Electropolymerization of Polysilanes with Functional Groups

Polysilane (PS) is a kind of polymers with backbone consisting of only silicon atoms. It arouses great attention for its special electronic and optical properties1, as well as SiC ceramic precursor2. The main method to synthesize polysilane is Wurth coupling reaction which rules out the production of many polysilanes with functional groups for the harsh reaction conditions3. Among the various methods for the synthesis of polysilanes4, the electrosynthesis technique is gaining importance for its mild and safe condition and aptness of making functional molecules5. Shono6 studied the effects of electrode materials, monomer concentration, amount of supplied electricity, and ultrasonic on the synthesis of polysilanes and related polymers. The concerned monomers bear inert side groups, such as methyl, ethyl, butyl, phenyl. The first attempt of attaching reactive vinyl groups to polysilane by electrosynthesis failed7. Our purpose to get high ceramic yield SiC precursor has prompted us searching for ways to introduce cross-linkable groups into polysilane molecules. Using allyl chloride and methyltrichlorosilane(MeSiCl3) as monomers, we managed to synthesize polysilane with double bonds8, an excellent SiC precursor9. To increase the anti-oxidative ablation property of SiC, one way is to adulterate refractory metals M, such as Ti, Zr, Hf, et al., into SiC ceramic10. Coordination of M into polysilane molecule needs ligands. Cyclopentadiene(Cp), which is served as ligand for transitional metals, was attached on PS chains, thus zirconium atom was chemically combined with polysilane chain. And a new route to make composite ceramic MC-SiC was invented.


Electropolymerization of polysilanes 2.1 The setup of electrosynthesis
To take advantage of electrosynthesis, many efforts are needed to design the electrolytic apparatus. The paper 6 briefly introduces the setup of electropolymerization. Here we present the details of the apparatus. The setup consists of four parts (Scheme1). The DC power can operate under two modes: the constant current mode and the constant voltage mode. We usually choose the constant current mode. The commutator can change the polarity of the electrodes in the range of 5 ~ 60 seconds. The supplied electricity is counted by a coulometer. When a predetermined electric volume is reached, it will cut off the electric supply to the electrolytic cell. A detailed description of the cell is plotted in Scheme 2. Scheme 1. The structure of the setup Scheme 2. The scheme of the electrolytic cell 1. N 2 bottle (99.99%) 2.drying tower (molecular sieve) 3.drying tower (CaCl 2 ) 4.ultrasonic 5.electrode 6. rubber plug 7. N 2 inlet 8.stirrer 9.motor 10.feeding hole 11.thermometer The rectangle electrodes are 40mm long, 30mm wide and 10mm thick. It is rubbed with emery cloth before use. The space between the two poles is fixed by inserting two hollow polyethylene tubes (Φ4mm), and the electrodes and the tubes are binded together with nylon wire. The power of the ultrasonic (20 kHz) is 100W.

Electropolymerization of polysilanes with double bonds
The introduction of double bonds will increase the ceramic yield, shorten the production period of the C/C-SiC composites, and diminish the mechanical damage to the composites. We will discuss in detail the influential factors in the reaction and characterize the reaction product.

Electropolymerization and isolation
The MeSiCl 3 and allyl chloride were put in a compartment cell equipped with Mg cathodes, mechanic stirrer and thermograph. Then THF and LiClO 4 (0.05M) were added. All reactions were carried out under dry nitrogen atmosphere in order to eliminate the oxygen. Constant current electrolysis was applied at room temperature. During the electroreduction the electrolysis cell was sonicated and the polarity of electrodes was periodically alternated. The supplied amount of electricity was counted by a coulometer. Electrolysis was terminated at the desired amount of electricity. Then a quantity of toluene was poured in the cell and adequate gaseous ammonia was introduced to react with the residual Si-Cl. The solution was filtrated to remove MgCl 2 precipitate, and then was vacuum distilled to remove THF in the solvent. Subsequently, the residue was filtrated again to remove insoluble material. Finally, toluene was evaporated at reduced pressure, in each case to yield the light yellow liquid, polysilane with double bonds (EPS1). The reaction scheme is shown as follows:

