Effect of electrode materials in the electroreductive formation of 1,1,2,2-tetramethyl-1,2-diphenyldisilane (
In contrast to the growing interest with the polysilane, the method of preparation hitherto known is highly limited. So far, the almost only practical method is the condensation of organodichlorosilane with alkali metal (Wurtz type condensation). This method, however, requires drastic reaction conditions and hence, is very much limited in the type of substituent that is allowed to be located on the monomer. Although several modified or alternative methods such as sonochemical coupling of dichlorosilane promoted by alkali metal [9-11], transition metal catalyzed reaction of hydrosilane [12, 13], anionic polymerization of masked disilene [14, 15], and ring opening polymerization of cyclic organosilane [16, 17] have been proposed, they are not always extensively effective as preparative methods.
The electroreductive coupling of dichlorosilanes with mercury electrode has been reported by Hengge in 1976 as a method to form disilanes , though this method was not effective in the preparation of polysilanes [19, 20].
On the other hand, we have recently found that the electroreduction of organic compounds with Mg electrode promotes a variety of unique reactions which can not be attained without using the Mg electrode. The use of Mg electrodes was highly effective to the formation of Si-Si bond and the synthesis of high molecular weight polysilanes [21, 22].
In this chapter, we describe the details of the electroreductive synthesis of high molecular weight polysilane and some types of functionalized polysilanes and also polygermanes, including the additional information about the effects of electrode material and monomer concentration. We also demonstrate that our electroreduction system is successfully applied for the synthesis of the sequence-ordered oligosilanes and polysilanes.
2. Formation of Si-Si bonds by electroreductive coupling of chlorosilanes [21, 22]
The electroreduction of chlorodimethylphenylsilane (
The material of electrode is one of the most important factors to control the formation of Si-Si bond (
The cathodic coupling of other organochlorosilanes was carried out under the optimized reaction condition, that is, Mg cathode and anode were alternated with the interval of 1 min., supporting electrolyte was LiClO4, solvent was THF, and the electricity passed was 2.0 F/mol (
|entry||anode||cathode||alternation||yield of |
|entry||chlorosilanes ||yield of |
Two types of mechanism may be proposed to this electroreductive Si-Si bond forming reaction. The first prosible mechanism is a radical coupling in which a silyl radical formed by one electron reduction of the starting chlorosilane couples with another silyl radical to give the disilane. In the second mechanism, two-electron reduction of the chlorosilane yields an active species equivalent to silyl anion which reacts with chlorosilane to give a dimer. In order to have an insight into the mechanism, the products obtained in the mixed system of chlorotrimethylsilane (
3. Stepwise synthesis of oligosilanes [22, 23]
This method is also applicable to the synthesis of trisilanes and tetrasilanes. For example, the electroreductive cross coupling of organodichlorosilanes (
The mildness of the reaction conditions of this electroreductive method is favorable for the synthesis of oligosilanes having various functionalities such as Si-H bonds which are known to be reactive under radical or anionic condition. The electroreductive cross-coupling reaction of chlorodimethylsilane (
4. Electroreductive polymerization of dichloromethylphenylsilane 
Electroreduction of dichloromethylphenylsilane (
|entry||alternation||sonication||yield of |
Mg is a remarkably effective material of electrode also for the formation of
|entry||electrode materials||yield of |
The effect of monomer concentration was investigated in order to obtain high molecular weight polysilane (
The most satisfactory result, in which material yield was 79 % and molecular weight (
|entry||monomer ||supplied electricity, F/mol||yield of |
The mechanism of electroreductive formation of polysilane is not always perfectly clear, though the initial step of reaction is obviously the reduction of
