ARTICLE IN PRESS - INSTRASnathan.instras.com/documentDB/paper-201.pdf · 1. Introduction...

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

0079-6700/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.progpolymsci.2003.08.003

Prog. Polym. Sci. xx (2004) xxx–xxx

www.elsevier.com/locate/ppolysci

* Corresponding author. Tel.: þ81-4-7124-1501x3608; fax: þ81-4-7123-9890.

E-mail address: [email protected] (Y. Abe).

ARTICLE IN PRESS

http://www.elsevier.com/locate/ppolysci

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

Polysiloxanes are versatile materials, many hav-

ing excellent chemical, physical, and electrical

properties; polydimethylsiloxane is an important

example of this class of polymers. The only

disadvantage, nevertheless it is one of the excellent

properties, of siloxanes is the fact that they are not

able to form fibers and films because of their low

interactions between molecules. In order to afford

new functions, the structure of polysiloxanes has

been modified by side chain functionalization or

change in the main chain. The dimension of the

macromolecular structure is a key factor in the

generation of new properties: ladder and sheet are

two-dimensional structures, while cross-linked cage,

cube, and spheres are three-dimensional structures.

Such structures may find use as materials with

characteristic thermal, optical, electrical, and mech-

anical properties. The state of association or

aggregation of the macromolecules must also be

considered. Recent research trends involve the

synthesis and properties of sila-functional oligo-

and polysiloxanes, silsesquioxanes such as cage,

cube, and ladder structures, and their applications as

functional materials.

Although research on siloxanes is attractive

because of the multitude of potential applications,

convenient and selective methods for the synthesis of

sila-functional silanes and oligosiloxanes are lacking

Even silanes with sila-functional groups such as

R42n 2 mSi(OR)nXm or (RO)42nSiXn (m ¼ 1; 2;

n ¼ 1 , 3, R ¼ alkyl, alkenyl, aryl; X ¼ halogen,

OR0, OH, NR2, OCOR, NCO) are not

versatile as starting materials. The synthesis and

uses of sila-functional oligosiloxanes are very limited,

although they can be potential building blocks for

polysiloxanes and polysilsesquioxanes. The key to

develop the methods for the synthesis of such

precursors should be the controlled reactions of

silanes with sila-functional groups, with special

attention to the reactivity of sila-functional groups.

Subsequently, di- and trifunctional silanes would be

precursors, with hydrolysis, condensation or elimin-

ation is as the preferred reactions to provide

oligosiloxanes.

This review article will focus on the syntheses,

properties, and applications of sila-functional oligo-

and polysiloxanes and silsesquioxanes with linear,

cyclic, ladder, cage, and cube structures.

2. Commercially available sila-functional

oligosiloxanes

A variety of organosilicon compounds are com-

mercially available. However, so far as oligosiloxanes

and silsesquioxanes are concerned, very limited

products are available as reagents including sila-

functional oligosiloxanes and oligosilsesquioxanes

like ladder, cage, and cube. The same is also true

for sila-functional silanes. Moreover, as illustrated by

the examples listed in Tables 1 and 2, based on

catalogues issued by several sources [1], the available

reagents are often expensive.

Sila-functional silanes R42nSiXn (n ¼ 2; 3: R ¼

alkyl, alkenyl, aryl; X ¼ H, halogen, OR) are versatile

reagents, whereas silanes R42nSiXn (n ¼ 2; 3;

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx2

ARTICLE IN PRESS

X ¼ OH, OCOCH3, NR2, NCO) and especially

(RO)42nSiXn (n ¼ 1 , 4; R ¼ Me, Et, Pri, But;

X ¼ halogen, OR0, OH, NR2, OCOCH3, NCO) are

so limited as to be supplied for commercial uses.

Many of them may be purchased as order-made or

obtained as a component product of reaction mixtures

or a by-product. Silanediols and triols R42nSi(OH)n

(R ¼ Me, Ph) are often used as a potential precursor

for the preparation of oligo- and polysiloxanes or

silsesquioxanes. Recently, isocyanato(methyl)silanes

Me42nSi(NCO)n ðn ¼ 1 , 4Þ have been used as

halogen-free coupling reagents, of which reactivity

is appreciably higher than that of alkoxysilanes, but

low compared with chlorosilanes [2]. On the other

hand, it is difficult to synthesize silanes with different

functional groups (RO)4-nSiXn. A few examples

(X ¼ halogen, OR0, OH, NR2) are listed in Table 1.

Linear and cyclic oligosiloxanes are listed in

Table 1. It may be noted that only siloxanes with

n ¼ 1 , 4 are commercially available, as shown at

the bottom of Table 1. Cyclic oligodimethylsiloxanes

D3,5 are the raw materials for the production of

silicone. Usually, the only variations of the functional

group found with sila-functional cyclosiloxanes

(SiMeXO)n are hydrogen and alkoxy groups

(n ¼ 3 , 5; X ¼ H, OR). While an appreciable

number of linear sila-functional oligosiloxanes are

listed, most are disiloxanes, as shown in Table 1, and

no homologues more than trimers are listed in the

catalogues.

Cubes can be a potential building block to

prepare silicon-based materials, but as shown in

Table 2, they are very expensive and limited as

commercial products. Therefore, convenient

methods to provide them as precursors have to be

developed.

Nomenclature

Bp boiling point

Bu butyl group, CH3CH2CH2CH2–

But tert-butyl group, (CH3)3C–

D3 hexamethylcyclotrisiloxane, (Si(CH3)2O)3

D4 octamethylcyclotetrasiloxane,

(Si(CH3)2O)4

D5 decamethylcyclopentasiloxane,

(Si(CH3)2O)3

DABCO 1,4-diazabicyclo[2.2.2]octane

DE degree of esterification

DS degree of silylation

Decomp. decomposition

Et ethyl, CH3CH2–

SEC size exclusion chromatography (or GPC,

gel permeation chromatography)

HDPE high density polyethylene

HPLC high performance liquid chromatography

IR spectrum infrared absorption spectrum

JIS K5400 Japan Industrial Standard No. K5400

MS mass spectroscopy

Me methyl group, CH3–

MeOH methanol

Mn number average molecular weight

Mp melting point

Mw weight average molecular weight

Mw/Mn polydispersity

NMR nuclear magnetic resonance

ORTEP Oak Ridge thermal ellipsoid plot

Oct octyl, (CH3)2(CH2)7–

PC polycarbonate

PEOS polyethoxysiloxanes

PET polyethylene terephthalate

PMSQ polymethylsilsesquioxane

PP polypropylene

PSSX partially silylated siloxane

PVSQ polyvinylsilsesquioxane

Ph phenyl, C6H5–

Pri isopropyl, (CH3)2CH–

Qn Siloxane unit, Si(OSi0.5)n(OR)42n ðn ¼

1–4Þ

SOG spin-on-glass

SUS304 stainless steel

TEOS tetraethoxysilane

THF tetrahydrofuran

TMOS tetramethoxysilane

Td (5%) temperature at the weight loss of 5%

Tn siloxane unit, RSi(OSi0.5)n(OR)32n ðn ¼

1–3Þ

Vi vinyl, CH2yCH–

m-CPBA meta-chloroperbenzoic acid

d solubility parameter

Y. Abe, T. Gunji / Prog. Polym. Sci. xx (2004) xxx–xxx 3

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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 polysilsesquiox-

anes with various structures which can be the

precursors for silicon-based materials. The hydrolysis

of tri- and tetrafunctional silanes usually gives

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.

