AU2003242399B2

AU2003242399B2 - Hindered Siloxanes

Info

Publication number: AU2003242399B2
Application number: AU2003242399A
Authority: AU
Inventors: Graham Roy Atkins; Dax Kukulj
Original assignee: RPO Pty Ltd
Current assignee: RPO Pty Ltd
Priority date: 2002-08-29
Filing date: 2003-08-29
Publication date: 2008-04-24
Anticipated expiration: 2023-08-29
Also published as: AU2003242399A1

Description

AUSTRALIA

PATENTS ACT 1990 PROVISIONAL SPECIFICATION FOR THE INVENTION ENTITLED- "HINDERED SILOXANES" The invention is described in the following statement:- Hindered siloxanes AU complete I1st draft 22-8-03/DK TECHNICAL FIELD The invention relates to improvements in the performance of low loss optical materials resulting from chemical modification thereto and to improved polymeric siloxanes.

BACKGROUND OF THE INVENTION Organically modified siloxanes (alternating Si-O backboned polymers) have a broad range of applications. In particular, they have good light transmission properties that make them ideal targets for use in optical materials such as optical waveguides and devices. They also generally possess good adhesion as well as mechanical and chemical stability over an extended temperature range.

Siloxane polymers can be divided into two broad classes polysiloxanes prepared by the sol-gel route and (ii) standard siloxane polymers of the polydiorganosiloxane type.

Polysiloxanes prepared by the sol-gel route are sometimes referred to as ORMOSILs (ORganically MOdified SILicates) or inorganic-organic hybrid polymers. These are formed from alkoxysilanes which are normally hydrolysed in the presence of base or acid to yield the corresponding silanol which then undergoes condensation to give a highly cross-linked polysiloxane.

Problematically, these polymers are difficult to process due to their high viscosity. While the condensation processes can be slowed down somewhat to assist in processing, there is always a tendency for such materials to condense so problems due to high viscosity are inevitable.

A further consequence of this unavoidable condensation is the formation of microgels.

These microgels make filtration difficult, particularly the passage through 0.2 gm filters, a step which is essential in preparing optical materials to avoid scattering losses.

WO 01/04186 discloses a method for the condensation of diaryl silanediols with trialkoxy silanes. This method produces a polycondensate with the concomitant elimination of alcohol, according to the following scheme: n Ar 2 Si(OH) 2 n RSi(OR') 3 Polycondensate 2n R'OH This synthetic route avoids the presence of large numbers of OH groups which have a high near IR absorption (3500cm 1 that impacts negatively upon optical transparency at 1550nm.

Uncondensed Si-OH groups can also continue a slow reaction over the service life of the polymeric material and lead to cracking and loss of adhesion.

It is desirable to cross-link polymer chains to provide greater chemical stability for the polymer matrix and more importantly to control and modify the physical properties of the polymer.

The most important of these is the ability to cross-link to control and modify rheology, which in practical terms represents the ability to cure the material from a relatively low viscosity workable Hindered siloxanes AU complete 1st draft 22-8-03/DK polymer to a polymer matrix with sufficient mechanical rigidity to allow use in applications such as optical devices.

A key aspect of the reaction in WO 01/04186 is that the silanediol is unable to selfcondense because of steric hindrance from the aromatic groups and therefore the reactive OH groups on the diaryl silanediol can only react with the RSi(OR') 3 compound.

The avoidance of self condensation is desirable because under the circumstances referred to above, water is evolved. For example, if a dimethyl silanediol rather than a diaryl silanediol were used, the following reaction would take place:

CH

3 CH 3

CH

3 I I I 2 HO-Si-OH HO-Si-O-Si-OH HO2 I I I

CH

3

CH

3

CH

3 It is known to those skilled in the art that the presence of water (either as starting material, reagent or product) should be avoided in materials for optical devices because water is difficult to remove and is a source of absorption in the infrared and near infrared region, which significantly impedes the optical qualities of the material.

The optical absorption properties of aromatic containing polycondensates themselves are, however, far from ideal. The aromatic groups present have significant and characteristic optical absorption patterns which, in many cases, are undesirable in optical materials.

