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Journal of Sol-Gel Science and Technology

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Open Access 27-07-2023 | Original Paper

Hydrolysis and condensation behavior of tetraethoxysilane, hexaethoxydisiloxane, and octaethoxytrisiloxane

Authors: Yohei Sato, Asahi Sugimoto, Tatsuya Iwashina, Ryohei Hayami, Kazuki Yamamoto, Takahiro Gunji

Published in: Journal of Sol-Gel Science and Technology | Issue 2/2023

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Abstract

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.

Graphical Abstract

Supporting Information

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1007/s10971-023-06159-x.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

1 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 [3‐9]. 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,: Qⁿ) in low-molecular-weight 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 ²⁹Si nuclear magnetic resonance (NMR) [15‐17], 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.

2 Experimental section

2.1 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 (¹H: 400 MHz, ¹³C: 100 MHz, ²⁹Si: 80 MHz). The chemical shifts were reported in ppm relative to the residual chloroform in chloroform-d (CDCl₃) (¹H: 7.26 ppm), chloroform (¹³C:77.16 ppm), and tetramethylsilane (²⁹Si{¹H}:0.00 ppm) as internal standards. For the ²⁹Si{¹H} NMR spectra, chromium(III) acetylacetonate was added to the sample as a paramagnetic relaxant. Fourier-transform 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 solvation-free energies for TEOS, HEDS, and OETS were determined at the B3LYP/6-31 G(d,p) level of theory using Gaussian 16 software [18].

2.2 Materials

Ethanol (EtOH), THF, toluene, and diethyl ether were purified using standard processes and stored over activated molecular sieves. Tetraisocyanatosilane was provided by Matsumoto Chemical Industry Co., Ltd. (Japan). Ammonium carbonate ((NH₄)₂CO₃) and molecular sieves were purchased from Kanto Chemical Co., Inc. (Japan). Anhydrous sodium sulfate, N,N-dimethylformamide (DMF), and 6 M hydrochloric acid (HCl aq.) were purchased from FUJIFILM Wako Pure Chemical Corporation (Japan). TEOS, thionyl chloride (SOCl₂), chlorodimethylsilane, and chromium(III) acetylacetonate were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). TEOS and tetraisocyanatosilane were purified by distillation (106.5–111.0 °C/160 mmHg and 133.5–136.5 °C/123 mmHg, respectively). Diethoxy(diisocyanato)silane (DEDIS) was prepared by the ethoxylation of tetraisocyanatosilane [19]. 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.

2.3 Synthesis of 1,1,3,3-tetraethoxy-1,3-diisocyanatodisiloxane (TEDIDS)

A solution of water (0.18 g, 0.01 mol) and THF (20 ml) was added dropwise into a solution of DEDIS (4.04 g, 0.02 mol) and THF (20 ml) at 0 °C. The solution was stirred at 0 °C for 2 h followed by stirring at room temperature or 85 °C for the prescribed time. The solution was dried over anhydrous sodium sulfate, filtered, and evaporated. TEDIDS was isolated by distillation.

TEDIDS: colorless liquid; yield 13–32%; b.p. 54.8–58.9 °C/0.25 mmHg; ¹H NMR (400 MHz, CDCl₃/ppm): δ 3.90–3.85 (m, 8H, CH₂), 1.28–1.23 (m, 12H, CH₃); ²⁹Si NMR (80 MHz, CDCl₃/ppm): δ − 94.6.

2.4 Synthesis of 1,1,1,5,5,5-hexaethoxy-3,3-diisocyanatotrisiloxane (HEDITS)

A solution of TESOL and THF (20 ml) was added dropwise to a solution of DEDIS (4.04 g, 0.02 mol) and THF (20 ml) at 0 °C. Then, the solution was stirred at 0 °C for 2 h, followed by stirring at room temperature for the prescribed time. After the completion of this reaction, the solvent was evaporated, and HEDITS was obtained by distillation.

HEDITS: colorless liquid; yield 24–44%; 100.8–104.8 °C/1.3 mmHg; ¹H NMR (400 MHz, CDCl₃ / ppm): δ 3.88–3.77 (m, 12H), 1.23–1.16 (m, 18H); ¹³C NMR (100 MHz, CDCl₃ / ppm): δ 122.5 (NCO), 59.0 (CH₂), 17.6 (CH₃); ²⁹Si NMR (80 MHz, CDCl₃ / ppm): δ −88.8, −89.0; IR (cm⁻¹): 2978, 2929, 2894, 2290, 1169, 1103, 1081; HRMS (HR-ESI-TOF) m/z: Calcd. for C₁₄H₃₁N₂O₁₀Si₃: 471.1208 [M + H]⁺, Found: 471.1459.

