Characterization and Some Insights into the Reaction Chemistry of Polymethylsilsesquioxane or Methyl Silicone Resins

Abstract

Structural characterization of a polymethylsilsesquioxane (PMSQ) and a DT-type methyl silicone resin (MeDT) has been carried out by various instrumental analyses including GPC, NMR, gas chromatography, and gas chromatography-mass spectrometry. Although the PMSQ had a Mw around 5000, the resin contained a significant amount of low molecular weight species consisting of T² [MeSi(OH)O_2/2] and T³ [MeSiO_3/2] units, ranging from T³₄T²₃ to T³₈T²₂ including many isomers. One isomer of T³₆T²₂ was isolated of which structure was determined as a cage structure. The species are supposed to consist mainly of cyclotetra- and cyclopentasiloxanes, but presence of strained rings such as cyclotrisiloxane rings also was suggested. In MeDT, species in which the T² units in the molecules from PMSQ is replaced with D² [Me₂SiO_2/2] were found, for example, T³₆D²₂, suggesting that general silicone resins consist of similar structures as silsesquioxanes. The Mark-Houwink exponent for these methyl resins was ~0.3, indicating the molecular shape to be compact. Investigation on the formation chemistry of the cubic octamers indicates that siloxane bond rearrangement is an important mechanism in the molecule build-up process.

1. Introduction

Silicone resins are a class of polysiloxane material primarily built from T (RSiO_3/2) and Q (SiO_4/2) units, thus have much higher crosslink density than elastomers that mainly consist of D unit (R₂SiO_2/2) [1, 2]. The M (R₃SiO_1/2) and D units are also used as components in silicone resins but usually at much lower concentrations. In silicone industry, silicone resins are defined as solvent-soluble materials that are stable at room temperature and have functionalities for further crosslinking to give insoluble materials in the final application forms like coatings. In this sense, silicone resins can be differentiated from sol-gel materials in which the final insoluble materials are formed in many cases directly from monomers, although the synthetic chemistry is essentially the same. One of the forms of silicone resins that consists exclusively from T units is called silsesquioxanes or polysilsesquioxanes [2–6]. Silicone resins or silsesquioxanes are known since the beginning of silicone industry in the 1930s [7], but traditionally these materials have been captured via simple parameters including the molar ratio of substituents to the silicon atom, R/Si ratio (1.0–1.7), molar ratio of phenyl substituent to methyl, and molecular weight. However, little has been known on what structural features are responsible for which property as well as what reaction mechanism results in what structure.

Polyphenylsilsesquioxane (PPSQ) has been often referred to as a ladder polymer since Brown Jr. et al. reported the polymer to have cis-syndiotactic conformation [8, 9]. They assigned the structure by X-ray diffractometry (XRD), IR, UV, bond angle calculation, and the exponent 𝑎 value in the Mark-Houwink equation. However, these data do not appear to be sufficient to claim such a defined structure. The XRD pattern is reported by Andrianov et al., which is questionable for a cis-syndiotactic structure [10, 11]. Some papers refer to the material as ladder polymers if there are no silanols left or simply by the IR spectra showing two bands at ~1150 and ~1050 cm⁻¹ [2], but these data cannot always be evidence for a ladder structure. Park and coworkers indicated by calculation that the IR band around 1150 cm⁻¹ is derived from the parallel asymmetric Si–O–Si stretch vibration mode while the lower-frequency band around 1050 cm⁻¹ is the asymmetric Si–O–Si stretch; thus incompletely condensed cages or T³₆ cage shown in Figure 1(a) can show these two bands [12]. Among these characterization techniques, the 𝑎 value in the Mark-Houwink equation is repeatedly reported to give values around 1 [13–15], implying that PPSQ has random coil to rod-like shapes. In their dilute solution study, Helminiak and Berry concluded that the conformation of PPSQ can be represented with a worm-like chain model with a persistence length of 75 Å [16]. Frye and Klosowski strongly opposed the ladder structure and suggested a more or less randomly linked array of polycyclic cages [17]. Real ladder structures can be found only for low molecular weight oligomers, up to heptacyclic ladder structures with isopropyl substituent as prepared by a stepwise synthesis [18–21] or by oxidation of ladder oligosilanes [22]. Preparation of ladder-like polymethylsilsesquioxanes (PMSQs) by spontaneous condensation of cis-trans-cis-tetrabromotetramethylcyclotetrasiloxane is suggested by Chang and coworkers [21]. It does not appear that the material is ultimately proven to have a ladder structure or it may not be known how one can prove a resin having a ladder structure, but an interesting observation in this work is that the IR spectrum of this PMSQ exhibits two absorption bands at 1130 and 1030 cm⁻¹, while tri- or pentacyclic ladder oligomers or a T³₆ cage (all isopropyl substituted) does not show such clear pair of the two bands. Seki et al. reported the preparation of ladder-like PMSQs by the hydrolytic polycondensation of an isocyanate-functional cyclic tetramer of methyl T² having the structure of Figure 1(l) [23]. The triple-detector GPC gave a 𝑀𝑤 of 285,000 and a 𝑀𝑛 of 110,000 with the Mark-Houwink 𝑎 value of 0.53. This 𝑎 value is much lower than what can be claimed as a rigid rod molecule, but is higher than common PMSQ (vide infra), suggesting that this PMSQ may have somewhat extended molecular shape. Another PMSQ with a 𝑀𝑤 of 214,000 and a 𝑀𝑛 of 71,000 prepared from a hydridofunctional cyclic tetramer showed the 𝑎 value of 0.38. Both these two PMSQs showed the distinct two bands at 1150 and 1130 cm⁻¹ in the IR spectra, indicating that the two IR bands are not directly associated with a ladder structure.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

(m)

Figure 1 

Possible structures of silsesquioxanes or silicone resins. Structure (f) was identified in the present study; (a), (e), and (i) have most probably no choice other than these structures; (b), (d), and (j) are identified with other substituents (see the text below); the presence of (h), (k), and (h) are likely; (c), (g), and (l) is possible structures from the chemical formulae.

