Article in Press

ARTICLE IN PRESS

Prog. Polym. Sci. xx (2004) xxx–xxx www.elsevier.com/locate/ppolysci

Oligo- and polysilo xanes

Yoshimoto Abe*, Takahiro Gunji

Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

Received 14 January 2003; revised 5 August 2003; accepted 21 August 2003

Abstract The article reviews the work on the synthesis, properties, and structure of curious oligo- and polysiloxanes which have been done mainly by the authors, referring the related papers. So far as oligosiloxanes, the topic is especially focused on sila- functional siloxanes as the building block for the synthesis of ladder or cube oligosiloxanes, while the another is the polysiloxanes derived from silicic acid and trimethoxysilanes RSi(OMe)3 which are able to form fibers and flexible free- standing films. The review also refers to the new routes for the selective synthesis of sila-functional oligosiloxanes in addition to the reaction control based on the relative reactivity of sila-functional groups. Finally, the application of polysiloxanes as the precursors to ceramics, high performance coatings, and interlayer low dielectric materials are described. q 2003 Elsevier Ltd. All rights reserved.

Keywords: Sila-functional oligosiloxanes; New synthetic routes; Siloxanenols; Cube; Ladder; Polysilicic acid esters; Partially silylated silicic acids; Polysilsesquioxanes; Spinnablility; Flexible free-standing ﬁlms; Ceramic precursor; Coatings; Interlayer low dielectrics

Contents 1. Introduction ...... 000 2. Commercially available sila-functional oligosiloxanes ...... 000 3. Formation of siloxanes ...... 000 3.1. Various oligo- and polysiloxanes ...... 000 3.2. Reactivity of sila-functional groups...... 000 3.3. Siloxane bond formation ...... 000 4. Polysiloxanes ...... 000 4.1. Polysilicic acid esters and their properties ...... 000 4.2. Polysiloxanes capable of forming fibers and films ...... 000 4.3. Highly polymerized TEOS stable to self-condensation ...... 000 4.4. Flexible free-standing films from RSi(OMe)3 ...... 000 4.5. Base-catalyzed hydrolytic polycondensation of RSi(OMe)3 ...... 000 5. Linear and cyclic sila-functional oligosiloxanes ...... 000 5.1. Facile synthesis routes ...... 000 5.1.1. Vapor phase hydrolysis...... 000 5.1.2. Oxidative condensation of dimethyldichlorosilane with dimethyl sulfoxide ...... 000

* Corresponding author. Tel.: þ81-4-7124-1501x3608; fax: þ81-4-7123-9890. E-mail address: [email protected] (Y. Abe).

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5.1.3. Linear siloxanes with deﬁnite chain length by ring opening reaction of Dn ...... 000 5.2. IR and NMR spectra ...... 000 5.3. Disiloxanols...... 000 5.4. Cyclotetrasiloxane tetrols ...... 000 6. Ladder oligosilsesquioxane ...... 000 7. Cube siloxanes ...... 000 8. Application of oligo- and polysiloxanes...... 000 8.1. Ceramic precursors ...... 000 8.2. High performance coatings ...... 000 8.3. Interlayer low dielectrics for electronic devices ...... 000 Acknowledgements ...... 000 References ...... 000

1. Introduction versatile as starting materials. The synthesis and uses of sila-functional oligosiloxanes are very limited, Polysiloxanes are versatile materials, many hav- although they can be potential building blocks for ing excellent chemical, physical, and electrical polysiloxanes and polysilsesquioxanes. The key to properties; polydimethylsiloxane is an important develop the methods for the synthesis of such example of this class of polymers. The only precursors should be the controlled reactions of disadvantage, nevertheless it is one of the excellent silanes with sila-functional groups, with special properties, of siloxanes is the fact that they are not attention to the reactivity of sila-functional groups. able to form fibers and films because of their low Subsequently, di- and trifunctional silanes would be interactions between molecules. In order to afford precursors, with hydrolysis, condensation or elimin- new functions, the structure of polysiloxanes has ation is as the preferred reactions to provide been modified by side chain functionalization or oligosiloxanes. change in the main chain. The dimension of the This review article will focus on the syntheses, macromolecular structure is a key factor in the properties, and applications of sila-functional oligo- generation of new properties: ladder and sheet are and polysiloxanes and silsesquioxanes with linear, two-dimensional structures, while cross-linked cage, cyclic, ladder, cage, and cube structures. cube, and spheres are three-dimensional structures. Such structures may find use as materials with characteristic thermal, optical, electrical, and mech- 2. Commercially available sila-functional anical properties. The state of association or oligosiloxanes aggregation of the macromolecules must also be considered. Recent research trends involve the A variety of organosilicon compounds are com- synthesis and properties of sila-functional oligo- mercially available. However, so far as oligosiloxanes and polysiloxanes, silsesquioxanes such as cage, and silsesquioxanes are concerned, very limited cube, and ladder structures, and their applications as products are available as reagents including sila- functional materials. functional oligosiloxanes and oligosilsesquioxanes Although research on siloxanes is attractive like ladder, cage, and cube. The same is also true because of the multitude of potential applications, for sila-functional silanes. Moreover, as illustrated by convenient and selective methods for the synthesis of the examples listed in Tables 1 and 2, based on sila-functional silanes and oligosiloxanes are lacking catalogues issued by several sources [1], the available Even silanes with sila-functional groups such as reagents are often expensive. R42n 2 mSi(OR)nXm or (RO)42nSiXn (m ¼ 1; 2; Sila-functional silanes R42nSiXn (n ¼ 2; 3: R ¼ n ¼ 1 , 3, R ¼ alkyl, alkenyl, aryl; X ¼ halogen, alkyl, alkenyl, aryl; X ¼ H, halogen, OR) are versatile 0 OR , OH, NR2, OCOR, NCO) are not reagents, whereas silanes R42nSiXn (n ¼ 2; 3; ARTICLE IN PRESS

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Nomenclature Mw/Mn polydispersity NMR nuclear magnetic resonance Bp boiling point ORTEP Oak Ridge thermal ellipsoid plot Bu butyl group, CH CH CH CH – 3 2 2 2 Oct octyl, (CH ) (CH ) – But tert-butyl group, (CH ) C– 3 2 2 7 3 3 PC polycarbonate D hexamethylcyclotrisiloxane, (Si(CH ) O) 3 3 2 3 PEOS polyethoxysiloxanes D octamethylcyclotetrasiloxane, 4 PET polyethylene terephthalate (Si(CH ) O) 3 2 4 PMSQ polymethylsilsesquioxane D decamethylcyclopentasiloxane, 5 PP polypropylene (Si(CH ) O) 3 2 3 PSSX partially silylated siloxane DABCO 1,4-diazabicyclo[2.2.2]octane PVSQ polyvinylsilsesquioxane DE degree of esteriﬁcation Ph phenyl, C H – DS degree of silylation 6 5 Pri isopropyl, (CH ) CH– Decomp. decomposition 3 2 Qn Siloxane unit, Si(OSi ) (OR) ðn ¼ Et ethyl, CH CH – 0.5 n 42n 3 2 1–4Þ SEC size exclusion chromatography (or GPC, SOG spin-on-glass gel permeation chromatography) SUS304 stainless steel HDPE high density polyethylene TEOS tetraethoxysilane HPLC high performance liquid chromatography THF tetrahydrofuran IR spectrum infrared absorption spectrum TMOS tetramethoxysilane JIS K5400 Japan Industrial Standard No. K5400 T (5%) temperature at the weight loss of 5% MS mass spectroscopy d Tn siloxane unit, RSi(OSi ) (OR) ðn ¼ Me methyl group, CH – 0.5 n 32n 3 1–3Þ MeOH methanol Vi vinyl, CH yCH– M number average molecular weight 2 n m-CPBA meta-chloroperbenzoic acid Mp melting point d solubility parameter Mw weight average molecular weight

X ¼ OH, OCOCH3,NR2, NCO) and especially Table 1. It may be noted that only siloxanes with i t (RO)42nSiXn (n ¼ 1 , 4; R ¼ Me, Et, Pr ,Bu; n ¼ 1 , 4 are commercially available, as shown at 0 X ¼ halogen, OR , OH, NR2, OCOCH3, NCO) are the bottom of Table 1. Cyclic oligodimethylsiloxanes so limited as to be supplied for commercial uses. D3,5 are the raw materials for the production of Many of them may be purchased as order-made or silicone. Usually, the only variations of the functional obtained as a component product of reaction mixtures group found with sila-functional cyclosiloxanes or a by-product. Silanediols and triols R42nSi(OH)n (SiMeXO)n are hydrogen and alkoxy groups (R ¼ Me, Ph) are often used as a potential precursor (n ¼ 3 , 5; X ¼ H, OR). While an appreciable for the preparation of oligo- and polysiloxanes or number of linear sila-functional oligosiloxanes are silsesquioxanes. Recently, isocyanato(methyl)silanes listed, most are disiloxanes, as shown in Table 1, and Me42nSi(NCO)n ðn ¼ 1 , 4Þ have been used as no homologues more than trimers are listed in the halogen-free coupling reagents, of which reactivity catalogues. is appreciably higher than that of alkoxysilanes, but Cubes can be a potential building block to low compared with chlorosilanes [2]. On the other prepare silicon-based materials, but as shown in hand, it is difﬁcult to synthesize silanes with different Table 2, they are very expensive and limited as functional groups (RO)4-nSiXn.Afewexamples commercial products. Therefore, convenient 0 (X ¼ halogen, OR , OH, NR2) are listed in Table 1. methods to provide them as precursors have to be Linear and cyclic oligosiloxanes are listed in developed. ARTICLE IN PRESS

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Table 1 Table 2 Price list of commercially available oligosiloxanes Price list of commercially available cubes

insoluble products, because base catalysis promotes hydrolysis to provide gels in the form of powders or bulk bodies. On the other hand, controlled acid- catalyzed hydrolysis followed by polycondensation of silanes provides polysiloxanes with various shapes, including cyclic, linear, pseudo-ladder, and even cubic structures, as shown in Schemes 1 and 2. Acid-catalyzed reactions are preferred to prepare linear polymers and branched polysiloxanes, from which silica glass and ﬁbers are prepared via precursor gels. Base catalysis or template tends to afford cube and pseudo-ladder silsesquioxanes.

