Synthesis and Properties of Silicon-Branched Organosilicon Polymers

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Downloaded by UNIV OF PITTSBURGH on March 10, 2015 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch027

Synthesis and Properties of SiliconBranched Organosilicon Polymers ‡

1

2

Yoichiro Nagai, Hamao Watanabe, Hideyuki Matsumoto, Yoshitake Naoi, and Naotake Sutou 2

Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan Two types of new silicon-branched organosilicon polymers, linear and ladder polysilane structures, were produced from dihalo- and tetrahalodisilane, respectively, via alkali-metal-mediated reactions. Further investigations disclosed that the polymers may be useful as photoresists, semiconductors, ceramic precursors, and composite materials in high-technology fields.

HE RADA ITO IN SENSITIVITY

JL of soluble alkyl- and aryl-substituted linear polysilanes with high molecular weights has been studied with much vigor recently (1-9). Because silicon branching in the silicon framework results in significant electronic stabilization (10-12), silicon-branched organosilicon polymers are expected to exhibit unique properties that are considerably different from those of existing organosilicon polymers. The structures of polysilanes are so far limited to the linear backbone. However, siliconbranched organosilicon polymers have been prepared (9). Because of the significance of silicon branching on electronic properties, we became interested in preparing the following two silicon-branched organosilicon polymers according equations 1 and 2. Linear Polysilane Polymer 1 was obtained (13, 14) by reductive condensation of 1,1-dichloro2-phenyl-l,2,2-trimethyldisilane (a silyl-substituted methyldichlorosilane) Deeeased. Author to whom correspondence should be addressed. 2 Current address: Yuki Gosei Kogyo Company, Itabashi, Tokyo 176, Japan # 1

0065-2393/90/0224-0505$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.

506

SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE

9

9

H-C—Si—CH-

J

O

3

H7C—Si—CH7

Na

CI—Si—CI

I


CHL

H,C C H , 3 ν 3 CH

H,G C H , CH ClCl-

I

-Si—CI

/ Li

I

—Si-

V -SiCH

!

-CH Si—CI H3C/CH3

/N

H C CH 3

3

with sodium dispersed in toluene at 50 °C. The starting dichlorodisilane was obtained readily by the reaction of l^l^-trichloro-l^^-trimethyldisilane with phenylmagnesium bromide in the presence of copper iodide (equation 3) with more than 99% regioselectivity. Cl(CH 3 ) 2 SiSiCH 3 Cl s

CeHsMgBr Cul

>

C e H 5 (CH3) 2 SiSiCH 3 Cl 2

(3)

Reductive condensation of the dichlorodisilane monomer with sodium yielded a pale-yellow semisolid crude product. Fractionation of the polymer was carried out by precipitation with ethanol from a T H F (tetrahydrofuran) solution. A polymer with a weight-average molecular weight (Mw) of 4 X 104 (determined by gel permeation chromatography [GPC] with polystyrene standards) was obtained as a nice white powder that was soluble in common organic solvents, fusible, and shapable. The polymer obtained showed no absorption bands due to S i - Η or Si-O-Si bonds in the IR region (Figure 1). The IR spectrum indicates that the polymer is of the expected poly[(phenyldimethylsilyl)methylsilylene] structure. The observed phenyl-tomethyl ratio (1:3) in the *H NMR spectrum also supports this structure, and obviously, the original Si-Si bond in the starting disilane was retained during the condensation reaction.


Ν AG AI ET AL.

Silicon-B ranched Organosilicon Polymers


27.

507

3

S y ôo 00 CCI

υ

1 to to IX ce

α; "Ή

ε




508

The polymer exhibited UV absorption bands near 245 and 310 nm (Fig ure 2). Irradiation of its solution in eyclohexane with a mercury lamp resulted in the rapid decrease of the molecular weight at the early stage of the reaction, but further irradiation gave rise to a considerable increase in mo lecular weight (Figure 3). Presumably, initial chain cleavage followed by cross-linking occurred. The polymer can be shaped into a film by the casting method. Irradiation (with a low-pressure lamp) of the polymer film in air gave an insoluble material. The IR spectrum of the irradiated film showed a strong band due to the Si-O-Si bond. The silicon-branched polymer also became semiconducting upon ex posure to iodine vapor; it is, therefore, closely related to "polysilastyrene", for which West et al. (2) used highly toxic pentafluoroarsine as dopant. Polymer 1 is originally insulating, but an iodine-doped sample, a black solid, showed a conductivity (σ) of ~ 1 0 " 3 / û - c m . The black solid was stable in air for several days and then gradually softened. The stability of the doped sample remains to be improved further.

200

250

300 Wavelength (nm)

Figure 2. UV spectrum of -[[(C H5)(CH3)2$i]SiCH ]n6

3

350

400

(eyclohexane solution).


27.

NAGAI ET AL.


20

Silicon-Branched

Organosilicon

Polymers

509

L

0

60 120 Irradiation time (min)

180

Figure 3. Photodegradation of-[[(C H )(CH )2Si]SiCH ] -. Key: · , irradiation with a high-pressure mercury lamp (100 W); A, irradiation with a lowpressure mercury lamp (30 W). 6

5

3

3 n

Ladder Polysilane Previously, a several-ring system polysilane, which is soluble in toluene, had been prepared by Baney et al. (15), starting from methylated polychlorodisilanes by heating. This system has a complex network structure. Our second silicon-branched organosilicon polymer, polymer 2, was obtained by the cocondensation of l,l,2,2-tetrachloro-l,2-diisopropyldisilane and 1,2dichloro-l,l,2,2-tetraisopropyldisilane. We have shown previously (16) that the reaction of a 1:3 mixture of the tetrachlorodisilane and the dichlorodisilane with lithium in T H F gives a series of annulated cyclotetrasilane ring such as structures 3-5 (Scheme I). As is shown in Figure 4, a high-pressure liquid chromatogram (HPLC) for a typical run gave major peaks, each of which can be assigned to one of the annulated cyclotetrasilane systems. Small peaks accompanying the major peaks could be due to linear oligomeric polysilanes or to a series of persilaprismanes. Nevertheless, Figure 4 clearly shows that a major part of the product mixture consists of the desired polycyclopolysilanes of ladder struc-


510


R R9Si—Si—SiR.