In contrast to more uncertain ladder structures, cage structures are identified in many reports. For instance, completely condensed cages around the cubic octamers with various substituents including methyl, n-hexyl, and phenyl are reported in old and new papers [3–6, 24, 25]. For a hydrogen silsesquioxane (HSQ) synthesized by the toluene-sulfuric acid method [26], Agaskar and Klemperer carefully fractionated cages [27]. They identified completely condensed ^HT³₈, ^HT³₁₀, ^HT³₁₂, ^HT³₁₄, and ^HT³₁₆ cage molecules using gas chromatography, ¹H and ²⁹Si NMR, elemental analysis, and mass spectrometry. Synthesis and identification of compounds with the structures of Figures 1(a), 1(c), 1(d), and 1(f) having cyclohexyl group as the substituent are presented by Feher and coworkers [28]. Most of these molecules consist of cycloterta- and cyclopentasiloxane rings, but the presence of strained cyclotrisiloxane rings in the structures of Figures 1(a) and 1(c) is also reported.

Methyltrichlorosilane is the most abundant low cost starting material in silicone industry as compared to phenyltrichlorosilane or trichlorosilane. Thus PMSQ or methyl silicone resins are one of the most important materials among silicone resins. In this work, structural characterization of a PMSQ will be described with some insight into the reaction chemistry. The materials were prepared by a simple hydrolytic polycondensation with excess water, which is not a very specific synthetic procedure to form characteristic structures. The interpretation of the ²⁹Si NMR spectra was revisited by trimethylsilyl-(Me₃Si-) capping of the silanol. The presence of low molecular weight compounds was then studied by gas chromatograph (GC) and GC-mass spectrometry (MS) analyses for Me₃Si-capped species. Isolation of a compound was tried to identify the structure by X-ray crystallography. The understanding of the chemistry for the formation of such molecules was explored by analyzing the reaction species at earlier stage of the hydrolytic polycondensation and by reacting the isolated molecule of known structure in a similar reaction condition trying to uncover the development of the structures during the course of the reaction.

In addition to the PMSQ, a DT-type methyl silicone resin, denoted as MeDT, having a small amount of D unit in addition to the T unit, was studied to compare with the PMSQ. This will show the structural characteristics of a more typical industrial silicone resin and will help verifying the generality of the findings for the PMSQ.

As usually practiced in silicone industry, the term “T” in this paper denotes species derived from RSiX₃ (X denotes Cl or OR) by hydrolytic condensation. Consequently, structures in the resin denoted as a T unit could contain RSi(OH)₃, RSi(OH)₂O_1/2, RSi(OH)O_2/2, and RSiO_3/2. These four structures are described as T⁰, T¹, T², and T³, respectively (the numbers in superscript denote the number of siloxane bonding). Likewise, a D unit represents those species derived from R₂SiCl₂ containing R₂Si(OH)₂, R₂Si(OH)O_1/2, and R₂SiO_2/2, abbreviated as D⁰, D¹, and D², respectively.

2. Experimental

2.1. Materials

Methyltrichlorosilane (Kanto Chemical), dimethyldichlorosilane, octamethylcyclotetrasiloxane (D₄) (Shin-Etsu Chemical), N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), methyl isobutyl ketone (MIBK, 4-methyl-2-pentanone), dichloromethane, 2-methylpentane, chloroform, distilled water, magnesium sulfate (Wako Pure Chemical), deuterated chloroform (CDCl₃), and tetrahydrofuran (THF) (Aldrich) were used without further purification.

2.2. Analytical Techniques

GPC curves were obtained using a Tosoh HLC-8020 gel permeation chromatograph equipped with a refractive index detector and two Tosoh TSKgel GMH_XL-L columns eluted by chloroform at the flow rate of 1.0 mL/min. The molecular weight was determined relative to polystyrene standards. The calibration curve was corrected for each run from the difference in the retention time of cyclohexane as the internal standard assuming that the retention times of the polystyrene standards and the resin molecules are proportional to the shift of the retention time of the internal standard.

The chromatographic equipment for the GPC with triple detector (light scattering, viscometric, and refractive index detectors) using THF was equipped with a Viscotek T60A light scattering/viscometric detector, a Waters 2410 refractive index detector, and two Polymer Laboratories Mixed E columns, programmed to run at 1.0 mL/min. Instrument calibration was based on polystyrene standard and the sample concentration was 0.5–1 wt%. The sample was prepared four times to verify reproducibility.

²⁹Si NMR spectra were recorded on a Bruker ACP-300 spectrometer in CDCl₃. The sample concentration was ~600 mg in 2.4 mL of the solvent for a 10 mm tube. Chromium acetylacetonate was used as a relaxation agent in ~32 mg quantity. A gated decoupling in which protons were irradiated only during the acquisition of FID (free induction decay) was employed with the pulse angle of 45° and the pulse delay of 7 s. The number of scans was >1000 and the line broadening of 2.0 Hz was employed on plotting. The chemical shift was adjusted using tetramethylsilane as either an internal or an external standard.

Gas chromatography-mass spectrometry (GC-MS) study was conducted using a Shimadzu gas chromatograph-quadrupole mass spectrometer, model QP-5050A, utilizing an electron impact ionization source with an ionizing electron energy of 70 eV. The interfacial temperature was 300°C and the ion source temperature was 250°C. The GC was a Shimadzu GC-17A gas chromatograph equipped with a capillary column (J&W Scientific, 30 m × 0.250 mm, coated with DB-5 in 0.10 mm thickness). The GC temperature program used was: initial temperature, 60°C; initial time, 2.00 minutes; program rate, 40°C/min; final temperature, 280°C; injection temperature, 280°C; detector temperature, 300°C. Helium was used as a carrier gas with its pressure of 150 kPa. A sample of ~25 wt% solution containing D₄ as an internal standard was injected in 0.1 μL amount. A Shimadzu GC-14A gas chromatograph equipped with the same capillary column as installed in the GC-MS and a flame ionization detector (FID) was used. The same carrier gas pressure, temperature program, and injection amount as in the GC-MS studies were employed for exactly the same sample. For PMSQ at earlier reaction stage, the following temperature program was used both for GC and GC-MS analyses: initial temperature, 60°C; initial time, 2.00 minutes; program rate, 10°C/min; final temperature, 280°C; He pressure of 100 kPa.

XRD study was performed with a JEOL JDX-3530 diffractometer using Ni-filtered Cu-K_α radiation. The intensity distribution (2^∘<2𝜃<35^∘) was observed in the reflecting mode using a goniometer equipped with a monochromator. The crystallinity was determined by the known method from the crystalline peaks and amorphous halo.

X-ray single crystal analysis was performed on a SIEMENS SMART diffractometer at a temperature of about −120°C. The structure was determined by direct methods using program SHELXS.