3. Formation of siloxanes

3.1. Various oligo- and polysiloxanes

Bifunctional silanes are used as starting materials for the preparation of linear and cyclic oligosiloxanes and polysiloxanes. By contrast, trifunctional silanes have not been used as raw materials. Now, they have been recognized as potential materials for the syntheses of interesting oligo- and polysiloxanes and polysilsesquioxanes. Controlled hydrolytic polycondensations with acid or base catalysis provide oligo- and polysilsesquioxanes with various structures which can be the precursors for silicon-based materials. The hydrolysis of tri- and tetrafunctional silanes usually gives Scheme 1. ARTICLE IN PRESS

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Scheme 2. Sol–Gel reactions of sila-functional silanes provide a form siloxane oligomers and polymers, especially with facile process to obtain the materials mentioned chloro and alkoxy groups. This is also the reaction in the above. sol–gel process, which is applicable to metallasiloxane or metalloxane bonds formation using metal-organic 3.2. Reactivity of sila-functional groups compounds. Hetero-functional condensations invol- ving silanol groups (Eq. (3.2)) are a potential route to Sila-functional groups, which are essential to the control reactions and/or construct siloxanes with well- formation of siloxane bonds and/or the synthesis of deﬁned structures, because the cleavage of siloxane siloxane compounds, make it possible to promote bond by acid or base catalyst is minimized. Therefore, various types of siloxane formation reactions. In the reactions shown in Eq. (3.2) provide tailor-made addition, the reactivity plays an important role to siloxanes, such as high molecular weight block, control reactions and subsequently to result in the random, and alternating copolysiloxanes, as well as structure control of siloxane compounds. A number of oligosiloxanes. Another type of hetero-functional sila-functional groups are shown below. Of these, the condensation is illustrated in Eq. (3.3). Similarly, following are particularly versatile: Halogen, NR2, condensation of silanes with alkoxy and chloro groups OCOR, NHCONR2, NCO, OH, OR, H, OCRyCR2, maytakeplace inthepresenceofasuitable catalyst.The ONyCR2, and ONR2. silicon–silicon bond can be oxidized with peroxides, There have been no reported quantitative, or even nitrosoamines, ozone, and hydrogen peroxide to form qualitative estimates of the reactivity of sila-func- siloxanes (Eq. (3.4)). It seems that almost no reports tional groups. Based on the experimental results [3], have been published about the reaction in Eq. (3.5). one would anticipate the following order: H, OR, Condensation followed by elimination takes place in OH , NCO, OCOCH3, ,NHCONR2,NR2, Cl. the presence of acid as a catalyst, if controlled, and would provide polysiloxanes with targeted structures. 3.3. Siloxane bond formation

Siloxane bond formation reactions are represented 4. Polysiloxanes by the Eqs. (3.1)–(3.5) in Scheme 3. Self-condensations followed by hydrolysis of silanes with sila- A wide variety of industrial applications have been functional groups (Eq. (3.1)) are a versatile means to developed for polysiloxanes, with excellent chemical, ARTICLE IN PRESS

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Scheme 3. physical, electrical properties. In recent years, the Silicic acids are potential starting materials, but requirements of advanced technologies have created they do not react with organic compounds in an as need for new high performance polysiloxanes. aqueous solution except by means of silylation by Here, an attention will be focused on polysiloxanes as Lentz [5]. However, alcohols, chlorosilane, acetyl precursors for coatings, binders, additives, and chloride [6] or even metal chloride [7,8] will react ceramics. with silicic acids to give derivatives, which may be extracted by organic solvents. Thus, silicic acid was 4.1. Polysilicic acid esters and their properties extracted with organic solvent from the resulted aqueous solution after neutralization of sodium Polyalkoxysiloxanes have been used as industrial metasilicate with hydrochloric acid followed by raw materials (so called SE 40, 48, and ME 52). salting out. In this procedure, acetone, alcohols such These are the mixture of oligomers prepared by as 1-propanol, 2-methyl-2-propanol, and 2-propanol, hydrolysis of tetraethoxysilane TEOS and tetra- and THF were used as solvent. When THF was used, methoxysilane TMOS. No high molecular weight silicic acid was extracted up to 90% as SiO2 [9].Asa polyalkoxysiloxanes stable to condensation have result, a silicic acid–THF solution of 1.2 mol/l was been synthesized. On the other hand, esterification conveniently prepared. of silicic acid, the reverse reaction of hydrolysis of According to Eq. (4.1), silicic acid is esterified by tetraalkoxysilane, is reported by Iler [4] to provide alcohol under azeotropic distillation to give poly- polysilicic acid esters and/or polyalkoxysiloxanes silicic acid esters over a range of molecular weight unstable to self-condensation, of which silica and degree of esterification (DE: relative degree of content was up to 66%. However, their properties, alkoxy group among the all functional groups). An structure, and esterification conditions were not apparatus for this purpose is shown in Fig. 1, and investigated in detail. example results with 1-butanol are summarized in ARTICLE IN PRESS

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hexane. Especially, gelation of ester was favored when the DE was less than 60%. Samples decompose before melting. Solubility and stability increase as the DE and carbon number of the alkyl group increase. The esters are soluble in organic solvents and stable to self-condensation, as silylates are, even the DE is higher than 60%. Subsequently, the esters of low DE undergo further condensation to give more high molecular weight polymers. Esters prepared by controlled condensation show silica contents up to 70%, and good spinnability, to form precursor fibers, as indicated in Table 4a. The structure was estimated by the spectral and elemental analysis together with the relationship between viscosity and Mn of esters by the Mark– Houwink–Sakurada equation (Eq. (4.3)). The esters are constructed of the structure (Scheme 4). The composition was estimated from the 1H NMR spectra to be ðx þ yÞ : z ¼ 0:7 , 0:9 : 1fortheesters (R ¼ Bu). In addition, the exponential a in the equation is determined to be 1.2. Consequently, the esters should be consisted of ladder-like structure as is supported by the thermal behavior of no melting point Fig. 1. Apparatus for the esterification of silicic acid. Silicic acid– THF solution is put in the flask at left side and alcohol and calcium but decomposition [10–14]. oxide are put in the flask at right side. On heating both flasks individually, the vapor of alcohol is introduced to the flask at left ð4:3Þ side, while the vapor of THF and water is introduced to the flask at the right side. The esterification of silicic acid is promoted by dehydration by calcium oxide. A silicic acid–THF solution is prepared by Table 3. These are characterized by SEC and 1H NMR extraction of an aqueous silicic acid solution with analysis after isolation as silylates, stable to conden- THF. A concentrated silicic acid solution, up to about sation according to Eq. (4.2) [10–14]. 6 mol/l, can be obtained by adding hexane into the silicic acid–THF solution. It was found that an about 6 mol/l silicic acid–THF solution diluted with methanol provides silica gels like silica glasses on ð4:1Þ aging in a sharle wrapped with a polyethylene film with several pin holes to permit slow evaporation of the solvent [15].

ð4:2Þ

4.2. Polysiloxanes capable of forming ﬁbers and ﬁlms The properties of polysilicic acid esters depend greatly on DE and the ester group. The esters gelled One of the outstanding properties of polysiloxanes just after the concentration of the reaction mixture or is the weak interaction between molecules, which isolation as a white powder by precipitation with affords them excellent physical properties, such ARTICLE IN PRESS

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Table 3 Table 4b Preparation of polysilicic acid butyl esters under atmospheric Solubility of various silicic acid esters pressure R DE (%) Spinnable timea (h) Length (cm) Temp (8C) Time (h) Mp (decomp., 8C) DE (%) Mn Et 50 28–32 10–20 135–145 0.5 235 54 13,200 Pri 40 160–168 50–100 2 230 63 10,000 Bu 43 38–42 50–100 12 220 90 11,500 Oct 26 70–90 5–10

140–145 0.5 – 63 22,000 a The time until a spinnability was observed and spinnable time 1 210 73 24,000 intervals on aging a solution of various esters (7.0 mol/l) at 40 8C. 6 215 85 18,000 Eq. (4.4). The silylates are soluble in organic solvents and isolated by precipitation with water/MeOH (5/1, Table 4a Variation of molecular weights of polysilicic acid butyl ester with v/v) followed by drying in vacuo. Despite a low DEs on aging molecular weight and dispersion as indicated in Table 5, concentrated silylate solutions show spinn- = DE (%) Aging time (h) Mn Mn Mw Mn ability that is highly dependent on the solvent such as

43 0 3390 4650 1.37 acetone, dibutyl ether, ethyl acetate, dioxane, and THF 12 4480 10,300 2.30 and also on the sila-functional groups of OH, OCOCH3, 19.5 4570 18,200 3.98 and OBu. Thus, remarkable spinnability is observed for 31.5 4300 29,500 6.86 the silylate with DS 70% when in dioxane solution. 38 4930 33,500 6.80 Further, reactions of silylates with acetyl chloride and 55 0 3850 5420 1.41 1-butanol provide the corresponding derivatives in 131 3300 5100 1.55 which the silanol groups are replaced with acetoxy and 226.5 3300 7360 2.23 butoxy groups. The relative spinnability is measured to 85 0 11,400 20,000 1.75 be the order: Bu , OCOCH3 , OH. No spinnability is 194 11,000 15,600 1.42 observed for the completely silylated polymer. 985 10,400 18,300 1.76 Obviously, spinnability highly depends on DS and solvent and/or the solubility parameter so that spinn- as a low viscosity coefficient and a small contact ability is correlated to the intermolecular interactions angle. However, this can also represent a potential resulted from the structure of polysiloxane. The limitation for their use as coating films or fibers. Such results suggest that structure modification of siloxane limitations could be overcome by modification or improvement of the polysiloxane structure. Table 5 As shown in Tables 4a and b, silicic acid esters Spinnability of partially silylated silicic acids PSSX undergo condensation to form high molecular weight a = c PSSX X Molar ratio Mn Mw Mw Mn n Spinnability polysilicic acid esters. Depending on DE and the ester b TMCS/SiO2 (cm) group, some of these show spinnability, and provide precursor fibers of silica. This may suggest that OH 1 980 1100 1.12 7.3 50 spinnability depends on the DE and/or silanol groups, 2 1150 1370 1.19 6.7 250 3 1270 1500 1.23 6.5 200 in addition to molecular weight. If so, partially silylated 5 1290 1720 1.33 6.5 100 silicic acids would be expected to show spinnability 7 1480 1990 1.34 6.6 80 [16–18]. Therefore, silylates with various degrees of 10 1500 2020 1.35 6.6 50 silylation (DS) were prepared by the reaction of OCOCH3 5 1340 1670 1.25 6.5 80 OBu 7 1330 2300 1.73 6.8 50 chloro(trimethyl)silane with silicic acid, according to a Functional group in PSSX. b Molar ratio of chloro(trimethyl)silane (TMCS) to silicic acid

(SiO2). Scheme 4. c Degree of condensation. ARTICLE IN PRESS

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx 9 backbone with sila-functional group as a pendant is a importance in sol–gel processing. Depending on key process to synthesize polysiloxanes capable of whether acid and base catalysis are utilized, linear or forming not only ﬁbers but also ﬁlms. cross-linkedpolymersolsareformedespeciallyonsol–