Table 1

Price list of commercially available oligosiloxanes

Table 2

Price list of commercially available cubes

Scheme 1.


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Sol–Gel reactions of sila-functional silanes provide a

facile process to obtain the materials mentioned

above.

3.2. Reactivity of sila-functional groups

Sila-functional groups, which are essential to the

formation of siloxane bonds and/or the synthesis of

siloxane compounds, make it possible to promote

various types of siloxane formation reactions. In

addition, the reactivity plays an important role to

control reactions and subsequently to result in the

structure control of siloxane compounds. A number of

sila-functional groups are shown below. Of these, the

following are particularly versatile: Halogen, NR2,

OCOR, NHCONR2, NCO, OH, OR, H, OCRyCR2,

ONyCR2, and ONR2.

There have been no reported quantitative, or even

qualitative estimates of the reactivity of sila-func-

tional groups. Based on the experimental results [3],

one would anticipate the following order: H, OR,

OH , NCO, OCOCH3, ,NHCONR2, NR2, Cl.

3.3. Siloxane bond formation

Siloxane bond formation reactions are represented

by the Eqs. (3.1)–(3.5) in Scheme 3. Self-conden-

sations followed by hydrolysis of silanes with sila-

functional groups (Eq. (3.1)) are a versatile means to

form siloxane oligomers and polymers, especially with

chloro and alkoxy groups. This is also the reaction in the

sol–gel process, which is applicable to metallasiloxane

or metalloxane bonds formation using metal-organic

compounds. Hetero-functional condensations invol-

ving silanol groups (Eq. (3.2)) are a potential route to

control reactions and/or construct siloxanes with well-

defined structures, because the cleavage of siloxane

bond by acid or base catalyst is minimized. Therefore,

the reactions shown in Eq. (3.2) provide tailor-made

siloxanes, such as high molecular weight block,

random, and alternating copolysiloxanes, as well as

oligosiloxanes. Another type of hetero-functional

condensation is illustrated in Eq. (3.3). Similarly,

condensation of silanes with alkoxy and chloro groups

may take place in thepresence ofa suitable catalyst. The

silicon–silicon bond can be oxidized with peroxides,

nitrosoamines, ozone, and hydrogen peroxide to form

siloxanes (Eq. (3.4)). It seems that almost no reports

have been published about the reaction in Eq. (3.5).

Condensation followed by elimination takes place in

the presence of acid as a catalyst, if controlled, and

would provide polysiloxanes with targeted structures.

4. Polysiloxanes

A wide variety of industrial applications have been

developed for polysiloxanes, with excellent chemical,

Scheme 2.


ARTICLE IN PRESS

physical, electrical properties. In recent years, the

requirements of advanced technologies have created

as need for new high performance polysiloxanes.

Here, an attention will be focused on polysiloxanes as

precursors for coatings, binders, additives, and

ceramics.

4.1. Polysilicic acid esters and their properties

Polyalkoxysiloxanes have been used as industrial

raw materials (so called SE 40, 48, and ME 52).

These are the mixture of oligomers prepared by

hydrolysis of tetraethoxysilane TEOS and tetra-

methoxysilane TMOS. No high molecular weight

polyalkoxysiloxanes stable to condensation have

been synthesized. On the other hand, esterification

of silicic acid, the reverse reaction of hydrolysis of

tetraalkoxysilane, is reported by Iler [4] to provide

polysilicic acid esters and/or polyalkoxysiloxanes

unstable to self-condensation, of which silica

content was up to 66%. However, their properties,

structure, and esterification conditions were not

investigated in detail.

Silicic acids are potential starting materials, but

they do not react with organic compounds in an

aqueous solution except by means of silylation by

Lentz [5]. However, alcohols, chlorosilane, acetyl

chloride [6] or even metal chloride [7,8] will react

with silicic acids to give derivatives, which may be

extracted by organic solvents. Thus, silicic acid was

extracted with organic solvent from the resulted

aqueous solution after neutralization of sodium

metasilicate with hydrochloric acid followed by

salting out. In this procedure, acetone, alcohols such

as 1-propanol, 2-methyl-2-propanol, and 2-propanol,

and THF were used as solvent. When THF was used,

silicic acid was extracted up to 90% as SiO2 [9]. As a

result, a silicic acid–THF solution of 1.2 mol/l was

conveniently prepared.

According to Eq. (4.1), silicic acid is esterified by

alcohol under azeotropic distillation to give poly-

silicic acid esters over a range of molecular weight

and degree of esterification (DE: relative degree of

alkoxy group among the all functional groups). An

apparatus for this purpose is shown in Fig. 1, and

example results with 1-butanol are summarized in

Scheme 3.


ARTICLE IN PRESS

Table 3. These are characterized by SEC and 1H NMR

analysis after isolation as silylates, stable to conden-

sation according to Eq. (4.2) [10–14].

ð4:1Þ

The properties of polysilicic acid esters depend

greatly on DE and the ester group. The esters gelled

just after the concentration of the reaction mixture or

isolation as a white powder by precipitation with

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 : 1 for the esters

(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

but decomposition [10–14].

ð4:3Þ

A silicic acid–THF solution is prepared by

extraction of an aqueous silicic acid solution with

THF. A concentrated silicic acid solution, up to about

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

aging in a sharle wrapped with a polyethylene film

with several pin holes to permit slow evaporation of

the solvent [15].

4.2. Polysiloxanes capable of forming fibers and films

One of the outstanding properties of polysiloxanes

is the weak interaction between molecules, which

affords them excellent physical properties, such

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

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

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.

ð4:2Þ


ARTICLE IN PRESS

as a low viscosity coefficient and a small contact

angle. However, this can also represent a potential

limitation for their use as coating films or fibers. Such

limitations could be overcome by modification or

improvement of the polysiloxane structure.

As shown in Tables 4a and b, silicic acid esters

undergo condensation to form high molecular weight

polysilicic acid esters. Depending on DE and the ester

group, some of these show spinnability, and provide

precursor fibers of silica. This may suggest that

spinnability depends on the DE and/or silanol groups,

in addition to molecular weight. If so, partially silylated

silicic acids would be expected to show spinnability

[16–18]. Therefore, silylates with various degrees of

silylation (DS) were prepared by the reaction of

chloro(trimethyl)silane with silicic acid, according to

Eq. (4.4). The silylates are soluble in organic solvents

and isolated by precipitation with water/MeOH (5/1,

v/v) followed by drying in vacuo. Despite a low

molecular weight and dispersion as indicated in

Table 5, concentrated silylate solutions show spinn-

ability that is highly dependent on the solvent such as

acetone, dibutyl ether, ethyl acetate, dioxane, and THF

and also on the sila-functional groups of OH, OCOCH3,

and OBu. Thus, remarkable spinnability is observed for

the silylate with DS 70% when in dioxane solution.