2.3. Preparation of PMSQ

Using the method described in a previous paper [29], the PMSQ was synthesized from 1196 g (8.00 mol) of methyltrichlorosilane in 3.20 L each of MIBK and water and heat aging at 50°C for 2 h 50 min. The PMSQ (PMSQ-1) was obtained as a white solid in 528 g quantity. Another batch, PMSQ-2, was synthesized in a 9 mol scale heat aging at 50°C for 4 h, providing 562 g of the material. To obtain a PMSQ without the heat aging, 74.7 g (0.500 mol) of methyltrichlorosilane was reacted by the same method using 200 mL each of MIBK and water, and the product was recovered immediately after the completion of the dropwise addition of methyltrichlorosilane to the mixture of MIBL and water with the maximum temperature during the addition over 26 min of 20.0°C (PMSQ-NA).

2.4. Preparation of MeDT

The MeDT resin was prepared in a similar manner using a mixture of 1155 g (7.7 mol) of methyltrichlorosilane and 165 g (1.3 mol) of dimethyldichlorosilane, and 4.00 L of MIBK and 3.42 L water. The heat aging after the hydrolysis was performed at 60°C for 2 h, giving a solid MeDT in 580 g yield. Another lot, MeDT-2, was synthesized in the same scale with the heat aging at 60°C for 3 h.

2.5. Isolation of a Molecule

To 100 g of PMSQ-1, a mixture of 1400 mL of hexane and 600 mL of chloroform, was added. After stirring for 30 min, the mixture was allowed to stand for 20 min, followed by decanting the supernatant solution. By stripping the solvent, Fraction 1, the lower molecular weight fraction, was obtained in 31% yield. Fraction 1 (19.99 g) was dissolved in 204 mL of acetone (0.098 g/mL). While stirring, 54 mL of distilled water was added to the solution, and the mixture was allowed to stand overnight, giving two separated layers. The top layer was separated by decantation, and 14 mL of water was added, which again gave two layers. The solvent in the top layer was stripped off, followed by redissolving in 77 mL of acetone (resin/acetone ratio of 0.098 g/mL). Removing the solvent from the supernatant solution gave the lowest molecular weight fraction, Fraction 4, as a solid (5.204 g). To 2.00 g of Fraction 4, 11.4 g of chloroform was added (15 wt%) and the solution was allowed to stand overnight at room temperature. Insoluble material remained in the solution, the amount of which increased by standing overnight. A crude cage material, Fraction 4-P1, was obtained by collecting the precipitates by filtration (0.068 g). By concentrating the filtrate followed by standing overnight, 0.067 g of a crude product (Fraction 4-P2) was isolated by filtration. Further concentration of the filtrate afforded a crude product, Fraction 4-P3, in a very small amount (0.008 g). Similarly, 0.10 g of a crude material (Fraction 4-P4 and 5) was obtained from 1.74 g of Fraction 4 and 0.48 g of a crude product (Fraction 4-P6 and 7) was obtained from 9.58 g of another Fraction 4. Recrystallization of 0.64 g of the crude products (1: mixture of Fraction 4-P1, 4, 5, 6, and 7) from an acetone/chloroform mixture gave crystals (X1, 0.43 g). Needle crystals, X2, were obtained from X1 by slow evaporation of the solution (acetone/chloroform = 3/1) covered with a heptane layer.

2.6. Trimethylsilyl-(Me₃Si-) Capping of Silanol in the Methyl Silicone Resins

A 25 mL three-neck flask with a septum, a three-way stopcock, a glass stopper, and a magnetic stir bar was charged with 3.01 g of PMSQ-1, followed by purging with nitrogen through the stopcock. After dissolving in 9.0 mL of dichloromethane, 1.7 mL of BSTFA was added using a syringe through the septum over 2 min while cooling on an ice-water bath. The amount of BSTFA was set at ~1.2 times of the SiOH present in the resin simply calculated from the T² region (−50 to −58 ppm) in the ²⁹Si NMR spectrum and the theoretical amount of water in the dichloromethane as calculated by the solubility of water of 0.198 wt%. After stirring at room temperature for ~10 minutes, ~10 mL of water was added, followed by washing the slightly basic organic phase with water to neutral. The organic phase was dried over magnesium sulfate and the solvent was stripped off under vacuum. Me₃Si-capping of the OH in MeDT was carried out in the same way.

2.7. Quantification Method of GC Peaks

The quantification procedure for the Me₃Si-capped PMSQ-1 is described below. Me₃Si-capped PMSQ-1 (73.8 mg) was dissolved in 0.17 mL of chloroform. A stock solution of D₄ was prepared by dissolving 31.7 mg of D₄ in 157.7 mg of chloroform. The stock solution (23.8 mg, 30 μL) was added to the solution of Me₃Si-capped PMSQ as an internal standard, which made the amount of D₄ in the PMSQ solution 3.98 mg (0.0134 mmol). The PMSQ solution was subjected to both the GC-MS and the GC analyses. After assigning the GC peaks from the GC-MS assignment, quantification was made assuming that the GC peak area with a FID detector was proportional to the number of methyl groups. The following shows an example taking peak x in Figure 4 for T³₆T²₂: Dividing the GC area for D₄ by the number of methyl group, 8, gives mole equivalent area for the D₄. In the present case, the D₄ GC peak at 3.372 min had an area of 7217; thus the mole equivalent area was 902.1. Since the amount of D₄ was 0.0134 mmole, the GC peak area for 1 mmol of D₄ is 67320 (902.1/0.0134). The peak x at 6.826 min for T³₆T²₂ had the peak area of 1478. Since the number of methyl group for a trimethylsilyl-capped T³₆T²₂ is 14, the mole equivalent area for this molecule was 105.6 (1478/14). From this value and the molar equivalent peak area for D₄, the amount of this molecule is calculated as 0.00157 mmole (105.6/67320). By multiplying the molecular weight of uncapped T³₆T²₂, 554, the amount of this molecule is calculated as 0.872 mg. Since the amount of the entire resin, 73.8 mg, was based on the Me₃Si-capped PMSQ, the weight of the uncapped resin was calculated by the ²⁹Si NMR data. The average molecular formula of PMSQ-1 was determined as T^2(OH)_0.045T^3(OSiMe3)_0.150T³_0.805 by simply assigning the resonances around 10 ppm as the M unit, those between −51 and −58 ppm as the T² unit, and those at −58 to −71 ppm as the T³ unit. This gives the average formula weight of 79.70, from which that for the uncapped resin was calculated as 68.88. From these values, the amount of the resin used for the GC quantification on uncapped basis was calculated as 63.8 mg (73.8 × 68.88/79.70). Thus the amount of the molecule of peak x is 1.37 wt% of the entire PMSQ. (The residual resonances in the T² region after the capping was assigned to T³ in strained rings as shown below, but in this calculation, the integration in the T² region was simply used as T² silicon, which will not cause major impact on the calculation.)