ð4:4Þ

Silylation of silicic acid with allyl(chloro)(di- gel process of TEOS. However, almost no work has methyl)silane in the presence of triethylamine gives been done on the isolation and characterization of sols, partially allyl(dimethyl)silylated silicic acids, of as they are unstable to self-condensation Moreover, which high molecular weight polymers (Mn most efforts are devoted to the preparation and 6000 , 34,000) are separated as a benzene-soluble characterization of ceramic materials. Therefore, the component from low molecular weight polymers (Mn structure is often investigated by in situ analysis in the 2400 , 4100). Interestingly, the silylates show not solution [20]. only spinnability, but film formation while no flexible Low molecular weight polymer sols are now films are prepared from partially trimethylsilylated supplied as commercially available products, so silicic acids. This appears to be the first finding of called SE 40, 48, and MS 52, but they are the flexible free-standing films prepared from polysilox- mixture of oligomers with low degrees of polym- anes as a precursor, other than Brown’s phenylsilses- erization. Investigation of the preparation and quioxane [19]. The results mentioned above reveal properties of polyethoxysiloxane sols stable to self- that spinnability and film formation are closely condensation by sol–gel reaction of TEOS is of associated with the structure of polysiloxanes: inter- interest and value to the basic chemistry of polymer molecular interactions resulted from backbone and sols, and their application as industrial raw pendant group are the key factor, and mechanical materials. strength of fibers and films is related to backbone As indicated in Section 4.2, the properties of structure in addition to molecular weight. polysilicic acid esters largely depends on the DE and ester groups. Controlled esterification is essential to 4.3. Highly polymerized TEOS stable to self- the preparation of esters with expected properties. The condensation same is true in the sol–gel process of TEOS, the reverse reaction of the esterification of silicic acid. Hydrolytic polycondensation of metal alkoxides is Thus, the sol–gel reaction, if controlled, would well knownasasol–gelmethod. It isapotentialrouteto provide polyethoxysiloxanes (PEOS) according to prepare oxide materials in a range of forms, including Eq. (4.5). The key factor to obtain polyethoxysilox- bulk bodies, particles, thin films, and fibers. The anes stable to condensation is structure control, based material properties can be strongly influenced by on the relative ratio of siloxane structural units Qn the sol and gel precursors. In this regard, control of the (Q2: x,Q3: y,Q4: z) together with the ratios of silanol hydrolysis and condensation reactions are of central to ethoxy and silanol groups [21].

ð4:5Þ ARTICLE IN PRESS

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Table 6 Preparation of polyethoxysiloxanes

= a No. Molar ratio H2O/TEOS Time (h) Yield (g) Mw Mw Mn Ratio of siloxane unit Spinnability at 25 8C (cm) (%)

Q1 Q2 Q3 Q4

1 1.50 7 17.4 1700 1.3 8 37 45 10 0 2 1.60 7 16.7 2300 1.5 3 24 54 19 0 3 1.70 7 15.9 5400 2.3 2 21 56 21 80 4 1.75 5 15.8 7300 2.9 2 17 58 23 120 5 1.80 2 15.3 11,700 3.8 2 13 62 23 120b 6 1.90 1.5 15.1 Gel – – – – – 0

Scale in operation: TEOS 34.8 g (0.167 mol). Molar ratios: EtOH/TEOS ¼ 2.07, HCI/TEOS ¼ 0.105. Temp.: 80 8C. a Calculated based on the 29Si NMR spectra. b At 80 8C.

In an investigation of these effects, reactions were analysis indicates silica contents up to 70%. This carried out in various molar ratios of H2O, HCl, and should be noted considering the fact that the silica EtOH to TEOS at 70 8C for 3 h with stirring under the contents of industrial ethyl silicates (SE 40, 48) and nitrogen stream, as shown in Table 6 [22]. The methyl silicate (SM 53) are 40, 48, and 53%, preparation process is featured by reaction under a respectively. Polyethoxysiloxanes as well as poly- nitrogen stream (Fig. 2) and evaporation of solvent, silicic acid esters, therefore, can be a potential hydrogen chloride and excess amount of water, to material as coating, additives, and binders. maintain steady progress in the condensation at the late stage, but PEOS with molecular weight up to 11,700. They are identified by 1H, 13C, and 29Si NMR spectra, IR spectra, and SEC analysis. From these results, the products were confirmed to be PEOS with siloxane backbone and side chains of ethoxy and silanol groups where the siloxane unit structures are consisted of Q2,Q3, and Q4 (Fig. 3) [21]. Since they mainly consist of Q3, the structures should not be linear, but branched-ladder. Polyethoxysiloxanes are soluble in organic solvents, except for hexane, and stable to self-condensation although they contain an appreciable amount of silanol groups (the molar ratio OH/Si ¼ 0.4), estimated from elemental analysis. Good spinnability was also observed by drawing a glass rod up from a concentrated viscous solution. They undergo condensation either in solution or neat, as illustrated by the results of an SEC analysis as shown in Fig. 4. Only Fig. 2. Apparatus for the preparation of polyethoxysiloxanes a slight increase in molecular weight is observed for (PEOS) by the hydrolytic polycondensation of tetraethoxysilane the controlled reactions at room temperature and (TEOS) under a nitrogen stream. TEOS, ethanol, and hydrochloric below, except for reactions in a THF solution, acid are placed in a four-necked flask and nitrogen is introduced trough the reaction. PEOS is produced as a highly viscous liquid. In showing unexpectedly stability compared with sols this process, the discharge of hydrochloric acid promotes the generally prepared via a sol–gel process. Elemental condensation reaction. ARTICLE IN PRESS

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4.4. Flexible free-standing ﬁlms from RSi(OMe)3

Much attention has been paid to hydrolytic polycondensation of trifunctional silanes RSiX3 (X ¼ Cl, OR0;R0 ¼ alkyl), as the controlled reaction provides oligo- and polysiloxanes with various structures, as shown in Scheme 2 in Section 3.1. These have potential as precursors to high performance silicon-based materials, as demonstrated by the properties of ladder polyphenylsilsesquioxanes, synthesized by hydrolytic condensation followed by base-catalyzed equilibration reaction of phenyltrichlorosilane or phenyltrimethoxysilane. High molecular weight polymethylsilsesquioxanes (PMSQ) soluble in solvents and capable of forming tough free-standing films had not been synthesized from the trifunctional silanes until 1992 [23] and 1995 [24]; Brown prepared soluble, film forming poly-T-phenylsilsesquioxane in 1960, [19], and allyl(dimethyl)silylated silicic acid was reported to give flexible free-standing films in 1988 [18]. Fig. 3. 29Si NMR spectra of polyethoxysiloxanes (PEOS). The Qn denotes the silicon atom substituted with n siloxy groups and 4 2 n According to Eq. (4.6), controlled acid-cata- alkoxy, aryloxy, or hydroxy groups, which is often described as lyzed hydrolytic polycondensation, characterized

Si(OSi)n(OR)42n (n ¼ 0–4).

Fig. 4. The variation of the molecular weight of polyethoxysiloxanes (PEOS) on aging as neat, THF solution, and EtOH solution at 0 8C (a) and 20 8C (b). ARTICLE IN PRESS

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Fig. 5. Free-standing ﬁlms prepared by casting a 20 wt% acetone– methanol (v/v ¼ 1) solution of polymethylsilsesquioxanes (PMSQ) and heating at 80 8C. Fig. 6. The variation of the tensile strength and elongation of polymethylsilsesquioxanes (PMSQ) free-standing ﬁlms as a func-

tion of molar ratio of H2O/trimethoxy(methyl)silane (MTS) on the by the reaction method described in Section 4.3, hydrolysis of MTS. provides PMSQ and polyvinylsilsesquioxanes (PVSQ), as summarized in Table 7. They are expected to be broken ladder since the backbone condensation of siloxane bond (DC) increases, consists of T2 and T3 structures. They are stable tensile strength increases and elongation decreases. to condensation and have excellent spinnability. At a DC of 70%, films become less flexible, and No gelation is observed, even after several are rather rigid and brittle. This is also indicated months in concentrated solution. Free-standing by the stress–strain curves of films: above DC of films are prepared by casting a 20 wt% acetone- 70%, films are broken at the maximum tensile methanol (v/v ¼ 1) solution and heating at 80 8C strength about 15 MPa. The results clearly reveal for 3 weeks (Fig. 5). Films show high transpar- that the mechanical properties of polysilsesquiox- ency up to 98% (at 500 nm), high thermal anes are closely associated with the structure in stability, with Td (5%) of 400 8C (PMSQ) and terms of DC and/or cross-linkage of siloxane 600 8C (PVSQ), and appreciable flexibility, with bond and molecular weight.

ð4:6Þ

tensile strength up to 27 (PMSQ) and 17 MPa Polysiloxanes such as polysilicic acid esters, (PVSQ) and elongation of 1.2 (PMSQ) and partially silylated silicic acids, and polymethyl(vi- 0.8 mm (PVSQ). The tensile strength and nyl)silsesquioxanes have fascinating properties as elongation of ﬁlms are represented in Fig. 6 advanced silicon-based materials, suggesting the as a function of the molar ratio of H2O/MTS on possibility of a wider range of applications than for the hydrolysis of MTS. As the degree of found with traditional organopolysiloxanes. ARTICLE IN PRESS

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx 13

4.5. Base-catalyzed hydrolytic polycondensation of substrate and the reaction temperature were increased. RSi(OMe)3 Volatile amines provide low molecular weight polysiloxanes, but non-volatile amine DABCO yields a Acid-catalyzed hydrolytic polycondensation of polysiloxane with fairly high molecular weight of 0 R42nSi(OR )n (n ¼ 3; 4) is a desirable way to obtain 6800. On the other hand, sodium hydroxide catalyzes polysiloxanes as polymer sols, because linear poly- hydrolytic polycondensation effectively, depending on siloxanes are formed according to the reaction the molar ratio of the base and H2O, to yield mechanism discussed in the previous sections. polysiloxanes with high molecular weight above Generally, base-catalyzed condensations provide 35,000. In the higher molar ratio of base, methacryloxy insoluble gels. Almost no reports have been published groups are hydrolyzed, while molecular weights on the preparation of soluble polysiloxanes, except by decrease as a white powder is formed. the hydrolysis of PhSiX3 (X ¼ Cl, OMe) followed by Thus, the reaction conditions investigated in detail equilibration with potassium hydroxide to form reveal that high molecular weight polysiloxanes are polysiloxanes with molecular weights around a attainable and, interestingly, consist of almost all T3 thousand [25]. structures, as shown in Table 8. The 29Si NMR spectra are noted, for the signal due to T3 has two shoulders at the ﬁelds below 266.5 ppm (264.0 and 265.5 ppm), as shown in Fig. 7. These signals may be ascribed to ð4:7Þ an irregular ladder structure due to three-, four-, and bridged four-membered rings. Polysiloxanes are soluble in organic solvents, In order to synthesize high molecular weight except for hexane and methanol Hence, thin ﬁlms polysiloxanes, 3-methacryloxypropyl(trimethoxy)si- are formed on various organic and inorganic sub- lane is hydrolyzed (Eq. (4.7)) with various base strates by dip coating with a 20 wt% acetone- catalysts such as ammonia, triethylamine, diazabicy- methanol (v/v ¼ 1) solution. clooctane (DABCO), and sodium hydroxide, using the Recently, tetraalkylammonium hydroxides have same apparatus as that for acid-catalyzed conden- found use in base catalysis of hydrolytic polyconden- 0 sations [26]. Table 8 shows the results of reactions with sation of TEOS and RSi(OR )3. It is well known that the bases, as well as acid-catalyzed reactions. For tetraalkylammonium hydroxides serve as a template reactions in the presence of HCl, soluble polysiloxanes to synthesize cube siloxane Q8(ONR4)8, but no work with molecular weighs higher than 2300 were not have been reported on their role as a base catalysis on prepared, even if the molar ratio of HCl and water to the hydrolytic polycondensation. Therefore, it is of