Scheme 3. Electrosynthesis of polysilane with double bonds
Polysilane without double bonds (EPS2) was synthesized in the same way using only MeSiCl 3 as monomer for comparison. The products were identified using GPC (Agilent1100 American) with THF as solvent, 1 H NMR (AV-500), XRD (D/Max-rc Japan) and FTIR (Thermo Nicolet-AVRTAR370FT-IR). FTIR spectra were measured using KBr method. The double bond content was determined by bromine addition method 11 . The measurement of monomer reaction rate is as follows: Half an hour later after the monomers are plunged into the reaction bottle, the electrodes are charged. A small amount of sample is drawn out at intervals. It is hydrolyzed, and then titrated with sodium hydroxide solution. Therefore, the content of Si-Cl bonds in the bottle is determined. By following the variation of the Si-Cl bonds, we can analyze the reaction rate of the electropolymerization.

Influential factors of electrosynthesis
In the following episodes, the various factors influencing the reaction will be discussed in detail.

Effect of molar ratios of monomers
The influence of the MeSiCl 3 /allyl chloride ratio on the yield, remaining double bonds, and Mw of polysilanes is shown in Table 1. The optimum molar ratio of MeSiCl 3 /H 2 C=CHCH 2 Cl was 3/1 where the highest yield, double bonds concentration and molecular weight were reached. The relatively low yield and molecular weight at ratio of 10/1 and 5/1, where allyl chloride was deficient, might be due to the intramolecular reaction of MeSiCl 3 . This reaction led to highly cross-linked structure, which was insoluble in solvent. Only the low molecular weight fraction was www.intechopen.com soluble and obtainable as product. As the molar ratio surpassed 3/1, the excessive allyl chloride, acting as end capping agent, blocked the propagation of polysilane chains, resulting in lower molecular weight.  (Fig.2) has four major peaks near δ 0.09, 0.25, 0.55 and 0.85 ppm, these are chemical shifts of Si-CH 3 . The broad and complication of these peaks are due to the complex structures around the Si-CH 3 groups. The observed peaks near δ 5.8, 5.1-5.2, and 3.8-3.9 are the characteristic chemical shifts for Si-allyl moiety 13 . This proves the presence of double bonds in polysilane. The following experiments were performed on MeSiCl 3 /Allyl Chloride ratio of 3:1.

Effect of monomer concentration
Under different monomer concentrations, the polysilanes were synthesized with 5% theoretical electricity. As shown in Table 2, the yield and molecular weight of the polysilanes became lower with increase of concentration. When the concentration was higher, MeSiCl 3 was apt to polymerize with itself, forming giant three dimensional networks, which would precipitate from solution.  Table 2. Effect of monomer concentration The following experiments were performed at 1M monomer concentration.

Effect of electrode materials
The electrode materials have a profound influence on the formation of polysilanes. As Table  3 shows, the entry using Mg anode and cathode gave the highest molecular weight and www.intechopen.com yield. Aluminum was less effective than Mg, and Cu the worst. It was said that Mg played some important roles in the formation of the Si-Si bond 6 . Although details of the role of Mg in the mechanism of formation of the Si-Si bond are not clear now, the unique reactivity of Mg electrode is undoubtedly shown in this reaction.  The following experiments were performed with Mg electrodes.

Effect of the amount of electricity
The molecular weight was monitored at the different amount of electricity during the reaction process (Fig. 3). The molecular weight went up rapidly before 3% theoretical electricity (1440C*mol -1 ) and entered a linear increasing period afterwards. This suggests that electrochemical synthesis of polysilane is a step-growth polymerization.  Table 4. With increase of the amount of electricity, the molecular weight rose stepwise and the molecular weight distributions became broader. The broader Mw /Mn might be due to the heterogeneous reaction condition and multiple reaction paths 14 Table 4. Effect of the amount of electricity