2.8. Quench Capping of Early Reaction Intermediates (PMSQ-QC)

A D₄ stock solution was prepared by dissolving 30.2 mg of D₄ in 294.5 mg of MIBK. In a 20 mL vial was placed 2.2 mL of 2-methylpentane and 0.8 mL (~0.78 g, ~3.0 mmol) of BSTFA, and the vial was placed in a dry ice-acetone bath of −70°C. In a 10 mL vial with a magnetic stir bar and a digital thermocouple thermometer, 0.7 mL of water, 0.35 mL of MIBK, and 30 μL (23.7 mg) of the D₄ stock solution (2.20 mg of D₄) were stirred vigorously in an ice-water bath. After the temperature of the mixture in the 10 mL vial reached 1.2°C, 0.1 mL (0.178 g, 1.19 mmol) of methyltrichlorosilane was added dropwise using a 1.0 mL syringe in 140 s (the maximum reaction temperature during the addition was 9.2°C), and the mixture was stirred another 35 s. The relative amount of MIBK to methyltrichlorosilane was the same as the above PMSQ synthesis, but the amount of water was twice the standard condition. The content of the reaction vial was then dumped into the 20 mL vial. The organic phase was separated from the frozen water phase and was washed with 3-4 mL of water 4 times to neutral. The 2-methylpentane was stripped off to obtain a solid product which was subjected to GC and GC-MS analyses in the same way as the Me₃Si-capped PMSQ. However, one difference was that the amount of the entire resin is not known. Thus, firstly the amount of a molecule was determined in mmol relative to the amount of D₄ (0.00742 mmol), which was multiplied by the number of silicon atoms in that molecule. This amount was expressed as percent of the feed methyltrichlorosilane (1.19 mmol) as Si mol% in Table 4.

2.9. Quantification of T³₈ Cubic Octamer

To a mixture of 45 mL of MIBK and 60 mL of water on an ice bath, 22.4 g (0.15 mol) of methyltrichlorosilane dissolved in 15 mL of MIBK was added dropwise. The reaction temperature was kept at 5–9°C during the addition over 1 h. After the completion of the addition, the reaction mixture was heated on an oil bath. It took 23 min for the reaction temperature to increase to 50°C, after which the heating was continued at 50°C for 3 h. After the MIBK phase was washed to neutral, the insoluble materials were collected from the MIBK phase by centrifugation (6000 rpm, 15 min). The precipitate was washed by dispersing in MIBK followed by centrifugation. The MIBK used for washing was combined with the MIBK phase. The precipitate was further washed with acetone and dried under vacuum for 3 h (284.4 mg). The water phase and the water used for this washing were combined and subjected to centrifugation. The precipitate was washed with acetone twice and dried. All the precipitates obtained were combined. The solvent was removed from the MIBK phase after removing the precipitate, giving a resin as a solid (10.2 g). The resin was Me₃Si-capped by the method described above and subjected to GC analysis using D₄ as an internal standard. In other two runs, the products were recovered immediately after the completion of the addition of the chlorosilane (0 h aging) and after 17 h aging at 50°C.

2.10. Reaction of the Isolated T³₆T²₂

The crude isolated T³₆T²₂ cage, 1, (50.0 mg, 0.0901 mmol) was dissolved in 0.864 mL of MIBK (the amount of MIBK relative to Si was three time of the PMSQ synthesis due to the solubility of T³₆T²₂). After adding 0.30 mL of 24 wt% hydrochloric acid, the mixture was heated at 50°C for 3 h. The MIBK phase was washed with water to neutral before recovering the precipitate by centrifugation. The amount of the precipitates was 9.7 mg (19 wt% of the starting material). From the MIBK phase, 37.5 mg of a resin was obtained by removing the solvent (1-R, 75 wt% of the starting material). The content of the T³₈ cage in the precipitated was determined by XRD crystallinity and that in the resin was determined by the aforementioned GC/GC-MS method after Me₃Si-capping.

3. Results and Discussion

3.1. GPC Analysis

Figure 2 represents the GPC curves for PMSQ-1 and MeDT-1 with the molecular weight data listed in Table 1. Both materials showed multimodal curves. The weight average molecular weight, Mw, were 6370 for PMSQ-1 and 4370 for MeDT-1. Both materials contained low molecular weight species, the peak at 17.9 min for PMSQ-1, 19.0% peak area with the 𝑀𝑤 of 390, and the peak at 18.5 min for MeDT-1, 22.9% peak area with the 𝑀𝑤 of 410. For PMSQ-2 and MeDT-2, GPC using coupled refractive index, viscosity, and light scattering detectors was conducted. Because the light scattering signal was too weak for realistic measurement, the universal calibration method was used as an alternative and the results are summarized in Table 2. The relationship between molecular weight M and intrinsic viscosity [𝜂] is typically represented by the Mark-Houwink equation: 𝜂=𝐾𝑀^𝑎, (1) where the coefficient 𝐾 and the exponent 𝑎 depend on the solute-solvent pair and temperature. The Mark-Houwink exponent can provide information with respect to molecular shape, where the limits are 0 for spheres and 2 for rod-like molecules. The 𝑎 values around 0.3 mean that the molecule is far from rigid-rod shapes.

Table 1 

GPC data for PMSQs and MeDT resins.

Table 2 

Universal GPC data.

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(b)

(c)

Figure 2 

GPC curves for (a) PMSQ-1, (b) MeDT-1, and (c) PMSQ-NA.