Further, reactions of silylates with acetyl chloride and

1-butanol provide the corresponding derivatives in

which the silanol groups are replaced with acetoxy and

butoxy groups. The relative spinnability is measured to

be the order: Bu , OCOCH3 , OH. No spinnability is

observed for the completely silylated polymer.

Obviously, spinnability highly depends on DS and

solvent and/or the solubility parameter so that spinn-

ability is correlated to the intermolecular interactions

resulted from the structure of polysiloxane. The

results suggest that structure modification of siloxane

Table 4a

Variation of molecular weights of polysilicic acid butyl ester with

DEs on aging

DE (%) Aging time (h) Mn Mn Mw=Mn

43 0 3390 4650 1.37

12 4480 10,300 2.30

19.5 4570 18,200 3.98

31.5 4300 29,500 6.86

38 4930 33,500 6.80

55 0 3850 5420 1.41

131 3300 5100 1.55

226.5 3300 7360 2.23

85 0 11,400 20,000 1.75

194 11,000 15,600 1.42

985 10,400 18,300 1.76

Table 3

Preparation of polysilicic acid butyl esters under atmospheric

pressure

Temp (8C) Time (h) Mp (decomp., 8C) DE (%) Mn

135–145 0.5 235 54 13,200

2 230 63 10,000

12 220 90 11,500

140–145 0.5 – 63 22,000

1 210 73 24,000

6 215 85 18,000

Scheme 4.

Table 4b

Solubility of various silicic acid esters

R DE (%) Spinnable timea (h) Length (cm)

Et 50 28–32 10–20

Pri 40 160–168 50–100

Bu 43 38–42 50–100

Oct 26 70–90 5–10

a The time until a spinnability was observed and spinnable time

intervals on aging a solution of various esters (7.0 mol/l) at 40 8C.

Table 5

Spinnability of partially silylated silicic acids PSSX

PSSX Xa Molar ratiob TMCS/SiO2

Mn Mw Mw=Mn nc Spinnability

(cm)

OH 1 980 1100 1.12 7.3 50

2 1150 1370 1.19 6.7 250

3 1270 1500 1.23 6.5 200

5 1290 1720 1.33 6.5 100

7 1480 1990 1.34 6.6 80

10 1500 2020 1.35 6.6 50

OCOCH3 5 1340 1670 1.25 6.5 80

OBu 7 1330 2300 1.73 6.8 50

a Functional group in PSSX.b Molar ratio of chloro(trimethyl)silane (TMCS) to silicic acid

(SiO2).c Degree of condensation.


ARTICLE IN PRESS

backbone with sila-functional group as a pendant is a

key process to synthesize polysiloxanes capable of

forming not only fibers but also films.

Silylation of silicic acid with allyl(chloro)(di-

methyl)silane in the presence of triethylamine gives

partially allyl(dimethyl)silylated silicic acids, of

which high molecular weight polymers (Mn

6000 , 34,000) are separated as a benzene-soluble

component from low molecular weight polymers (Mn

2400 , 4100). Interestingly, the silylates show not

only spinnability, but film formation while no flexible

films are prepared from partially trimethylsilylated

silicic acids. This appears to be the first finding of

flexible free-standing films prepared from polysilox-

anes as a precursor, other than Brown’s phenylsilses-

quioxane [19]. The results mentioned above reveal

that spinnability and film formation are closely

associated with the structure of polysiloxanes: inter-

molecular interactions resulted from backbone and

pendant group are the key factor, and mechanical

strength of fibers and films is related to backbone

structure in addition to molecular weight.

4.3. Highly polymerized TEOS stable to self-

condensation

Hydrolytic polycondensation of metal alkoxides is

well known as a sol–gel method. It is a potential route to

prepare oxide materials in a range of forms, including

bulk bodies, particles, thin films, and fibers. The

material properties can be strongly influenced by

the sol and gel precursors. In this regard, control of the

hydrolysis and condensation reactions are of central

importance in sol–gel processing. Depending on

whether acid and base catalysis are utilized, linear or

cross-linkedpolymersolsare formedespeciallyonsol–

gel process of TEOS. However, almost no work has

been done on the isolation and characterization of sols,

as they are unstable to self-condensation Moreover,

most efforts are devoted to the preparation and

characterization of ceramic materials. Therefore, the

structure is often investigated by in situ analysis in the

solution [20].

Low molecular weight polymer sols are now

supplied as commercially available products, so

called SE 40, 48, and MS 52, but they are the

mixture of oligomers with low degrees of polym-

erization. Investigation of the preparation and

properties of polyethoxysiloxane sols stable to self-

condensation by sol–gel reaction of TEOS is of

interest and value to the basic chemistry of polymer

sols, and their application as industrial raw

materials.

As indicated in Section 4.2, the properties of

polysilicic acid esters largely depends on the DE and

ester groups. Controlled esterification is essential to

the preparation of esters with expected properties. The

same is true in the sol–gel process of TEOS, the

reverse reaction of the esterification of silicic acid.

Thus, the sol–gel reaction, if controlled, would

provide polyethoxysiloxanes (PEOS) according to

Eq. (4.5). The key factor to obtain polyethoxysilox-

anes stable to condensation is structure control, based

on the relative ratio of siloxane structural units Qn

(Q2: x, Q3: y, Q4: z) together with the ratios of silanol

to ethoxy and silanol groups [21].

ð4:4Þ

ð4:5Þ


ARTICLE IN PRESS

In an investigation of these effects, reactions were

carried out in various molar ratios of H2O, HCl, and

EtOH to TEOS at 70 8C for 3 h with stirring under the

nitrogen stream, as shown in Table 6 [22]. The

preparation process is featured by reaction under a

nitrogen stream (Fig. 2) and evaporation of solvent,

hydrogen chloride and excess amount of water, to

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 sol-

vents, except for hexane, and stable to self-conden-

sation although they contain an appreciable amount of

silanol groups (the molar ratio OH/Si ¼ 0.4), esti-

mated from elemental analysis. Good spinnability was

also observed by drawing a glass rod up from a

concentrated viscous solution. They undergo conden-

sation either in solution or neat, as illustrated by

the results of an SEC analysis as shown in Fig. 4. Only

a slight increase in molecular weight is observed for

the controlled reactions at room temperature and

below, except for reactions in a THF solution,

showing unexpectedly stability compared with sols

generally prepared via a sol–gel process. Elemental

analysis indicates silica contents up to 70%. This

should be noted considering the fact that the silica

contents of industrial ethyl silicates (SE 40, 48) and

methyl silicate (SM 53) are 40, 48, and 53%,

respectively. Polyethoxysiloxanes as well as poly-

silicic acid esters, therefore, can be a potential

material as coating, additives, and binders.

