Hydrolysis and condensation behavior of tetraethoxysilane, hexaethoxydisiloxane, and octaethoxytrisiloxane

In the initial stage of the hydrolysis–condensation of tetraethoxysilane (TEOS), hexaethoxydisiloxane (HEDS) and octaethoxytrisiloxane (OETS) are formed. However, little is known about the hydrolysis–condensation of HEDS and OETS. In this study, the hydrolysis–condensation of TEOS, HEDS, and OETS was investigated. HEDS and OETS were synthesized from diethoxy(diisocyanato)silane, a raw material with controllable functionality. The hydrolysis of TEOS, HEDS, and OETS was analyzed by mass spectroscopy, gel permeation chromatography, and nuclear magnetic resonance. The hydrolysis–condensation product of TEOS was a three-dimensional network-type polysiloxane. The hydrolysis–condensation product of HEDS consisted mainly of four-membered cyclic siloxane. The hydrolysis–condensation product of OETS consisted mainly of various membered cyclic siloxanes. Hexaethoxydisiloxane (HEDS) and octaethoxytrisiloxane (OETS) were synthesized from diethoxy(diisocyanato)silane. Trimethylsilylates of hydrolyzates in the initial stage of tetraethoxysilane (TEOS), HEDS, and OETS hydrolysis were analyzed by FTIR, MS, and NMR. Hydrolysis behaviors of TEOS, HEDS, and OETS were monitored by GPC and NMR. Hexaethoxydisiloxane (HEDS) and octaethoxytrisiloxane (OETS) were synthesized from diethoxy(diisocyanato)silane. Trimethylsilylates of hydrolyzates in the initial stage of tetraethoxysilane (TEOS), HEDS, and OETS hydrolysis were analyzed by FTIR, MS, and NMR. Hydrolysis behaviors of TEOS, HEDS, and OETS were monitored by GPC and NMR.


Introduction
Hydrolysis-condensation of alkoxysilanes is a representative synthesis method for siloxanes [1,2].Tetraethoxysilane (TEOS) is the most versatile raw material for preparing silica and silicate glasses.Several studies on TEOS regarding its kinetic rate constant and hydrolysis behavior under various pH, temperature, and catalytic conditions have been conducted [3][4][5][6][7][8][9].Regarding the kinetic rate constant of hydrolysis-condensation reaction of tetramethoxysilane under acidic condition, Kay and Assink reported as follows: 1) hydrolysis reaction is faster than condensation reaction; 2) water producing condensation reaction is faster than methanol producing condensation reaction (hydrolysis reaction: 0.2 mol −1 min −1 , water producing condensation reaction: 0.006 mol −1 min −1 , methanol producing condensation reaction: 0.001 mol −1 min −1 ) [10].This kinetic rate constant is widely accepted.In contrast, we recently reported that ethanol producing condensation reaction is faster than water producing condensation reaction in the condensation reaction between triethoxysilane and triethoxysilanol (ethanol producing condensation reaction: 0.12 mol −1 min −1 , water producing condensation reaction: 0.08 mol −1 min −1 ) [11,12].These results indicate that steric hindrance dominates the hydrolysis-condensation reaction of alkoxysilane.In the initial hydrolysis-condensation reaction of TEOS, linear oligosiloxanes such as disiloxane and trisiloxane are formed under acidic conditions.Subsequently, long linear components, cyclic components, and randomly condensed structural oligomers and polymers are formed.The properties of siloxanes also differ according to the molecular weight and the ratio of Si(OSi) n (OR) 4-n (n = 1-4,: Q n ) in low-molecularweight polysiloxanes [13,14].Therefore, it is not difficult to imagine that hydrolysis and condensation behaviors depend on the structure of the raw material.Although hydrolyzates of hexaethoxydisiloxane (HEDS) and octaethoxytrisiloxane (OETS) have been identified by 29 Si nuclear magnetic resonance (NMR) [15][16][17], the behavior of hydrolysis-condensation of HEDS and OETS has not been investigated.Therefore, in this work, we report the hydrolysis-condensation behavior of TEOS, HEDS, and OETS in detail based on instrumental analysis.Moreover, the reactivity and the synthesis of an alkoxy-substituted isocyanatosilane from tetraisocyanatosilane is reported using a new synthetic method for alkoxydisiloxanes and trisiloxanes, as shown in Scheme 1.

