Alternatives to Thermal Curing in Diacetylene-Containing

Chapter 28

Alternatives to Thermal Curing in DiacetyleneContaining Carboranylenesiloxanes Downloaded by UNIV MASSACHUSETTS AMHERST on September 27, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch028

ManojK.Kolel-Veetiland TeddyM.Keller Advanced Materials Section, Code 6127, Chemistry Division, Naval Research Laboratory, Washington, DC 20375

The dilutions in crosslinking density in the thermally and thermo-oxidatively stable, diacetylene-containing inorganic-organic hybrid oligomers of poly(carboranylenesiloxanes) have resulted in rendering the network polymers derived from them elastomeric in nature. While the crosslinking reactions of the diacetylenes require high temperatures and protracted curing times, avenues of diacetylene-curing with less demanding conditions should add to the possibilities of their potential applications. Several alternative avenues for the curing of the diacetylenes have been explored and the preliminary results are reported.

366

U.S. government work. Published 2006 American Chemical Society

In Fire and Polymers IV; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Downloaded by UNIV MASSACHUSETTS AMHERST on September 27, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch028

The recent rapid advance of modern technology has resulted in an increasing demand for new high performance materials in a wide variety of engineering applications. Such materials are increasingly expected to function under unusual service conditions. In the aerospace industry, the need especially for high temperature elastomers, plastics, and ceramics that have thermal,

Figure 1: Poly(carboranylenesiloxane)s. thermo-oxidative, and hydrolytic stability and that can also maintain flexibility to well below ambient temperatures is severe. In this regard, the linear polymers of carboranylenesiloxanes (Figure 1) stand out as excellent candidates due to their exceptional thermal, thermo-oxidative, and elastic properties (7). The properties of these linear polymers may be enhanced further by their conversion into extended network polymers. During the early development of the carboranylenesiloxane chemistry, the available method for the production of a network polymer from a precursor carboranylenesiloxane was by the polymerization in air (at 315°C for 300h) of the vinyl groups of a pendant vinylcontaining carboranylenesiloxane by organic peroxides (2). However, in recent times, research in this area has resulted in the development of extended network systems of carboranylenesiloxanes that were produced either by the thermal polymerization of the diacetylene groups (PCSA networks) (Figure 2) (5) or by the hydrosilation of vinyl or ethynyl groups (Figure 3) (4). While the hydrosilation reaction proceeded at ambient conditions, the thermal curing required the exposure of the materials to temperatures in excess of 250°C for several hours. The networks produced from the thermal reactions, however, were observed to be tougher than the networks obtained from the hydrosilation reactions. The parent precursor, 1 (Figure 2), of the diacetylene-cured network contained the disiloxyl unit as the constituent siloxane moiety and produced networks that were plastic in nature on thermal curing (5). The crosslinked materials were observed to have high weight retention on thermal treatment to 1000°C in both N and air (weight retention in N = 87% and in air = 92%) and consequently were found to possess exceptional ceramic characteristics, thereby allowing their applications as both high temperature plastics and ceramics. Subsequent research to produce high temperature elastomeric versions of these materials resulted in the recent development of crosslinked network carboranylenesiloxane systems by the manipulation of the crosslinking density in 1 (6). A reduction in the crosslinking density in 1 by the lowering of the concentration of constituent diacetylene units yielded products with decreased 2

2


368 H QPH H < y H 3

3

3

3

H (^f H H ^ f H 3

3

3

3


1 Poly(carborane-siloxane-diacetylene) (PCSA) N

2

Thermally-crosslinked PCSA network Figure 2: Thermal curing of a PCSA oligomer into a network system.

HjCfHjHj^Hj

f

Hj^HjHj^Hj

Hi

l,7-Bis(divinyltetramethyldisiloxyl)-m-carborane

Q

i

H

H

Tetrakis(dimethylsiloxy)silane

Hexane, RT Karstedt catalyst

Hydrosilatively-crosslinked network Figure 3: Network formation by the hydrosilation reaction of a vinylcarboranylenesiloxane with a branched crosslinking siloxane.

glass transition temperatures. However, the products were still observed to predominantly possess a plastic nature at ambient conditions. In order to further improve the elasticity of these diacetylene-diluted products, the constituent siloxyl unit was changed from disiloxyl to the more flexible trisiloxyl unit. As expected, the substitution rendered the network products derived from these altered precursors elastomeric at ambient conditions (Figure