Effect of interval time
The electroreduction was carried out under ultrasound with different interval times of 8s, 17s and 26s, respectively. As shown in Fig. 4, the molecular weights of the products became higher with decreasing of the interval time. It could be explained that the alternation of anode and cathode might overcome the difficulty of keeping the electric current at a suitable level due to the increase of the voltage between anode and cathode with progress of the reaction 6 . The maximum voltage was relatively lower at shorter interval time. The lower voltage reduced the possibility of side reactions, thus benefiting the propagation of Si-Si bonds. www.intechopen.com

Effect of the concentration of supporting electrolyte
The increase in the concentration of supporting electrolyte resulted in the growth of molecular weight and yield (Table 5). Higher concentration of supporting electrolyte brought on lower electrode voltage to maintain constant current, thus reducing side reaction. The 0.05M concentration seemed reasonable because the acquired molecular weight and yield were satisfying, and even higher concentration leads to higher cost as the recovery of costly LiClO 4 is difficult.

Comparison of the rates of electrosynthesis with and without electricity
The result above indicates polymerization can happen without electricity. To evaluate the role of electricity in the synthesis, the synthetic reactions of EPS1 and EPS2 with and without electricity were pursued (Fig. 6). The Si-Cl content in the case of electricity (curve b and d) is 5% ~ 13% less than that without electricity (curve a and c). In the whole electrosynthesis, the Grignard reaction between monomers and Mg ingot is in the majority, its ratio being as high as 80% ~ 90%. This result implies that the amount of charge passed could be much lower than that of the theoretical one to fulfill the electrosynthesis.

On the reaction mechanism
The reaction mechanism for synthesis of polysilane through Wurtz route has been studied in the past few decade 15 . However, the mechanism by electrochemical route is scarcely reported 16 . According to above-mentioned, we speculate the mechanism might be divided into two parts. The first part is the Grignard reaction mechanism (Scheme 4). The monomers can react with Mg to form Grignard reagent, which again react with CH 3 SiCl 3 to produce a large molecule. The process repeats to form polysilane. Another part is the electropolymerization mechanism (Scheme 5). The reactions include the electrode reaction and thermal reaction. First, the monomer gains an electron, being reduced to anion. The anion changes to silicon radical by eliminating chlorine anion, which can react www.intechopen.com with magnesium ion from sacrificing anode. The formed silicon radical might gain another electron to change into silicon anion intermediate. This intermediate reacts with other monomers by nucleophilic substitution to form oligomers, which can also be reduced to silicon anions. In the process of chain elongation, the delocalization of the electron along the chain makes it easier for the reduction of the oligomer. The thermal process includes the nucleophilic substitution of silicon anion with monomer or oligomer, and the coupling reaction between radicals. The nucleophilic substitution may happen intramolecularly or intermolecularly. The intramolecular one leads to cross-linking, while the intermolucular makes the chain to grow.

Electropolymerization of polysilanes with refractory metals
The study of refractory-metals-containing SiC through polymer-derived ceramic (PDC) method is of interest 17  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 5 π bonds.

Electrosynthetic procedure of polysilane containing zirconium
Into the electrolytic cell were plunged MeSiCl 3 , allyl chloride, and cyclopentadiene in the molar ratio of 1:1/3:1/5. THF was used as solvent, LiClO 4 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. ZrCl 4 /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 ZrCl 4 /THF solution. The reaction is schemed as follows:

Characterization of the Zr-containing polysilane
The EPS3, being synthesized with the monomers of MeSiCl 3 , allyl chloride, cyclopentadiene and ZrCl 4 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-Cp 23 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. The chemical shift δ=4.9-5.1, 5.8 and 1.5 on the 1 H-NMR spectrum of EPS3 (Fig.9) correspond to the primary, secondary and tertiary H of the allyl group 13 . The spectral peak at δ=5.5 and 6.0 belongs to the H of Cp group attaching to Zr elements 24,25 .
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.

The procedure of cross-linking and ceramization
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 (m 0 ) and after (m) cross-linking were measured so that the mass retaining ratio (m/m 0 ) 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 cross-linking and pyrolysis of polysilanes with functional groups
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 yield 26 . 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.