Measurements
Gas chromatography (GC) was performed using a GC-390 instrument (GL Science, Japan) packed with an SE-30 capillary column (Agilent, USA) and a thermal conductivity detector.Helium was used as the carrier gas.The column temperature was programmed as follows: injection temperature of 250 °C, isothermal state at 80 °C for 2 min, then heating up to 200 °C at a rate of 10 °C min −1 , followed by heating up to 280 °C at a rate of 20 °C min −1 , and held at the final temperature of 280 °C for 2 min.Gas chromatography/ mass spectroscopy (GC/MS) was performed using a GCmate™ GC/MS double-focusing mass spectrometer (JEOL, Japan).Helium was used as the carrier gas.The column temperature was programmed as follows: injection temperature of 280 °C, isothermal state at 80 °C for 2 min, then heating up to 200 °C at a rate of 10 °C min −1 , followed by heating up to 280 °C at a rate of 20 °C min −1 , and held at the final temperature of 280 °C for 20 min.Nuclear magnetic resonance (NMR) spectra were recorded using a JEOL Resonance JNM-ECZ 400 spectrometer ( 1 H: 400 MHz, 13 C: 100 MHz, 29 Si: 80 MHz).The chemical shifts were reported in ppm relative to the residual chloroform in chloroform-d (CDCl 3 ) ( 1 H: 7.26 ppm), chloroform ( 13 C:77.16ppm), and tetramethylsilane ( 29 Si{ 1 H}:0.00ppm) as internal standards.For the 29 Si{ 1 H} NMR spectra, chromium(III) acetylacetonate was added to the sample as a paramagnetic relaxant.Fouriertransform infrared (FTIR) spectra were recorded on an FT/IR-6100 spectrophotometer (JASCO, Japan) using the neat method, in which the sample was sandwiched between two KBr crystal disks.High-resolution electrospray ionization time-of-flight mass spectrometry (HR-ESI-TOF MS) was performed using JEOL JMS-T100CS AccuTOF CS.The molecular weights of the oligomers and polymers were determined by gel permeation chromatography (GPC) using an LC-20AD HPLC prominent liquid chromatograph (Shimadzu, Japan) attached to a PLgel 5-μm Mixed-D column.Tetrahydrofuran (THF) was used as an eluent (1 mL min −1 ), and a RID-20A was used as the detector at 40 °C.Molecular simulations of ethoxysilane oligomers were performed using quantum chemical calculations.Optimized structures, electrostatic potential (ESP) maps, dipole moments, and solvationfree energies for TEOS, HEDS, and OETS were determined at the B3LYP/6-31 G(d,p) level of theory using Gaussian 16 software [18].
Chloro(triethoxy)silane (CTES) was prepared by the chlorination of TEOS, and triethoxysilanol (TESOL) was prepared by the hydrolysis of CTES [20,21].The preparation methods and characterization are provided in Supporting Information.

Initial hydrolysis reaction of ethoxysilane monomers
HCl aq. was slowly added to an EtOH solution of the ethoxysilane monomer (TEOS, HEDS, or OETS) in an ice bath, and the molar ratio of HCl:H 2 O:EtOH:monomer was 0.1:2:10:1.After stirring the mixture in an ice bath for 10 min, chloro(trimethyl)silane (equal to the mol amount of H 2 O) was added to the mixture to stop hydrolysis.The mixture was then poured into a mixture of hexane/THF (1/ 2 v/v) and washed twice with water and twice with brine.
The organic layer was dried over anhydrous sodium sulfate, filtered, and evaporated.The residue was characterized by mass spectroscopy and 29 Si{ 1 H} NMR.