2.5 Synthesis of hexaethoxydisiloxane (HEDS) and octaethoxytrisiloxane (OETS)

Based on the alcoholysis of isocyanatosilane [22], TEDIDS (2 mmol) or HEDITS (1.5 mmol) were dissolved in THF (1 ml). Subsequently, 20 equiv. EtOH against oligosiloxane was added to the solution and refluxed overnight. After the reaction, the solvents were evaporated and distilled to obtain HEDS or OETS.

HEDS: colorless liquid; yield 12%; b.p. 51.4–54.2 °C/0.5 mmHg; ¹H NMR (400 MHz, CDCl₃ / ppm): δ 3.83 (q, J = 6.0 Hz, 12H, CH₂), 1.20 (t, J = 6.0 Hz, 18H, CH₃); ¹³C NMR (100 MHz, CDCl₃ / ppm): δ 59.2 (CH₂), 18.1 (CH₃); ²⁹Si NMR (80 MHz, CDCl₃ / ppm): δ −88.8.

OETS: colorless liquid; yield 42%; b.p. 90.2–90.5 °C/0.3 mmHg; ¹H NMR (400 MHz, CDCl₃ / ppm): δ 3.79–3.74 (m, 16H), 1.15–1.11 (m, 24H); ¹³C NMR (100 MHz, CDCl₃ / ppm): δ 59.3 (CH₂), 59.2 (CH₂), 18.1 (CH₃), 18.6 (CH₃); ²⁹Si NMR (80 MHz, CDCl₃ / ppm): δ −89.0, −96.2.

2.6 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:H₂O: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 H₂O) 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 ²⁹Si{¹H} NMR.

2.7 Polymerization behavior of ethoxysilane monomers

3 Results and discussion

3.1 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 [23‐25]. 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.

Table 1

Synthetic conditions and yield of TEDIDS

Entry	Temperature^a (°C)	Time^b (h)	Yield (%)
1	85	3	12
2	23	3	32
3	23	16	28

^aCorresponding to “X” in Scheme 2

^bCorresponding to “Y” in Scheme 2

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 HEDITS and ethanol, as shown in Scheme 3. Because the generated ethanol attacks Si–NCO, the reaction mixture was complex because of the formation of various compounds during the ethanolysis reaction. HEDITS in the reaction mixture was isolated by distillation because the reactive isocyanato group was protected by two bulky triethoxysiloxy groups. The isolated yield of HEDITS depended on the reaction time and the additional molar amount of TESOL, as shown in Table 2. The isolated yield of HEDITS first increased (Entries 1 and 2) and then decreased (Entry 3) because of ethanolysis and siloxane bond formation. When the molar ratio was set to 2 (Entry 3), HEDITS could not be isolated by distillation because the distillate contained many byproducts. HEDS and OETS were yielded in 12 and 42% by the ethanolysis of TEDIDS and HEDITS, respectively.

Table 2

Isolated yield of HEDITS by the reaction of DEDIS with TESOL

Entry	TESOL	Time (day)	Isolated yield of HEDITS (%)
Entry	(equiv.)	Time (day)	Based on DEDIS	Based on TESOL
1	1	0.67	18	36
2	1	2.00	22	44
3	1	3.00	12	24
4	2	0.67	– ^a

^aComplicated mixture

The ²⁹Si NMR spectra of tetraisocyanatosilane, DEDIS, TEDIDS, HEDITS, HEDS, and OETS are shown in Fig. 1. The chemical shift of Q¹ unit for –OSi(OEt)₃ appeared at −88.8 ppm. In contrast, the chemical shift of the silicon atom bound to the isocyanate group was irregular. In general, the chemical shift is influenced by the shielding effect of p(X)π → d(Si)π and p(X)π → σ*(Si–Y), and bonding angles 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 [31‐35]. 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.