Fig. 2. Apparatus for the preparation of polyethoxysiloxanes

(PEOS) by the hydrolytic polycondensation of tetraethoxysilane

(TEOS) under a nitrogen stream. TEOS, ethanol, and hydrochloric

acid are placed in a four-necked flask and nitrogen is introduced

trough the reaction. PEOS is produced as a highly viscous liquid. In

this process, the discharge of hydrochloric acid promotes the

condensation reaction.

Table 6

Preparation of polyethoxysiloxanes

No. Molar ratio H2O/TEOS Time (h) Yield (g) Mw Mw=Mn Ratio of siloxane unita

(%)

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.


ARTICLE IN PRESS

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 poly-

phenylsilsesquioxanes, synthesized by hydrolytic

condensation followed by base-catalyzed equili-

bration reaction of phenyltrichlorosilane or

phenyltrimethoxysilane.

High molecular weight polymethylsilsesquiox-

anes (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].

According to Eq. (4.6), controlled acid-cata-

lyzed hydrolytic polycondensation, characterized

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).

Fig. 3. 29Si NMR spectra of polyethoxysiloxanes (PEOS). The Qn

denotes the silicon atom substituted with n siloxy groups and 4 2 n

alkoxy, aryloxy, or hydroxy groups, which is often described as

Si(OSi)n(OR)42n (n ¼ 0–4).


ARTICLE IN PRESS

by the reaction method described in Section 4.3,

provides PMSQ and polyvinylsilsesquioxanes

(PVSQ), as summarized in Table 7. They are

expected to be broken ladder since the backbone

consists of T2 and T3 structures. They are stable

to condensation and have excellent spinnability.

No gelation is observed, even after several

months in concentrated solution. Free-standing

films are prepared by casting a 20 wt% acetone-

methanol (v/v ¼ 1) solution and heating at 80 8C

for 3 weeks (Fig. 5). Films show high transpar-

ency up to 98% (at 500 nm), high thermal

stability, with Td (5%) of 400 8C (PMSQ) and

600 8C (PVSQ), and appreciable flexibility, with

tensile strength up to 27 (PMSQ) and 17 MPa

(PVSQ) and elongation of 1.2 (PMSQ) and

0.8 mm (PVSQ). The tensile strength and

elongation of films are represented in Fig. 6

as a function of the molar ratio of H2O/MTS on

the hydrolysis of MTS. As the degree of

condensation of siloxane bond (DC) increases,

tensile strength increases and elongation decreases.

At a DC of 70%, films become less flexible, and

are rather rigid and brittle. This is also indicated

by the stress–strain curves of films: above DC of

70%, films are broken at the maximum tensile

strength about 15 MPa. The results clearly reveal

that the mechanical properties of polysilsesquiox-

anes are closely associated with the structure in

terms of DC and/or cross-linkage of siloxane

bond and molecular weight.

Polysiloxanes such as polysilicic acid esters,

partially silylated silicic acids, and polymethyl(vi-

nyl)silsesquioxanes have fascinating properties as

advanced silicon-based materials, suggesting the

possibility of a wider range of applications than for

found with traditional organopolysiloxanes.

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

hydrolysis of MTS.

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.

ð4:6Þ


ARTICLE IN PRESS

4.5. Base-catalyzed hydrolytic polycondensation of

RSi(OMe)3

Acid-catalyzed hydrolytic polycondensation of

R42nSi(OR0)n (n ¼ 3; 4) is a desirable way to obtain

polysiloxanes as polymer sols, because linear poly-

siloxanes are formed according to the reaction

mechanism discussed in the previous sections.

Generally, base-catalyzed condensations provide

insoluble gels. Almost no reports have been published

on the preparation of soluble polysiloxanes, except by

the hydrolysis of PhSiX3 (X ¼ Cl, OMe) followed by

equilibration with potassium hydroxide to form

polysiloxanes with molecular weights around a

thousand [25].

ð4:7Þ

In order to synthesize high molecular weight

polysiloxanes, 3-methacryloxypropyl(trimethoxy)si-

lane is hydrolyzed (Eq. (4.7)) with various base

catalysts such as ammonia, triethylamine, diazabicy-

clooctane (DABCO), and sodium hydroxide, using the

same apparatus as that for acid-catalyzed conden-

sations [26]. Table 8 shows the results of reactions with

the bases, as well as acid-catalyzed reactions. For

reactions in the presence of HCl, soluble polysiloxanes

with molecular weighs higher than 2300 were not

prepared, even if the molar ratio of HCl and water to the

substrate and the reaction temperature were increased.

Volatile amines provide low molecular weight poly-

siloxanes, but non-volatile amine DABCO yields a

polysiloxane with fairly high molecular weight of

6800. On the other hand, sodium hydroxide catalyzes

hydrolytic polycondensation effectively, depending on

the molar ratio of the base and H2O, to yield

polysiloxanes with high molecular weight above

35,000. In the higher molar ratio of base, methacryloxy

groups are hydrolyzed, while molecular weights

decrease as a white powder is formed.

Thus, the reaction conditions investigated in detail

reveal that high molecular weight polysiloxanes are

attainable and, interestingly, consist of almost all T3

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

an irregular ladder structure due to three-, four-, and

bridged four-membered rings.

Polysiloxanes are soluble in organic solvents,

except for hexane and methanol Hence, thin films

are formed on various organic and inorganic sub-

strates by dip coating with a 20 wt% acetone-

methanol (v/v ¼ 1) solution.

Recently, tetraalkylammonium hydroxides have

found use in base catalysis of hydrolytic polyconden-

sation of TEOS and RSi(OR0)3. It is well known that

tetraalkylammonium hydroxides serve as a template

to synthesize cube siloxane Q8(ONR4)8, but no work

have been reported on their role as a base catalysis on

hydrolytic polycondensation. Therefore, it is of

Table 7

Preparation of polymethylsilsesquioxanes and polyvinylsilsesquioxanes

Molar ratio H2O/TEOS Spinnability (cm) Mw £ 103 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

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 oligosilox-

anes are expensive and limited. In addition,

synthetic routes for these are also limited. Certainly,

hydrolytic condensation of sila-functional silanes

R42nSiXn ðn ¼ 3; 4Þ is versatile, but neither attrac-

tive nor preferable for selective synthesis in

appreciable yields. Tedious and complicated separ-

ation processes, resulting in low yields, often follow

it. Therefore, it would be desirable to develop

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)si-

lane using base catalyst. The Tn denotes the silicon atom substituted

with n siloxy groups and 3 2 n alkoxy, aryloxy, or hydroxy groups,

which is often described as RSi(OSi)n(OR0)32n (n ¼ 0–3).

Fig. 8. Apparatus for the vapor-phase hydrolysis. Four-necked flask

is attached with reflux condenser and flowmeter, that connects to a

flask. Trifunctional silane, RSiX3 (R ¼ Me, vinyl, Ph; X ¼ Cl,

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

reduced pressure, a mixed vapor of 1,4-dioxane and steam is

introduced into the four-necked flask to hydrolyze trifunctional

silane.


ARTICLE IN PRESS

described in detail. The method would provide a

convenient route to obtain sila-functional oligosilox-

anes, if the reaction can be carried out under

controlled conditions.