Polymerization behavior of ethoxysilane monomers
HCl aq. was slowly added to an EtOH solution of the ethoxysilane monomer (TEOS, HEDS, or OETS) in an ice bath, and the molar ratio of HCl:H 2 O:EtOH:monomer was 0.1:2:10:1.After stirring in an ice bath for 10 min, the mixture was heated to room temperature.The mixture was then aged for GPC and 29 Si{ 1 H} NMR monitoring.
3 Results and discussion

Reactivity of diethoxy(diisocyanato)silane
Diethoxy (diisocyanato)silane (DEDIS) was used as the raw material for the selective synthesis of HEDS and OETS.The reactivity of isocyanatosilanes is lower than that of chlorosilanes and higher than that of the corresponding alkoxysilanes [23][24][25].We previously reported the synthesis and isolation of partially alkoxy-substituted isocyanatosilanes by the reaction of tetraisocyanatosilane with alcohol [19], as isocyanatosilanes are easier to handle than chlorosilanes.However, the reactivities of alkoxy-substituted isocyanatosilanes have not yet been investigated.In this section, the reactivity of DEDIS with water and silanol is discussed.
To synthesize tetraethoxy(diisocyanato)disiloxane (TEDIDS), the hydrolysis-condensation of DEDIS was performed according to Scheme 2; the results are summarized in Table 1.The yield of TEDIDS depended on the temperature and time.When condensation reaction was performed at 85 °C for 3 h (Entry 1), the yield of TEDIDS was 13% because condensation reaction was favored to form oligosiloxanes.When the reaction was performed at room temperature (23 °C) for 3 h (Entry 2), the yield of TEDIDS was maximum at 32%.However, the yield decreased to 28% when the reaction time was 16 h (Entry 3).The yield of TEDIDS via ethanolysis of hexa(isocyanato)disiloxane from tetraisocyanatosilane was 11% [26]; in contrast, the yield of TEDIDS via ethanolysis of DEDIS from tetra(isocyanate)silane was 29%.Hence, this method is useful for synthesizing disiloxanes containing isocyanate and alkoxy groups.
The reaction between DEDIS and TESOL was monitored for 8 h by GC/MS (Supplementary Fig. S1), and a strong signal attributed to ethanol was observed, indicating that the reaction proceeded between Si-OH and Si-OEt rather than between Si-OH and Si-NCO.DEDIS, triethoxy(isocyanato)silane (TEIS), and TEOS were observed, implying the reaction of DEDIS with ethanol.Additionally, 1,3,3,3-Tetraethoxy-1,1-diisocyanatodisiloxane (TE-1,1-DIDS) and hexaethoxydisiloxane (HEDS) were identified, supporting the ethanolysis of TE-1,1-DIDS and/or the condensation reaction between TEOS and TESOL or between TEIS and TESOL.Hexaethoxy(diisocyanato)trisiloxane (HEDITS) and octaethoxytrisiloxane (OETS) were also identified.Moreover, various other signals were observed, but the structures could not be determined because the molecular ion peak could not be identified or because there were many plausible isomers.The reaction between DEDIS and TESOL produced TE-1,1-DIDS and ethanol, whereas the reaction between TE-1,1-DIDS and TESOL produced around Si atom [27,28].The signal of tetraisocyanatosilane appeared at high magnetic field due to the shielding effect of p(N)π → d(Si)π [19,29,30].The chemical shifts of DEDIS, TEDIDS, and HEDITS were observed at lower magnetic fields than that of tetraisocyanatosilane.The FTIR spectra of DEDIS, TEDIDS, HEDITS, HEDS, and OETS are shown in Fig. 2, and the assignments are summarized in Table 3.The absorption bands were assigned based on the vibrational frequencies of tetraisocyanatosilane, triethoxysilane, and tetraethoxysilane [31][32][33][34][35].All siloxanes except for DEDIS exhibited the υSiOSi peak at approximately 795 cm −1 .DEDIS, TEDIDS, and HEDITS exhibited a band corresponding to υNCO at approximately 2285 cm −1 , which was not observed for HEDS or OETS.