Cl0.RSiSiRCl«,

I

Li THF

ClR2SiSiR2Cl

I

R*2 S i — R Si—SiR2

(CH3)2CH

R

R


R~Si—Si—Si—SiR2 | 1 I i 2 R„Si—Si—Si—SiR,

Scheme I

ture. Thus, a 10:1 mixture of the tetraehlorodisilane and the dichlorodisilane was treated with lithium or sodium in T H F to give a reddish yellow semisolid in 80-100% yields. The average molecular weight of the crude polymer was 2000, but fractions of M w = 25,000 were observed by GPC. Iodine doping converted the crude polymer to a semiconducting black solid with σ = ~10-2/ft-cm. Thermogravimetric analysis of the polymer was carried out under very nearly the same conditions as previously reported for permethylpolysilane and polysilastyrene (17, 18) (Figure 5). Unfractionated polymer 2 lost 35% of its weight upon heating under nitrogen at a 10 ° C / min rise in temperature, up to 800 °C. The theoretical loss according to equation 4 or 5 is 27 or 40%, respectively, and the results indicate that practically no volatile silicon com pounds have been driven off. (C3H7Si) -+ C H 4 + 3/2H 2 + C + SiC

(4)

(C3H7Si)

(5)

C 2 H 4 + 3/2H 2 4- SiC

West et al. (17) have reported that permethylpolysilane is completely lost at 650 °C, whereas 70% of a sample of polysilastyrene was lost as volatile



2

2

4

3

2

4

2

2

2

2

n

4

2

2

3

Figure 4. High-pressure liquid chrornatogram of cyclic [(R Si ) (R Si ) ] (R = isopropyl; n > 1) from the cocondensation ofR Si Cl (4.6 mmol) and R Si Cl (4.6 mmol) with Li. The following chromatographic conditions were used: eluent, THF-CH OH 3:7 (υIυ); column, ODS (silica gel treated with octadecyldimethylchlorosilane); detection wavelength, 300 nm.

n=l

R



512


800 Temperature (°C) Figure 5. Thermogravimetric curves for polysilanes. Key: —, polysilastyrene; , permethylpolysilane; and , ladder polysilane. (Reproduced with permission from reference 17. Copyright 1983 American Ceramic Society.)

silicon compounds at 800 °C. The ladder silicon network in polymer 2 seems to be responsible for the observed highly efficient ceramic yield.

References 1. Trujillo, R. E. J. Organomet. Chem. 1980, 198, C27.

2. West, R.; David, L. D.; Djurovich, P. I.; Stearley, K. L; Srinivasan, K. S. V.; Yu, H . J. Am. Chem. Soc. 1981, 105, 7352.

3. Trefonas, P., III; Djurovich, P. I.; Zhang, X. H . ; West, R.; Miller, R. D.; Hofer, D. C. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 819. 4. Zhang, X. H . ; West, R. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 159. 5. Zhang, X. H . ; West, R. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 225.

6. Todesco, R. V.; Basher, R. J. Polym. Sci., Part A 1986, 24, 1943.

7. West, R. J. Organomet. Chem. 1986, 300, 327. 8. Miller, R. D.; Sooriyakumaran, R. J. Polym. Sci., Polym. Lett. Ed. 1987, 25,

321. 9. Harrah, L. Α.; Zeigler, J. M . Macromolecules 1987, 20, 2037. 10. Matsumoto, H . ; Yokoyama, N . ; Sakamoto, Α.; Aramaki, Y.; Endo, R.; Nagai, Y. Chem. Lett. 1986, 1643.

11. Ishikawa, M . ; Watanabe, M . ; Iyoda, M . ; Ikeda, H . ; Kumada, M . Organome tallics 1987, 1, 317.

12. Plinka, T. B.; West, R. Organometallics 1986, 5, 128.

13. Watanabe, H . ; Akutsu, Y ; Shinohara, Α.; Ohta, Α.; Onozuka, M . ; Nagai, Y. 52nd Annual Meeting of the Chemical Society of Japan, Kyoto, Japan; Chemical Society of Japan: Tokyo, 1986; Abstract 1K15.


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Silicon-Branched

Organosilicon

Polymers

513

14. Watanabe, H . ; Akutsu, Y.; Shinohara, Α.; Shinohara, S.; Yamaguchi, Y.; Ohta, Α.; Onozuka, M . ; Nagai, Y. Chem. Lett. 1988, 1883. 15. Baney, R. H . ; Gaul, J. H . , Jr.; Hilty, T. K. Organometallics 1983, 2, 859. 16. Matsumoto, H . ; Miyamoto, H . ; Kojima, N . ; Nagai, Y. J. Chem. Soc. Chem., Chem. Commun. 1987,

1316.

17. West, R.; David, L. D.; Djurovich, P. I.; Yu, Y.; Sinclair, S. Ceram. Bull. 1983, 62, 899. 18. Yajima, S.; Hasegawa, Y.; Hayashi, J.; Iimura, M . J. Mater. Sci. 1978, 13, 2569.


RECEIVED for review May 27, 1988. ACCEPTED revised manuscript July 31, 1989.


Synthesis and Properties of Silicon-Branched Organosilicon Polymers

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