3.2. NMR Spectroscopy

Figure 3(a) shows the ²⁹Si NMR spectrum of PMSQ-1 together with the T unit region magnified. The integration data are summarized in Table 3. The spectrum consists of three major envelopes: (i) −51 to −58 ppm, (ii) −58 to −60.5 ppm, and (iii) −60.5 to −70 ppm. Region (i) will be assigned to T² silicon and (ii) and (iii) to T³ silicon [30–32]. These broad resonances suggest the presence of a variety of environments. At the same time, sharp peaks are observed at −52.3, −52.9, −54.2, −54.8, −55.1, −63.2, −63.4, −64.3, −64.6, −64.9, −66.0 ppm, and so forth, indicating that specific environments are heavily populated or preferred. Figure 3(b) illustrates the ²⁹Si NMR spectrum of Me₃Si-capped PMSQ-1. The resonances in the T² region decreased and a new resonance appeared around 9 ppm for the vinyl dimethyl-M unit. The NMR integration for the M region, the decrease in the T² region, and the increase in the T³ region indicated that no condensation essentially took place during the capping reaction. However, the resonances including the two sharp peaks at −52.9 and −55.2 ppm remained in the −51 to −58 ppm region. A deuteration technique in IR spectroscopy by Lipp to detect trace silanol in PDMS revealed that the silanol content of the Me₃Si-capped PMSQ was 0.12 wt%, implying that the capping efficiency was around 95% [33]. Feher and coworkers reported that an incompletely condensed cyclohexylsilsesquioxane of the structure (b) in Figure 1 showed the ²⁹Si NMR peaks at −55.57, −56.94, −57.11, and −66.40 ppm in the intensity ratio of 1 : 2 : 2 : 1 [34]. Since there are only two T² silicon atoms among the six, this clearly indicates that the T³ silicon in the strained cyclotrisiloxane ring gives resonances around −56 ppm for this silsesquioxane structure with an aliphatic substituent. Unno and coworkers reported the chemical shift for hexasilsesquioxane (Figure 1(a)) with substituents of 1,1,2-trimethylpropyl and t-butyl giving peaks at −55.1 and −54.3 ppm, respectively [35]. All these pieces of information imply that an aliphatic substituted T³ silicon in a cyclotrisiloxane ring gives ~10 ppm downfield shift. Therefore it is highly likely that the major part of the residual resonances in the −51 to −58 ppm region for the Me₃Si-capped PMSQ represent silicon atoms of T³ in strained rings. It is safe to say that only 75–80 mol% of the silicon in the −51 to −58 ppm region can be assigned to T². In other words, the T²/T³ molar ratio of the PMSQ from traditional ²⁹Si NMR assignment is 0.19/0.81, but the actual composition is roughly T²_0.15T³_0.85.

Table 3 

²⁹Si NMR data for PMSQ-1 and MeDT-1.

Table 4 

QR-ES-Metflex GC quantification summary.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3 

29Si NMR spectra of (a) PMSQ-1, (b) Me3Si-capped PMSQ-1, (c) PPMSQ-NA, (d) Me3Si-capped PMSQ-NA, (e) MeDT-1, and (f) Me3Si-capped MeDT-1.

Figure 4 

Gas chromatograph of Me3Si-capped PMSQ with the peak assignment by the GC-MS. The molecular compositions are described as uncapped PMSQ.

Table 3 and Figures 3(e) and 3(f) show the data and spectra for the ²⁹Si NMR spectrum of MeDT-1. Essentially the same phenomenon that there are residual resonances in the −51 to −58 ppm region after Me₃Si-capping was seen. However, the integration of the D silicon region around −20 ppm is not affected by the capping, indicating that there is no D¹ silicon in MeDT-1. Usually D² silicon, represented by those in polydimethylsiloxane, appears around −22 ppm [36]. Those in −15 to −19 ppm region denoted as D²-a in Table 3 will be D² silicon next to a T unit. The sharp peak at −14.4 ppm which was not affected by the capping could be a D² silicon in strained ring structures. In a similar manner to the PMSQ, the composition of MeDT-1 could be described as D²_0.15T²_0.10T³_0.75.

3.3. GC-MS and GC Analysis

PMSQ-1 was subjected to GC and GC-MS analyses using the Me₃Si-capped material to avoid silanol condensation in the high-temperature GC injection port and the detector. Figure 4 shows the GC chart using an FID detector. GC-MS analysis was carried out using the same capillary column as the GC in which molecular weights of 18 peaks were identified. The molecular weights are assigned based upon the common knowledge that a methyl radical (molecular mass of 15) is readily cleaved from neutral species [37] which are subjected to 70 eV electron ionization. As a reference data, T³₈ was directly placed in the ionization chamber of the GC-MS instrument, which gave the expected [M-Me]⁺ ion at 521 daltons, arising from the loss of a CH₃ radical from the odd electron molecular ion of the neutral (nominal mass = 536 Daltons). As can be seen in Figure 4, seven species with their isomers were detected. Mass spectrometry of PMSQ materials is reported for matrix-assisted laser desorption/ionization time-of-flight (MALDI) MS [38], graphite plate laser desorption/ionization time-of-flight (GPLDI-TOF) MS [39, 40], and electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) MS [38]. These mass spectroscopic methods can directly analyze silanol-functional PMSQs without the Me₃Si-capping, but the GC-MS method can identify the presence of isomers and quantify the identified material in combination with a GC. As described in the experimental section, the peaks in Figure 4 were quantified using D₄ as an internal standard as listed in Table 4. The molecular weight observed in the GC-MS is for the capped species. For instance, [MeSiO_3/2]₆[MeSi(OSiMe₃)_2/2]₂ was actually observed instead of the original T³₆T²₂, but the amount of each species shown in Table 4 was calculated on the uncapped molecules as described in the experimental section. The sum of the seven types of species, 18 compounds all together including isomers, was 7.9 wt% of the entire PMSQ, in which the most abundant compound was T³₆T²₂.

The GC-MS and GC study of MeDT-1 was carried out by the same method. For the PMSQ, one molecular weight detected by the GC-MS corresponded to one combination of T³ and T². For MeDT, however, there were multiple compositions which corresponded to a given molecular weight for the capped resin. The molecular weight of 550 could be interpreted as a capped T³₆T² or a T³₆D²₂ (not capped being completely condensed), and the molecular weight of 564 could be a capped T³₄T²₂ or a capped T³₄T²D²₂. To help determine which composition was the correct assignment to the GC-MS molecular weight, an uncapped resin was subjected to GC-MS, which gave the same peaks of molecular weight of 550 (m/e of 535) as the capped material. Thus the molecular weight of 550 for the capped species was assigned as T³₆D²₂, not T³₆T². Based upon the appearance in electrospray mass spectrometry, the capped molecular weight of 564 could be T³₄T²D²₂ [41]. Based on these considerations, the assignment and quantification were made as summarized in Table 5. Eight species ranging from T³₄D²₂ to T³₈T² (20 compounds all together including isomers) were detected with their sum of 8.5 wt% on an uncapped resin basis.

Table 5 

GC quantification of assignable low molecular weight species in MeDT-1.