Hydrolysis and condensation of TEOS, HEDS, and OETS
The reaction between alkoxysilanes and water is a wellknown competitive reaction involving hydrolysis and condensation.The rate of hydrolysis reaction is faster than that of condensation reaction [10,36], and condensation reaction can be slowed down at low temperatures (<0 °C) [37].We performed the hydrolysis reaction of ethoxysilane monomers (TEOS, HEDS, OETS) with the molar ratio of HCl: H 2 O: EtOH: monomer = 0.1: 2: 10: 1 at 0 °C for 10 min.To investigate the hydrolysis reaction in the duration of 10 min at 0 °C, the silanol in the hydrolyzed ethoxysilanes was capped using chlorotrimethylsilane [38,39], and the structures of TEOS-TMS, HEDS-TMS, OETS-TMS were estimated by mass spectroscopy and 29 Si{ 1 H} NMR.In addition, to investigate the progress of condensation reaction, the hydrolysis reaction was performed for ethoxysilane monomers at 0 °C for 10 min followed by aging at room temperature (23 °C).The condensation of the ethoxysilane oligomers was analyzed by 29 Si NMR and GPC.These processes are illustrated in Fig. 3.

Hydrolysis of TEOS, HEDS, and OETS
Progression of trimethylsilylation was confirmed by FTIR (Supplementary Fig. S2), by the appearance of the adsorption bands corresponding to δSi-Me (approximately 1250 cm −1 ) and ρSi-Me (approximately 840 cm −1 ), and the no appearance of the band corresponding to υSi-OH (approximately 950 cm −1 ) [34,40,41].No υSi-O-Si adsorption band was observed at 1200-1100 or 1040-1000 cm −1 ; hence, the main structures of ethoxysilane-TMSs could be attributed to their linear, branched, and cyclic forms [34,42].The mass spectra of the ethoxysilane-TMSs are shown in Fig. 4 (results are summarized in Supplementary Table S1).TEOS-TMS was confirmed to be a mixture of linear (trimer to octamer) and cyclic (tetramer to heptamer) oligosiloxanes, with the main components being the trimers, tetramers, and pentamers.This result was similar to that of the general hydrolysis of TOES under acidic conditions [38,43].HEDS-TMS was confirmed to be a mixture of linear (dimer to octamer) and cyclic (tetramer to heptamer) oligosiloxanes, with the main component being the tetramers.The peak intensity of the mass spectrum of the oligosiloxanes with even-numbered silicon atoms in the main structure was higher than that with odd-numbered silicon atoms, indicating that the rearrangement reaction hardly proceeded.The main structure of the OETS-TMS consisted of trimers.The 29 Si{ 1 H} NMR spectra of the ethoxysilane-TMSs are shown in Fig. 5, and the assignments are summarized in Table 4.The M 1 /Q ratio was in the order of TEOS-TMS>HEDS-TMS>OETS-TMS, which was consistent with the relationship between the amount of water added and the number of alkoxy groups (H 2 O/OEt = 0.50 for TEOS, 0.33 for HEDS, and 0.25 for OETS).The degrees of M 1 for TEOS-, HEDS-, and OETS-TMSs were 47.0, 56.3, and 72.8%, respectively.The compositions of the siloxane unit structures a, b, c, d, and e (Table 5) were calculated based on the signal area of 29 Si{ 1 H} NMR spectra of ethoxysilane-TMSs, as shown in Fig. 6.Unit a was attributed to the end group, whereas Units b, c, and d were attributed to the linear or cyclic structures.Unit e was attributed to the branched structure.From these results, ethoxysilane-TMSs were confirmed to be mainly consisting of unit a, as shown in Fig. 5 (TEOS-TMS: 58.1%, HEDS-TMS: 70.6%, OETS-TMS: 56.7%) without Q 4 signal.The spectrum of the HEDS oligomer after 14 days was similar to that after 7 days, and the Q 4 unit was barely formed.The molecular weight of the HEDS oligomer from 1 h to 14 days changed from 1300 to 1800 Da, and the HEDS oligomer after 14 days was composed of mainly 4-membered cyclic Q 2 , cyclic Q 3 , and Q 3 structures; linear Q 2 and 6-membered cyclic Q 2 was also incorporated in the oligomer.The hydrolysis-condensation behavior of OETS differed from that of TEOS and HEDS.The hydrolyzate of OETS after 1 h showed the signals corresponding to 3-membered cyclic Q 2 (OEt)(OH) (−85.5, −85.7 ppm), Q 1 (OEt) 2 (OH) (−86.2 ppm), 3-membered cyclic Q 2 (OEt) 2 (−87.7,−87.9 ppm), Q 1 (OEt) 3 (−88.7,−88.9 ppm), cyclic Q 2 (OEt)(OH) (−92.7 ppm), Q 2 (OEt)(OH) (−93.6 ppm), 4-membered cyclic Q 2 (OEt) 2 (−95.1 ppm), and linear Q 2 (−96.2ppm).After 24 h, the intensity of 4-, 5-, or 6-membered cyclic Q 2 (OEt) 2 (−95.1, −95.4,−95.6 ppm) increased, and intensity of Q 1 and 3-membered cyclic Q 2 decreased.After seven days, cyclic Q 3 -OH (−100.5 ppm), Q 3 -OH (−101.5 ppm), cage-type Q 3 such as open cage and complete cage (−102.3ppm), cyclic Q 3 -OEt (−103.2ppm), and Q 3 -OEt (−104.5 ppm) were formed, and the content of Q 1 -OH and 3-membered cyclic Q 2 decreased.After 14 days, Q 1 -OH and 3-membered cyclic Q 2 disappeared, Q 3 structures grew, and the content of 4-membered cyclic Q 2 decreased.Moreover, end-terminated Q 1 (OEt) 3 was observed.After 14 days, the OETS oligomer was mainly composed of 4-, 5-, or 6-membered cyclic Q 2 , linear Q 2 , various Q 3 , and end-terminated Q 1 (OEt) 3 .These assignments are summarized in Table 6.
Figure 7 shows the changes in the area ratios of the structures in the 29 Si{ 1 H} NMR spectra of the TEOS, HEDS, and OETS oligomers over time.The contents of cyclic and linear structures reached up to approximately 50 and 20%, respectively Fig. 8.The area ratios of these structures decreased with an increasing area ratio of the branched structure.Schematics of the estimated structures generated by the linear ethoxysilane oligomers are shown in Fig. 9.

