New Condensation Polymer Precursors Containing Consecutive Silicon Atoms—Decaisopropoxycyclopentasilane and Dodecaethoxyneopentasilane—And Their Sol–Gel Polymerization

The sol–gel polymerization of alkoxysilanes is a convenient and widely used method for the synthesis of silicon polymers and silicon–organic composites. The development of new sol–gel precursors is very important for obtaining new types of sol–gel products. New condensation polymer precursors containing consecutive silicon atoms—decaisopropoxycyclopentasilane (CPS) and dodecaethoxyneopentasilane (NPS)—were synthesized for the preparation of polysilane–polysiloxane material. The CPS and NPS xerogels were prepared by the sol–gel polymerization of CPS and NPS under three reaction conditions (acidic, basic and neutral). The CPS and NPS xerogels were characterized using N2 physisorption measurements (Brunauer–Emmett–Teller; BET and Brunauer-Joyner-Halenda; BJH), solid-state CP/MAS (cross-polarization/magic angle spinning) NMRs (nuclear magnetic resonances), TEM, and SEM. Their porosity and morphology were strongly affected by the structure of the precursors, and partial oxidative cleavage of Si-Si bonds occurred during the sol–gel process. The new condensation polymer precursors are expected to expand the choice of approaches for new polysilane–polysiloxane.

Quite recently, our group found interesting reactivities of peralkoxy-, perchlorocyclopentasilane, and perchloroneopentasilane: (i) Si-Si bond breaking of a soluble polymer synthesized from decaethoxycyclopentasilane with methyltrimethoxysilane occurred even at one minute-baking conditions (200 • C) [14], while a polymer obtained from hexaethoxyhexamethylcyclohexasilane with methyltrimethoxysilane was thermally stable; (ii) adding a secondary amine into minute-baking conditions (200 °C) [14], while a polymer obtained from hexaethoxyhexamethylcyclohexasilane with methyltrimethoxysilane was thermally stable; ii) adding a secondary amine into decachlorocyclopentasilane and dodecachloroneopentasilane [15] induced Si-Si bond cleavages [16][17][18][19][20]. The interesting cleavage reactivity of the consecutive silicon bonds in the oligosilanes and the lack of study on the sol-gel process of peralkoxyoligosilanes prompted us to study their sol-gel reaction. To the best of our knowledge, sol-gel reactions of oligosilanes have not been studied. Only commercially available simple alkoxydisilanes-(OEt)3SiSi(OEt)3 and (OMe)3SiSi(OMe)3-were used for the synthesis of sol-gel silicon oxide materials [21,22]. We expected that the new core structures and Si-Si bond breaking may affect the morphology and textural property of the resulting sol-gel polymer.
In this report, we describe the synthesis of new condensation polymer precursors containing consecutive silicon atoms-decaisopropoxycyclopentasilane (CPS) and dodecaethoxyneopentasilane (NPS)-as well as their sol-gel reactions and structures of the resulting CPS and NPS xerogels, representing the first sol-gel study of peralkoxyoligosilane.

Materials
In all of the reactions in which air-sensitive chemicals were used, the reagents and solvents were dried prior to use. Diethyl ether, triethylamine, and n-hexane were distilled from Na/Ph2CO. EtOH and i PrOH were distilled from calcium hydride. Other starting materials were purchased as reagent grade and were used without further purification. Glassware was flame-dried with nitrogen or argon flushing prior to use. All of the manipulations were performed using the standard Schlenk techniques in nitrogen or argon atmosphere and using a glove box (MBraun, Garching, Germany).

