxanes into various engineering polymers offers an opportunity for many improvements, such as lower temperatures for the ductile-to-brittle transitions and improved impact strength. Siloxane-containing block copolymers are often prepared by step-growth or condensation polymerization of preformed difunctionalized siloxane oligomers with other difunctionalized monomers or oligomers. Our current work (3, 4, 6-8) on siloxane chemistry includes the preparation of a number of functionalized oligomers, with emphasis on equilibration processes with the commonly available cyclic tetramer, octamethylcyclotetrasiloxane (D4), in the presence of a functionalized disiloxane or end blocker. Despite the importance and synthetic utility of these siloxane equilibration reactions, relatively little has been reported with respect to the detailed kinetics and mechanisms involved, especially in the presence of functionalized end blockers. A major focus of our efforts (3, 4, 6-8) is the investigation of various aspects of siloxane equilibration reactions to establish the exact nature of the active polymerization species and the effect of various reaction parameters on the preparation of well-defined difunctionalized siloxane oligomers. The synthesis and equilibration reaction kinetics involved in the preparation of aminopropyl-terminated polysiloxanes has been studied most extensively because of the utility of the amino-terminated species as components of a large number of segmented copolymers such as imides, amides, and ureas. 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.



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Because of the random nature of the equilibration processes, the resulting siloxane oligomers generally possess a Gaussian molecular weight distribution. In addition, a ring-chain equilibrium results in the presence of a certain amount of low-molecular-weight cyclic species at the end of the reaction. In addition to the synthesis of difunctionalized siloxane oligomers, a number of studies have been conducted to explore the usefulness of supercritical-fluid-fractionation techniques for the preparation of well-defined aminopropyl-functionalized polysiloxanes of narrow polydispersity (9, 10). The isolation of oligomer fractions with narrow molecular weight distributions (MWD) was interesting from a fundamental viewpoint, because incorporation of these narrow-MWD fractions into segmented copolymers would allow the determination of the effect of the polydispersity of the individual blocks on the properties of the resulting copolymer systems. Studies of the mechanical and morphological properties of such systems would result in a better understanding of structure-property relationships in multiphase copolymer systems.

Experimental Procedures Materials. Oetamethyleyclotetrasiloxane, D 4 , was generously supplied by 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 chargingfinelycrushed potassium hydroxide, D 4 , and toluene to aflaskequipped 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 D 4 /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, D 4 , and an azeotropic agent to theflaskand 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-

American Chemical Society Library 16th St., Zeigler, N.W. J., et al.; In Silicon-Based1155 Polymer Science; Advances in Chemistry; American Chemical Society: Washington, DC, 1989. Washington, O.C. 20036

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geneous tetrabutylphosphonium siloxanolate. The number of moles of siloxanolate catalyst per gram of catalyst solution in all cases was determined by titration with 0.10 Ν HC1. Equilibration Reactions. D 4 and the aminopropyldisiloxane were charged into a three-necked flask equipped with a magnetic stirring bar, a reflux condenser, an argon inlet, and a rubber septum for the removal of samples via a syringe during the course of the equilibration reaction. The flask was heated in a controlled-temperature bath until the temperature of the reaction mixture had stabilized, and the desired amount of siloxanolate catalyst was added. Samples were removed at various reaction times and quenched in a dry-ice-isopropyl alcohol bath for later chromato graphic analysis. After 24-48 h, the equilibration reaction was terminated for analysis and isolation of the resulting siloxane oligomer. The transient tetramethylammonium and tetrabutylphosphonium siloxanolate catalysts were readily decomposed by heat ing for 30 min at >145 °C. The potassium siloxanolate catalyst required neutralization with acetic acid, which was followed by washing with water and drying of the oli gomer. The cyclic species were removed by vacuum distillation at 100 °C and 40 Pa. Characterization. Number-average molecular weights (Mns) of the stripped oligomers were determined by potentiometric titration of the primary-amine end groups with 0.10 Ν HC1. The D 4 content of the samples was determined by reverse-phase high-pressure liquid chromatography (HPLC) with a Varian 5500 liquid chromatograph. A DuPont Zorbax ODS (Ci8) column was used with a Wilmad infrared detector set at 12.45 μπι to monitor the S i - C H 3 vibration. The mobile phase was an 83:17 mixture of acetonitrile and acetone at a flow rate of 0.8 mL/min. A Rheodyne injector valve operating on compressed air was used with a ΙΟ-μί. sample loop for reproducible injection volumes. Ethyl acetate was used to dissolve the samples for analysis. Capillary gas chromatography (GC) was used to determine the aminopropyl disiloxane concentration. An 11-m column (0.2-mm internal diameter) coated with a dimethylsiloxane 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. Gel permeation chromatographic (GPC) analysis was not possible directly on the primary-amine-functionalized oligomers because of their tendency to be adsorbed on the GPC columns rather than to elute. A method has been developed in our laboratories (9, JO) for derivatization of the amine end groups with benzophenone to form an imine functionality. This method allows FPC (fixed-partial-charge) analysis of the oligomers. The titration-determined molecular weights of the individual samples were used to calculate the amount of benzophenone required to completely derivatize the amine functionalities. A 10 mol % excess of benzophenone was used to ensure complete derivatization. The reaction was done in bulk at 80 °C by heating the siloxane oligomer and benzophenone for 24 h in the presence of 3-A molecular sieves. GPC analysis was then carried out at 30 °C with a Waters 590 GPC apparatus^ equipped with ultrastyragel columns with particle sizes of 500, ΙΟ3, 104, and 104 A. A Waters 490 programmable wavelength detector set at 218 nm was used with T H F (tetrahydrofuran) as the solvent.


8.

ELSBERND ET AL.

Studies of Functionalized

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Synthesis and Fractionation Studies of Functionalized Organosiloxanes

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