Thus, the hydrolysis of isocyanatosilanes R42n

Si(NCO)n (R ¼ Me, vinyl, Ph; n ¼ 3; 4) was inves-

tigated 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-

necked flask, the silane, vaporized on heating under

reduced pressure, undergoes hydrolysis/condensation

with a water–dioxane vapor introduced into a flask

through a flow meter, to form the oligosiloxane.

Since the oligosiloxane is not vaporized at the

temperature and pressure, only the starting silane is

vaporized and selectively hydrolyzed to give

oligosiloxanes. The reaction in the vapor phase

proceeds according to Eq. (5.1).

The method is applied to the synthesis of linear

oligosiloxanes 2 , 6 and 8 with an even number of

silicon atom, but not those with odd number of silicon

atoms, nor cyclic oligosiloxanes 30 , 60. Therefore,

they are synthesized by hydrolysis of linear oligosi-

loxanes in the liquid phase according to Schemes 5

and 6 [29]. Tables 9 and 10 summarize the results of

the synthesis of linear and cyclic oligosiloxanes.

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

shown by the ORTEP drawing in Fig. 9. The

propellane is assumed form via an intermediate on

the hydrolysis of 50 according to Scheme 7.

ð5:1Þ

5.1.2. Oxidative condensation of

dimethyldichlorosilane with dimethyl sulfoxide

Recently, a new method for the preparation of

cyclosiloxanes D3 and D4 by ‘anhydrous hydrolysis’

was reported [30]. It is the reaction of dichloro(di-

methyl)silane with dimethylsulfoxide, which acts as

oxygen donor or oxidizing agent for dichloro(di-

methyl)silane according to Scheme 8. This may be

preferable to provide D3.

5.1.3. Linear siloxanes with definite chain length by

ring opening reaction of Dn

Hydrolysis of difunctional silanes with chloro and

alkoxy group is not useful to prepare high molecular

weight polysiloxanes. Thus, acid or base catalyzed

ring-opening polymerization of cyclic dimethylsilox-

anes D3, D4, and D5 are the practical and industrial

methods to produce polydimethylsiloxanes.

Scheme 5.

Scheme 6.


ARTICLE IN PRESS

If a ring opening reaction of Dn without equili-

bration is conducted, linear oligosiloxanes with a

limited chain length may be derived. One-way to

obtain linear oligo- and polysiloxanes with definite

and various chain lengths is partial hydrolysis to form

silanol, followed by condensation with a suitable

chlorosilane. This is also used as a method to prepare

dendrimers [31,32].

5.2. IR and NMR spectra

It seems that almost no investigations have

addressed the IR or 29Si NMR spectra of oligosilox-

anes. The asymmetric stretching vibration nSi – O – Si of

linear and cyclic dimethylsiloxanes is reported: the

signals appear at 1018 ðn ¼ 3Þ; 1076 ðn ¼ 4Þ; 1081

ðn ¼ 5Þ; 1068 ðn ¼ 6Þ; 1060 ðn ¼ 7Þ; and 1056 cm21

ðn ¼ 8Þ for cyclic dimethylsiloxanes (Me2SiO)n, and

also at 1000 , 1100 cm21 for linear dimethylsilox-

anes (Me2SiO)n The absorption band is increased,

overlapped, and broaden as the increase of n more

than three. On the other hand, no work has been

reported on the IR spectra of sila-functional oligosi-

loxanes such as those in Tables 9 and 10. Similar

trends to behavior for polydimethylsiloxane are

observed, as shown in Figs. 10 and 11, but the

following aspects are revealed: in Fig. 10, a sharp

peak at 1041 cm21 for 30 shifts to 1100 cm21 for

40 , 60. No further shifts are observed while the

shoulder appears at 1040 cm21 in addition to the peak

broadening. In Fig. 11, on the other hand, the sharp

peak at 1099 cm21 for 2 is broaden for 3 , 6 and 8

and splits into two at the peak top. The band at

1099 cm21 shows no shift, but the others a low

wavenumber shift from 1079 cm21 (trisiloxane) to

1053 cm21 (octasiloxane).

The pattern of NMR signals for cyclic oligosilox-

anes depends on the structure and/or stereochemical

configuration, and often shows complicated splittings,

as expected from the molecular structures. On the

other hand, a similar pattern of signal splittings is

observed for the 1H, 13C, and 29Si NMR spectra of

linear oligosiloxanes 2 , 5: the signals due to the

proton, carbon, and silicon attached to the methylsilyl

groups are shifted to low and high field, respectively.

The signals at low field due to the terminal groups do

not shift regardless of length of siloxane bond, while

those at high field are split into several peaks, and

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

5 NH3 1.0 £ 1021 3.0 70 620 1.9 9.39 –

6 3.2 £ 1021 780 2.1 9.19 –

7 NEt3 1.0 £ 1021 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

Scale of operation: 3-methacryloxypropyl(trimethoxy)silane, 10.4 g (4.2 £ 1022 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.


ARTICLE IN PRESS

coalesce in a narrow region of several ppms

261.3 , 2 62.2 for oligosiloxanes 2 , 6 and 8.

From the results, it is expected that no further shifts

are appeared for higher homologues. A similar trend

is observed for the 1H and 13C NMR spectra.

5.3. Disiloxanols

Disiloxanols (disiloxane polyols) such as disilox-

anediols and even tetrols are essentially synthesized

by hydrolysis of the corresponding chlorosilanes. The

reaction has to be carried out under mild and carefully

controlled conditions, because the products easily

undergo condensation, especially in the presence of a

trace amount of acid and base, or on heating. A key

factor to isolate silanediol, and especially disiloxane

polyols, is the steric hindrance of substituents and

additional intramolecular hydrogen bonding, by

which the condensation is prevented from forming

silanols with an appreciable stability to self-conden-

sation. The synthesis and properties of various

silanols are reviewed by Lickiss [33].

Schemes 9 and 10 show 1,3-disiloxanediols

(a) , (c) [33] and (d) [34], together with 1,5-

trisiloxanediols ((a), n ¼ 2) and also disiloxanetetrols

(e) and (f). Some of the diols (a) in Scheme 9 are

commercially available. Brown reported 1,1,3,3-

diphenyldisiloxanetetrol (e) in Scheme 10 by hydroly-

sis of phenyltriacetoxysilane [35]. Bulky organic

groups: such as phenyl, t-butyl, hexyl, octyl, decyl

[36], and 1,1,2-trimethylpropyl, are essential to isolate

the tetrols [37]. The disiloxanetetrol with an aryl

(trimethylsilyl)amino group (f) by hydrolysis of

arylamino(trichloro)silane is noted [38], because in

general silylamino groups are easily substituted with a

hydroxyl group, but this is not true in this case,

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

30 79.4–76.5/0.80 0.371 (9H, s, CH3–Si):cis 22.07 (CH3–Si): trans 251.2: cis 1041 ðnSiOSiÞ 287 (Mþ-15)