Table 7 Preparation of polymethylsilsesquioxanes and polyvinylsilsesquioxanes

3 = Molar ratio H2O/TEOS Spinnability (cm) Mw £ 10 Mw Mn State

As prepared After 30 min

PMSQ 1.28 2 40 31.0 9.2 Viscous liquid 1.30 200 300 42.0 11.6 Viscous liquid 1.32 300 0 – – White resin 1.64 – – – – White powder PVSQ 1.30 0 1 2.4 1.8 Transparent liquid 1.44 20 40 3.8 2.3 Viscous liquid 1.60 200 300 19.0 5.0 Viscous liquid 1.64 300 0 – – White resin

Scale in operation: MTS and VTS 0.167 mol. MeOH: 14 ml. Molar ratio: HCI/TEOS ¼ 0.105. Temp.: 70 8C. Stirring: 150 rpm. N2 ﬂow rate: 360 ml/min. ARTICLE IN PRESS

14 Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx

convenient and selective synthesis routes of sila- functional oligosiloxanes.

5.1. Facile synthesis routes

5.1.1. Vapor phase hydrolysis The vapor phase hydrolysis reported by Andrianov is expected to form disiloxanes [27], but the reaction conditions, procedure, and apparatus were not

Fig. 7. 29Si NMR spectra of poly(3-methacryloxypropyl)siloxanes prepared by the hydrolysis 3-methacryloxypropyl(trimethoxy)silane using base catalyst. The Tn denotes the silicon atom substituted with n siloxy groups and 3 2 n alkoxy, aryloxy, or hydroxy groups, 0 which is often described as RSi(OSi)n(OR )32n (n ¼ 0–3). importance to investigate the reaction mechanisms as a template and also base catalysis.

5. Linear and cyclic sila-functional oligosiloxanes

Sila-functional oligosiloxanes, as well as silanes, can be potential building blocks for synthesis of linear, cyclic, ladder, and cubic siloxanes, although oligomers without sila-functional groups are less useful. As indicated in Table 1 in Section 2, commercially available sila-functional oligosiloxanes are expensive and limited. In addition, synthetic routes for these are also limited. Certainly, Fig. 8. Apparatus for the vapor-phase hydrolysis. Four-necked flask hydrolytic condensation of sila-functional silanes is attached with reflux condenser and flowmeter, that connects to a R42nSiXn ðn ¼ 3; 4Þ is versatile, but neither attrac- flask. Trifunctional silane, RSiX3 (R ¼ Me, vinyl, Ph; X ¼ Cl, tive nor preferable for selective synthesis in NCO), is placed in the four-necked flask and water and 1,4-dioxane are placed in the other flask. On refluxing trifunctional silane under appreciable yields. Tedious and complicated separ- reduced pressure, a mixed vapor of 1,4-dioxane and steam is ation processes, resulting in low yields, often follow introduced into the four-necked flask to hydrolyze trifunctional it. Therefore, it would be desirable to develop silane. ARTICLE IN PRESS

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Scheme 5. described in detail. The method would provide a the hydrolysis of 50 according to Scheme 7. convenient route to obtain sila-functional oligosiloxanes, if the reaction can be carried out under controlled conditions. ð5:1Þ Thus, the hydrolysis of isocyanatosilanes R42n Si(NCO)n (R ¼ Me, vinyl, Ph; n ¼ 3; 4) was investigated using a simple apparatus [28]. Fig. 8 shows the vapor phase hydrolysis apparatus, equipped with a reflux condenser, a thermometer, a flow meter, and a flask containing water and dioxane. In a four- 5.1.2. Oxidative condensation of necked flask, the silane, vaporized on heating under dimethyldichlorosilane with dimethyl sulfoxide reduced pressure, undergoes hydrolysis/condensation Recently, a new method for the preparation of with a water–dioxane vapor introduced into a flask cyclosiloxanes D3 and D4 by ‘anhydrous hydrolysis’ through a flow meter, to form the oligosiloxane. was reported [30]. It is the reaction of dichloro(di- Since the oligosiloxane is not vaporized at the methyl)silane with dimethylsulfoxide, which acts as temperature and pressure, only the starting silane is oxygen donor or oxidizing agent for dichloro(di- vaporized and selectively hydrolyzed to give methyl)silane according to Scheme 8. This may be oligosiloxanes. The reaction in the vapor phase preferable to provide D . proceeds according to Eq. (5.1). 3 The method is applied to the synthesis of linear 5.1.3. Linear siloxanes with definite chain length by oligosiloxanes 2 , 6 and 8 with an even number of ring opening reaction of Dn silicon atom, but not those with odd number of silicon Hydrolysis of difunctional silanes with chloro and 0 , 0 atoms, nor cyclic oligosiloxanes 3 6 . Therefore, alkoxy group is not useful to prepare high molecular they are synthesized by hydrolysis of linear oligosi- weight polysiloxanes. Thus, acid or base catalyzed loxanes in the liquid phase according to Schemes 5 ring-opening polymerization of cyclic dimethylsilox- and 6 [29]. Tables 9 and 10 summarize the results of anes D3,D4, and D5 are the practical and industrial the synthesis of linear and cyclic oligosiloxanes. methods to produce polydimethylsiloxanes. Except for cyclotetrasiloxane 40, which solidifies after distillation, they are isolated as liquids by distillation under reduced pressure, and identified by NMR, IR, and MS spectral analysis. During distillation of 50 from a reaction mixture, the distillate with low boiling point solidifies. The structure was determined to be a propellane 500 by X-ray analysis, with the results shownbytheORTEPdrawinginFig. 9.The propellane is assumed form via an intermediate on Scheme 6. ARTICLE IN PRESS

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Table 8 Hydrolytic polycondensation of 3-methacryloxypropyl(trimethoxy)silane

Run Catalyst Molar ratios Temp. (8C) Molecular weight Crude yields (g)

= Cat./MAS H2O/MAS Mn Mw Mn Polysiloxane Powder

1 HCl 1.05 £ 1021 1.5 70 2200 1.6 2 100 2200 1.6 3 150 2300 1.7 4 200 Gel Gel

21 5NH3 1.0 £ 10 3.0 70 620 1.9 9.39 – 6 3.2 £ 1021 780 2.1 9.19 –

21 7 NEt3 1.0 £ 10 3.0 70 2100 2.0 7.89 – 8 2.7 £ 1021 2400 2.0 7.79 – 9 DABCO 2.7 £ 1021 3.0 70 6800 6.1 7.43 –

10 NaOH 5.4 £ 1025 3.0 70 2700 1.6 7.76 – 11 2.7 £ 1023 .35,000a – a 7.27 – 12b 2.7 £ 1022 9000 5.9 6.40 0.28 13b 5.7 £ 1022 2900 2.1 6.10 0.27 14b 2.7 £ 1021 390 1.0 0.18 4.97

22 Scale of operation: 3-methacryloxypropyl(trimethoxy)silane, 10.4 g (4.2 £ 10 mol); MeOH, 14 ml. Temp.: 70 8C. Time: 3 h. N2 ﬂow rate: 360 ml/min. a Over the exclusion limit of the column. b Methacryloxy group was hydrolyzed.

If a ring opening reaction of Dn without equili- trends to behavior for polydimethylsiloxane are bration is conducted, linear oligosiloxanes with a observed, as shown in Figs. 10 and 11, but the limited chain length may be derived. One-way to following aspects are revealed: in Fig. 10, a sharp obtain linear oligo- and polysiloxanes with definite peak at 1041 cm21 for 30 shifts to 1100 cm21 for and various chain lengths is partial hydrolysis to form 40 , 60. No further shifts are observed while the silanol, followed by condensation with a suitable shoulder appears at 1040 cm21 in addition to the peak chlorosilane. This is also used as a method to prepare broadening. In Fig. 11, on the other hand, the sharp dendrimers [31,32]. peak at 1099 cm21 for 2 is broaden for 3 , 6 and 8 and splits into two at the peak top. The band at 5.2. IR and NMR spectra 1099 cm21 shows no shift, but the others a low wavenumber shift from 1079 cm21 (trisiloxane) to It seems that almost no investigations have 1053 cm21 (octasiloxane). addressed the IR or 29Si NMR spectra of oligosilox- The pattern of NMR signals for cyclic oligosiloxanes. The asymmetric stretching vibration nSi–O–Si of anes depends on the structure and/or stereochemical linear and cyclic dimethylsiloxanes is reported: the configuration, and often shows complicated splittings, signals appear at 1018 ðn ¼ 3Þ; 1076 ðn ¼ 4Þ; 1081 as expected from the molecular structures. On the ðn ¼ 5Þ; 1068 ðn ¼ 6Þ; 1060 ðn ¼ 7Þ; and 1056 cm21 other hand, a similar pattern of signal splittings is 1 13 29 ðn ¼ 8Þ for cyclic dimethylsiloxanes (Me2SiO)n, and observed for the H, C, and Si NMR spectra of also at 1000 , 1100 cm21 for linear dimethylsilox- linear oligosiloxanes 2 , 5: the signals due to the anes (Me2SiO)n The absorption band is increased, proton, carbon, and silicon attached to the methylsilyl overlapped, and broaden as the increase of n more groups are shifted to low and high field, respectively. than three. On the other hand, no work has been The signals at low field due to the terminal groups do reported on the IR spectra of sila-functional oligosi- not shift regardless of length of siloxane bond, while loxanes such as those in Tables 9 and 10. Similar those at high field are split into several peaks, and ARTICLE IN PRESS

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Table 9 Spectral and analytical data of cyclic oligosiloxanes 30 –60 and 500