Comparing the PMSQ and the MeDT resin, the number of the species detected by GC and GC-MS and their sum were similar. But it is noted that many of the T² units [CH₃Si(OH)O_2/2] in the PMSQ are replaced with D² unit [(CH₃)₂SiO_2/2], the units with two siloxane bonds and two substituents. While T³₆T²₂ was the most abundant species in the PMSQ, T³₆T²D² (exemplified in Figure 1(h)) was the most abundant in the methyl-DT resin. Most of the molecules in the PMSQ were incompletely condensed silsesquioxanes, but 35% of the molecules were completely condensed in the MeDT. It should be noted that these structures are not specific to silsesquioxanes, but common to silicone resins.

3.4. Isolation of a Compound and Consideration for the Structures

One compound 1 was able to be isolated from PMSQ-1 by solvent fractionation and recrystallization as described in the experimental section. GC analysis of the Me₃Si-capped material showed that crude crystal of 1 mainly consisted of peak z in Figure 4 with the relative GC peak area of 90%. The GC purity increased to 98% after recrystallization (X1). Figure 5 shows the X-ray single crystal structure of X2 which proves that the molecule is T³₆T²₂. The molecule consisted of two identical parts related by a crystallographic twofold axis passing through atom O6. Schematically, this compound is described as shown in Figure 1(f). Tables 6 and 7 show the bond distances and the bond angles, respectively. The Si–O bond lengths range from 1.601(3) to 1.630(3) Å, which are comparable to values found in the other Si–O ring systems [28, 42]. Figure 6 shows the unit cell structure of X2. The molecules showed no short intramolecular contact distances and were connected by intermolecular hydrogen bonding between the hydroxyl groups. Four neighboring hydroxyl groups were arranged about a crystallographic fourfold axis. There were no other significant intermolecular interactions, but a chloroform molecule was statically distributed over four symmetry equivalent positions.

Table 6 

Bond distances for 1, X2.

Table 7 

Bond angles for 1, X2.

Figure 5 

The structure of X2.

Figure 6 

The unit cell structure of X2.

Figure 7(a) shows the ²⁹Si NMR spectrum of crude 1 mainly consisting of four sharp resonances at −55.7, −63.6, −64.5, and −65.1 ppm with the integration ratio of 1 : 1 : 1 : 1. The peak at −55.7 ppm is of the T² silicon and the other three peaks are of the T³ silicon atoms. The pattern was similar to that of the cyclohexylsilsesquioxane of the same structure (pyridine/C₆D₆, −58.46, −65.66, −67.51, −68.58 ppm, 1 : 1 : 1 : 1) [28]. The spectrum verifies that T³ silicon atoms in a cyclotetrasiloxane or a cyclopentasiloxane ring, not in a strained cyclotrisiloxane, appear in the −65 ppm region, not in the −55 ppm region. The ²⁹Si NMR peak assignment was further verified by Me₃Si-capping as shown in Figure 7(b). The spectrum shows four resonances at –64.0, −64.1, −64.8, and −65.4 ppm with the integration ratio of 1 : 1 : 1 : 1, reflecting the conversion of the T² silicon to T³ by the capping. The 𝑀/𝑇 ratio was 1/4 as expected.

(a)

(b)

Figure 7 

29Si NMR spectrum of (a) crude 1 and (b) Me3Si-capped X1.

The isolation of this molecule and the identification of the structure clarify the presence of cage molecules in the PMSQ. As shown in Figure 4 and Table 4, there are three other isomers for the composition of T³₆T²₂, peaks w, x, and e2. Since the structure of these species is unknown, it is possible to draw a ladder-like structure shown in Figure 1(g). For the chemical formulae like T³₆T² or T³₈T², however, one cannot draw such ladder-like structures. T³₈ and T³₁₀ are surely cage structures. Thus it is safe to say that the presence of cage molecules in the PMSQ is evident. Since Feher and coworkers reported spontaneous formation of structures (a), (d), and (f) for cyclohexyl substituent [28], the major structure of T³₄T²₃ could be (d) in Figure 1. The presence of cage molecules may be in line with the low Mark-Houwink exponent values. Another observation is that the structure of T³₆T² cannot be drawn without including at least one cyclotrisiloxane ring as shown in Figure 1(c) as an example. This structure was reported by Feher and coworkers as the dehydration product from the material having the structure of Figure 1(d) for a cyclohexylsilsesquioxane. If a ladder structure as in Figure 1(g) exists, that also contains cyclotrisiloxane rings. The presence of T³₆T² and the residual resonance in the −55 ppm region after Me₃Si-capping in the ²⁹Si NMR spectrum verifies the presence of strained cyclotrisiloxane rings.

3.5. Early Stage of the Reaction

To obtain insight into the formation of these low molecular weight species in PMSQ-1, reaction intermediates at much earlier reaction stage were captured. PMSQ-1 was prepared by adding 8.00 mol (~1200 g) of methyltrichlorosilane to a mixture of water and MIBK over 3 h 4 min allowing the reaction temperature to increase to 23°C, followed by heating up to 50°C taking 53 min and aging at 50°C for ~3 h. In contrast, to capture the reaction intermediates at earlier stage, firstly a quench capping approach was carried out. Methyltrichlorosilane (0.2 g) was added to a stirred mixture of water and MIBK over 140 s suppressing the reaction temperature only up to 9°C. After stirring for 35 s, the reaction mixture was poured into 2-methylpentane containing BSTFA in a dry ice-acetone bath to freeze the acid-water phase and cap the silanol by trimethylsilyl group. The reaction product, PMSQ-QC, was subjected to GC and GC-MS analyses in a similar way to the analysis of the Me₃Si-capped PMSQ-1.