0.398 (6H, s, CH3–Si):

trans

22.19 (CH3–Si): trans 251.1: trans 1274 ðdSiCH3Þ

0.425 (3H, s, CH3–Si):

trans

22.32 (CH3–Si): cis 250.8: trans 1457 ðdCH3Þ

123 (Si–NCO) 2285 ðnNCOÞ

40 133.0–134.2/3.0 0.36 (12H, m, CH3–Si) 22.20 (CH3–Si) 260.4 to 261.1 1030–1107 ðnSiOSiÞ 404 (Mþ-15)

123.0 (Si–NCO) 1270 ðdSiCH3Þ

1457 ðdCH3Þ

2290 ðnNCOÞ

50 134.7–136.8/0.45 0.233–0.501 (15H,

br, CH3–Si)

21.74 (CH3–Si) 262.4 to 262.0 1031–1106 ðnSiOSiÞ 490 (Mþ-15)

123 (Si–NCO) 261.1 to 261.0 1274 ðdSiCH3Þ

260.8 to 260.5 1457 ðdCH3Þ

2288 ðnNCOÞ

60 136.3–140.2/0.34 0.234–0.471 (18H, br,

CH3–Si)

22.04 (CH3–Si) 262.4 to 262.2 1040–1103 ðnSiOSiÞ 591 (Mþ-15)

123 (Si–NCO) 260.4 1275 ðdSiCH3

Þ

260.1 1456 ðdCH3Þ

2290 ðnNCOÞ

500c – 0.281 (6H, s, CH3–Si) 21.73 (CH3–Si) 258.0 to 258.5 1033–1114 ðnSiOSiÞ 491 (Mþ-15)

0.349 (3H, s, CH3–Si) 21.95 (CH3–Si) 1273 ðdSiCH3Þ

0.398 (3H, s, CH3–Si) 22.17 (CH3–Si) 1454 ðdCH3Þ

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.


ARTICLE IN PRESS

probably due to steric hindrance as well as the low

basicity of the amino group.

5.4. Cyclotetrasiloxane tetrols

Cyclotetrasiloxanetetrols are potential precursors

for the preparation of bead, cage, and cube siloxanes.

However, it is not easy to synthesize them selectively

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 con-

veniently synthesized by the reaction of sila-func-

tional disiloxanes or sila-functional linear

tetrasiloxanes.

Hydrolysis of 1,3-diisopropyl-1,1,3,3-tetrachloro-

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

tetrol and water molecules to form a silanol cluster.

The structure is identified by X-ray crystallographic

analysis. It can be a starting material for

the preparation of cage polysiloxanes by a conden-

sation reaction, with cyclohexylcarbodiimide

Scheme 7.

Fig. 9. ORTEP drawing of propellane 500.


ARTICLE IN PRESS

as a dehydrating agent.

ð5:2Þ

6. Ladder oligosilsesquioxane

Ladder polyphenylsilsesquioxane has excellent

properties, such as enhanced thermal stability, film

formation, and mechanical properties, in addition to

those of usual polysiloxanes of chemical, physical,

and electrical properties. If the structure is completely

ladder, it consist of the T3 structural unit. However, so

called ladder-T-phenylsilsesquioxane virtually con-

tains the T2 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

then subjected to base-catalyzed equilibration reac-

tion 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-con-

densed 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 and1H 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,

three, and five-ring system [43,44] have been

Scheme 8.

ð5:3Þ

Fig. 10. IR spectra of cyclic oligosiloxanes 30 , 60.


ARTICLE IN PRESS

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 commer-

cially available chloro- and isocyanatosilanes con-

veniently 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-

disiloxanes or tetraisocyanatocyclotetrasiloxane with

Scheme 10.

Fig. 11. IR spectra of linear oligosiloxanes 2 , 6 and 8.

Scheme 9.


ARTICLE IN PRESS

disiloxanediols, as is shown in Eqs. (6.5) and (6.6).

This method should be versatile, for the starting

materials are simply and selectively synthesized by

vapor phase hydrolysis, as it is described in Section 5,

and a number of ring systems with various pendant

groups would be obtained in addition to the preparation

of more larger ring systems. The products are obtained

by distillation in vacuo and the results of spectral

analysis indicate ladder oligosilsesquioxanes.

7. Cube siloxanes

Oligosilsesquioxanes consist of the (RSiO3/2)n

structural unit; the homologues with n ¼ 6; 8, 10,

and 12 have been isolated [47]. Of these, Q8X

( ¼ Q8X8, X denotes substituted group to Q unit)

and T8R ( ¼ T8R8, R denotes substituted group to T

unit), abbreviated ‘cubes’ hereafter, have attracted

considerable attentions from the stand point of

synthesis and applications: they have a nano-sized

three-dimensional structure consisted of almost inor-

ganic silica backbone with an angstrom level cavity,

high thermal stability, and reactive functional groups.

The first cube T8H was obtained in 1959 by Muller

et al. as a intermediate, in yield less than 1%, on the

synthesis of poly(hydridosilsesquioxane) [48]. Frye

et al. [49] modified the hydrolysis of HSiCl3 to

prepare T8H in 13% yield. Later, the cube was obtained

in 27% yield in 1991 by Agaskar [50]. The reaction is

represented in Eq. (7.1). On the other hand, hydrolysis

of TEOS to give water-soluble silicate ions, followed

by condensation, yields complicated condensed

species. Hoebbel [51] found another cube in 1971:

in the presence of quaternary ammonium ions, the

silicate ion Si8O20O82 is templated to give an

ammonium salt Q8(NR4)8 in almost quantitative

yields, according to Eq. (7.2).

To date, many cubes have been synthesized, as

listed in Table 11 [52]. These are classified into three

groups, (a) alkyl and aryl substituted cubes, (b) carbo-

functional cubes, (c) sila-functional cubes. A large

numbers of cubes belong to the groups (a) and (b)

while the residues are the group (c). In the group (a),

the cubes are prepared by the hydrolysis of trichloro-

or trialkoxysilanes (Eq. (7.3)). The cubes in the group

(b) are synthesized by the hydrosilylation of the

corresponding carbo-functional olefines with T8H (Eq.

(7.4)) prepared by the hydrolysis of trichlorosilane

HSiCl3 (Eq. (7.1)). The group (c) is the cubes with the

sila-functional groups of Cl and OMe together with H

and ONR4. The cube T8(OMe) is synthesized by the

reaction of methyl formate with T8Cl which is derived

by chlorination of T8H (Eq. (7.5)).

Thus, the synthetic route and the starting materials

are limited. Moreover, it is a serious problem that

regardless of the groups, cubes are obtained in low

yields after long time reactions. In particular, only a

few cubes with sila-functional groups are prepared,

ð6:5Þ

ð6:6Þ


ARTICLE IN PRESS

as mentioned above and also shown in Table 11 (see

also Table 2 in Section 2). Most of them are poorly

soluble in organic solvents and have no melting point,

but sublime or decompose at elevated temperature. As

shown in Table 12, there are few reports on the

physical properties of cubes, and the reported data are

sometimes inconsistent among different authors.

Cubes of tetraammonium salt are prepared as hydrates

with tens of molecules of water, and are soluble in

water, methanol, and ethanol. The synthesis must be

improved in order to apply the cubes as a precursor for

the preparation of silicon-based new materials.