Compound Bp (8C/mm Hg) NMRa (ppm) IRb (cm1) MS/m=z

1H 13C 29Si

0 þ 3 79.4–76.5/0.80 0.371 (9H, s, CH3 –Si):cis 22.07 (CH3–Si): trans 251.2: cis 1041 ðnSiOSiÞ 287 (M -15) ð Þ 0.398 (6H, s, CH3 –Si): 22.19 (CH3–Si): trans 251.1: trans 1274 dSiCH3 trans ð Þ 0.425 (3H, s, CH3 –Si): 22.32 (CH3–Si): cis 250.8: trans 1457 dCH3 trans 123 (Si–NCO) 2285 ðnNCOÞ 0 þ 4 133.0–134.2/3.0 0.36 (12H, m, CH3 –Si) 22.20 (CH3–Si) 260.4 to 261.1 1030–1107 ðnSiOSiÞ 404 (M -15) ð Þ 123.0 (Si–NCO) 1270 dSiCH3 ð Þ 1457 dCH3 2290 ðnNCOÞ 0 þ 5 134.7–136.8/0.45 0.233–0.501 (15H, 21.74 (CH3–Si) 262.4 to 262.0 1031–1106 ðnSiOSiÞ 490 (M -15) br, CH3 –Si) ð Þ 123 (Si–NCO) 261.1 to 261.0 1274 dSiCH3 ð Þ 260.8 to 260.5 1457 dCH3 2288 ðnNCOÞ 0 þ 6 136.3–140.2/0.34 0.234–0.471 (18H, br, 22.04 (CH3–Si) 262.4 to 262.2 1040–1103 ðnSiOSiÞ 591 (M -15) CH3 –Si) ð Þ 123 (Si–NCO) 260.4 1275 dSiCH ð 3Þ 260.1 1456 dCH3 2290 ðnNCOÞ 00c þ 5 – 0.281 (6H, s, CH3 –Si) 21.73 (CH3–Si) 258.0 to 258.5 1033–1114 ðnSiOSiÞ 491 (M -15) ð Þ 0.349 (3H, s, CH3 –Si) 21.95 (CH3–Si) 1273 dSiCH3 ð Þ 0.398 (3H, s, CH3 –Si) 22.17 (CH3–Si) 1454 dCH3 0.404 (3H, s, CH3 –Si) 25.06 (CH3–Si) 2288 ðnNCOÞ 123 (Si–NCO)

a Solv. CDCl3. Ref.: TMS. b CCl4 soln. method. c Mp: 73.6–75.5 8C. coalesce in a narrow region of several ppms which the condensation is prevented from forming 261.3 , 2 62.2 for oligosiloxanes 2 , 6 and 8. silanols with an appreciable stability to self-conden- From the results, it is expected that no further shifts sation. The synthesis and properties of various are appeared for higher homologues. A similar trend silanols are reviewed by Lickiss [33]. is observed for the 1H and 13C NMR spectra. Schemes 9 and 10 show 1,3-disiloxanediols (a) , (c) [33] and (d) [34], together with 1,5- 5.3. Disiloxanols trisiloxanediols ((a), n ¼ 2) and also disiloxanetetrols (e) and (f). Some of the diols (a) in Scheme 9 are Disiloxanols (disiloxane polyols) such as disilox- commercially available. Brown reported 1,1,3,3- anediols and even tetrols are essentially synthesized diphenyldisiloxanetetrol (e) in Scheme 10 by hydroly- by hydrolysis of the corresponding chlorosilanes. The sis of phenyltriacetoxysilane [35]. Bulky organic reaction has to be carried out under mild and carefully groups: such as phenyl, t-butyl, hexyl, octyl, decyl controlled conditions, because the products easily [36], and 1,1,2-trimethylpropyl, are essential to isolate undergo condensation, especially in the presence of a the tetrols [37]. The disiloxanetetrol with an aryl trace amount of acid and base, or on heating. A key (trimethylsilyl)amino group (f) by hydrolysis of factor to isolate silanediol, and especially disiloxane arylamino(trichloro)silane is noted [38], because in polyols, is the steric hindrance of substituents and general silylamino groups are easily substituted with a additional intramolecular hydrogen bonding, by hydroxyl group, but this is not true in this case, ARTICLE IN PRESS

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in good yields, for they easily undergo condensation to form polymerized products, making it difficult to isolate tetrols. Therefore, few reports have been published on their preparation and crystal structure. The method to synthesize tetrols is the hydrolysis of trichlorosilanes with organic groups such as phenyl [39], isopropyl and cyclopentyl [40]. The reaction proceeds via silane triols, where the steric hindrance and inductive effects of organic groups should play an important role directing the condensation to form cyclic tetrols. Hydrolysis is preferably carried out in water–acetone or methyl ethyl ketone, as is shown in Eq. (5.2). This is a simple and convenient way, although the yields are low, around 40% [39]. Another route is the stepwise synthesis shown in Eq. (5.3). This is a fine way, but the three-step reaction results in a total yield of 25% [40]. Sila-functional cyclic tetrasiloxanes can be a starting material if conveniently synthesized by the reaction of sila-functional disiloxanes or sila-functional linear 00 Fig. 9. ORTEP drawing of propellane 5 . tetrasiloxanes. probably due to steric hindrance as well as the low Hydrolysis of 1,3-diisopropyl-1,1,3,3-tetrachloro- basicity of the amino group. disiloxane in acetone as a solvent [40] yields cyclic tetrasiloxane tetrol, where all silanol groups arrange in a cis configuration by hydrogen bonding between the 5.4. Cyclotetrasiloxane tetrols tetrol and water molecules to form a silanol cluster. The structure is identified by X-ray crystallographic Cyclotetrasiloxanetetrols are potential precursors analysis. It can be a starting material for for the preparation of bead, cage, and cube siloxanes. the preparation of cage polysiloxanes by a conden- However, it is not easy to synthesize them selectively sation reaction, with cyclohexylcarbodiimide

Scheme 7. ARTICLE IN PRESS

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6. Ladder oligosilsesquioxane

Ladder polyphenylsilsesquioxane has excellent properties, such as enhanced thermal stability, ﬁlm formation, and mechanical properties, in addition to Scheme 8. those of usual polysiloxanes of chemical, physical, and electrical properties. If the structure is completely as a dehydrating agent. ladder, it consist of the T3 structural unit. However, so called ladder-T-phenylsilsesquioxane virtually con- 2 ð5:2Þ tains the T unit, which means that the structure has broken ladder defects. Another problem is the synthesis: at ﬁrst, oligosiloxanes are prepared by the hydrolysis of phenyltrichloro- or trialkoxysilanes and

ð5:3Þ

then subjected to base-catalyzed equilibration reaction to provide polyphenylsilsesquioxanes. Therefore, the reactions may not be controlled, and may not provide products with definite physical properties. If the reaction is controlled, high performance ladder polysilsesquioxanes would be obtained, and it would be of great interest to investigate their properties. Since it is difficult to obtain a perfect ladder product, the present research target is the synthesis of ladder oligosilsesquioxanes as a model compound. Brown reported the first synthesis of ladder oligosilsesquioxane of cyclotetrasiloxane unit-condensed three ring system in 1965, by the hetero- functional condensation of cyclotetrasiloxane tetrol with dichlorodisiloxane [41], as shown by Eq. (6.1) in Scheme 11. The results on the measurement of IR and 1H NMR spectra and elemental analysis in addition to Mn (815), mp (124 8C), and yield (24%) are cited, but the structures in solution and crystal were not described. Later, the two ring systems are also synthesized by the reaction of tetrachlorodisiloxane with disiloxane diols in 1973 according to Eq. (6.2) [42]. No spectral and analytical data were given, except for IR spectral data and melting point. Recently, ladder oligosilsesquioxanes with two, Fig. 10. IR spectra of cyclic oligosiloxanes 30 , 60. three, and five-ring system [43,44] have been ARTICLE IN PRESS

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synthesized and isolated as one component of several stereoisomers [44], by means of HPLC through effective, but tedious, stepwise reaction processes (Eq. (6.3)). The X-ray crystal structure analysis reveals the ring system is not linear, but bent to be face to face against the both terminal sites, which may result from the reaction with cyclotetrasiloxane tetrol, where all silanol groups are in the cis conﬁguration. Interestingly, oxidation of ladder oligosilanes was reported to give bi- and tricyclic siloxanes (Eq. (6.4)). This is a new route, which uses not oligosiloxanes, but ladder oligosilanes as a starting material [45]. The cyclic siloxanes mentioned above bear bulky organic groups, which means that the starting materials are not versatile and have to be prepared via several steps. Vapor phase hydrolysis of commercially available chloro- and isocyanatosilanes conveniently provides the di- and tetrasiloxanes. If they can be used as a precursor, cyclic siloxanes with usual organic groups of methyl, vinyl, phenyl as a pendant would be obtained. A convenient potential route for the synthesis of two and three ring systems with all methyl or vinyl group as a pendant has been realized [46]: the hetero-functional condensation of disiloxane diols with tetraisocyanato- Fig. 11. IR spectra of linear oligosiloxanes 2 , 6 and 8. disiloxanes or tetraisocyanatocyclotetrasiloxane with

Scheme 9.

Scheme 10. ARTICLE IN PRESS

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx 21 disiloxanediols, as is shown in Eqs. (6.5) and (6.6). represented in Eq. (7.1). On the other hand, hydrolysis This method should be versatile, for the starting of TEOS to give water-soluble silicate ions, followed materials are simply and selectively synthesized by by condensation, yields complicated condensed vapor phase hydrolysis, as it is described in Section 5, species. Hoebbel [51] found another cube in 1971: and a number of ring systems with various pendant in the presence of quaternary ammonium ions, the 82 groups would be obtained in addition to the preparation silicate ion Si8O20O is templated to give an of more larger ring systems. The products are obtained ammonium salt Q8(NR4)8 in almost quantitative by distillation in vacuo and the results of spectral yields, according to Eq. (7.2). analysis indicate ladder oligosilsesquioxanes. To date, many cubes have been synthesized, as listed in Table 11 [52]. These are classiﬁed into three

ð6:5Þ

ð6:6Þ

7. Cube siloxanes groups, (a) alkyl and aryl substituted cubes, (b) carbo- functional cubes, (c) sila-functional cubes. A large

Oligosilsesquioxanes consist of the (RSiO3/2)n numbers of cubes belong to the groups (a) and (b) structural unit; the homologues with n ¼ 6; 8, 10, while the residues are the group (c). In the group (a), X and 12 have been isolated [47].Ofthese,Q8 the cubes are prepared by the hydrolysis of trichloro- ( ¼ Q8X8, X denotes substituted group to Q unit) or trialkoxysilanes (Eq. (7.3)). The cubes in the group R and T8 ( ¼ T8R8, R denotes substituted group to T (b) are synthesized by the hydrosilylation of the H unit), abbreviated ‘cubes’ hereafter, have attracted corresponding carbo-functional olefines with T8 (Eq. considerable attentions from the stand point of (7.4)) prepared by the hydrolysis of trichlorosilane synthesis and applications: they have a nano-sized HSiCl3 (Eq. (7.1)). The group (c) is the cubes with the three-dimensional structure consisted of almost inor- sila-functional groups of Cl and OMe together with H (OMe) ganic silica backbone with an angstrom level cavity, and ONR4. The cube T8 is synthesized by the Cl high thermal stability, and reactive functional groups. reaction of methyl formate with T8 which is derived H H The first cube T8 was obtained in 1959 by Muller by chlorination of T8 (Eq. (7.5)). et al. as a intermediate, in yield less than 1%, on the Thus, the synthetic route and the starting materials synthesis of poly(hydridosilsesquioxane) [48]. Frye are limited. Moreover, it is a serious problem that et al. [49] modified the hydrolysis of HSiCl3 to regardless of the groups, cubes are obtained in low H prepare T8 in 13% yield. Later, the cube was obtained yields after long time reactions. In particular, only a in 27% yield in 1991 by Agaskar [50]. The reaction is few cubes with sila-functional groups are prepared, ARTICLE IN PRESS