Figure 8 shows the FID-GC chart with the peak assignment from GC-MS for PMSQ-QC. The GC quantitative analysis data are summarized in Table 4. Since the amount of the entire resin is not known for PMSQ-QC, the amount of each species relative to the entire resin on the uncapped basis was calculated as silicon mol% (see the Experimental section), which is close to weight percent. Many species that were not present in PMSQ-1 were observed: T⁰ [CH₃Si(OH)₃], T¹₂ [HO(CH₃)₂Si-O-Si(CH₃)₂OH], T²₃ [cyclic trimer of CH₃Si(OH)O_2/2], T²₄ [cyclic tetramer, two isomers], T²₅ [cyclic pentamer, two isomers], T²₄T² [one isomer], T²₂T²₃ [five isomers], T²₂T²₄ [four isomers], and T²₄T²₂ [three isomers]. Including those common with the PMSQ, which are T²₆T², T²₆T²₂, and T²₄T²₃, the sum of the identified species was 16.4%, indicating that more than 80% of the silicon is already in higher molecular weight species. Most of the cage structures found in PMSQ-1 or in the literature [3–6, 27, 28] consist of cyclotetra- or cyclopentasiloxanes. The abundance of T²₄ and T²₅ in PMSQ-QC may be in line with this. The amount of the cyclic trimer, T²₃ exemplified in Figure 1(i), was very small but clearly identified, while a linear trimer, T¹₂T² illustrated in Figure 1(j), was not detected. Thus the formation of strained cyclic trimer was evidenced. In a computer simulation of condensation of HSi(OH)₃ and trisilanol with other substituents under excess water condition [43, 44], Kudo and Gordon suggested that the energy for the formation of cyclic trimer is not very much higher than that for cyclic tetramer. For the formation mechanism of cyclic tetramer, they indicated that insertion of T⁰ to cyclic trimer is even more favored than the condensation of two T¹₂ molecules or T⁰ and a linear trimer. Kelts and Armstrong observed the formation of cyclic trimers of Q², [Si(OH)₂O_2/2], by ²⁹Si NMR spectroscopy for the hydrolytic polycondensation of tetraethyl orthosilicate [45]. Brunet reported the formation of cyclic trimers by the acidic sol-gel reaction of methyltriethoxysilane as observed by DEPT ²⁹Si NMR spectroscopy [46]. Among the detected species in the present study, the structures of T²₂T²₃ and T²₄T²₂ also cannot be drawn without including cyclotrisiloxane rings as exemplified in Figure 1(m) (the structures represent one possible isomer but it is not verified that these exact structures are real). All these observations imply that the condensation favors the formation of such strained rings at this stage of the reaction. For cyclic tetramer, T²₄, all cis isomer shown in Figure 1(k) is reported for phenyl [47] and isopropyl [18] substituents. Kudo and Gordon simulated that the formation of the all cis isomer is most favored due to hydrogen bonding among the silanols in the transition state [43]. They calculated that the next stable isomer is the structure of Figure 1(l).

Figure 8 

Gas chromatograph of PMSQ-QC with the peak assignment by the GC-MS. The molecular compositions are described as uncapped PMSQ.

As the reaction stage in between PMSQ-1 and PMSQ-QC, a PMSQ was recovered immediately after the completion of the addition of methyltrichlorosilane (0.500 mol) to a mixture of water and MIBK without heat aging, PMSQ-NA. Figure 2(c) shows the GPC curve. The low molecular weight end overlaps with the solvent peak and the relative area of the peak around 18 min reached 53%. The 𝑀𝑤 and the 𝑀𝑛 are much lower than those of PMSQ-1. Figure 3(c) represents the ²⁹Si NMR spectrum. As summarized in Table 3, the resonance in the T¹ region (−46 to −49 ppm) was present and the resonances in the T¹ and the T² region were twice of those in the T² region in PMSQ-1. By Me₃Si-capping as shown in Figure 3(d), the resonances in the T² region again remained of which relative area was greater than that for PMSQ-1.

The GC and the GC-MS data for PMSQ-NA by the same method described before are listed in Table 4. The change in PMSQ-QC, PMSQ-NA, and PMSQ-1 can be summarized as follows.(1)Species present in PMSQ-QC but reduced or extinct in PMSQ-NA and extinct in PMSQ-1 are T⁰, T²₃, T¹₂, T²₄ (both 2 isomers), T²₅ (both 2 isomers), T³₂T²₃ (all the 5 isomers), T³₄T², T³₂T²₄ (all the 4 isomers), T³₄T²₃ (1 out of the 6 isomers), and T³₆T²₂ (1 out of the 6 isomers). (2)Species that increase monotonously (including those not present in PMSQ-QC and PMSQ-NA) are T³₈, T³₁₀, T³₆T² (2 out of the 5 isomers), T³₈T² (2 out of the 4 isomers), and T³₆T²₂ (3 out of the 6 isomers). The isolated isomer of T³₆T²₂, 𝑧, is one of these. (3)Species that increase from PMSQ-QC to PMSQ-NA, but then decrease or become extinct in PMSQ-1, are T³₆T² (3 out of the 5 isomers), T³₄T²₂ (both 2 isomers), T³₆T²₂ (2 out of the 6 isomers), T³₈T² (2 out of the 4 isomers), and T³₄T²₃ (5 out of the 6 isomers). Many of the species of which structures cannot be drawn without a cyclotrisiloxane ring are in this category.

It should be noted that the sum of the material detected by the GC-MS analysis increased from 16.4% in PMSQ-QC to 19.6% in PMSQ-NA. This suggests that the buildup of the molecules during the course of the reaction is not simply proceeding by continued condensation.

3.6. Reaction Chemistry for the Formation of the T³₈ Cubic Octamer

In an effort to obtain insights into the chemistry for the structure buildup in silsesquioxanes or silicone resins, formation of T³₈ cubic octamers was pursued. Three separate runs were carried out: after completing the dropwise addition of methyltrichlorosilane to a mixture of MIBK and water, (i) the product was immediately recovered (0 h aging), (ii) aged at 50°C for 3 h, and (iii) aged at 50°C for 17 h. For each run, precipitates were collected both from the MIBK and the water phases, followed by determining the amount of T³₈ by X-ray diffraction crystallinity, which was around 80%. The soluble resins were subjected to Me₃Si-capping by BSTFA, for which the GC quantification was carried out in the same way as mentioned above. The amounts of T³₈ by these methods are listed in Table 8. The amount of T³₈ in the resin at 0 h aging was not able to be determined because the GC peak of T³₈ overlapped with that of T³₄T²₂, preventing from the quantitative analysis. However, it is safe to say that the amount of T³₈ increased with increasing aging time, because the major part of the material is found in the precipitate.

Table 8 

The amount of T³₈ formed during the hydrolytic polycondensation of MeSiCl₃.