Instruments and Measurements
1 H, 13 C, and 29 Si NMR spectra were recorded using a Bruker Avance II + BBO 400 MHz S1 spectrometer (Bruker, Billerica, MA, US). The chemical shifts were referenced to internal C6D6 or CDCl3, or external tetramethylsilane. All of the solid-state NMR experiments were conducted with NMR instruments (Varian unity NOVA, Varian, Palo Alto, CA, US) using 5 and 2.5 mm double resonance MAS probe heads for 29 Si and 1 H at 14.1 T ( 1 H resonance frequency 600 MHz, wide bore) at room temperature. Mass spectra were recorded using a low-resolution (Agilent Technologies GC/MS: 6890N, 5973N mass selective detector) EI (electron ionization) mass spectrometer and a highresolution (JEOL JMS-600W Agilent 6890 Series, Agilent, Santa Clara, CA, US) instrument. The morphology of the xerogels was monitored by scanning electron microscopy (SEM, Quanta 250FEG, FEI, Hillsboro, OR, US). High-resolution transmission electron microscopy (HRTEM) was performed with a JEOL JEM2100F (200 kV) instrument (JEOL, Tokyo, Japan) using a carbon-coated 200-mesh copper grid. Nitrogen adsorption-desorption isotherms were obtained using a surface area analyzer (BELSORP-max and Micromeritics, ASAP 2010, BEL, Osaka, Japan). Thermogravimetric analysis (TGA) curves were recorded using a simultaneous thermal analyzer (STA, STA 8000, Perkin Elmer, Waltham, MA, US) in N2 (50.0 mL/min) atmosphere with heating from 30.0 to 1000.0 °C (10.0 °C/min).

Materials
In all of the reactions in which air-sensitive chemicals were used, the reagents and solvents were dried prior to use. Diethyl ether, triethylamine, and n-hexane were distilled from Na/Ph 2 CO. EtOH and i PrOH were distilled from calcium hydride. Other starting materials were purchased as reagent grade and were used without further purification. Glassware was flame-dried with nitrogen or argon flushing prior to use. All of the manipulations were performed using the standard Schlenk techniques in nitrogen or argon atmosphere and using a glove box (MBraun, Garching, Germany).

Instruments and Measurements
1 H, 13 C, and 29 Si NMR spectra were recorded using a Bruker Avance II + BBO 400 MHz S1 spectrometer (Bruker, Billerica, MA, USA). The chemical shifts were referenced to internal C 6 D 6 or CDCl 3 , or external tetramethylsilane. All of the solid-state NMR experiments were conducted with NMR instruments (Varian unity NOVA, Varian, Palo Alto, CA, USA) using 5 and 2.5 mm double resonance MAS probe heads for 29 Si and 1 H at 14.1 T ( 1 H resonance frequency 600 MHz, wide bore) at room temperature. Mass spectra were recorded using a low-resolution (Agilent Technologies GC/MS: 6890N, 5973N mass selective detector) EI (electron ionization) mass spectrometer and a high-resolution (JEOL JMS-600W Agilent 6890 Series, Agilent, Santa Clara, CA, USA) instrument. The morphology of the xerogels was monitored by scanning electron microscopy (SEM, Quanta 250FEG, FEI, Hillsboro, OR, USA). High-resolution transmission electron microscopy (HRTEM) was performed with a JEOL JEM2100F (200 kV) instrument (JEOL, Tokyo, Japan) using a carbon-coated 200-mesh copper grid. Nitrogen adsorption-desorption isotherms were obtained using a surface area analyzer (BELSORP-max and Micromeritics, ASAP 2010, BEL, Osaka, Japan). Thermogravimetric analysis (TGA) curves were recorded using a simultaneous thermal analyzer (STA, STA 8000, Perkin Elmer, Waltham, MA, USA) in N 2 (50.0 mL/min) atmosphere with heating from 30.0 to 1000.0 • C (10.0 • C/min). −78 • C. After the mixture was stirred for 6 h at −78 • C and slowly warmed to room temperature, the solution was stirred for 12 h at room temperature. The Et 3 N + Cl − salt was removed by filtration and washing with n-hexane in argon atmosphere in a glove box, and volatiles were distilled using vacuum distillation. Solid CPS (4.0 g, 5.5 mmol) was obtained at 88% yield. Si 5 (O i Pr) 10