22 Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx

Scheme 11. as mentioned above and also shown in Table 11 (see Recently, new synthetic routes to cages and poly- also Table 2 in Section 2). Most of them are poorly silsesquioxanes have been investigated using tetra- soluble in organic solvents and have no melting point, alkylammonium salt as a catalyst: hydrolytic but sublime or decompose at elevated temperature. As condensation of alkyl(triethoxy)silane in the presence shown in Table 12, there are few reports on the of tetrabutylammonium ﬂuoride provides octasilses- physical properties of cubes, and the reported data are quioxane cage and higher homologues with short sometimes inconsistent among different authors. reaction times and in very high yield (90%) [53]. The Cubes of tetraammonium salt are prepared as hydrates redistribution reaction of oligohydridosilsesquioxane with tens of molecules of water, and are soluble in using tetraalkylammonium hydroxide yields polyhy- water, methanol, and ethanol. The synthesis must be dridosilsesquioxanes. Hydrolytic condensation of improved in order to apply the cubes as a precursor for TEOS using a stoichiometric amount of water to the preparation of silicon-based new materials. TEOS using tetraalkylammonium hydroxide also ARTICLE IN PRESS

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Table 10 Spectral and analytical data of linear oligosiloxanes 2–6 and 8

Compd Bp (8C/mm Hg) NMRa (ppm) IRb (cm1) MS/m=z

1H 13C 29Si

þ 2 128.1–129.9/13 0.50 (6H, s, CH3 –Si) 20.50 (C H3–Si) 259.4 1099 ðnSiOSiÞ 255 (M -15) ð Þ 123 (NCO) 1280 dSiCH3 ð Þ 1457 dCH3 2310 ðnNCOÞ þ 3 111.3–112.1/1.0 0.389 (6H, s, CH3 –Si) 21.90 (CH3 –Si) 261.3 1079–1115 ðnSiOSiÞ 457 (M -15) ð Þ 0.474 (6H, s, CH3 –Si) 20.60 (CH3 –Si) 259.8 1274 dSiCH3 ð Þ 123 (Si–NCO) 1457 dCH3 2275 ðnNCOÞ þ 4 141.0–143.1/2.0 0.389 (9H, s, CH3 –Si) 21.97 (CH3 –Si) 261.8 1063–1109 ðnSiOSiÞ 559 (M -15) ð Þ 0.475 (6H, s, CH3 –Si) 21.92 (CH3 –Si) 260.1 1275 dSiCH3 ð Þ 20.52 (CH3 –Si) 1455 dCH3 123 (Si–NCO) 2275 ðnNCOÞ þ 5 68.1–71.6/0.45–0.50 0.389 (9H, s, CH3 –Si) 21.97 (CH3 –Si) 261.8 1063–1109 ðnSiOSiÞ 559 (M -15) ð Þ 0.477 (6H, s, CH3 –Si) 21.92 (CH3 –Si) 260.1 1275 dSiCH3 ð Þ 20.52 (CH3 –Si) 1455 dCH3 123 (Si–NCO) 2275 ðnNCOÞ þ 6 184.5–186.9/0.56 0.393 (6H, s, CH3 –Si) 21.93 (CH3 –Si) 261.7 1078–1113 ðnSiOSiÞ 660 (M -15) ð Þ 0.397 (6H, s, CH3 –Si) 21.90 (CH3 –Si) 260.0 1275 dSiCH3 ð Þ 0.484 (6H, s, CH3 –Si) 20.49 (CH3 –Si) 1454 dCH3 123 (Si–NCO) 2277 ðnNCOÞ þ 8 203.5–210.0/0.10 0.393 (24H, s, CH3 –Si) 21.90 (CH3 –Si) 262.2 1032–1113 ðnSiOSiÞ 793 (M -15) ð Þ 0.483 (6H, s, CH3 –Si) 20.45 (CH3 –Si) 260.1 1275 dSiCH3 ð Þ 123 (Si–NCO) 1455 dCH3 2285 ðnNCOÞ Mp: 73.6–75.5 8C. a Solv. CDCl3. Ref.: TMS. b CCl4 soln. method. provides polyethoxysiloxanes [54].

ð7:1Þ

ð7:3Þ

ð7:2Þ ð7:4Þ ARTICLE IN PRESS

24 Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx

Table 11 Syntheses of cube siloxanes

No. Reagent Substituent (R) Yield (%) References

1 HSiCl3, hexane, toulene, MeOH, FeCl3, HCl –H 23 [48–50,58–61] 2 No. 1, Cl2, CCl4, hn –Cl .95 [62–67] 3 No. 2, HC(OMe)3, heptane –OCH3 68 [62,63,68,69] 4 MeSiCl3 –CH3 37 [70–72] 5 TEOS, Me4NOH 5H20 –ONME4 76 [61,73–77] 6 No. 5, Me3SiCl –OSiMe3 72 [51,73,78] 7 No. 5, HMe2SiCl –OSiMe2H83[61,78,79,80] 8 No. 5, CH2yCHMe2SiCl –OSiMe2CHyCH2 61 [78,81–84] 9 No. 5, (Allyl)Me2SiCl –OSiMe2CH2 –CHyCH2 [85] 10 No. 5, C6H5Me2SiCl –OSiMe2C6H5 54 [83] 11 No. 5, ClCH2Me2SiCl –OSiMe2CH2Cl 53 [83,86] 12 EtSiCl3 –Et 37 [72] 13 PrSiCl3 –Pr 44 [72] 14 BuSiCl3 –Bu 38 [72] 15 PhSiCl3,C6H5NMe3OH –C6H5 [72,87,88] 16 p-MeC6H4SiCl3 –C6H4Me 19 [88] 17 (1-naphthyl)Si(OMe)3 –C10H7 60 [88] 18 ViSiCl3 –CHyCH2 21 [61,81,89–91] 19 (Allyl)SiCl3 –CH2CHyCH2 13 [92] 20 HS(CH2)3Si(OMe) –(CH2)3SH [93] 21 No. 1, 1-hexane –C6H13 90 [59] 22 No. 1, 1-octane –C8H17 [60] 23 No. 1, dec-1-ene –C10H21 [59] 24 No. 1, tetradec-1-ene –C14H29 [59] 25 No. 1, octadec-1-ene –C18H37 [59] 26 No. 1, CH2yCH6H9 –(CH2)2C6H9 [60] 27 No. 1, hydrosilylation –(CH2)3C6H4OCH3 [93] 28 No. 1, hydrosilylation –(CH2)3C6H5 [93] 29 No. 1, hydrosilylation –(CH2)3CN [93] 30 No. 1, hydrosilylation –(CH2)3 –O–CH2CH(O)CH2 [93] 31 No. 1, hydrosilylation –(CH2)3C6F5 [93] 32 No. 1, hydrosilylation –(CH2)3OC6H5 [93] 33 No. 1, hydrosilylation –(CH2)3Si(CH3)3 [93] 34 CH2yCHMe2CH(O)CH2 –(CH2)3CH(O)CH2 [60] 35 CH2yCHOMe2CHOCH2C6H9 –(CH2)3 –O–(CH2)2 –O–CH2C6H9 [60] 36 No. 48, (vinyl)MgCl –(CH2)3Si(CHyCH2)3 [91] 37 No. 7, CH2yCHCH2OH –OSiMe2(CH2)3OH 86 [80] 38 No. 7, 2-allyloxyethanol –OSiMe2(CH2)3OCH2CH2OH 87 [80] 39 No. 15, HNO3 –C6H4NO2 [88] 40 (3-ClC3H6)8Si8O12, NaI –(CH2)3I [93] 41 (3-ClC3H6)8Si8O12, NaSCN –(CH2)3SCN [93] 42 (3-ClC3H6)8Si8O12, KP(C6H5)2 –(CH2)3P(C6H5)2 [93] 43 (3-ClC3H6)8Si8O12,CH3SNa –(CH2)3SCH3 [93] 44 No. 35, m-CPBA –(CH2)–O–(CH3)2 –O–CH2C6H8(O) [60] 45 No. 18, m-CPBA –CH(O)CH2 50 [81] 46 No. 8, m-CPBA –OSi(CH3)2CH(O)CH2 80 [81] 47 No. 26, oxone, acetone, CH2Cl2 –(CH2)C6H8(O) [60] 48 No. 18, HSiCl3 –(CH2)2SiCl3 [91] 49 No. 8, HSiCl3 –OSi(CH3)2CH2CH2SiCl3 95 [84] 50 No. 7, Rh(acac)3 –OSi(CH3)2Br 99 [61] 51 No. 7, Co2(CO)8 –OSi(CH3)2Co(CO)4 99 [61] Continued on next page ARTICLE IN PRESS

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx 25

Table 11 (continued) No. Reagent Substituent (R) Yield (%) References

52 No. 7, CH2yCH(CH2)4O(C6H4)2CN –OSi(CH3)2(CH2)4O(C6H4)2CN 85 [79] 53 No. 7, CH2yCH(CH2)6O(C6H4)2CN –OSi(CH3)2(CH2)6O(C6H4)2CN [79] 54 No. 7, CH2yCH(CH2)11O(C6H4)2CN –OSi(CH3)2(CH2)11O(C6H4)2CN [79] 55 No. 1, CH2yCH(CH2)3OSiMe3(OSiMe3)2 –(CH2)5OSiCH3(OSiMe3)2 [94] 56 Me3SnCl –OSnMe3 [95] 57 Me4SbCl –OSbMe4 [95] 58 No. 7 –OSiMe2(CH2)17CH3 [96] 59 No. 7 –OSiMe2(CH2)2C6H5 [96] 60 No. 7 –OSiMe2CH2CH(CH3)C(O)OMe [96] 61 No. 7 –OSiMe2(CH2)2Si(C2H5)3 [97] 62 No. 7 –OSiMe2(CH2)2SiMe(OSiMe3)2 [97] 63 No. 7 –OSiMe2CHyCH–C6H5 [98] 64 No. 7 –OSiMe2CHyCH–C3H7 [98] 65 C6H11Si(OH)2OSi(OH)2C6H11 –C6H11 13 [43]