Further, the refractive index of polymeric material containing aromatic groups is often difficult to match to the refractive index of commonly used silica-based optical fibres, the former being too high. A poor refractive index match between connected optical materials results in reflection, which translates into optical loss.

It is an object of the present invention to overcome or ameliorate at least one of the abovementioned disadvantages of the prior art, or to at least provide a useful alternative.

SUMMARY OF THE INVENTION According to a first aspect, the invention provides a polycondensate prepared from the reaction of a silanediol of formula and or a derived precondensate thereof

B

HO- Si-OH

B

(I)

with one or more organically modified silanes of formula (IIa), (IIb) or mixtures thereof Hindered siloxanes AU complete 1st draft 22-8-03/DK

OR"

I

Si- OR"

OR"

(IIa)

OR"

Si-R'

OR"

(IIb) wherein B is a sterically bulky non-aromatic group, and R' and R" are independently an alkyl, aralkyl or aryl group with up to 20 carbon atoms.

Preferably, B is an aliphatic, cycloaliphatic or branched aliphatic group. In some alternative embodiments, B bears at least one fluorine atom.

In a particularly preferred embodiment, B is one or more oftert-butyl, isopropyl, cyclopentyl, cyclohexyl, norbornyl or adamantyl, as illustrated below.

CH3

CH

3 CH3

CH

3 CH3

CH

3 Other groups, such as but-2-yl, pent-2-yl, 2-methylpent-2-yl and the like may also provide significant steric bulk.

Those skilled in the art will appreciate that these substituents are given by way of nonlimiting example only. A skilled person will appreciate that there are a vast number of such groups and that the functional definition employed here, namely that B is an aliphatic group of sufficient steric bulk to avoid or limit self condensation to no more than trace levels, is adequate to identify the types of group which may be suitable.

In the present invention, R' or B may be substituted with cross-linkable groups, such as C=C double bonds or epoxy groups.

Alternatively, the presence of small quantities of cross-linkable agents, such as a di-styryl silanediol or styrylphenyl silanediol, may be added to allow cross-linking.

Hindered siloxanes AU complete 1st draft 22-8-03/DK Quantities of other condensable units may also be added, eg units which have aromatic rings, such as diphenyl silanediol, and which function to alter the bulk properties of the polycondensate, for example, by raising the refractive index.

According to a second aspect, the invention provides a polycondensate prepared from the reaction of a mixture ofN mol% of a silanediol of formula and/or a derived precondensate thereof

B

HO-Si-OH

B

(I)

and (100-N) mol% of a diaryl silanediol of formula (III) and/or a derived precondensate thereof Ar HO- Si-OH Ar

(III)

with one or more organically modified silanes of formula (IIa), (IIb) or mixtures thereof OR" OR" I I Si- OR" Si- R' OR" OR" (Ia) (IIb) wherein B is independently a sterically bulky non-aromatic group, Ar is independently an aromatic group, R' and R" are independently an alkyl, aralkyl or aryl group with up to 20 carbon atoms.

The aromatic group, Ar, may be any aryl or heteroaryl group for instance phenyl or naphthyl, and may be substituted or unsubstituted.

Preferably, N and M are selected to provide a predetermined property in the polycondensate, such as refractive index, absorption in the L, S or C bands, mechanical hardness or the like.

Any balance of N and M may be selected, for example, 50 mol% M with 50 mol% N, mol% M with 5 mol% N, 90 mol% M with 10 mol% N, 75 mol% M with 25 mol% N, 95 mol% N with 5 mol% M, 90 mol% N with 10 mol% M, 75 mol% N with 25 mol% M or the like.

It is also possible that some or all of the compound of formula can be replaced by compounds which have one aromatic group and one B group, ie as shown in formula (IV) Hindered siloxanes AU complete 1st draft 22-8-03/DK

B

I

HO-Si-OH

I

Ar

(IV)

According to a third aspect, the invention provides a silanediol with reduced silicon reactivity according to formula (I)

B

HO-Si-OH

B

(I)

wherein B is a sterically bulky non-aromatic group.