5. Electroreductive synthesis of functionalized polysilanes 
The mildness of the polymerization conditions of the electroreductive method is favorable for the synthesis of the polysilanes having a variety of hydroxyl-related functional groups. The electroreduction of a mixture of
The modification or the property of polysilane must be achieved by using the hydroxyl group located on the polymer
|entry||charged mol% of ||yield of ||Mne||Mw/Mne|
|1||7 (||79 (7)||9900||1.9|
|2||10 (||57 (12)||6900||1.7|
|3||100 (||28 (100)||1100||1.2|
|4||10 (||36 (11)||6100||1.5|
|5||100 (||-f (100)||1100||1.2|
|6||10 (||50 (6)||4500||1.3|
|7||50 (||22 (46)||4600||1.3|
|8||100 (||-f (100)||1700||1.3|
|9||10 (||56 (17)||4600||1.3|
|10||100 (||57 (100)||4000||1.1|
6. Electroreductive Polymerization of Dichlorooligosilanes 
The electroreductive polymerization of the dichlorooligosilanes is highly promising for the synthesis of sequence-ordered polysilanes. The electroreduction of dicholodisilane
|entry||dichlorosilane ||supplied electricity, F/mol||polysilane |
Dichlorooligosilanes, such as dichlorotrisilane 9 was found to be good monomers for the electroreductive synthesis of the polysilanes having longer sequence units (Schemes 16). The temperature control is found to be very important in the electroreductive polymerization of 9 (Table 8). The reaction at higher temperature, the backbiting reaction of the propagating polymer proceeded forming cyclohexasilane as a by-product (entry 1). This side reaction was successfully suppressed when the reaction was carried out below 0°C, and polysilanes 19 having relatively high molecular weight were obtained (entries 3, 4). In the optimized reaction conditions, the electroreduction of dichlorotetrasilane 12 gave the corresponding polysilane 20, units of which were ordered in four sequences in satisfactory yield (Scheme 17). The polymerizability of dichlorooligosilanes under the electroreduction conditions seems to be mainly affected by the substituents on the chlorinated terminal silicon atom, and this fact provides a wide possibility to design the oligosilane sequences of the inner silicon atoms.
|entry||polymerization temperature, °C||yield of |
7. Electroreductive polymerization of dichlorosilanes in the presence of disilane additives 
The disilane additives, which are generated
The mechanism for the triphenylsilyl substituted disilane to control the electroreductive polymerization suggested is as follows (
|entry||disilane additives||yield of |
triphenylsilyl group terminated polysilane. The resulting polysilane is isolable but does not lose its polymerizability completely since the triphenylsilyl group at the terminal position acts as an activator, that is, it probably reacts as a macroinitiator.
8. Electroreductive block copolymerization using triphenylsilyl group-terminated polysilane 
The triphenylsilyl group terminated polysilanes have been synthesized by the electroreductive polymerization of dichloromethylphenylsilane (
|disilane additives||yield, %|
|1||Me3SiSiMe3||3740||1.9||5530||1.4||83 : 17||70 : 30||16|
|2||Me3SiSiPh3||3000||1.3||4080||1.4||66 : 34||77 : 23||28|
|3||Ph3SiSiPh3||3350||1.4||4730||1.2||75 : 25||74 : 26||38|
The UV absorption spectra of the resulting polysilane
The formation of Si-Si bonds was achievable by the electroreductive condensation of organochlorosilanes with Mg sacrificial electrode. Disilanes, trisilanes, and tetrasilanes were readily obtained in good to moderate yield. Moreover, this method was also remarkably effective to the synthesis of polysilanes. The molecular weight and yield of the polymers was controlled by the concentration of monomers and the supplied electricity. The mildness of the reaction conditions allowed to use a wide variety of monomers, and enabled the synthesis of the functionalized polysilanes and the structure-controlled polysilanes. The electroreductive polymerization of the dichlorooligosilanes was highly useful for the synthesis of sequence-ordered polysilanes. Moreover, this electroreductive method also provided a new procedure to synthesize well-controlled di-block polysilane copolymers. Since the present electroreductive polymerization requires only a single compartment cell, it is undoubtedly one of the simplest and most powerful tools for synthesis of polysilanes.
