**2.3 Electropolymerization of polysilanes with refractory metals**

The study of refractory-metals-containing SiC through polymer-derived ceramic (PDC) method is of interest17, because the introduction of refractory metals M (such as Ta, Ti, Zr, Hf, Nb, Mo et al.) can enhance the heat-resistance property and the anti-oxidative ablation property of SiC ceramic. The methods of introducing M metals can be divided into two classes. One way to obtain M containing ceramic precursors is by pyrolysis of polycarbosilane (PCS) or polysilacarbosilane (PSCS) with organometallic compounds, such as M(acac)n, M(OR)n, Cp2MCl2 18-20. Another way is by condensation of Si-H containing polymethylsilane with MCln21, or by copyrolysis of polydimethylsilane with M(OR)n22. The polymers used in above methods are synthesized by Wurtz reaction, which is featured by the relatively high cost and harsh reaction conditions. Our success in synthesis of polysilane with double bonds has prompted us to fabricate polysilane with refractory metals. By attaching cyclopentadienyl (Cp) ligand to the side chains of PS, the M atoms can be chemically combined with the Cp through η<sup>5</sup> π bonds.

#### **2.3.1 Electrosynthetic procedure of polysilane containing zirconium**

Into the electrolytic cell were plunged MeSiCl3, allyl chloride, and cyclopentadiene in the molar ratio of 1:1/3:1/5. THF was used as solvent, LiClO4 as supporting electrolyte, and magnesium ingot as electrodes. After deairing and inputing nitrogen three times, start stirring and ultrasonic. Set the interval time 10 seconds. The reaction was carried out at constant current mode. ZrCl4/THF solution was charged into the cell as the passed electricity reached 90% of the predetermined electrical amount. It took about 8 hours to finish the electrosynthesis. Then 200ml toluene was poured into the cell, and ammonia gas was introduced to eliminate the remaining Si-Cl groups. After pressure filtration plus vacuum distillation two times, the Zr-containing polysilane EPS3, a black liquid, was obtained. As a comparison, the polysilane EPS4 was synthesized in the same procedure without adding ZrCl4/THF solution.

The reaction is schemed as follows:

The study of refractory-metals-containing SiC through polymer-derived ceramic (PDC) method is of interest17, because the introduction of refractory metals M (such as Ta, Ti, Zr, Hf, Nb, Mo et al.) can enhance the heat-resistance property and the anti-oxidative ablation property of SiC ceramic. The methods of introducing M metals can be divided into two classes. One way to obtain M containing ceramic precursors is by pyrolysis of polycarbosilane (PCS) or polysilacarbosilane (PSCS) with organometallic compounds, such as M(acac)n, M(OR)n, Cp2MCl218-20. Another way is by condensation of Si-H containing polymethylsilane with MCln21, or by copyrolysis of polydimethylsilane with M(OR)n22. The polymers used in above methods are synthesized by Wurtz reaction, which is featured by the relatively high cost and harsh reaction conditions. Our success in synthesis of polysilane with double bonds has prompted us to fabricate polysilane with refractory metals. By attaching cyclopentadienyl (Cp) ligand to the side chains of PS, the M atoms can be

Into the electrolytic cell were plunged MeSiCl3, allyl chloride, and cyclopentadiene in the molar ratio of 1:1/3:1/5. THF was used as solvent, LiClO4 as supporting electrolyte, and magnesium ingot as electrodes. After deairing and inputing nitrogen three times, start stirring and ultrasonic. Set the interval time 10 seconds. The reaction was carried out at constant current mode. ZrCl4/THF solution was charged into the cell as the passed electricity reached 90% of the predetermined electrical amount. It took about 8 hours to finish the electrosynthesis. Then 200ml toluene was poured into the cell, and ammonia gas was introduced to eliminate the remaining Si-Cl groups. After pressure filtration plus vacuum distillation two times, the Zr-containing polysilane EPS3, a black liquid, was obtained. As a comparison, the polysilane EPS4 was synthesized in the same procedure

Cl CH2 CHCH2Cl + x Si Si Si

CH3

CH2 CHCH2

CH3

EPS4

y

**2.3 Electropolymerization of polysilanes with refractory metals** 

chemically combined with the Cp through η<sup>5</sup> π bonds.

without adding ZrCl4/THF solution. The reaction is schemed as follows:

n Si CH3 + m

Cl

Cl

**2.3.1 Electrosynthetic procedure of polysilane containing zirconium** 

EPS3

Scheme 6. Electrosynthesis of polysilane containing refractory metals

### **2.3.2 Characterization of the Zr-containing polysilane**

The EPS3, being synthesized with the monomers of MeSiCl3, allyl chloride, cyclopentadiene and ZrCl4 in the ratio of 1:1/3:1/5:1/5, had the element contents (Wt%): 48.8%Si, 42.6%C, 4.1%Zr and a little oxygen. The retaining ratio of double bonds was 10.8% and the product yield was 71.2%.