Preferably, B is selected to produce a compound which is not self condensable. More preferably, B is an aliphatic, cycloaliphatic or branched aliphatic group, and most preferably B is as described above.

According to a fourth aspect, the invention provides a method of preparing a polycondensate including the step of condensing one or more silanediols of formula and/or derived precondensates thereof

B

HO- Si- OH

B

(I)

with one or more organically modified silanes of formula (IIa), (IIb) or mixtures thereof OR" OR" I I Si- OR" Si- R' OR" OR" (IIa) (IIb) wherein B is a sterically bulky group, R' and R" is alkyl, aralkyl or aryl group with up to 20 carbon atoms.

According to a fifth aspect, the invention provides a polycondensate having a general structure Hindered siloxanes AU complete 1st draft 22-8-03/DK B OR" B R' I I or I I or B R' B R m m where m is at least 1, more preferably at least 10, or may be 102 to 106 or higher.

The use of indicates that either the material is in oligomeric form, in which case will represent HO- or or may be end terminated or reacted with a trace impurity, or may even indicate that the polycondensate is cyclic. Those skilled in the art will appreciate that it is the repeating nature of the polymer which provides the functional characteristics of the polymer material, rather than the end groups, the importance of which becomes vanishingly small in larger molecules.

The polycondensate of the present invention may also contain traces of cross-linkable moieties, such as epoxides or C=C double bonds, for example, in the form of styryl groups, which can assist in curing. These may be for example, present in place of one or both B groups, or in the place of They may also be present on R" although this would be less desirable given that there is a statistical elimination of two R" groups during condensation.

The polycondensates of the present invention have a lower optical absorption in the "L band" of the communications window around 1550nm than polycondensates produced according to WO/0104186. There are three bands in this window: the "S band" (1465 to 1500nm); the "C band" (1525 to 1565nm); and the "L band" (1568 to 1610nm).

Without wishing to be bound by theory, it is believed that the new polycondensates of the present invention have a larger absorption in the S and C bands because of the aliphatic C-H stretch/bend overtone at 1400nm, but a lower absorption in the L band because the aromatic C-H overtone at 1680nm is replaced by the aliphatic C-H overtone at 1740nm.

Polymers produced according to WO/0104186 in any case exhibit some aliphatic C-H stretch/bend, because of the R group and the remnant OR' group from the R'Si(OR") 3 silane. The effects of the C-H bond are generally regarded as inevitable given the flexibility afforded by carbon based molecules and the costs and efforts involved in preparing material which avoids this structure.

An example of a polycondensate based on aromatic blocking groups, such as when diphenylsilanediol is used, is described in WO/0104186 and has a refractive index of 1.532. To minimise reflection losses at interfaces with silicate optical fibres, it is desirable that the refractive index be in the region 1.45 to 1.46 1.55 tm, as can be achieved with aliphatic non-aromatic) radicals.

Hindered siloxanes AU complete 1st draft 22-8-03/DK In those cases where the refractive index of the modified siloxane polymer is "over reduced", by using B groups rather than Ar groups, ie when the refractive index falls below that of the silica based fibres to which it may be attached, the refractive index can easily be increased as required by including a small amount of aromatic functionality, for example, by adding a phenyl trialkoxysilane as one of the R'Si(OR") 3 or R' 2 Si(OR") 2 compounds, trace amounts or diarylsilanediols, or some compound of formula (IV) or an alkoxide of a refractive indexincreasing metal, e.g. titanium or zirconium.

A useful starting compound in the present invention from the point of view of availability is dicyclopentyldimethoxysilane, available from Gelest, which may be converted relatively easily to dicyclopentylsilanediol by hydrolysis in acidified water.

9 9

H

3 CO-Si-OCH 3 HO-Si-OH 6 6 It is preferred if the polycondensates according the present invention are capable of being photo-structured in layers up to 150gm in thickness.

Thus, the present invention provides cured condensates and a method of preparing such cured polycondensates including the step of treating one or more of the polycondensates of the present invention, where curable, such as with the inclusion of cross-linkable groups, with a curing agent.