369 4) (7). Thus, the network systems of carboranylenesiloxanes that are currently available encompass the gamut of elastomers, plastics, and ceramics. However, the conditions for the thermal curing of the diacetylene-diluted versions of both 1 and its trisiloxyl derivatives still require thermal treatment at high temperature (250°C and above) as required for 1. Hence, in an attempt to make these systems more amenable to applications at ambient conditions, four avenues were explored that required much milder conditions for the curing of diacetylenes. The explored avenues included the curing of the diacetylenes in 1 by UV irradiation, TaCl -catalyzed polymerization, W(CO) -catalyzed photo polymerization and Rh (|Li-Cl)2(COD)2-catalyzed hydrosilation reaction. The preliminary results of the studies are reported. 5

6

2

-0.4-0.8-

-3.6 H -100

1

1 -50

1

1 0

1

1 50

«

1 100

«

1 150

1

Temperature (°C)

Figure 4: DSC thermograms of three diacetylene-diluted systems of poly(carborane-trisiloxane-diacetylene) systems with varying ratios of siloxane: carborane: diacetylene.


370

Experimental The synthesis of 1 was performed by following a published procedure (5). All of the reactions were carried out under inert conditions using standard Schlenk line techniques. Toluene (anhydrous, 99.8%) and diethyl ether (Et 0, anhydrous, 99.9%) were used as received from Aldrich. 1,1,3,3,5,5,7,7Octamethyltetrasiloxane was used as received from Gelest. The metal catalysts, tantalum(V) chloride (TaCl , 99.99%), tungsten hexacarbonyl (W(CO) , 99.9+%), and chloro(l,5-cyclooctadiene)rhodium(I) dimer (Rh (^i-C1) (C0D) , 98%) were used as received from Aldrich. (Cp) Ta=CH(CMe) Cl was synthesized following a published procedure (8). Thermogravimetric analyses (TGA) were performed on a SDT 2960 Simultaneous DTA-TGA analyzer. Differential scanning calorimetry (DSC) studies were performed on a DSC 2920 modulated DSC instrument. All thermal experiments were carried out at a heating rate of 10°C/min and a nitrogen flow rate of lOOcc/min. The UV-curing studies were performed using a Model 22-UV lamp (115 V, 60 Hz, 4w) obtained from Chemical Engineering Inc., Santa Rosa, CA. Infrared (IR) spectra were obtained on a Nicolet Magna 750 Fourier transform infrared spectrometer. 2


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6

2

2

2

2

3

Polymerization reactions: (a) UV-curing reactions: A 1M solution of compound 1 in Et 0 was placed on a NaCl infrared disk. On evaporation of the solvent, a fine film was deposited on the surface of the disk. An IR spectrum of this sample was obtained as a reference and the film was exposed to the radiation of a UV lamp. The FTIR spectrum of the film was monitored periodically for several days. 2

(b) TaCls-catalyzed curing of the diacetylenes in 1: To a flame-dried Schlenk flask was added 0.450 g (1.000 mmol) of 1 and 5 mL of toluene under argon. The resulting brown solution was placed in an oil bath at 85°C. In a separate flame-dried Schlenk flask, 0.023 g (0.066 mmol) (polymer to catalyst ratio = 15:1) of TaCl was dissolved in 2.5 mL of toluene under argon at 80°C yielding a bright yellow solution. The Ta solution was then transferred to the polymer solution via cannula. In about an hour, a darkening of the reaction solution was observed. The reaction was allowed to proceed for 3 days. After this period, the solvent was removed under vacuum to yield a rubbery dark blackish-brown product. The reactions involving polymer:TaCl and polymer:Cl Ta=CH(CMe) Cl at ratios of 1:1 and 15:1, respectively, were carried out following the procedure described above. 5

5

2


3

371 (c) W(CO) -catalyzed photo polymerization of 1: A mixture of 0.450 g (1.000 mmol) of 1 and 0.023 g (0.066 mmol) of W(CO) was placed in a quartz photo reactor under argon. The solids were dissolved in 5 mL of hexane at room temperature and the solution was exposed to wavelengths >300nm for a day. After this period, the solvent was removed under vacuum to yield a rubbery blackish-brown product. 6

6

(d) (Rh (M,-Cl) (COD) -catalyzed hydrosilation of 1: To a flame-dried Schlenk flask was added 0.225 g (0.500 mmol) of 1 and 0.330 mL (1.000 mmol) of 1,1,3,3,5,5,7,7-octamethyltetrasiloxane in 1 mL of toluene under argon. The golden brown solution was placed in an oil bath at 70°C. A solution of 0.010 g (0.020 mmol) of Rh (|i-C1) (C0D) in 0.50 mL of toluene was prepared in another flame-dried Schlenk flask and the contents were cannulated into the former flask under argon. The solution turned blackish-brown immediately. The progress of the reaction was monitored periodically by infrared spectroscopy. After an hour of reaction, the solvent was removed under vacuum to leave behind a rubbery black product.


