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 [3,4,5,6,7,8,9]. 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,: 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) [15,16,17], 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.

Scheme 1
scheme 1

Synthesis of alkoxydisiloxanes and trisiloxanes

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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

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 in an ice bath for 10 min, the mixture was heated to room temperature. The mixture was then aged for GPC and 29Si{1H} NMR monitoring.

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 [23,24,25]. 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.

Scheme 2
scheme 2

Synthesis of TEDIDS via the hydrolysis of DEDIS

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Table 1 Synthetic conditions and yield of TEDIDS
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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.

Scheme 3
scheme 3

Reaction between DEDIS and TESOL

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Table 2 Isolated yield of HEDITS by the reaction of DEDIS with TESOL
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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.

Fig. 1
figure 1

29Si NMR spectra of tetraisocyanatosilane, DEDIS, TEDIDS, HEDITS, HEDS, and OETS

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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.

Fig. 2
figure 2

FTIR spectra of DEDIS, TEDIDS, HEDITS, HEDS, and OETS using neat method

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Table 3 FTIR assignment of DEDIS, TEDIDS, HEDITS, HEDS, and OETS
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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.

Fig. 3
figure 3

Experimental processes of the hydrolysis and condensation of ethoxyoligosiloxanes

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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.

Fig. 4
figure 4

ESI-MS of a TEOS–TMS, b HEDS–TMS, and c OETS–TMS detected as [M + Na]+

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Fig. 5
figure 5

29Si{1H} NMR of TEOS-, HEDS-, and OETS-TMSs a 30 to −110 ppm region, and b −80 to −110 ppm region

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Table 4 Assignments for 29Si{1H} NMR spectra of ethoxysilane–TMSs
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Table 5 Relative ratios of siloxane unit structures for ethoxysilanes–TMS
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Fig. 6
figure 6

Structure of ethoxysilane–TMSs

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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, 44,45,46,47,48]. 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.

Fig. 7
figure 7

29Si{1H} NMR spectra of the hydrolyzates of a TEOS, b HEDS, and c OETS

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Fig. 8
figure 8

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

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Fig. 9
figure 9

Schematics of estimated oligomer structures using a TEOS, b HEDS, and c OETS

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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 [49,50,51,52].

Fig. 10
figure 10

ESP maps and dipole moments for TEOS, HEDS, and OETS calculated using the DFT method (B3LYP/6-31G(d,p))

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Table 6 Assignments for the 29Si{1H} NMR spectra of ethoxysiloxane oligomers
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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.