In highly preferred embodiments the curing agent is light. A photoinitiator may be added.

Preferably, the light is UV light and the photoinitiator is selected from the group consisting of: 1hydroxycyclohexylphenyl ketone, benzophenone, 2-chlorothioxanthone, 2-methylthioxanthone, 2iso-propylthioxanthone, benzoin, 4,4'-dimethoxybenzoin and mixtures thereof.

In an alternative preferred embodiment, the light is visible light and the photoinitiator may be, for example, camphorquinone.

In further alternative embodiments, other initiators may be added. These may be for example dibenzoyl peroxide, t-butyl perbenzoate and azobisisobutyronitrile.

Furthermore the resin can also be thermally cured using no initiator whatsoever. The curing temperature is between 80 250 0 C and more preferably between 170 210°C In the present invention, up to 90 mol% of compound (II) can be replaced by one or more co-condensable compounds of boron, aluminium, silicon, germanium, titanium and zirconium.

Hindered siloxanes AU complete 1st draft 22-8-03/DK In the present invention, one or more of the groups may bear cross-linking functionalities, most commonly a double bond, such as that in a styrene or acrylate (where they are more reactive by conjugation), or epoxides.

The cross-linking group may be attached to the aliphatic group B or R' by any intervening moiety.

Substitution of a hydrogen on any of the components with fluorine may take place in order to enhance the optical properties of the polycondensate and subsequently cured matrix.

Other reactive species, such as -OH, -SH and -NH 2 may also be present on one or more of the substituents.

The polycondensates of the present invention are described herein with reference to idealised structural representations, ie they are shown as alternating units. Those skilled in the art will appreciate that, in reality, the polymers themselves are statistical polymers and as such, will be unlikely to have only repeating units. It is not necessary that the monomer precursors are present in a 1:1 ratio although this is preferred.

For production of the polymers of the present invention, at least a portion (up to 90%) of compounds of the general structure R'Si(OR") 3 or R' 2 Si(OR") 2 can be replaced by one or more cocondensable compounds of boron or aluminium of general formula M(OR") 3 These substitutions may have the advantage of increasing chemical stability and mechanical hardness.

The groups R" are identical or different, M signifies boron or aluminium and R" represents an alkyl group with 1 to 4 carbon atoms. In the general formula M(OR") 3 all three alkoxy groups can condense with compounds of general formula so that only 2/3 of the molar quantity is required. The replacement compounds can be quite highly branched before crosslinking. Examples of compounds of general formula M(OR")3 are Al(OCH 3 3 Al(OC 2

H

5 3 Al(On-C 3

H

7 3 Al(O-i-C 3

H

7 3 Al(O-n-C 4

H

9 3 Al(O-i-C 4

H

9 3 Al(O-s-C 4

H

9 3 B(O-n-C 4

H

9 3 B(O-t-

C

4

H

9 3 B(O-n-C 3

H

7 3 B(O-i-C 3

H

7 3

B(OCH

3 3 and B(OC 2 Hs) 3 Alternatively, at least a portion (up to 90%) of compounds of general structure R'Si(OR") 3 or R' 2 Si(OR") 2 can be replaced by one or more co-condensable compounds of silicon, germanium, titanium or zirconium of general formula M'(OR") 4 The groups R" are identical or different, M' signifies silicon, germanium, titanium or zirconium and R" represents an alkyl group with 1 to 4 carbon atoms. In the general formula

M'(OR")

4 all four alkoxy groups can condense with compounds of general formula so only one half the molar quantity is required. Examples of compounds of general formula M'(OR") 4 include Si(OCH 3 4 Si(OC 2