Sol-Gel Polymerization of CPS and NPS
The sol-gel polymerization of CPS and NPS were carried out at 0.4 M concentration in EtOH or i PrOH according to the substituent groups of the precursors, respectively. The monomers were dissolved in anhydrous alcohols in 25.0 mL vials. An aqueous catalyst (0.01 wt % HCl or NaOH per water) was added to the vial. The solutions were sealed and shaken at room temperature. Gelation was determined at a point where the solution did not flow as a liquid. After gelation, the monoliths were aged for 2 weeks at 30 • C prior to further processing. The gels were crushed in water (100 mL), and filtered and washed with water (2 × 100 mL) and alcohols (EtOH or i PrOH, 2 × 100 mL). After drying overnight in air, the xerogels were prepared by vacuum drying at 50 • C for 24 h.

Preparation of CPSH
CPS (1.7 g, 2.32 mmol) was diluted in anhydrous i PrOH (4.5 mL) to adjust to 0.4 M concentration in an oven-dried vial (25 mL). A prepared aqueous acid catalyst, namely, 0.01 wt % HCl dissolved in 10 eq of H 2 O (0.42 mL, 23 mmol), was added to the vial, and then the vial was sealed and shaken. Gelation occurred within 5 min. After aging for 2 weeks at 30 • C, the wet gel was crushed in distilled H 2 O (100 mL) and filtered. The solids were washed twice with distilled H 2 O (100 mL) and anhydrous i PrOH (100 mL). After drying for 12 h in air, a white solid CPSH xerogel (0.54 g) was obtained by vacuum drying for 24 h at 50 • C. CPSH: 1

Preparation of CPSOH
Similarly to the procedure for the preparation of CPSH, CPSOH was prepared using an aqueous base catalyst, namely, 0.01 wt % NaOH dissolved in 10 eq of H 2 O (0.42 mL, 23 mmol), instead of the acid catalyst. Gelation occurred within 3 min. After aging, washing and drying were carried out in the same manner as described above. A white solid CPSOH xerogel (0.51 g) was obtained. CPSOH: 1

Preparation of CPSN
Similarly to the procedure used for the preparation of CPSH, CPSN was prepared using 10 eq of distilled H 2 O (0.42 mL, 23 mmol) without any acid or base catalyst. Gelation occurred within 10 min. After aging, washing and drying were carried out in the same manner as described above. A white solid CPSN xerogel (0.53 g) was obtained. CPSN: 1

Preparation of NPSOH
Similarly to the procedure used for the preparation of NPSH, NPSOH was prepared using an aqueous base catalyst, namely, 0.01 wt % NaOH dissolved in 12 eq of H 2 O (0.56 mL, 31 mmol), instead of the acid catalyst. Gelation occurred within 1 day. After aging, washing and drying were carried out in the same manner as described above. A white solid NPSOH xerogel (0.67 g) was obtained. NPSOH: 1

Preparation of NPSN
Similarly to the procedure for the preparation of NPSH, NPSOH was prepared using 12 eq of distilled H 2 O (0.56 mL, 31.0 mmol) without any acid or base catalyst. Gelation occurred within 2 days. After aging, washing and drying were carried out in the same manner as described above. A white solid NPSN xerogel (0.62 g) was obtained. NPSN: 1

Synthesis of the New Condensation Polymer Precursors
To investigate the sol-gel reaction of the oligosilane with consecutive silicon bonds by the molecular designed precursors, cyclic and branched peralkoxyoligosilanes were synthesized. The decaisopropoxycyclopentasilane (CPS)-Si 5 (O i Pr) 10  at −78 • C in Et 2 O for 6 h, and then the reaction mixture was slowly warmed to room temperature. After 12 h of stirring at room temperature, the formed white salt (Et 3 N + Cl − ) was removed by filtration without contact with air and moisture. The CPS cyclic compound was obtained as a solid at 88% yield. Using a method similar to the method described above, branched precursor NPS was obtained as an oil at 82% yield. The two products were characterized by 1 H NMR (Figures S1 and S4), 13 C NMR, 29 Si NMR (Figures S2 and S5), and HRMS ( Figures S3 and S6). The 29 Si NMR spectrum of CPS displayed a resonance at −14.4 ppm. In the HRMS data, one i Pr group removed fragment of CPS was detected. Two resonances were observed at −40.5 and −152.2 ppm in the 29 Si NMR spectrum of NPS and were assigned to the four terminal Si atoms and the central Si atom, respectively. as an oil at 82% yield. The two products were characterized by 1 H NMR ( Figures S1 and S4), 13 C NMR, 29 Si NMR ( Figures S2 and S5), and HRMS ( Figures S3 and S6). The 29 Si NMR spectrum of CPS displayed a resonance at −14.4 ppm. In the HRMS data, one i Pr group removed fragment of CPS was detected. Two resonances were observed at −40.5 and −152.2 ppm in the 29 Si NMR spectrum of NPS and were assigned to the four terminal Si atoms and the central Si atom, respectively.