West R. 1986 The polysilane high polymersJ. Organomet. Chem. 300(1-2): 327 EOF 346 EOF
Yajima S. Hasegawa Y. Hayashi J. Iimura M. 1978 Synthesis of continuous silicon carbide fiber with high tensile strength and high Young’s modulusPart 1. Synthesis of polycarbosilane as precursor. J. Mater. Sci. 13 12 2569 2576
Hasegawa Y. Okamura K. 1985Silicon carbide-carbon composite materials synthesized by pyrolysis of polycarbosilane. J. Mater. Sci. Lett. 4 3 356 8
Miller R. D. Willson C. G. Wallroff G. M. Clecak N. Sooriyakumaran R. Michl J. Karatsu T. Mc Kinley A. J. Klingensmith K. A. Downing J. 1989Polysilanes: photochemistry and deep-UV lithography. Polym Eng Sci 29 13 882 886
Miller R. D. Michl J. 1989Polysilane high polymers. Chem. Rev. 89 6 1359 1410
West R. David L. D. Djurovich P. I. Stearley K. L. Srinivasan K. S. V. Yu H. 1981 Phenylmethylpolysilanes: formable silane copolymers with potential semiconducting propertiesJ. Am. Chem. Soc. 103 24 7352 7354
Kepler RG, Zeigler JM, Harrah LA, Kurtz SR 1987Photocarrier generation and transport in σ-bonded polysilanes. Phys. Rev. B 35 6 2818 2822
Baumert J. C. Bjorklund G. C. Jundt D. H. Jurich M. C. Looser H. Miller R. D. Rabolt J. Soorijakumaran R. JD Swalen Twing. R. J. 1988Temperature dependence of the third-order nonlinear optical susceptibilities in polysilanes and polygermanes. Appl. Phys. Lett. 53 13 1147 1149
Matyjaszewski K. Greszta D. Hrkach J. S. Kim H. K. 1995 Sonochemical synthesis of polysilylenes by reductive coupling of disubstituted dichlorosilanes with alkali metalsMacromolecules 28 1 59 72
Jones RD, Holder SJ 2006 High-yield controlled syntheses of polysilanes by the Wurtz-type reductive coupling reactionPolym. Int. 55 7 711 718
Koe J. 2008 Contemporary polysilane synthesis and functionalisationPolym. Int. 58 3 255 260
Tilley TD 1993The coordination polymerization of silanes to polysilanes by a " σ-bond metathesis" mechanism. Implications for linear chain growth. Acc. Chem. Res. 26 1 22 29
Minato M. Matsumoto T. Ichikawa M. Ito T. 2003 Dehydropolymerization of arylsilanes catalyzed by a novel silylmolybdenum complex.Chem. Commun. (24): 2968 EOF 9 EOF
Sanji T. Kawabata K. Sakurai H. 2000 Alkoxide initiation of anionic polymerization of masked disilenes to polysilanesJ. Organomet. Chem. 611(1-2): 32 EOF
Sanji T. Isozaki S. Yoshida M. Sakamoto K. Sakurai H. 2003Functional transformation of poly(dialkylaminotrimethyldisilene) prepared by anionic polymeriztion of the masked disilenes. The preparation of a true polysilastyrene. J. Organomet. Chem. 685 (1-2): 65-69.
Cypryk M. Gupta Y. Matyjaszewski K. 1991 Anionic ring-opening polymerization of 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilaneJ. Am. Chem. Soc. 113 3 1046 7
Suzuki M. Kotani J. Gyobu S. Kaneko T. Saegusa T. 1994Synthesis of sequence-ordered polysilane by anionic ring-opening polymerization of phenylnonamethylcyclo-pentasilane. Macromolecules 27 8 2360 2363
Hengge E. Litscher G. 1976A new electrochemical method for the formation of silicon-silicon bonds. Angew] Chem] 88(12): 414.