Computational study of ethoxysiloxane monomers
The optimized structures with their ESP maps and dipole moments were calculated using DFT to supplement the reactivity of the ethoxysilane monomers (Fig. 10).When siloxane bonding was extended, the dipole moment of optimized structure increased (TEOS: 0.11 debye, HEDS: 1.07 debye, OETS: 3.86 debye).The optimized HEDS and OETS structures showed that the electron density was high for the oxygen atoms of the siloxane bond and alkoxy group.In particular, OETS had high electron density on the oxygen atom of the alkoxy group arranged on one side of the molecule.These electron densities may affect the reactivity of the monomers [49][50][51][52].

Conclusion
HEDS and OETS were synthesized in 3.5 and 8.4% of the total respectively, from DEDIS by controlling the functionality.The hydrolysates of TEOS, HEDS, and OETS were trimethylsilylated and analyzed by MS, GPC, and NMR.Linear and cyclic oligosiloxanes were generated during the initial stage.The structure was estimated to be a network-type polymer containing a cyclic siloxane for TEOS.The polymers mainly consisted of fourmembered cyclic siloxanes for HEDS.The polymers consisted mainly of cyclic siloxanes for OETS.In addition, the electron densities of the ethoxysilane oligomers calculated using DFT were investigated.The optimized HEDS and OETS structures showed a high electron density on the oxygen atoms in the siloxane bond and alkoxy group.These results suggest that electron density may affect the reactivities of TEOS, HEDS, and OETS.

Fig. 3
Fig. 3 Experimental processes of the hydrolysis and condensation of ethoxyoligosiloxanes

Fig. 8
Fig. 8 Variation of the peak area ratio of structures with time on the 29 Si{ 1 H} NMR spectra of the hydrolyzates of a TEOS, b HEDS, and c OETS (△: End group, ◇: Linear structure, □: Cyclic structure, ×: Branch structure)

Table 6
Assignments for the29Si{ 1 H} NMR spectra of ethoxysiloxane oligomersQ n unit Chemical shift of Q n unit / ppm