Figure 9 illustrates various potential routes for the formation of T³₈. The natural thought would be simple condensation of precursor molecules, represented by routes (i)–(iv). The species T⁰, T¹₂, T²₄, and T²₂T²₄ appearing in routes (i), (ii), and (iv) are present at the very early stage exemplified by PMSQ-QC. However, T⁰, T¹₂, and T²₄ are extinct in PMSQ-NA (0 h aging) and in PMSQ-1 (3 h aging) and T²₂T²₄ decreased in PMSQ-NA and is extinct in PMSQ-1. Considering the fact that the amount of T³₈ continues to increase during the 17-hour aging, there appear to be no clear evidence that routes (i), (ii), and (iv) are the major mechanism of T³₈ formation. Route (iii) does not require such early reaction stage intermediates, but there is no evidence of the presence of this structure among the isomers of T³₆T²₂.

Figure 9 

Potential routes for the formation of the T38 cubic octamer.

To investigate if siloxane bond rearrangement was involved in forming T³₈, aging of the isolated T³₆T²₂, crude 1, was conducted at 50°C for 3 h in MIBK with hydrochloric acid of a similar concentration as the PMSQ synthesis. The solution after the reaction was cloudy, for which centrifugation was carried out after washing the MIBK phase with water. WAXD analysis of the precipitates showed that they were T³₈ with the crystallinity of 82%. The amount of the precipitates was 19 wt% of the starting material; hence the amount of T³₈ was 16 wt%. From the MIBK phase after centrifugation, a resin was obtained by removing the solvent (1-R, 75 wt% of the starting material), which was subjected to GC analysis after Me₃Si-capping (1-R-cap). As summarized in Table 9, the amount of the starting material decreased to 17 wt%, while two other isomers of T³₆T²₂ formed in 5 and 20 wt% yields by the reaction. T³₆T² and T³₄T²₃ were also detected but the amounts of these molecules were much smaller compared with that of the T³₆T²₂ isomers. GPC of 1-R-cap revealed the formation of higher molecular weight species too. These findings prove that siloxane bond rearrangement of low molecular weight molecules is one of the mechanisms for T³₈ formation, route (v) in Figure 9. We can speculate that such reaction mechanism can be applied to molecules other than T³₈. In the hydrolytic polycondensation of phenyltrimethoxysilane in alkaline media, Lee and Kawakami reported that the T³₄T²₃ structure, Figure 1(d), is first formed, which was then consumed to provide the so-called double-decker structure, T³₄T²₄, that need siloxane bond rearrangement [48].

Table 9 

Formation of T³₈ from T³₆T²₂ Cage 1 compound.

4. Concluding Remarks

The present study focused on the characterization of a PMSQ and a methyl-DT silicone resin that were synthesized by hydrolytic polycondensation of chlorosilanes without specific control of the reaction. The key findings can be summarized as follows.

Firstly, as opposed to the proposal by Brown and coworkers that PPSQs have ladder structures [8, 9], the PMSQ was found to contain cage molecules. Presence of intense low molecular weight peak in the GPC curve, several sharp peaks on the broad envelope of resonances in the ²⁹Si NMR spectrum, and low Mark-Houwink exponent value are the indirect evidences. More direct proof was obtained by the GC and GC-MS study which revealed the presence of ~20 low molecular weight species with their sum of ~8 wt%. Isolated T³₆T²₂ molecule, Presence of T³₈ and T³₁₀ cage molecules, and some species of which structures can be drawn only as cages from the chemical formulae determined by the GC-MS clarified the presence of cage structures. Cyclization plays a critical role in the formation of these materials to deviate from Flory’s random branching theory as pointed out by McCormick and coworkers for the sol-gel reaction of tetraethyl orthosilicate [49] and methyltriethoxysilane [50].

Secondly substantial evidences for the presence of strained cyclotrisiloxane rings were found. Among the chemical formulae identified by the GC-MS analysis, the structures of T³₆T² cannot be drawn without including at least one cyclotrisiloxane ring (see Figure 1(c)). The ²⁹Si NMR spectrum of the PMSQ of which silanols were capped by trimethylsilyl group suggested the presence of strained T³ structure by the residual resonances in the classical T² region, −50 to −58 ppm. A cyclic trimer of T² was directly observed at the very early stage of the reaction (PMSQ-QC). In addition, 2 out of the 5 isomers of T³₆T² increased monotonously from PMSQ-QC to the PMSQ, and the other 3 isomers increased from PMSQ-QC to PMSQ-NA, then decreased in the PMSQ. This suggests that the cyclotrisiloxane rings are not always unstable species.

In the study to clarify the formation mechanism of T³₈ cubic octamer, no clear evidence was identified that T³₈ was formed by simple condensation of precursor molecules. In contrast, when the isolated T³₆T²₂ (90% purity) was subjected to the same heat aging condition in acidic media as the synthesis of the PMSQ, the amount of the starting material was reduced to 17%, and T³₈ and two other isomers of T³₆T²₂ were found in 16, 20, and 5% amounts, respectively. Thus, siloxane bond rearrangement is an important mechanism in the formation of cage molecules or low molecular weight species. The amount of the species detected by the GC-MS analysis in PMSQ-QC and PMSQ-NA could be in line with this observation. Some species increased from PMSQ-QC to PMSQ-NA then decreased in the PMSQ, and some molecules monotonously increased from PMSQ-QC to the PMSQ. This could be indicating that the reaction is not proceeding only by simple continuation of condensation.

Finally the methyl-DT resin containing 15 mol% of a D² unit [(CH₃)₂SiO_2/2] showed essentially the same tendency as the PMSQ, showing the presence of cage molecules as analyzed by GPC, ²⁹Si NMR spectroscopy, and GC/GC-MS studies (the reaction chemistry was not pursued). One feature was that many of the T² unit [CH₃Si(OH)O_2/2] in the PMSQ were replaced with D² unit [(CH₃)₂SiO_2/2] in the methyl-DT resin. Thus the form of the molecule as an incompletely condensed cage in the PMSQ, for example, T³₆T²₂, was a completely condensed cage, for example, T³₆D²₂. The presence of cyclotrisiloxane rings was suggested in a similar manner by the residual resonance in the −50 to −58 ppm region in the ²⁹Si NMR spectrum after trimethylsilyl-capping and the presence of species that one cannot draw a structure without including a cyclotrisiloxane ring, for example, T³₆D. This has clarified that a methyl-DT resin, which is a more common industrial silicone resin, consists of similar structures as silsesquioxanes and the generality of the trend found in the PMSQ.

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Acknowledgments

The authors thank Drs. Akihito Saito of Dow Corning (currently Canon Inc.), Ronald Tecklenburg, Elmer Lipp, Larry Wood, Russel King, Katsuya Eguchi, Gregg Zank, and Dimitris Katsoulis of Dow Corning for their great support and helpful discussions.