Recently, new synthetic routes to cages and poly-

silsesquioxanes have been investigated using tetra-

alkylammonium salt as a catalyst: hydrolytic

condensation of alkyl(triethoxy)silane in the presence

of tetrabutylammonium fluoride provides octasilses-

quioxane cage and higher homologues with short

reaction times and in very high yield (90%) [53]. The

redistribution reaction of oligohydridosilsesquioxane

using tetraalkylammonium hydroxide yields polyhy-

dridosilsesquioxanes. Hydrolytic condensation of

TEOS using a stoichiometric amount of water to

TEOS using tetraalkylammonium hydroxide also

Scheme 11.


ARTICLE IN PRESS

provides polyethoxysiloxanes [54].

ð7:1Þ

ð7:2Þ

ð7:3Þ

ð7:4Þ

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

2 128.1–129.9/13 0.50 (6H, s, CH3–Si) 20.50 (C H3–Si) 259.4 1099 ðnSiOSiÞ 255 (Mþ-15)

123 (NCO) 1280 ðdSiCH3Þ

1457 ðdCH3Þ

2310 ðnNCOÞ

3 111.3–112.1/1.0 0.389 (6H, s, CH3–Si) 21.90 (CH3–Si) 261.3 1079–1115 ðnSiOSiÞ 457 (Mþ-15)

0.474 (6H, s, CH3–Si) 20.60 (CH3–Si) 259.8 1274 ðdSiCH3Þ

123 (Si–NCO) 1457 ðdCH3Þ

2275 ðnNCOÞ



20.52 (CH3–Si) 1455 ðdCH3Þ


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 (Mþ-15)


20.52 (CH3–Si) 1455 ðdCH3Þ




0.484 (6H, s, CH3–Si) 20.49 (CH3–Si) 1454 ðdCH3Þ




123 (Si–NCO) 1455 ðdCH3Þ

2285 ðnNCOÞ

Mp: 73.6–75.5 8C.a Solv. CDCl3. Ref.: TMS.b CCl4 soln. method.


ARTICLE IN PRESS

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 –OSiMe2H 83 [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

ð7:5Þ

In another class of cubes, one pendant group differs

from the others. These are prepared by the reaction of

various trichlorosilanes with the silane triols

T7R5(OH)3 (Eq. (7.6)), where most of the organic

substituents are cyclopentyl or cyclohexyl groups. An

appreciable numbers of derivatives have been syn-

thesized as summarized in Table 13. The silane triol

[55] and diol [56] are obtained as intermediates during

the preparation of cubes, but only in fairly low yields,

around 30%. Recently, It was a preparation of the triol

as the sodium silanolate was reported, with almost

quantitative yields by the hydrolysis or the trichlor-

osilane with a stoichiometric amount of water and

sodium hydroxide (Eq. (7.7)) [56].

ð7:6Þ

ð7:7Þ

8. Application of oligo- and polysiloxanes

The sila-functional oligosiloxanes described so far

are potential candidates for surface modifier, coupling

agents, additives, and building blocks for ladder and

cube oligosiloxanes, polysiloxanes with well-con-

trolled structure and silicon-based materials. It should

be noted that they are a potential precursor as well for

the synthesis of silicon-based organic–inorganic

hybrids, closely related to the polysiloxanes discussed

above as additives and binders in combination with

organic polymers and inorganic materials such as

glasses, oxides, and ceramics.

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

problems to be improved such as the stability of sols

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]


ARTICLE IN PRESS

to condensation regardless of the optical properties of

silica fibers. On the other hand, esterification of silicic

acid provides spinnable polysiloxanes [13,14], for

which the stability and/or self-condensation are

dependent on the degree of esterification (DE) and

the alkyl group, so that the esters with appropriate

DEs can be used as a good precursor for the

preparation of silica fibers. Thus, esters with DE less

than around 60% undergo further condensation which,

on precipitation with a solvent such as hexane, form

insoluble powders which are the precursors for the

preparation of submicrometer-sized silica particles

[108]. Silicic acid itself is also used as a precursor for

the preparation of bulk silica glasses, which are

prepared by aging a concentrated silicic acid in

organic solvents, followed by gradual evaporation of

the solvent at room temperatures [15,109].

Polysilsesquioxanes can be a precursor for the

preparation of silicon oxycarbide SiOC or SiC

[110]. A mixture of the two components of

(RSiO3/2)n (R ¼ Me, Pr, Ph) provides silicon

oxycarbide fibers on heating the precursor fibers

in N2 or argon [111]. Black glasses, silicon

oxycarbide, are formed by heat-treatment of

polysilsesquioxane gels prepared by hydrolytic

polycondensation. They are also obtained by

heating the polysiloxanes prepared by hydrolytic

polycondensation of TEOS/a,v-polydimethylsiloxa-

nediol [112] or TEOS/Me2Si(OEt)2 [113].

As described in Section 6, controlled hydrolytic

polycondensation of methyl- and vinyltrimethoxy-

silane yields polymethyl- and vinylsilsesquioxanes,

providing access to bulk gels and flexible free

standing films. Black glasses are formed on

pyrolysis of the precursor films at 1400 8C under

N2 atmosphere [114].

Pyrolysis of polymethylsilsesquioxane films

forms broken pieces of black glass. However,

Table 12

Spectral and analytical data of cube siloxanes T8R8

R in T8R8

–H –Cl –OMe –CHyCH2 –ONMe4 –OSiMe2H

NMR [d/ppm] 1H 4.20 – 3.36 – 4.80 (H2O),

3.19 (CH3)4 N

4.7 (Si H),

0.2 (SiCH3)13C – – 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

IR (cm21) (nSi – O – Si

is shown in bold)

2290(s) 1142(s) 1155(s) 3064(vw) 3420 2961(w)

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

found (calcd)

H: 2.0 (1.9) Cl: 40.75

(40.50)

C: 14.37 (14.45);

H: 3.64 (3.64)

C: 29.19 (30.35);

H: 3.88 (3.83)

– C: 18.74 (18.88);

H: 5.04 (5.54)


ARTICLE IN PRESS

polyvinylsilsesquioxane films provides ceramic films

without cracks, though they shrink about 15% both in

length and width. In addition, a little weight loss

(about 10%) is observed for both films. The ceramics

are identified to be silicon oxycarbide with the

composition of SiOxCy ðx ¼ 0:71 , 2:0; y ¼ 1:40 ,1:60Þ: The black glass ceramic films have a free

carbon content up to 90%, in contrast to the free

carbon content of only several wt% into silica glasses

by the usual melt method. This may result from the

fact that the structure of the precursor films is well-

controlled, and subsequently converted into ceramics

via an organic–inorganic hybrid in which the vinyl

groups are incorporated into silica matrices through

addition polymerization.

8.2. High performance coatings

Among the various shapes of materials, thin films

have found wide applications, especially as functional

materials for protectors, optics, electronics and

membranes for separation or gas permeation. Thus,

high performance coatings having excellent proper-

ties are possible with polysiloxanes if they show film

formation. The interactions between polysiloxane

molecules and also between the molecules and

substrate in the coating are of central importance.