H

5 4 Si(O-n-C 3

H

7 4 Si(O-i-C 3

H

7 4 Si(O-n-C 4

H

9 4 Si(O-i-C 4

H

9 4 Si(Os-C 4

H

9 4 Ge(OCH 3 4 Ge(OC 2 Hs) 4 Ge(O-n-C 3

H

7 4 Ge(O-i-C 3

H

7 4 Ge(O-n-C 4

H

9 4 Ge(O-i-C 4

H

9 4 Ge(O-s-C 4

H

9 4 Ti(OCH 3 4 Ti(OC 2 Hs) 4 Ti(O-n-C 3

H

7 4 Ti(O-i-C 3

H

7 4 Ti(O-n-C 4

H

9 4 Ti(O-i-

C

4

H

9 4 Ti(O-s-C 4

H

9 4 Zr(OCH 3 4 Zr(OC 2

H

5 4 Zr(O-n-C 3

H

7 4 Zr(O-i-C 3

H

7 4 Zr(O-n-C 4

H

9 4 Zr(O-i-C 4

H

9 4 and Zr(O-s-C 4

H

9 4 Hindered siloxanes AU complete 1st draft 22-8-03/DK The present invention allows for the substitution of these groups into the polycondensate without the requirement that they also provide cross-linking functionality, because this is provided via the functionalities pendant on the aliphatic group B or R'.

By utilising compounds of general formula M(OR") 3 or M'(OR") 4 the refractive index and optical attenuation of the resultant polycondensate can be tuned to a specific application. For example at certain wavelengths, alkyl-substituted components cause a reduction in refractive index while simultaneously increasing the attenuation, while aryl-substituted components cause an increase in refractive index without significantly increasing the attenuation of the inventive material except in the L band. Fluorination, by contrast, decreases both the refractive index and the attenuation of the inventive polycondensates.

Other resins, oligomers or monomers or particulate matter or other functional material may be added to the reaction mixture to modify the physical (refractive index), mechanical (hardness, thermal expansion profile) or chemical (introduction of reactive moieties) properties of the resulting polycondensate.

To initiate or accelerate the condensation, Lewis or Bronstead bases can be added. Some examples are amines, e.g. N-methyl imidazole, benzyldimethylamine, triethylamine, ammonium fluoride or one or more alkaline earth hydroxides. The alkaline earth hydroxide barium hydroxide is particularly preferred. Insoluble bases are recommended because they have the advantage that they can be readily removed from the mixture by filtration after condensation. Aluminium or zirconium alkoxides can be used in place of the abovementioned bases for the condensation.

The polycondensates of the present invention have good storage stability, ie they do not gel or cross-link when maintained in the appropriate conditions (ie away from polymerisation sources).

The polycondensates of the present invention may be made UV curable and transparent in the NIR, especially at the wavelengths of 1310 nm and 1550 nm which are critical for optical applications. Curing, i.e. cross-linking proceeds with little associated shrinkage, meaning cracking in the bulk cured material can be avoided (cracking causes discontinuities in the material, making it unsuitable for optical data transmission).

The polycondensates of the present invention are photo-structurable in layers of thickness up to 150 tm without loss of quality, making them suitable for application as photoresists, negative resists, dielectrics, light guides, transparent materials, planar waveguides or as photo-structurable materials.

Before curing and further processing, a solvent can be added to the polycondensate if desired and, if necessary, a suitable initiator can be added. In the curing processes, the C=C double bonds or the epoxy groups are linked together, many from different polycondensate chains, and the organic polymer matrix is constructed. Because of the relatively high molecular weight of the inventive polycondensates, curing proceeds with only minimal shrinkage.

Hindered siloxanes AU complete 1st draft 22-8-03/DK -11- It is also possible to add further polymerisable components before curing, for example, acrylates or methacrylates, or styrene compounds (to space polymer chains) where the polymerisation proceeds across the C=C double bonds, or compounds containing ring systems that are polymerisable by cationic ring opening.

Photoinitiators or thermal initiators may be added to increase the rate of curing.

Commercially available photoinitiators include 1 -hydroxycyclohexylphenyl ketone, benzophenone, 2-chlorothioxanthone, 2-methylthioxanthone, 2-isopropylthioxanthone, benzoin, 4,4'dimethoxybenzoin etc. For curing with visible light, the initiator may be for example camphorquinone.

For thermal initiators, organic peroxides in the form of peroxides dibenzoyl peroxide), peroxydicarbonates, peresters (t-butyl perbenzoate), perketals, hydroperoxides may also be used.