Preparation of CPS and NPS Xerogels
To prepare polysilane-polysiloxane materials, sol-gel polymerizations of CPS and NPS (0.4 M) were carried out at room temperature in isopropanol or ethanol according to the substituent groups of the precursors. Hydrolysis and condensation reactions were carried out in neutral, acidic (0.01 wt % HCl per H2O), and basic (0.01 wt % NaOH per H2O) conditions. For the hydrolysis, water was used with the number of moles of the equivalent of alkoxy groups of each precursor (10 eq for CPS and 12 eq for NPS). The CPS solution became a viscous gel within a few minutes, while the gelation of NPS required a few days. After gelation, all of the gels were aged for 2 weeks at room temperature to obtain condensation that was as complete as possible prior to further processing. The wet gels were washed with water and the corresponding solvent to remove the catalyst. Xerogels were obtained as opaque white brittle solids by vacuum drying for 24 h at 50 °C. The xerogels were ground into fine powder for analyses (Scheme 2).

Preparation of CPS and NPS Xerogels
To prepare polysilane-polysiloxane materials, sol-gel polymerizations of CPS and NPS (0.4 M) were carried out at room temperature in isopropanol or ethanol according to the substituent groups of the precursors. Hydrolysis and condensation reactions were carried out in neutral, acidic (0.01 wt % HCl per H 2 O), and basic (0.01 wt % NaOH per H 2 O) conditions. For the hydrolysis, water was used with the number of moles of the equivalent of alkoxy groups of each precursor (10 eq for CPS and 12 eq for NPS). The CPS solution became a viscous gel within a few minutes, while the gelation of NPS required a few days. After gelation, all of the gels were aged for 2 weeks at room temperature to obtain condensation that was as complete as possible prior to further processing. The wet gels were washed with water and the corresponding solvent to remove the catalyst. Xerogels were obtained as opaque white brittle solids by vacuum drying for 24 h at 50 • C. The xerogels were ground into fine powder for analyses (Scheme 2). as an oil at 82% yield. The two products were characterized by 1 H NMR ( Figures S1 and S4), 13 C NMR, 29 Si NMR ( Figures S2 and S5), and HRMS ( Figures S3 and S6). The 29 Si NMR spectrum of CPS displayed a resonance at −14.4 ppm. In the HRMS data, one i Pr group removed fragment of CPS was detected. Two resonances were observed at −40.5 and −152.2 ppm in the 29 Si NMR spectrum of NPS and were assigned to the four terminal Si atoms and the central Si atom, respectively.

Preparation of CPS and NPS Xerogels
To prepare polysilane-polysiloxane materials, sol-gel polymerizations of CPS and NPS (0.4 M) were carried out at room temperature in isopropanol or ethanol according to the substituent groups of the precursors. Hydrolysis and condensation reactions were carried out in neutral, acidic (0.01 wt % HCl per H2O), and basic (0.01 wt % NaOH per H2O) conditions. For the hydrolysis, water was used with the number of moles of the equivalent of alkoxy groups of each precursor (10 eq for CPS and 12 eq for NPS). The CPS solution became a viscous gel within a few minutes, while the gelation of NPS required a few days. After gelation, all of the gels were aged for 2 weeks at room temperature to obtain condensation that was as complete as possible prior to further processing. The wet gels were washed with water and the corresponding solvent to remove the catalyst. Xerogels were obtained as opaque white brittle solids by vacuum drying for 24 h at 50 °C. The xerogels were ground into fine powder for analyses (Scheme 2).