Table 3

FTIR assignment of DEDIS, TEDIDS, HEDITS, HEDS, and OETS

Wavenumber (cm⁻¹)^a					Assignment^b
DEDIS	TEDIDS	HEDITS	HEDS	OETS	Assignment^b
2983m	2980m	2980m	2978m	2977m	υ_asCH₃
2940w	2930w	2930w	2930w	2930w	υ_sCH₂
2901w	2897w	2895w	2893w	2893w	υ_sCH₃
2281s	2289s	2291s	–	–	υNCO
1453w	1447w	1449w	1445w	1443w	δCH₃
1394vw	1394vw	1394vw	1394vw	1394vw	δCH₂
1299vw	1297vw	1295vw	1297vw	1297vw	δCH₂
1172m	1171m	1169m	1170m	1169m	ρCH₃
1105s	1104s	1103s	1106s	1105s	υCO
1086s	1081s	1082s	1082s	1080s	υCO
980w	975w	971w	970m	970m	υCC, ρCH₃
–	796m	795m	795m	796m	υ_sSiOSi
771m	–	–	–	–
–	740w	746vw	–	–
–	–	–	688vw	703vw
618w	619vw	618vw	–	–	δNCO
–	459w	468w	473w	477w	δSiOSi
448w	447sh	448sh	447sh	448sh	δCCO

^as strong, m middle, w weak, vw very weak, sh shoulder

^bυ stretching, δ scissoring, ρ rocking, as asymmetric, s symmetric

3.2 Hydrolysis and condensation of TEOS, HEDS, and OETS

The reaction between alkoxysilanes and water is a well-known 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: H₂O: 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 ²⁹Si{¹H} 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 ²⁹Si NMR and GPC. These processes are illustrated in Fig. 3.

3.2.1 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 ²⁹Si{¹H} NMR spectra of the ethoxysilane–TMSs are shown in Fig. 5, and the assignments are summarized in Table 4. The M¹/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 (H₂O/OEt = 0.50 for TEOS, 0.33 for HEDS, and 0.25 for OETS). The degrees of M¹ 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 ²⁹Si{¹H} 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 Q⁴ signal.

Table 4

Assignments for ²⁹Si{¹H} NMR spectra of ethoxysilane–TMSs

Qⁿ unit	Chemical shift of Qⁿ unit prepared from linear-oligoethoxysiloxanes/ppm
Qⁿ unit	TEOS–TMS	HEDS–TMS	OETS–TMS
monomer	−81.9	−88.8	−88.9, −96.2
M¹	10.7 to 8.4	11.1 to 10.0	13.3 to 9.9
Q¹(OEt)₃	−88.8	−88.7 to 88.9	−88.9
Q¹–Qⁿ(TMS)_n	−86.9	−88.0
Q²(OEt)₂	−95.9 to −96.5	−96.6 to −97.5	−96.2 to −96.5
Q²–TMS	−94.3 to −94.6, −93.8, −92.5	−94.3 to −94.5, −93.9	−94.6 to −94.7, 93.8
Q³(OEt)	−102.4 to −102.6	−101.2 to −102.5	−102.3 to −102.5
Q³–(TMS)	−100.2 to −100.6	−100.2 to −100.5	−100.1 to −100.5
Q³–(TMS)₂	−98.4

Table 5

Relative ratios of siloxane unit structures for ethoxysilanes–TMS

ethoxysilanes–TMS	M¹/ M¹Q^a	Ratio of structure units^b (%)
ethoxysilanes–TMS	ratio (%)	a	b	c	d	e
TEOS–TMS	47.0	58.1	9.1	9.0	16.4	7.5
HEDS–TMS	56.3	70.6	5.5	0	15.7	8.2
OETS–TMS	72.8	56.7	19.3	0	11.0	13.0

^aCalculated based on ²⁹Si{¹H} NMR spectrab Calculated based on the signal area of ²⁹Si{¹H} NMR spectra