Hengge E. Litscher G. 1978Electrochemical formation of di-, oligo- and polysilanes. Monatsh. Chem. 109 5 1217 1225
Hengge E. Firgo H. 1981An electrochemical method for the synthesis of silicon-silicon bonds. J. Organomet. Chem. 212 2 155 161
Shono T. Kashimura S. Ishifune M. Nishida R. 1990 Electroreductive formation of polysilanesJ. Chem. Soc. Chem. Commun. (17): 1160 EOF
Kashimura S. Ishifune M. Yamashita N. Bu H. B. Takebayashi M. Kitajima S. Yoshihara D. Kataoka Y. Nishida R. Kawasaki S. Murase H. Shono T. 1999Electroreductive synthesis of polysilanes, polygermanes, and related polymers with magnesium electrodes. J. Org. Chem. 64 18 6615 6621
Ishifune M. Kashimura S. Kogai Y. Fukuhara Y. Kato T. Bu H. B. Yamashita N. Murai Y. Murase H. Nishida R. 2000Electroreductive synthesis of oligosilanes and polysilanes with ordered sequences. J. Organomet. Chem. 611(1-2): 26-31.
Umezawa M. Takeda M. Ichikawa H. Ishikawa T. Koizumi T. Nonaka T. 1991Electroreductive polymerization of mixtures of chloromonosilanes. Electrochim. Acta 36(3-4): 621-624.
Biran C. Bordeau M. Pons P. Leger M. P. Dunogues J. 1990Electrosynthesis, a convenient route to di- and polysilanes. J. Organomet. Chem. 382 (3): C 17C20.
( Kunai A. Kawakami T. Toyoda E. Ishikawa M. 1991) Electrochemistry of organosilicon compounds. 2. Synthesis of polysilane oligomers by a copper electrode system. Organometallics 10 6 2001 2003.
Okano M. Takeda K. Toriumi T. Hamano H. 1998 Electrochemical synthesis of polygermanesElectrochim. Acta 44 4 659 666
Yamada K. Okano M. 2006Electrochemical synthesis of poly(cyclotetramethylene-silylene) Electrochemistry 74 8 668 671
Kashimura S. Ishifune M. Bu H. B. Takebayashi M. Kitajima S. Yoshihara D. Nishida R. Kawasaki S. Murase H. Shono T. 1997Electroorganic chemistry. 153. Electroreductive synthesis of some functionalized polysilanes and related polymers. Tetrahedron Lett. 38 26 4607 4610
Ishifune M. Kogai Y. Uchida K. 2005 Effect of disilane additives on the electroreductive polymerization of organodichlorosilanesJ. Macromol. Sci. Part A Pure and Appl. Chem. 42 7 921 929
Chen S. M. David L. D. Haller K. J. Wadsworth C. L. West R. 1983Isomers of (PhMeSi)6 and (PhMeSi)5 Organometallics 2 3 409 414
Ishifune M. Sana C. Ando M. Tsuyama Y. 2011 Electroreductive block copolymerization of dichlorosilanes in the presence of disilane additivesPolym. Int. 60 8 1208 1214
Terunuma D. Nagumo K. Kamata N. Matsuoka K. Kuzuhara H. 2000 Preparation and characterization of water-soluble polysilanes bearing chiral pendant ammonium moietiesPolymer Journal 32 113 117
Herzog U. West R. 1999Hererosubstituted polysilanes. Macromolecules 32 2210 2214
Hu Z. Zhang F. Huang H. Zhang M. He T. 2004Morphology and structure of poly(di-n-butylsilane) single crystals prepared by controlling kinetic process of solvent evaporation. Macromolecules 37 3310 3318
Chunwachirasiri W. Kanaglekar I. MJ Winokur Koe. J. C. West R. 2001Structure and chain conformation in poly(methyl-n-alkyl)silanes. Macromolecules 34 6719 6726