Synthesis and Properties of Silicones and Silicone-Modified Materials

Chapter 31

Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 13, 2014 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch031

Diffusion-Controlled Titanate-Catalyzed Condensation of Alkoxysilanes in Nonpolar Solvents Xiaobing Zhou, Sanlin H u , Nick E. Shephard, and Dongchan A h n Dow Corning Corporation, 2200 West Salzburg Road, Midland, MI 48686-0994

The diffusion-controlled titanate-catalyzed condensation of alkoxysilanes has been studied in non-polar solvents using O and O labeling techniques. Incorporation of the oxygen isotopes into reaction intermediates and products via labeled water vapor provided a tag for direct detection of TiOSi, SiOSi and TiOTi species by O NMR and FT-IR. By monitoring reactions between alkoxysilanes and titanium t-butoxide (TtBT), the fates of the reactants were determined. The key intermediate linkage that facilitates formation of siloxanes was found to be the TiOSi bond. Studies with model compounds have shown that TiOSi groups can be converted to SiOSi species through either transesterification or redistribution routes, suggesting that the reaction of TiOSi species with silanols may be insignificant in the formation of siloxanes. Based upon these results, a catalytic cycle has been proposed for diffusion-controlled titanate-catalyzed condensation of alkoxysilanes. 17

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© 2003 American Chemical Society

In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Titanate-catalyzed condensation reactions of alkoxysilanes to form siloxane materials have been used in many industrial processes that utilize sol-gel chemistry. Sol-gel processing is used to prepare primers, protective coatings, photocatalytic zeolites, fillers and numerous other intermediates for ceramic manufacture. Motivated by such applications, most fundamental studies of solgel process chemistry have been carried out in wet polar solvents (7). Alternatively, our research focuses on such reactions conducted in dry nonpolar solvents exposed to moisture. These conditions arefrequentlyencountered in the cross linking of hydroxyl functional polymers. Commercially important examples utilizing such chemistry include silicone networks for protective coatings, primers, adhesives and sealants (2). The catalytic mechanism, however, still remains somewhat unclear due to the lack of convincing experimental data (3). In this study, we utilize the latest labeling techniques and a novel sample arrangement to investigate the reaction mechanism under diffusion-limited conditions. Mechanistic studies of the hydrolysis and condensation chemistry of organic titanates and other systems including alkoxysilanes have recently been made easier by the development of both solution and solid state 0 NMR techniques (4-9). 0 NMR analysis often requires enrichment of the 0 isotope in hydrolyzed or condensed products such as metal oxo complexes or siloxanes due to the low sensitivity (relative sensitivity = 2.91 χ 10") and low natural abundance (0.037%) of the nucleus. In some cases, isotopic enrichment can be achieved in situ simply by using 0 enriched water as a starting material. Instead of the direct addition of liquid water, in this study water is allowed to diffuse into reaction mixtures of organic titanates and alkoxysilanes so that condensation reactions can be monitored over a long period of time. This approach allows improved control of the rates of hydrolysis and condensation of hydrolytically unstable titanates. Direct addition of water results in rapid formation of insoluble Ti-O-Ti species that are difficult to characterize due to their low solubilities and polymeric nature. This combination of diffusioncontrolled experiments with labeled water has enabled a direct mechanistic study of the titanate catalyzed alkoxysilane condensation reaction. 17

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Experimental Materials. Titanium f-butoxide (TtBT), methytrimethoxysilane (MTMS), and octamethylcyclotetrasiloxane (D ) were purchased from Aldrich. Octamethyltrisiloxane (OS-20) is a Dow Corning product. MTMS, D and OS20 were dried over 4 Â molecular sieve before use. Titanium trimethylsiloxide 4