ð7:7Þ ð7:5Þ 8. Application of oligo- and polysiloxanes In another class of cubes, one pendant group differs from the others. These are prepared by the reaction of The sila-functional oligosiloxanes described so far various trichlorosilanes with the silane triols are potential candidates for surface modiﬁer, coupling T7R5(OH)3 (Eq. (7.6)), where most of the organic agents, additives, and building blocks for ladder and substituents are cyclopentyl or cyclohexyl groups. An cube oligosiloxanes, polysiloxanes with well-con- appreciable numbers of derivatives have been syn- trolled structure and silicon-based materials. It should thesized as summarized in Table 13. The silane triol be noted that they are a potential precursor as well for [55] and diol [56] are obtained as intermediates during the synthesis of silicon-based organic–inorganic the preparation of cubes, but only in fairly low yields, hybrids, closely related to the polysiloxanes discussed around 30%. Recently, It was a preparation of the triol above as additives and binders in combination with as the sodium silanolate was reported, with almost organic polymers and inorganic materials such as quantitative yields by the hydrolysis or the trichlor- glasses, oxides, and ceramics. osilane with a stoichiometric amount of water and sodium hydroxide (Eq. (7.7)) [56]. 8.1. Ceramic precursors

As discussed in the preceding, the sol–gel process with TEOS forms polysiloxanes with sufficient spinnability to form fibers that are transformed to silica fibers by a subsequent pyrolysis. This is an effective process to obtain silica fibers at an appreciable low temperature although there are the ð7:6Þ problems to be improved such as the stability of sols ARTICLE IN PRESS

26 Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx

Table 12

Spectral and analytical data of cube siloxanes T8R8

RinT8R8 –H –Cl –OMe –CHyCH2 –ONMe4 –OSiMe2H

1 NMR [d/ppm] H 4.20 – 3.36 – 4.80 (H2O), 4.7 (Si H), 3.19 (CH3)4 N 0.2 (SiCH3) 13 C – – 51.3 128.1 (SiC HyCH2), –– 137.7 (CHyC H2) 29Si 284.4 291.1 2101.4 280.2 299.0 0.5 (SiMeH)

–108.8Si(O)4

21 IR (cm )(nSi – O –Si 2290(s) 1142(s) 1155(s) 3064(vw) 3420 2961(w) is shown in bold) 1140(vs) 1090(sh) 1090(vs) 3025(vw) 3019 2920(w) 918(w) 795(vw) 848(m) 2982(vw) 1643 2140(m), 1245(m) 885(sh) 712(s) 795(w) 2959(vw) 1489 1169(m) 870(s) 515(s) 720(w) 1601(w) 1404 1093(s) 500(sh) 450(m) 570(vs) 1404(w) 1019 897(s) 470(w) 335(m) 470(w) 1273(w) 949 744(w) 395(m) 395(m) 1144(m) 725(w) 1105(s) 644(w) 1000(w) 550(w) 965(w) 774(w) 579(m) MS 423 (M–H)þ 665 (M–Cl)þ 664 (Mþ)– – – Mp (8C) 250 173 (decomp.) 161 (decomp.) 278 Sublim. p. (8C/Torr) 130/0.5 135/0.5 Yield (%) 23 .95 68 20.5 75.6 82.6 Elemental analysis H: 2.0 (1.9) Cl: 40.75 C: 14.37 (14.45); C: 29.19 (30.35); – C: 18.74 (18.88); found (calcd) (40.50) H: 3.64 (3.64) H: 3.88 (3.83) H: 5.04 (5.54) to condensation regardless of the optical properties of [110]. A mixture of the two components of silica fibers. On the other hand, esterification of silicic (RSiO3/2)n (R ¼ Me, Pr, Ph) provides silicon acid provides spinnable polysiloxanes [13,14], for oxycarbide fibers on heating the precursor fibers which the stability and/or self-condensation are in N2 or argon [111]. Black glasses, silicon dependent on the degree of esterification (DE) and oxycarbide, are formed by heat-treatment of the alkyl group, so that the esters with appropriate polysilsesquioxane gels prepared by hydrolytic DEs can be used as a good precursor for the polycondensation. They are also obtained by preparation of silica fibers. Thus, esters with DE less heating the polysiloxanes prepared by hydrolytic than around 60% undergo further condensation which, polycondensation of TEOS/a,v-polydimethylsiloxa- on precipitation with a solvent such as hexane, form nediol [112] or TEOS/Me2Si(OEt)2 [113]. insoluble powders which are the precursors for the As described in Section 6, controlled hydrolytic preparation of submicrometer-sized silica particles polycondensation of methyl- and vinyltrimethoxy- [108]. Silicic acid itself is also used as a precursor for silane yields polymethyl- and vinylsilsesquioxanes, the preparation of bulk silica glasses, which are providing access to bulk gels and flexible free prepared by aging a concentrated silicic acid in standing films. Black glasses are formed on organic solvents, followed by gradual evaporation of pyrolysis of the precursor films at 1400 8C under the solvent at room temperatures [15,109]. N2 atmosphere [114]. Polysilsesquioxanes can be a precursor for the Pyrolysis of polymethylsilsesquioxane films preparation of silicon oxycarbide SiOC or SiC forms broken pieces of black glass. However, ARTICLE IN PRESS

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx 27 polyvinylsilsesquioxane films provides ceramic films molecules and also between the molecules and without cracks, though they shrink about 15% both in substrate in the coating are of central importance. length and width. In addition, a little weight loss These interactions depend on the molecular structure (about 10%) is observed for both films. The ceramics and molecular weight: the silanol groups as a pendant are identified to be silicon oxycarbide with the afford an intermolecular force and an interaction with composition of SiOxCy ðx ¼ 0:71 , 2:0; y ¼ 1:40 , substrates, and in some cases, forms a chemical 1:60Þ: The black glass ceramic films have a free bonding. As it is well known, polysiloxane sols carbon content up to 90%, in contrast to the free prepared by a sol–gel process with TEOS provide carbon content of only several wt% into silica glasses good precursors for coatings, although they produce by the usual melt method. This may result from the silica thin films. Correspondingly, polysilicic acid fact that the structure of the precursor films is well- esters (polyalkoxysiloxanes) [11–14] and polysilses- controlled, and subsequently converted into ceramics quioxanes [22–24] can be the potential candidates via an organic–inorganic hybrid in which the vinyl because they forms fibers and flexible free-standing groups are incorporated into silica matrices through films. addition polymerization. In general, the chemical coating process is conveniently performed by dip or spin coating 8.2. High performance coatings methods using rather simple apparatuses (Fig. 12) Here, the results on coatings with PMSQ and PVSQ Among the various shapes of materials, thin films will be described [115]. The coating solutions are have found wide applications, especially as functional 20 wt% PMSQ or PVSQ acetone–methanol (v/v) materials for protectors, optics, electronics and solutions, and organic or inorganic substrates are membranes for separation or gas permeation. Thus, used: high density polyethylene (HDPE), polypro- high performance coatings having excellent proper- pylene (PP), polycarbonate (PC), polyethylene ties are possible with polysiloxanes if they show film terephthalate (PET), 6-nylon, Aluminum, stainless formation. The interactions between polysiloxane steel (SUS304), soda-lime glass, quartz glass, and

Fig. 12. Apparatus for dip-coating (a) and spin-coating (b). ARTICLE IN PRESS

28 Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx

Table 13 PC , PET , 6-Nylon, but the coatings do not Mono-substituted cubes adhere to HDPE or PP. Strong adherence to the No. Substituent (R0) References inorganic substrates may be due to the formation of metallasiloxane bonds, in addition to physical 66 –H [99] interaction with the surfaces, while the different 67 –Cl [92] adhesion among the organic substrates is associated 68 –OH [92] with the solubility parameter ðdÞ of coatings and y 69 –CH CH2 [100–104] substrates. Table 15 gives the solubility parameter 70 –CH CHyCH [105] 2 2 of PMSQ and some of the substrates. A high 71 –CH2CH(O)CH2 [105] 72 –(CH2)6CHyCH2 [100–104] adhesive strength is observed when the solubility y y 73 –CH CH(CH2)8CH CH2 [100–104] parameters of polymer and substrate are close in y 74 –(CH2)2C6H4CH CH2 [100–104] value. Adhesive strength by the crosscut tape test 75 –(CH ) Si(CH )Cl [100–104] 2 2 3 2 based on JIS K5400 is a qualitative test. The 76 –(CH2)2SiC6H4Cl2 [100–104] 77 –(CH2)3OCOCH–CH2 [106] adhesive strength shown in Fig. 13 was evaluated 78 –(CH2)3OCOC(CH3)yCH2 [107] quantitatively by the stud-pull method, commonly known as the Sevastian method. Values in the silicon wafers. Films are prepared by dipping the ranges 50 , 100 and 80 , 130 kg/cm2 for PMSQ substrates into the solution and pulling up followed and PVSQ, respectively, correspond to values in by heating at 80 8C for 24 h and then 100 8C for Table 14. The pencil hardness of PMSQ, which several hours. was measured using the Japanese Industrial Dip-coating of 1–10 times provides films of Standard K5400 protocol, also depends on the thickness 0.25 , 0.85 mm(PMSQ)and heating time and molecular weight of the PMSQ, 0.40 , 0.95 mm (PVSQ). The film thickness depends as is shown in Table 16. Similar, but lower, results on the molecular weight of the polymer. Higher the were observed for the adherence and hardness of molecular weight lead to thicker the films, and PVSQ PVSQ. (Fig. 14) provides thicker films than PMSQ. Thin films are highly transparent, with transmit- The adhesive strength of PMSQ was measured tance of more than 98% at 500 nm. As shown in using the Japanese Industrial Standard K5400 Fig. 13, both PMSQ and PVSQ are thermally stable up protocol, with results given in Table 14. to 450 , 500 8C, as evaluated by measuring the Obviously, the properties depend on heating time adherence at various temperatures. The adhesive and the PMSQ molecular weight. Polysilsesquiox- strength increases with the increasing temperature anes adhere more strongly to the inorganic up to 800 8C, indicating that a film still remains. The substrates than to the organics. Different degrees unexpected thermal stability may be due to the fact of adhesion are observed for the organic substrates: that polysilsesquioxanes are converted into ceramics adhesive strength increases in the order: films via an organic–inorganic hybrid.