2

2

2

2

2

2

Results and Discussion UV-curing reactions: Solid state photo polymerization of diacetylenes was first discovered by G. Wegner in 1969 (9). Innumerable examples have appeared in the literature on this area (10). The efficacy of photo polymerization by UV exposure in 1 was found to be poor compared to the solid-state polymerization of diacetylenes. The progress of the curing was monitored by IR spectroscopy. On complete curing, the diacetylene absorption at around 2079 cm" in the FTIR spectrum of the uncured sample was found to completely disappear from the corresponding spectrum of the cured sample which was dark brown in color (Figure 5). To achieve any appreciable extent of curing, the films (cast on a NaCl IR disk) had to be irradiated with UV radiation for an extended period (several days). The same level of cure could be achieved more rapidly (in about 2 hours) under harsher thermal cure conditions (250-400°C) (7). The slowness of the UV-curing could be attributed to a lack of proximal ordering/stacking of the diacetylene centers, which results in the enhancement of the rate of solid state polymerization by irradiation in the solid phase (//). 1

TaCI -cataIyzed curing of the diacetylene units in 1: In the literature, a plethora of examples of transition-metal catalyzed metathetical polymerization of olefins and alkynes involving metallacyclobutadiene intermediates are known (12). Among the catalysts, TaCl is known to be especially effective for the 5

5


372 polymerization of acetylenes (/J). Hence, TaCl was chosen for the polymerization reaction studies of 1. A reaction at an oligomer to catalyst ratio of 15:1 was allowed to proceed for 3 days since periodic FTIR analysis of the reaction mixture exhibited the remnance of the diacetylene stretch at 2079 cm' during the first two days of the reaction. The reaction was repeated at an oligomer to catalyst ratio of 1:1 and resulted in the disappearance of the diacetylene absorption in 2h. Hence, in the former reaction it is believed that, at the low catalyst concentration, the availability and ability of the diacetylene units to polymerize at the Ta centers are greatly reduced due to the initial 5


1

i — • — i — • — i — > — i — • — i — • — i — • — i —

4000

3500

3000

2500

2000

1500

1000

1

Wavenumbers ( c m )

Figure 5: IR spectra of the cured and uncured samples of 1.

formation of a viscous gel during the reaction. Thus, a rapid curing of the diacetylene units is thwarted under such a condition. This was also apparent in the DSC thermogram of the intermediate reaction product after 2 days (Figure 6). There were two endothermic transitions centered at -44°C and -30°C within the developing networked system attributed to glass transition temperatures of distinct regions in the product. The production of a Ta=C species during the polymerization could be argued based on the observation that a similar polymerization reaction catalyzed by (Cp) Ta=CH(CMe) Cl which contained an alkylidene species was found to accelerate the reaction. The polymerization reaction using this catalyst was complete in 4h at 80°C. In the TaCl -catalyzed 2

3

5



373 reaction, the initial formation of a catalytically active Ta=C species, derived from the diacetylene units in 1, perhaps has a long induction period thereby causing the polymerization to be slow. Thus a probable mechanism for the reaction could involve the initial formation of a Ta=C (Ta-vinylidene) species from a diacetylene unit followed by the metallacyclobutadiene formation and the propagation steps (Figure 7). Such a mechanism has been proposed previously in transition metal-catalyzed polymerization of alkynes (14). The polymerization products were determined to have high thermal stabilities on treatment to 1000°C (weight retention in N = 85% and in air = 91%). 2

W(CO) -catalyzed photo polymerization of 1: In 1985, Landon et al. reported the room temperature photo polymerization of alkynes in the presence of W(CO) (15). A tungsten-vinylidene/alkylidene intermediate was postulated as the reactive intermediate in the polymerization. As 1 was susceptible to a metathetical polymerization involving a Ta-alkylidene intermediate, a photo polymerization of 1 using W(CO) seemed reasonable. In a reaction conducted at room temperature at a polymerxatalyst ratio of 15:1 in the presence of wavelengths >300nm, a complete polymerization was observed in 24h as determined by the absence of a diacetylene stretch at 2079cm" in the FTIR spectrum of the product. The product from the polymerization was determined to have similar thermal stabilities on treatment to 1000°C as observed for the TaCl -catalyzed product (weight retention in N = 86% and in air = 92%). It seems reasonable that the rate of the polymerization could be enhanced if the 6

6

6

1

5

2

-0.1

^-0.2-

.0.4-1 ' -75

'

'

1 ' -25

'

i ' ' 25 Temperature (°C)

1 ' 75

'

• r 125

Figure 6: DSC thermogram of the network producedfrom the TaCl -catalyzed polymerization of 1. 5



374

R CljTa-" ||

Cl Ttf=C=

Alternatives to Thermal Curing in Diacetylene-Containing

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