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(Ti(OSiMe ) ) and silanol terminated polydimethylsiloxane (MW = 400 - 700) were purchased from Gelest and used as received. 20% 0 enriched water and 95% 0 enriched water were purchased from Isotec and used without further purification. Pentane was dried and distilled from sodiunVbenzophenone. The 0.1 Ν NaOH solution was purchasedfromAldrich. Fourier Transform Infrared Spectroscopy (FT-IR). Infrared spectra were recorded at a resolution of 4 cm" on a Nicolet 5SXB FT-IR spectrometer with purge of dry nitrogen. KBr disks were prepared with solid samples. Relative peak areas for semi-quantitative analysis of TiOSi redistribution kinetics were obtained by normalizing the integrated area of the 923 cm" band by the nearby Si-Me band at 1270 cm" . Nuclear magnetic resonance spectroscopy (NMR). The solution NMR data were collected on a Varian INOVA 400 spectrometer operating at 54.203 MHz for 0, at 79.402 MHz for Si, and at 100.507 MHz for C . A Nalorac 10 mm dual broadband probe was used with H 0 - ' 0 as an external reference for 0 NMR. A special capacitor stick insert was used to make the probe tunable to 0 frequency. To optimize spectral resolution, the probe was shimmed with D 0 before running test samples. All subsequent runs were conducted neat with samples unlocked. The experimental parameters were optimized to enable accumulation of FIDs within two minutes for a spectrum with sufficient signalto-noise ratio for kinetic studies of fast reactions. A standard single pulse sequence with Waltz-16 composite pulse decoupling was employed with a 10 ps 90° pulse and an acquisition delay of 200 ms. In most cases, the spectra were recorded after 500 scans. Solid state C (spin rate = 5,000 Hz) and Si (spin rate = 4000 Hz) NMR data were collected using Varian's CP/MAS probe in a 7mm Zr0 rotor. Diffusion controlled hydrolysis and condensation reactions. Reactions were run in 10 mm screw thread NMR tubes. In a typical experiment at ambient temperature, TtBT (0.50 ml, 1.3 mmol), MTMS (0.56 ml, 3.9 mmol) and D (1.5 ml) were pre-mixed in a tube. A small ball of cotton (0.042 g) was lodged in the headspace of the tube about 15 cm above the meniscus of the solution (Figure 1). The tube was then capped with a Teflon/Silicone septum and 0 or O labeled water (0.15 ml, 8.5 mmol) was injected into the cotton. The reactions were complete after two weeks at 25 °C. The reactions were monitored with NMR without spinning the NMR tube. Titanium siloxide resin. Silanol terminated polydimethylsiloxane (20 g, containing approximately 60 mmol OH groups) in 100 ml anhydrous pentane was added dropwise to TtBT (5.62 ml, 15 mmol) in 50 ml anhydrous pentane. After stirring for 10 minutes, volatile species were removed in vacuo (25°C, 10" mrnHg) to leave a viscous liquid that thickened further with time. 3

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Cotton wetted with water

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Figure 1. Experimental setup for the diffusion controlled condensation of alkyltrimethoxysilanes Condensed MTMS Gel. The base-catalyzed condensed MTMS gel was prepared by quickly mixing MTMS and excess 0.1 Ν NaOH solution. A clear gel was immediately formed and dried in vacuo (25°C, 10" mrnHg) for 2 hr. 2

Results and Discussion

Reactions Observed with NMR 17

Direct addition of water. Attempts to add 0 enriched water directly into the reaction mixture of TtBT, MTMS and OS-20 led to immediate formation of a white precipitate. In the liquid phase, a strong 0 NMR signal (0.0 ppm) for bulk water and a smaller peak (-12 ppm) for water in solution were detected. Since most organic titanates are much more reactive with water than alkoxysilanes the white precipitate is attributed to hydrolyzed and condensed titanium oxo alkoxides, Τί(0) (Ο Βυ) , derivedfromTtBT. Thus, water added to the reaction mixture as a separate liquid phase leads to a very rapid interfacial reaction with titanates yielding insoluble titanium oxo alkoxides that are no longer good catalysts for the condensation of alkoxysilanes. Vapor phase water addition. Condensation appeared to begin shortly after the labeled water was added to initiate the diffusion of moisture, as a clear gel was formed within two hours. In the first 0 NMR spectrum recorded at ambient temperature (25°C), three peaks appear at 300.4, 55.2 and -15.6 ppm and are assigned to TtBT, D and MTMS respectively (Figure 2). Sharp new signals at around 240 ppm in subsequent spectra can be assigned to titanium siloxides (TiOSi), of which the exact structures are not known at this time (7, 10). The narrow linewidths of both new signals suggest the initial titanium I7