The GPC analysis result and viscosity of EPS2 and EPS3 is listed in Table 6. As a result of the introduction of Zr increased the size of the polysilane. Both the EPS2 and EPS3 can dissolve in toluene, THF and chloroform.


Table 6. Molecular weight and Shear viscosity of EPS2 and EPS3

The IR absorption of EPS3 at 1405cm-1 corresponds to Si-Cp23 and the 1640cm-1 peak corresponds to the C=C stretching(Fig.7).

The UV maximum absorptions of EPS3 and EPS4 shift to red, being broadening and higher, compared with EPS2 (Fig.8). The first reason for this phenomenon is the delocalization of the double bonds of the allyl groups, which increases the σ-π conjugation between the backbone and the side groups. Next the Cp group can conjugate with the main chain, making it bigger the whole conjugation system of the molecule. Thus the electron-transfer energy is lowered, causing significant spectral red shifts into the accessible UV region. Furthermore, the big π bond formed between Zr and Cp makes the conjugation system even wider, leading to still broader adsorption peak.

Electropolymerization of Polysilanes with Functional Groups 15

The samples were loaded onto the sample holders. The cross-linking was carried out under dry nitrogen atmosphere at 130°С/12h plus 200°С/3h. The samples' weights before (m0) and after (m) cross-linking were measured so that the mass retaining ratio (m/m0) can be obtained. Ceramization was carried out in a sealed alumina tube furnace in flowing argon gas at 25- 1300°С, with a heating rate 1°С/min and a dwell time of 3h. The crucible loaded with sample was weighed before and after pyrolysis so that ceramic yield may be calculated.

The mass retaining ratio of EPS1 was much higher than that of EPS2 (Table 7), and was close to that of EPS1/DVB (1/0.5). It could be inferred that the double bonds on EPS1 had played important role in the cross-linking processes and were the cause of high mass retaining ratio. The ceramic yields of self-cross linking samples were dramatically higher, compared with specimens having DVB as curing agent. DVB might form inhomogeneous structure during curing process, leading to lower ceramic yield26. Therefore, it is not necessary to add curing agent to polysilane with double bonds, which is on its own a good ceramic precursor. The Zr-containing EPS3 also had a high mass retaining ratio and a moderate ceramic yield.

Sample Mass retaining ratio Ceramic yield

EPS1(self-cross linking) 97.5% 73.4% EPS1+DVB (1:0.5) 99.1% 47.3% EPS2(self-cross linking) 66.1% 70% EPS2+DVB (1:0.5) 76.7% 40% EPS3(self-crosslinking) 86.5% 65.5% EPS3+DVB (1:0.5) 89.1% 60.1%

Table 7. Mass retaining ratio and Ceramic yield of EPS1, EPS2 and EPS3

**3.2 The cross-linking and pyrolysis of polysilanes with functional groups** 

Fig. 9. 1H-NMR spectra of EPS2 and EPS3

DVB: Divinylbenzene , as curing agent.

**3. Ceramization of polysilanes with functional groups** 

**3.1 The procedure of cross-linking and ceramization** 

Fig. 7. FT- IR spectrum of EPS2 and EPS3

#### Fig. 8. UV spectra of EPS2, EPS3 and EPS4

The chemical shift δ=4.9-5.1, 5.8 and 1.5 on the 1H-NMR spectrum of EPS3 (Fig.9) correspond to the primary, secondary and tertiary H of the allyl group13. The spectral peak at δ=5.5 and 6.0 belongs to the H of Cp group attaching to Zr elements24, 25.

The above results prove that EPS3 has the predesigned structure. With Cp groups as a bridge, the refractory metals can be chemically attached to the backbone of the chain. As a result, the polysilane with both double bonds and hetero-elements has been invented.

EPS3

Fig. 7. FT- IR spectrum of EPS2 and EPS3

EPS2

Fig. 8. UV spectra of EPS2, EPS3 and EPS4

EPS2

3500 3000 2500 2000 1500 1000 500 wavenumber(cm-1

200 400 600 800

The chemical shift δ=4.9-5.1, 5.8 and 1.5 on the 1H-NMR spectrum of EPS3 (Fig.9) correspond to the primary, secondary and tertiary H of the allyl group13. The spectral peak

The above results prove that EPS3 has the predesigned structure. With Cp groups as a bridge, the refractory metals can be chemically attached to the backbone of the chain. As a result, the polysilane with both double bonds and hetero-elements has been invented.

wavenumber(nm)

at δ=5.5 and 6.0 belongs to the H of Cp group attaching to Zr elements24, 25.

EPS4

EPS3

)

C=C Si-Cp

Fig. 9. 1H-NMR spectra of EPS2 and EPS3