^bCalculated based on the signal area of ²⁹Si{¹H} NMR spectra

3.2.2 ²⁹Si{¹H} NMR spectra of ethoxysiloxane oligomers

GPC traces of the hydrolyzates after 1 h since reaction started are shown in Supplementary Fig. S3. The traces differed for each monomer. The ²⁹Si{¹H} NMR spectra of ethoxysiloxane oligomers are shown in Fig. 5. The structures of the oligomers were estimated based on the hydrolysis–condensation of ethoxysilane and isolated specific structural monomer [7, 9, 17, 34, 44‐48]. The hydrolyzate of TEOS after 1 h showed main signals attributed to Q¹(OEt)₂(OH) (−86.2 ppm), 4-membered cyclic Q²(OEt)(OH) (−92.7 ppm), 4-membered cyclic Q²(OEt)₂ (−95.1 ppm), and 5- or 6-membered cyclic Q²(OEt)₂ (−95.4 ppm), and minor signals attributed to linear Q² (−96 ppm) and silanol-terminated species (−85.5 ppm). The Q¹ unit disappeared after 24 h. After 7 days, the intensity of 4-membered cyclic Q² signals decreased, and the signals appeared from −99 to −105 ppm in the Q³ unit region corresponding to cyclic Q³–OH (−99.7 ppm), Q³–OH (−100.5 ppm), cyclic Q³ (−101.5 ppm), cage-type Q³ such as open cage and complete cage (−102.3 ppm), and Q³–OEt (−103.2 ppm). After 14 days, the intensities of the signals decreased and unclear broad signals were observed in the Q² (−92 to−97 ppm), Q³ (−99 to−105 ppm), and Q⁴ (−110 ppm) regions. The molecular weight of the TEOS oligomer increased over time (from 1600 to 2400 Da), and after 14 days, the TEOS oligomer was mainly composed of Q³ structures.

The hydrolyzate of HEDS after 1 h showed the signals attributed to Q¹(OEt)(OH)₂ (−84.0 ppm), Q¹(OEt)₂(OH) (−86.1, −86.2 ppm), Q¹(OEt)₃ (−88.7 ppm), cyclic Q²(OEt)(OH) (−92.7 ppm), 4-membered cyclic Q²(OEt)₂ (−95.1 ppm), 5- or 6-membered cyclic Q²(OEt)₂ (−95.4 ppm), and linear Q² (−96.3 ppm), confirming that the HEDS oligomer after 1 h was mainly composed of 4-membered cyclic Q² and end-terminated Q¹. After 24 h, the signals corresponding to Q¹ units disappeared, and the 4-membered cyclic Q² signal was strong. The small signals due to cyclic Q³ (−101.2, −101.5 ppm) were observed. After seven days, the signal intensity of the 4-membered cyclic Q² decreased, and Q³ unit was grown. The spectrum of the HEDS oligomer after 14 days was similar to that after 7 days, and the Q⁴ 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 Q², cyclic Q³, and Q³ structures; linear Q² and 6-membered cyclic Q² 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 Q²(OEt)(OH) (−85.5, −85.7 ppm), Q¹(OEt)₂(OH) (−86.2 ppm), 3-membered cyclic Q²(OEt)₂ (−87.7, −87.9 ppm), Q¹(OEt)₃ (−88.7, −88.9 ppm), cyclic Q²(OEt)(OH) (−92.7 ppm), Q²(OEt)(OH) (−93.6 ppm), 4-membered cyclic Q²(OEt)₂ (−95.1 ppm), and linear Q² (−96.2 ppm). After 24 h, the intensity of 4-, 5-, or 6-membered cyclic Q²(OEt)₂ (−95.1, −95.4, −95.6 ppm) increased, and intensity of Q¹ and 3-membered cyclic Q² decreased. After seven days, cyclic Q³–OH (−100.5 ppm), Q³–OH (−101.5 ppm), cage-type Q³ such as open cage and complete cage (−102.3 ppm), cyclic Q³–OEt (−103.2 ppm), and Q³–OEt (−104.5 ppm) were formed, and the content of Q¹–OH and 3-membered cyclic Q² decreased. After 14 days, Q¹–OH and 3-membered cyclic Q² disappeared, Q³ structures grew, and the content of 4-membered cyclic Q² decreased. Moreover, end-terminated Q¹(OEt)₃ was observed. After 14 days, the OETS oligomer was mainly composed of 4-, 5-, or 6-membered cyclic Q², linear Q², various Q³, and end-terminated Q¹(OEt)₃. These assignments are summarized in Table 6.

Figure 7 shows the changes in the area ratios of the structures in the ²⁹Si{¹H} 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.

3.2.3 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 [49‐52].

Table 6

Assignments for the ²⁹Si{¹H} NMR spectra of ethoxysiloxane oligomers

Qⁿ unit	Chemical shift of Qⁿ unit / ppm
Qⁿ unit	TEOS oligomer	HEDS oligomer	OETS oligomer
monomer	−81.9	−88.8	−88.9, −96.2
Q₁(OEt)₃		−88.7	−88.7, −88.9
Q₁(OEt)₂(OH)	−85.5, −86.2	−86.1, −86.2	−86.2
Q₁(OEt)(OH)₂		−84.0
Q²_linear	−96.1, −96.4	−96.3	−96.2
Q²_5,6-cyclic	−95.4	−95.4	−95.4, −95.6
Q²_4-cyclic	−95.1	−95.1	−95.1
Q²_linear(OEt)(OH)			−93.6
Q²_n-cyclic(OEt)(OH)	−92.7	−92.7	−92.7
Q²_3-cyclic(OEt)₂			−87.7, −87.9
Q²_3-cyclic(OEt)(OH)			−85.7, −85.5
Q³(OEt)	−101.5 to −103.8	−102.4 to −103.8	−102.5 to −103.4
Q³(OH)	−99.4 to −101.5	−100.4 to −101.5	−100.2 to −101.6
Q⁴	−110.7