AIBN (azobisisobutyronitrile) may also be used.

Radiation cure, for example by gamma rays or electron beam, is also possible.

EXAMPLES

Example 1.

a) Synthesis of dicyclopentylsilanediol g of dicyclopentyldimethoxysilane (Gelest) was mixed with 75 mL of iso-propanol and 25 mL of 1 mol/L acetic acid forming a white mixture. The mixture was stirred until a clear solution was obtained (approximately 30 minutes) and then left to crystallise for 18 h. The crystals of dicyclopentylsilanediol were isolated by filtration and dried in a vacuum oven at 45 °C for 24 h.

The filtrate was placed in a freezer at -18 0 C overnight to recover another crop of crystals. These crystals were also isolated by filtration and dried in a vacuum oven at 45 °C for 24 h. The yield of dicyclopentylsilanediol was 35 g.

b) Synthesis of polymer based on dicyclopentylsilanediol 0.1 mol of dicyclopentylsilanediol and 0.1 mol of methacryloxypropyltrimethoxysilane (Gelest) were placed in a flask and heated to 80 OC. After 30 min 0.0002 mol of Ba(OH) 2

.H

2 0 (Aldrich) was added and the mixture stirred for 30 min. The methanol that is liberated was removed by heating under vacuum at 80 oC/650 mbar for 30 min, 80 °C/400 mbar for 30 min and then 90 mbar for 1 hour. The resin was then filtered through a 0.2 gm filter in order to remove the insoluble catalyst.

c) Synthesis of polymer based on dicyclopentylsilanediol 0.1 mol of dicyclopentylsilanediol, 0.05 mol ofmethacryloxypropyltrimethoxysilane (Gelest) and 0.05 mol of 3,3,3-trifluoropropyltrimethoxysilane (Gelest) were placed in a flask and heated to After 30 min 0.0002 mol of Ba(OH) 2

.H

2 0 (Aldrich) was added and the mixture stirred for min. The methanol that is liberated was removed by heating under vacuum at 80 oC/650 mbar for min, 80 °C/400 mbar for 30 min and then 90 °C/5 mbar for 1 hour. The resin was then filtered through a 0.2 gm filter in order to remove the insoluble catalyst.

Hindered siloxanes AU complete 1st draft 22-8-03/DK 12- Example 2.

a) Synthesis of dicyclohexylsilanediol g of dicyclohexyldichlorosilane (Gelest) was added to a flask and a mixture of 24 g of trimethyl orthoformate (Aldrich) and 1.6 g of methanol was added dropwise over 1 hour at room temperature. The solution was left to stir for 18 h forming a dicyclohexyldimethoxysilane solution.

The dicyclohexyldimethoxysilane solution was mixed with 65 mL of iso-propanol and 22 mL of 1 mol/L acetic acid forming a white mixture. The mixture was stirred until a clear solution was obtained (approximately 30 minutes) and then left to crystallise for 18 h. The crystals of dicyclohexylsilanediol were isolated by filtration and dried in a vacuum oven at 45 °C for 24 h.

The filtrate was placed in a freezer at -18 0 C overnight to recover another crop of crystals. These crystals were also isolated by filtration and dried in a vacuum oven at 45 °C for 24 h. The yield of dicyclohexylsilanediol was 28 g.

b) Synthesis of polymer based on dicyclohexylsilanediol 0.1 mol of dicyclohexylsilanediol and 0.1 mol of methacryloxypropyltrimethoxysilane (Gelest) were placed in a flask and heated to 80 OC. After 30 min 0.0002 mol of Ba(OH) 2

.H

c) Synthesis of polymer based on dicyclohexylsilanediol 0.1 mol of dicyclohexylsilanediol, 0.05 mol of methacryloxypropyltrimethoxysilane (Gelest) and 0.05 mol of 3,3,3-trifluoropropyltrimethoxysilane (Gelest) were placed in a flask and heated to After 30 min 0.0002 mol of Ba(OH) 2

.H

Example 3.

a) Synthesis of di-tert-butylsilanediol 50 g of di-tert-butyldichlorosilane (Gelest) was added to a flask and a mixture of 30 g of trimethyl orthoformate (Aldrich) and 2 g of methanol was added dropwise over 1 hour at room temperature.