Improvement of the anti-oxidative ablation property of C/C composites by using polysilanes with functional groups
The poor oxidation resistance of C/C composites restricts its usage in oxidized environments. To improve its oxidative ablation resistance, the matrix of C/C composites is www.intechopen.com modified with SiC ceramic 27 . The introduction of SiC can be achieved by the chemical vapor infiltration (CVI) and the polymer infiltration and pyrolysis (PIP) 28 . PIP has aroused much interest due to its low manufacturing temperature, ability for the design of precursor molecules, simplicity of formation and feasibility of fabricating complex components. The main precursors for SiC at present are polycarbosilane and polysilane (PS) 4 . The C/C-SiC composites were manufactured using PS with double bonds by the PIP technique 9 . The C/C-ZrC-SiC composites were prepared using Zr-containing polysilane. The ablation behavior of the C/C-SiC composites and C/C-ZrC-SiC composites were measured. The used C/C composites with a density of 1.29 g/cm 3 were made by CVI of bulk needled carbon felt and temperature treatment at 2 300 ℃. The C/C-SiC composites and C/C-ZrC-SiC composites were densified by the following procedure: C/C composites → vacuum + pressure infiltration of precursor → cross-linking → pyrolysis → sintering. The process was repeated several times. The maximum density of the obtained C/C-SiC and C/C-ZrC-SiC composites were 1.75 g/cm 3 and 1.69 g/cm 3 , respectively. The specimens, with a size of ø30 mm×10 mm, were flushed vertically with a H 2 -O 2 torch flame. The inner diameter of the nozzle tip was 4.68 mm. The distance between the nozzle tip and specimen surface was 15.8 mm. The pressure and flux of oxygen were 1.55 MPa and 2.1 L/min, and for hydrogen, they were 0.18 MPa and 1.68 L/min, respectively. The ablation time was 180 s. The thickness of the specimen before ablation (d1) was measured with a micrometer (precision 0.01 mm), and the mass (m1) was measured with an analytical balance (precision 0.1 mg). After ablation, the thickness at the lowest point (d2) and mass (m2) were measured. For the ablation time of t, the linear ablation rate was calculated by (d1-d2)/t, and the mass ablation rate was calculated by (m1-m2)/t. The ablation test results are depicted in Fig.12 and Fig.13, which show that the linear and mass ablation rate of the C/C-SiC and C/C-ZrC-SiC were lower than that of the C/C composites at the same density. With the increase of infiltration times, the density of C/C-SiC specimens increased, and the ablation rate decreased. The linear ablation rates of the 1.75 g/cm 3 C/C-SiC and 1.69 g/cm 3 C/C-ZrC-SiC specimen were 21.7% and 20.6% those of www.intechopen.com the 1.78 g/cm 3 C/C specimen, respectively. And the mass ablation rates of the 1.75 g/cm 3 C/C-SiC and 1.69 g/cm 3 C/C-ZrC-SiC specimen were 78.6% and 31.6% those of the 1.78 g/cm 3 C/C specimen, respectively. Therefore, introduction of SiC into C/C composite greatly improved its oxidative ablation-resistive property, and the adulteration of Zr element enhances the effect even more. It is worthwhile to investigate the impact of attaching refractory metals, such as Ta, Zr, Hf, Th, et al. solely or combinatorially to polysilanes on the anti-oxidative ablation property of the C/C composites.

Conclusions
The electroreduction MeSiCl 3 and Allyl chloride monomers carried out with Mg electrodes in a single compartment cell gave polysilane with double bonds. The preferred MeSiCl 3 /Allyl chloride ratio was 3:1. The molecular weight and yield of the products were affected by the concentration of monomer, the amount of electricity, the electrode's interval time and concentration of supporting electrolyte. By introducing cyclopendienyl ligands as side groups of polysilanes, the polysilanes with refractory metals were obtained by the combination of Cp with the metal elements. The synthesized polysilanes with self-curable ability and high ceramic yields are excellent ceramic precursors.