These interactions depend on the molecular structure

and molecular weight: the silanol groups as a pendant

afford an intermolecular force and an interaction with

substrates, and in some cases, forms a chemical

bonding. As it is well known, polysiloxane sols

prepared by a sol–gel process with TEOS provide

good precursors for coatings, although they produce

silica thin films. Correspondingly, polysilicic acid

esters (polyalkoxysiloxanes) [11–14] and polysilses-

quioxanes [22–24] can be the potential candidates

because they forms fibers and flexible free-standing

films.

In general, the chemical coating process is

conveniently performed by dip or spin coating

methods using rather simple apparatuses (Fig. 12)

Here, the results on coatings with PMSQ and PVSQ

will be described [115]. The coating solutions are

20 wt% PMSQ or PVSQ acetone–methanol (v/v)

solutions, and organic or inorganic substrates are

used: high density polyethylene (HDPE), polypro-

pylene (PP), polycarbonate (PC), polyethylene

terephthalate (PET), 6-nylon, Aluminum, stainless

steel (SUS304), soda-lime glass, quartz glass, and

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


ARTICLE IN PRESS

silicon wafers. Films are prepared by dipping the

substrates into the solution and pulling up followed

by heating at 80 8C for 24 h and then 100 8C for

several hours.

Dip-coating of 1–10 times provides films of

thickness 0.25 , 0.85 mm (PMSQ) and

0.40 , 0.95 mm (PVSQ). The film thickness depends

on the molecular weight of the polymer. Higher the

molecular weight lead to thicker the films, and PVSQ

provides thicker films than PMSQ.

The adhesive strength of PMSQ was measured

using the Japanese Industrial Standard K5400

protocol, with results given in Table 14.

Obviously, the properties depend on heating time

and the PMSQ molecular weight. Polysilsesquiox-

anes adhere more strongly to the inorganic

substrates than to the organics. Different degrees

of adhesion are observed for the organic substrates:

adhesive strength increases in the order:

PC , PET , 6-Nylon, but the coatings do not

adhere to HDPE or PP. Strong adherence to the

inorganic substrates may be due to the formation

of metallasiloxane bonds, in addition to physical

interaction with the surfaces, while the different

adhesion among the organic substrates is associated

with the solubility parameter ðdÞ of coatings and

substrates. Table 15 gives the solubility parameter

of PMSQ and some of the substrates. A high

adhesive strength is observed when the solubility

parameters of polymer and substrate are close in

value. Adhesive strength by the crosscut tape test

based on JIS K5400 is a qualitative test. The

adhesive strength shown in Fig. 13 was evaluated

quantitatively by the stud-pull method, commonly

known as the Sevastian method. Values in the

ranges 50 , 100 and 80 , 130 kg/cm2 for PMSQ

and PVSQ, respectively, correspond to values in

Table 14. The pencil hardness of PMSQ, which

was measured using the Japanese Industrial

Standard K5400 protocol, also depends on the

heating time and molecular weight of the PMSQ,

as is shown in Table 16. Similar, but lower, results

were observed for the adherence and hardness of

PVSQ. (Fig. 14)

Thin films are highly transparent, with transmit-

tance of more than 98% at 500 nm. As shown in

Fig. 13, both PMSQ and PVSQ are thermally stable up

to 450 , 500 8C, as evaluated by measuring the

adherence at various temperatures. The adhesive

strength increases with the increasing temperature

up to 800 8C, indicating that a film still remains. The

unexpected thermal stability may be due to the fact

that polysilsesquioxanes are converted into ceramics

films via an organic–inorganic hybrid.

Table 13

Mono-substituted cubes

No. Substituent (R0) References

66 –H [99]

67 –Cl [92]

68 –OH [92]

69 –CHyCH2 [100–104]

70 –CH2CHyCH2 [105]

71 –CH2CH(O)CH2 [105]

72 –(CH2)6CHyCH2 [100–104]

73 –CHyCH(CH2)8CHyCH2 [100–104]

74 –(CH2)2C6H4CHyCH2 [100–104]

75 –(CH2)2Si(CH3)Cl2 [100–104]

76 –(CH2)2SiC6H4Cl2 [100–104]

77 –(CH2)3OCOCH–CH2 [106]

78 –(CH2)3OCOC(CH3)yCH2 [107]

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) 0 3 6 12 0 3 6 12 0 3 6 12

PC 2 6 6 6 4 8 8 8 6 8 8 8

PET 4 6 8 8 8 10 10 10 10 10 10 10

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

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 dielec-

trics, organic compounds can not be applied to the

present device preparation process at high tempera-

tures around 400 , 450 8C. A candidate to over-

come this limitation is offered by polysiloxanes,

with their excellent chemical, physical, and elec-

trical properties, but only if they are capable of

forming thin films. Suitable polysiloxanes could

include sols by sol– gel process with TEOS,

copolymers of TEOS and Me42nSi(OMe)n (n ¼ 2

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)

process to provide films with the k values in the

range 2.9 , 3.0. On the other hand, polysilsesquiox-

anes are expected to be a potential candidate as

mentioned in Section 8.2, for they provide high

performance coatings. At present, no investigations

have been made to evaluate dielectric constant k of

polysilsesquioxanes for correlation with the molecu-

lar structure [115,116].

Table 17 shows the results on the measurement

of dielectric constant of polymethylsilsesquioxanes,

consisting of T3 units of 47 , 58%, with various

molecular weights. The dielectric constants appear

to be fairly low, around 2.5, compared with those

of practical films, which are in the range

2.9 , 3.0.

Fig. 13. The variation of tensile strength of polymethyl silsequiox-

ane (PMSQ) and polyvinylsilsesquioxane (PVSQ) coating films by

stud-pull method, commonly known as the Sevastian method, as a

function of heating temperature. Coating films were prepared on

silicon wafer by dip coating and then heated at 808C for 24 h under

nitrogen atmosphere followed by further heating at various

temperatures for 1 h.

Table 15

Solubility parameters ðdÞ

Substrate d (calcd)a Note

PP 8.01 Non-polar/crystalline

HDPE 8.56

PC 10.5 Polar/non-crystalline

PET 11.7 Polar/crystalline

6-Nylon 11.9–12.5

PMS ðMw ¼ 5500Þ 12.01 13.14b

(Mw ¼ 55,000) 12.17 13.17b

PVS ðMw ¼ 2600Þ 12.09 13.15b

(Mw ¼ 21,000) 12.71 13.28b

a d (cal/cm3)1/2 ¼ ðDE=DVÞ1=2 ¼ ðPDei=

PDviÞ

1=2; Dei and Dvi :

the additive atomic and group contribution for the energy of

vaporization and volume, respectively. Fedors RF. Polym Engng

Sci 1974;14:147.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) 0 3 6 12 0 3 6 12 0 3 6 12

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

Acknowledgements

The paper will be presented to a memorial issue for

the continuous and great contributions of Professor

Otto Vogl to the progress and development in polymer

science. The authors are very proud of the present

publication of our work on the siloxane compounds.

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