Surface Area and Porosity
The porosity of the xerogels was determined by N 2 physisorption measurements. The textural properties of xerogels are summarized in Table 1. The BET (Brunauer-Emmett-Teller) method was applied to determine the surface area of the xerogels (Figures S7 and S8). CPS xerogels showed a larger surface area than the NPS xerogels. CPSOH exhibited the largest surface area of 408 m 2 ·g −1 and a total pore volume (at P/P 0 = 0.99) of 0.74 cm 3 ·g −1 . CPSH showed a quite large surface area of 317 m 2 ·g −1 and the largest total pore volume of 0.83 cm 3 ·g −1 . By contrast, the NPS xerogels were either non-porous or exhibited negligible porosity. These results indicated that the designed structure of the precursors affected the surface area and the porosity of the xerogels. Considering that the core size of CPS is larger than that of NPS, it was assumed that the higher porosity of CPS may be due to the difference in the core size. Usually, bridged polysilsequioxanes, which are large-core silsesquixoanes, are highly porous and have high surface areas [10][11][12][28][29][30][31][32]. Table 1. Textural properties of the xerogels (BET and BJH).

Solid-State CP/MAS NMR Analysis
The xerogels were characterized by solid-state 1 H, 13 C, and 29 Si CP/MAS NMR spectroscopy. Shorthand notations have been commonly used in the organosilicon literature for the Si-O bonding configurations: The structures of (SiO) 1 Si, (SiO) 2 Si, (SiO) 3 Si, and (SiO) 4 Si are referred to as M, D, T, and Q, as this relates to the number of O atoms bonded to a Si atom, respectively. 29 Si CP/MAS NMR spectra of the CPS xerogels show the presence of D (R 2 SiO 2 ) (δ from −20 to −40 ppm), T (RSiO 3 ) (δ from −60 to −80 ppm), and Q (SiO 4 ) units (δ from −90 to −120 ppm) [33][34][35][36][37]. If there was no Si-Si bond cleavage of CPS during the sol-gel process, only the D unit should be observed. However, T and even Q units were observed in Figure 1. The spectra indicate that the oxidative cleavage of Si-Si bonds in cyclopentasilane partially occurred during the sol-gel reaction to give Si-O-Si bonds [38][39][40][41][42][43][44]. Generally, when materials with Si-Si bonds on the surface are immersed into water or exposed to dry air, the Si-Si bonds are readily oxidized, and chemically stabilized Si-O-Si bonds are formed. The surface Si-Si bonds of the xerogels may be oxidized since NPS and CPS were exposed to water and air for a long time. Similarly to the 29 Si CP/MAS NMR spectrum of CPS xerogels, the Q unit is also found in the spectra of NPS xerogels. Without Si-Si bond breaking of NPS during the sol-gel process, only the T unit should be observed. The resonances at the T and Q units of NPS xerogels observed in Figure 2 imply the partial oxidation of the Si-Si bonds of neopentasilane.