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siloxides may still be small molecules. Both peaks gradually evolved into broad featureless bands within 2 days. Simultaneously, another group of new signals at 60 ppm also grew with time. These can be assigned to various siloxane (SiOSi) structures (10). Over the next 3 days, the siloxanes became dominant in the reaction mixture while the titanium siloxides only grew moderately, and a third group of new species, titanium oxo (TiOTi) compounds, were formed. The titanium oxo compounds show unique 0 NMR bands consistent with prior assignments of / / (OTi , 760 ppm), ]i (OTi , 530 ppm) and / / (OTi , 380 ppm) structures (4). 0 spectra collected from 100 h to 140 h at 60°C clearly reflect these changes (Figure 3). These titanium oxo compounds are evidently complex macromolecular structures, as indicated by the broad multimodal peaks. In the later stages of the experiment at 25°C, the NMR tube began to fill with a white precipitate, and the solution 0 NMR signals broadened and decayed relative to the D signal. This signal decay is attributed to the precipitation of 0 enriched TiOTi, TiOSi and SiOSi solids that have peaks too broad for detection by the solution NMR setup.


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Figure 2. O NMR data - diffusion controlled condensation of MTMS at 25°C The diffusion-controlled condensation reactions were also conducted at elevated temperatures to obtain information on the relative rates of reactions. At 90°C, for example, 0 NMR data show that fewer titanium siloxides and more siloxanes were formed within the first 2 hours than at 25°C (Figure 4). These trends continued until the reactions were complete. This phenomenon implies that higher temperatures favor the conversion of titanium siloxides to siloxanes. Titanium siloxides can be converted to siloxanes via either transesterification with methoxysilanes or redistribution into TiOTi and SiOSi species (6). Both routes have been further investigated and relative reaction rates have been measured. It appears that transesterification is more favored at elevated 17


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termperature since TiOTi species were not detected along with the siloxanes in the early stage at 90°C. Further, at 90°C the reaction mixture eventually gelled after the reaction was stopped, forming a small amount of clear precipitate above the reaction mixture.

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Figure 3. 0 NMR data - diffusion controlled condensation of MTMS at 60°C Siloxane formation. As observed in Figures 2, 3 and 4, the siloxane products have a complex 0 NMR pattern. Closer examination of the SiOSi region reveals that MTMS is condensed stepwise, although the reaction rate for each step is comparable such that a mixture of siloxane products is generated (Figure 5). At 90°C, the 0 NMR bands of the siloxane products were initially dominated by a strong signal at 55 ppm in the first 4 hours. Over time, the dominant peak position shifts to 60 ppm (from 6 to 16 h), and then to 65 ppm after 18 hours. Similar transitions have been observed when the reactions were conducted at 25°C and 60°C. Thus it appears that MTMS first condenses to the T unit, which was then further condenses to the T and T structures. The sequential formation of Τ , T and T structures has been verified with Si NMR, a method commonly used to differentiate siloxane structures (//). 17

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Further, a semi-quantitative analysis of the 0 NMR data over the first 7 h reveals that the rates of siloxane formation from TiOSi intermediates increased dramatically with temperature (Figure 6. A). However, the temperature dependence of TiOSi bond formation appears more complex (Figure 6. B). In these diffusion-controlled titanate-catalyzed reactions, TiOSi peaks were observedfromthefirstspectrum after water vapor introduction and continuously



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thereafter. These are believed to be the key intermediates in the formation of the siloxanes (3). There are several competing reactions that determine the transient concentration of TiOSi species, including 1) formationfromthe reactants TiOC (e.g., TtBT) and SiOC (e.g., MTMS), 2) transesterification with SiOC to SiOSi and TiOC and 3) redistribution to SiOSi and TiOTi. The relative effects of these competing reactions at any given time and temperature may be related to the non-monotonic temperature dependence of the TiOSi concentration shown in Figure 6 B). SiOSi

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Synthesis and Properties of Silicones and Silicone-Modified Materials

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