4 Conclusion

HEDS and OETS were synthesized in 3.5 and 8.4% of the total yield, 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 four-membered 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.

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1007/s10971-023-06159-x.

Acknowledgements

The authors thank Assistant Prof. Takuya Sagawa at the Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, for helpful suggestions on DFT.

Author contributions

CRediT authorship contribution statement: YS: Investigation, conceptualization, visualization, methodology, writing–original draft. AS: Investigation. TI: Investigation. RH: Writing–review and editing, investigation, visualization. KY: Supervision. TG: Writing–review and editing, conceptualization, and supervision.

Compliance with ethical standards

Conflict of interest

The authors declare no competing interests.

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Title: Hydrolysis and condensation behavior of tetraethoxysilane, hexaethoxydisiloxane, and octaethoxytrisiloxane
Authors: Yohei Sato
Asahi Sugimoto
Tatsuya Iwashina
Ryohei Hayami
Kazuki Yamamoto
Takahiro Gunji
Publication date: 27-07-2023
Publisher: Springer US
Published in: Journal of Sol-Gel Science and Technology / Issue 2/2023
Print ISSN: 0928-0707
Electronic ISSN: 1573-4846
DOI: https://doi.org/10.1007/s10971-023-06159-x

Springer Professional

Hydrolysis and condensation behavior of tetraethoxysilane, hexaethoxydisiloxane, and octaethoxytrisiloxane

Abstract

Graphical Abstract

Supplementary information

1 Introduction

2 Experimental section

2.1 Measurements

2.2 Materials

2.3 Synthesis of 1,1,3,3-tetraethoxy-1,3-diisocyanatodisiloxane (TEDIDS)

2.4 Synthesis of 1,1,1,5,5,5-hexaethoxy-3,3-diisocyanatotrisiloxane (HEDITS)

2.5 Synthesis of hexaethoxydisiloxane (HEDS) and octaethoxytrisiloxane (OETS)

2.6 Initial hydrolysis reaction of ethoxysilane monomers

2.7 Polymerization behavior of ethoxysilane monomers

3 Results and discussion

3.1 Reactivity of diethoxy(diisocyanato)silane

3.2 Hydrolysis and condensation of TEOS, HEDS, and OETS

3.2.1 Hydrolysis of TEOS, HEDS, and OETS

3.2.2 ²⁹Si{¹H} NMR spectra of ethoxysiloxane oligomers

3.2.3 Computational study of ethoxysiloxane monomers

4 Conclusion

Supplementary information

Acknowledgements

Author contributions

Compliance with ethical standards

Conflict of interest

Supplementary information

Premium Partners

Springer Professional

Abstract

Graphical Abstract

Supplementary information

1 Introduction

2 Experimental section

2.1 Measurements

2.2 Materials

2.3 Synthesis of 1,1,3,3-tetraethoxy-1,3-diisocyanatodisiloxane (TEDIDS)

2.4 Synthesis of 1,1,1,5,5,5-hexaethoxy-3,3-diisocyanatotrisiloxane (HEDITS)

2.5 Synthesis of hexaethoxydisiloxane (HEDS) and octaethoxytrisiloxane (OETS)

2.6 Initial hydrolysis reaction of ethoxysilane monomers

2.7 Polymerization behavior of ethoxysilane monomers

3 Results and discussion

3.1 Reactivity of diethoxy(diisocyanato)silane

3.2 Hydrolysis and condensation of TEOS, HEDS, and OETS

3.2.1 Hydrolysis of TEOS, HEDS, and OETS

3.2.2 29Si{1H} NMR spectra of ethoxysiloxane oligomers

3.2.3 Computational study of ethoxysiloxane monomers

4 Conclusion

Supplementary information

Acknowledgements

Author contributions

Compliance with ethical standards

Conflict of interest

Supplementary information

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3.2.2 ²⁹Si{¹H} NMR spectra of ethoxysiloxane oligomers