Table 14 Adhesive strength of PMS coating ﬁlms

Mw (H2O/MTS) 5500 (1.10) 21,000 (1.23) 55,000 (1.30)

Heating timea (h)036120361203612 PC 266648886888 PET 4688810101010101010 6-Nylon 6 8 8 10 10 10 10 10 10 10 10 10 Aluminum 8 10 10 10 10 10 10 10 10 10 10 10 SUS304 10 10 10 10 10 10 10 10 10 10 10 10 Glass 10 10 10 10 10 10 10 10 10 10 10 10

Coating conditions: the number of dipping, one time; winding speed, 80 mm/min. No adhesions to PP and HDPR. a At 100 8C after drying at 80 8C for 24 h. ARTICLE IN PRESS

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx 29

8.3. Interlayer low dielectrics for electronic devices

One research target for silicon-based materials is the development of interlayer low dielectrics for semiconductor electronic devices. In the near future, dielectric materials with the k values lower than 2.0 will be needed for practical uses in very large scale integrated circuits. Despite their being low dielectrics, organic compounds can not be applied to the present device preparation process at high temperatures around 400 , 450 8C. A candidate to overcome this limitation is offered by polysiloxanes, Fig. 13. The variation of tensile strength of polymethyl silsequiox- with their excellent chemical, physical, and elec- ane (PMSQ) and polyvinylsilsesquioxane (PVSQ) coating films by stud-pull method, commonly known as the Sevastian method, as a trical properties, but only if they are capable of function of heating temperature. Coating films were prepared on forming thin films. Suitable polysiloxanes could silicon wafer by dip coating and then heated at 808C for 24 h under include sols by sol–gel process with TEOS, nitrogen atmosphere followed by further heating at various copolymers of TEOS and Me42nSi(OMe)n (n ¼ 2 temperatures for 1 h. or 3), and polysilsesquioxanes. The sols from TEOS provide films with k around 4. At present, copolymers are used in the spin-on-glass (SOG) Table 15 process to provide films with the k values in the Solubility parameters ðdÞ range 2.9 , 3.0. On the other hand, polysilsesquiox- Substrate d (calcd)a Note anes are expected to be a potential candidate as mentioned in Section 8.2, for they provide high PP 8.01 Non-polar/crystalline performance coatings. At present, no investigations HDPE 8.56 have been made to evaluate dielectric constant k of PC 10.5 Polar/non-crystalline polysilsesquioxanes for correlation with the molecu- PET 11.7 Polar/crystalline 6-Nylon 11.9–12.5 lar structure [115,116]. b PMS ðMw ¼ 5500Þ 12.01 13.14 Table 17 shows the results on the measurement b (Mw ¼ 55,000) 12.17 13.17 of dielectric constant of polymethylsilsesquioxanes, b PVS ðMw ¼ 2600Þ 12.09 13.15 3 b consisting of T units of 47 , 58%, with various (Mw ¼ 21,000) 12.71 13.28 P P molecular weights. The dielectric constants appear a 3 1/2 = 1=2 = 1=2; d (cal/cm ) ¼ðDE DVÞ ¼ð Dei DviÞ Dei and Dvi : to be fairly low, around 2.5, compared with those the additive atomic and group contribution for the energy of of practical films, which are in the range vaporization and volume, respectively. Fedors RF. Polym Engng Sci 1974;14:147. 2.9 , 3.0. b SP value of PMS (or PVS) 20 wt% acetone–methanol solution.

Table 16 Pencil-hardness of PMS coating ﬁlms on soda-lime glass

Mw 5500 21,000 55,000

Heating timea (h)036120361203612 Pencil-hardness 5H 5H 6H 9H 5H 5H 6H 9H 6H 6H 7H 9H

Coating conditions: number of dipping, one time; winding speed, 80 mm/min. a At 100 8C after drying at 80 8C for 24 h. ARTICLE IN PRESS

30 Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx

tional silanes from 3-chloropropyltrialkoxysilane. Przemysl Chemiczny 1998;77:288–90. (b) Abe Y. Synthesis of isocyanatosilanes and their uses as versatile reagents 1995; 40. (c) Abe Y. Fac Sci Technol (Senryo to Yakuhin) 1995; 40:141–7. [3] Sakurai H. Application of organosilicon compounds to organic synthesis. In: Nozaki H, Yamamoto S, Tsuji J, Noyori R, editors. Kagaku Zokan, vol. 105. Kagaku Dojin. 1985. p. 33–40. [4] (a) Iler RK, Pinkney PS. Polysilicic acid esters-preparation from sodium silicate. Ind Engng Chem 1947;39:1379. (b) Iler RK. The chemistry of silica. New York: Wiley; 1979. p. 140. (c) Lentz CW. Silicate structures extracted intact. Chem Engng News 1963;Sep 23:44. [5] Lentz WC. Silica minerals as sources of trimethylsilyl silicates Fig. 14. Tensile strength of polymethylsilsesquioxane (PMSQ) and and silicate structure analysis of sodium silicate solutions. polyvinylsilsesquioxane (PVSQ) coating films by stud-pull method, Inorg Chem 1964;3:574. commonly known as the Sevastian method, as a function of heating [6] Abe Y, Nojiri F, Misono T. Preparation of polysiloxanes from time at 100 8C. on pyrolysis. Coating films were prepared on silicon silicic acid. IV. The preparation of acetylated silicic acid by wafer by dip coating and then heated at 80 8C for 24 h under nitrogen atmosphere followed by further heating at 100 8C for various hours. the reaction of silicic acid with acetyl chloride. J Chem Soc Jpn Chem Ind Chem 1983;1277. Table 17 [7] Kas¸go¨z A, Misono T, Abe Y. Preparation of silica–MxOy thin The effect of Mw and thickness on specific dielectric constant 1 films and gels by sol–gel method using silicic acid and metal halides. J Ceram Soc Jpn 1992;100:763. Mw Run no. 1 Run no. 2 [8] Kasgo¨z A, Yoshimura K, Misono T, Abe Y. Preparation and ˚ ˚ 1 Thickness (A) 1 Thickness (A) properties of SiO2 –TiO2 thin films from silicic acid and titanium tetrachloride. J Sol–Gel Sci Technolnol 1994;1:185. 55,000 2.3 2420 2.5 2250 [9] Gunji T, Toyota K, Arai K, Abe Y. Syntheses and 21,000 2.4 2330 2.6 4300 characterization of polymetallosiloxanes from silicic acid 5500 2.4 2150 2.7 4410 and metal chlorides. J Sol–Gel Sci Technol 1997;10:139. a X 2.6 2200 2.9 4480 [10] Abe Y, Misono T. Preparation of polysiloxanes from silicic acid. 1. Preparation of polysiloxanes by the silylation of a X: polysiloxanes consisted of the units D2,T3, and Q4 silicic acid extracted with tetrahydrofuran. J Chem Soc Jpn (commercial product). Chem Ind Chem 1981;1:1152–8. Acknowledgements [11] Abe Y, Misono T. Preparation of polysiloxanes from silicic acid. II. Esterification of silicic acid with various alcohols and isolation of esterification products by silylation. J Polym The paper will be presented to a memorial issue for Sci, Polym Lett Ed 1982;20:205–10. the continuous and great contributions of Professor [12] Abe Y, Misono T. Preparation of polysiloxanes from Otto Vogl to the progress and development in polymer silicic acid. III. Preparation and properties of polysilicic science. The authors are very proud of the present acid butyl esters. J Polym Sci, Polym Chem Ed 1983;21: publication of our work on the siloxane compounds. 41–53. [13] Abe Y, Sekiguchi T, Misono T. Preparation of polysiloxanes from silicic acid. V. Condensation of silicic acid butyl esters and formation of silica fiber from the ester solutions. J Polym References Sci, Polym Chem Ed 1984;22:761–7. [14] Abe Y, Shintani N, Misono T. Preparation of polysiloxanes [1] (a) Metal-organics for metal and polymer technology. Japan: from silicic acid. VII. Effects of the degree of esterification Gelest, Inc., Azmax Co. Ltd. Metal-organics including silianes and alkyl groups on condensations of silicic acid esters and and silicones. Japan: Gelest, Inc., Azmax Co. Ltd; (b) formation of fibrous silica. J Polym Sci, Polym Chem Ed Silylating agents.4 Fluka; (c) Organosilane, reagents and 1984;22:3759–69. speciality silicones. Chisso Co., Ltd; (d) Silicon compounds. [15] Abe Y, Misono T. Preparation of polysiloxanes from silicic Aldrich Chemical Company, Inc.; (e) Silicon compound acid. VI. Formation of a gel-like silica glass from silicic acid. reagents. Shin-Etsu Co., Ltd. J Polym Sci, Polym Lett Ed 1984;22:565–7. [2] (a) Gulinski J, Maciejewski H, Marciniec B, Wydzial C. [16] Abe Y, Shintani N, Magome T, Misono T. Preparation of New low-tonnage processes for synthesis of organofunc- polysiloxanes from silicic acid. VIII. A polysiloxane with ARTICLE IN PRESS

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a good spinnability. J Polym Sci, Polym Lett Ed 1985;23: spectra of linear and cyclic isocyanato(methyl)oligosilox- 497–501. anes. Silicon Chem; in press [17] Abe Y, Kaijou A, Shintani N, Nagao Y, Misono T. [30] Roux CL, Yang H, Wenzel S, Grigoras S, Brook MA. Sing Preparation of polysiloxanes from silicic acid. IX. Partially anhydrous hydrolysis to favor formation of hexamethylcy- silylated silicic acid and its spinnability. J Polym Sci, Polym clotrisiloxane from dimethyldichlorosilane. Organometallics Chem Ed 1987;25:1671–9. 1998;17:556. [18] Abe Y, Kaijou A, Nagao Y, Misono T. Preparation of [31] Yoshino K, Kawamata A, Uchida H, Kabe Y. Convenient polysiloxanes from silicic acid. X. Preparation and properties synthesis of a,v-difunctionalized linear dimethylsiloxanes of allyldimethylsilylated silicic acids. J Polym Sci, Polym with deﬁnite chain lengths. Chem Lett 1990;2133. Chem Ed 1988;26:419–27. [32] Uchida H, Kabe Y, Yoshino K, Kawamata A, Tumuraya T, [19] (a) Brown Jr JF, Vogt Jr L, Katchman HA, Eustane JW, Kiser Masamune S. General strategy for the systematic synthesis of KM, Krants KW. Double chain polymers of phenylsilses- oligosiloxanes. Silicone dendrimers. J Am Chem Soc 1990; quioxane. J Am Chem Soc 1960;82:6194. (b) Brown Jr JF. 112:7077. Double chain polymers and nonrandom crosslinking. J Polym [33] Lickiss PD. In: Sykes AG, editor. Advances in inorganic Sci C 1963;1:83. chemistry. London: Academic Press; 1995. [20] Pope EJA, Sakka S, Klein LC. Sol–Gel science and [34] Gunji T, Kubota K, Kishiki S, Abe Y. Syntheses of twelve- technology. The American Chemical Society 1995;51–101. membered ring titana- and zirconasiloxane compounds and [21] Abe Y, Shimano R, Gunji T. Preparation and properties of their properties as ceramic precursor. Bull Chem Soc Jpn high molecular weight polyethoxysiloxanes stable to self- 2002;75:357. condensation by acid-catalyzed hydrolytic polycondensation [35] Brown Jr JF, Slusarczuk GMJ. 1,3-Diphenyldisiloxanetetrol. of tetraethoxysilane. J Polym Sci, Part A: Polym Chem 2003; J Org Chem 1964;29:2809. 41:2250–5. [36] (a) Lickiss PD, Litster SA, Redhouse AD, Wisener CJ. [22] Takamura N, Gunji T, Hatano H, Abe Y. Preparation Isolation of a tetrahydroxydisiloxane formed during hydroly- and properties of polysilsesquioxanes: polysilsesquioxanes sis of an alkyltrichlorosilane: crystal and molecular structure

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