Microscopy
The morphology and pore texture of xerogels were investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). For CPS xerogels, Figures 3-5 show disordered meso-and macroporous structures consisting of spherical nanoparticles with other shapes. The sizes of the spherical nanoparticles were measured as 10-20 nm (CPSH), 20-30 nm (CPSOH), and 30-90 nm (CPSN). The different sizes of the particles can be attributed to the different conditions of the sol-gel reactions. While CPSOH consisted of spherical nanoparticles only (Figure 4a,b), aggregated nanorods sized 400-500 nm in CPSH (Figure 3b) and a huge particle sized approximately 1.5 µm in CPSN (Figure 5b) were observed along with spherical nanoparticles. These noncrystalline composites may have contributed to the reduction of the surface areas of CPSH and CPSN (Table 1). TEM images display the mesoporous textures of the CPS xerogels (Figures 3c-5c and 3d-5d).  (Table 1). TEM images display the mesoporous textures of the CPS xerogels (Figures 3c,d-5c,d).     The morphologies of NPS xerogels were observed to be significantly different from those of the CPS xerogels. NPSH and NPSN are flat plates connected to each other as stacked films, and NPSOH is an irregularly wrinkled plate. In the TEM images, the textures consist of highly crystalline composites with no pores (Figures 6-8).  The morphologies of NPS xerogels were observed to be significantly different from those of the CPS xerogels. NPSH and NPSN are flat plates connected to each other as stacked films, and NPSOH is an irregularly wrinkled plate. In the TEM images, the textures consist of highly crystalline composites with no pores (Figures 6-8). The morphologies of NPS xerogels were observed to be significantly different from those of the CPS xerogels. NPSH and NPSN are flat plates connected to each other as stacked films, and NPSOH is an irregularly wrinkled plate. In the TEM images, the textures consist of highly crystalline composites with no pores (Figures 6-8).

Conclusions
The new condensation polymer precursors containing consecutive silicon atoms-CPS and NPS-were successfully synthesized at high yields. CPS and NPS xerogels were prepared by sol-gel polymerization under three reaction conditions (acidic, basic, and neutral) using the new precursors. The characterizations of CPS and NPS xerogels using BET, BJH, and microscopy showed that their porosity and morphology were strongly affected by the structure of precursors. The CPS xerogels exhibited relatively large surface areas and total pore volumes, while the NPS xerogels exhibited nonporosity or negligible porosity. The relatively high porosity of CPS xerogels was achieved without any organic substituent, template, or porogen, and they showed 16~19% of weight loss in TGA analysis. Bridged polysilsesquioxanes exhibited higher porosity, however, the weight loss

Conclusions
The new condensation polymer precursors containing consecutive silicon atoms-CPS and NPS-were successfully synthesized at high yields. CPS and NPS xerogels were prepared by sol-gel polymerization under three reaction conditions (acidic, basic, and neutral) using the new precursors. The characterizations of CPS and NPS xerogels using BET, BJH, and microscopy showed that their porosity and morphology were strongly affected by the structure of precursors. The CPS xerogels exhibited relatively large surface areas and total pore volumes, while the NPS xerogels exhibited nonporosity or negligible porosity. The relatively high porosity of CPS xerogels was achieved without any organic substituent, template, or porogen, and they showed 16~19% of weight loss in TGA analysis. Bridged polysilsesquioxanes exhibited higher porosity, however, the weight loss

Conclusions
The new condensation polymer precursors containing consecutive silicon atoms-CPS and NPS-were successfully synthesized at high yields. CPS and NPS xerogels were prepared by sol-gel polymerization under three reaction conditions (acidic, basic, and neutral) using the new precursors. The characterizations of CPS and NPS xerogels using BET, BJH, and microscopy showed that their porosity and morphology were strongly affected by the structure of precursors. The CPS xerogels exhibited relatively large surface areas and total pore volumes, while the NPS xerogels exhibited nonporosity or negligible porosity. The relatively high porosity of CPS xerogels was achieved without any organic substituent, template, or porogen, and they showed 16~19% of weight loss in TGA analysis. Bridged polysilsesquioxanes exhibited higher porosity, however, the weight loss reached around 45% [10,29]. 29 Si CP/MAS NMR study of the xerogels indicated that a partial oxidative cleavage of the Si-Si bonds in cyclopentasilane and neopentasilane occurred during the sol-gel process. The structural features-a hybrid of Si-Si and Si-O-Si bonds-might provide a new optical property since consecutive silicon bonds absorb UV-visible light [14], and porous silica shows photoluminescence. The mixing of different skeletal bonds-Si-O-Si in an insulator and Si-Si in a semiconductor-provided interesting thermal and electrical conductivity. We hope that the new condensation polymer precursors for polysilane-polysiloxane materials will be used in many applications such as core-shell particle, anode materials for Li-ion batteries and non-metal thermal conductors.