The solution was left to stir for 18 h forming a di-tert-butyldimethoxysilane solution. The di-tertbutyldimethoxysilane solution was mixed with 80 mL of iso-propanol and 27 mL of 1 mol/L acetic acid forming a white mixture. The mixture was stirred until a clear solution was obtained (approximately 30 minutes) and then left to crystallise for 18 h. The crystals of di-tertbutylsilanediol were isolated by filtration and dried in a vacuum oven at 45 °C for 24 h. The filtrate was placed in a freezer at -18 0 C overnight to recover another crop of crystals. These Hindered siloxanes AU complete 1st draft 22-8-03/DK 13crystals were also isolated by filtration and dried in a vacuum oven at 45 °C for 24 h. The yield of di-tert-butylsilanediol was 30 g.

b) Synthesis of polymer based on di-tert-butylsilanediol 0.1 mol of di-tert-butylsilanediol and 0.1 mol of methacryloxypropyltrimethoxysilane were placed in a flask and heated to 80 OC. After 30 min 0.0002 mol of Ba(OH) 2

.H

2 0 (Aldrich) was added and the mixture stirred for 30 min. The methanol that is liberated was removed by heating under vacuum at 80 °C/650 mbar for 30 min, 80 °C/400 mbar for 30 min and then 90 °C/5 mbar for 1 hour. The resin was then filtered through a 0.2 gm filter in order to remove the insoluble catalyst.

c) Synthesis of polymer based on di-tert-butylsilanediol 0.1 mol of di-tert-butylsilanediol, 0.05 mol of methacryloxypropyltrimethoxysilane (Gelest) and 0.05 mol of 3,3,3-trifluoropropyltrimethoxysilane (Gelest) were placed in a flask and heated to After 30 min 0.0002 mol of Ba(OH) 2

.H

Sample preparation and measurement.

All polycondensates described above were filtered through a 0.2 um filter after preparation.

The optical loss was measured with a SHIMADZU UV-VIS-NIR spectrophotometer (UV- 3101 PC) using a 0.5 cm quartz cuvette. Since the resins are colourless the absorption was calibrated using the zero absorption area 700 nm as baseline. The absorption spectrum from the resin was measured from 3200 nm-200 nm. The lowest absorption value (usually the absorption between 700 and 550 nm is a straight line if there is no scattering as a result of particles and if the resin is colourless) is set as 0 absorption. The loss in dB/cm is calculated from the optical density of the resin at 1310 and 1550nm, multiplied by 10 and divided by the thickness of the cuvette in cm (whereas the optical density equals the log to the base 10 of the reciprocal of the transmittance).

The loss was estimated from the un-cured resin only.

The refractive index was estimated by a standard refractometer using daylight as the light source.

Curing The polycondensate was mixed with 2% Irgacure 1000 as photoinitiator and stirred under the exclusion of light for 24 hours. 2ml of this mixture was spun onto a 10cm Si-wafer at 4000 rpm for The wafer was exposed to UV-light (280-350 nm) using a Hg arc lamp with 8mW/cm 2 intensity for 60s under a nitrogen atmosphere.

The invention has been described by reference to certain preferred embodiments; however, it should be understood that it may be embodied in other specific forms or variations thereof without departing from its spirit or essential characteristics. The embodiments described above are Hindered siloxanes AU complete 1st draft 22-8-03/DK 14therefore considered to be illustrative in all respects and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description.

Hindered siloxanes AU complete 1st draft 22-8-03/DK

Claims

2. A polycondensate according to claim 1 wherein the B groups have sufficient steric bulk to prevent self-condensation of the silanediols of formula
3. A polycondensate according to claim 1 or claim 2 wherein each B is independently a linear aliphatic, cycloaliphatic or branched aliphatic group.
4. A polycondensate according to any one of the preceding claims wherein at least one of B, R' and R" bears a cross-linkable group. A polycondensate produced by condensation of a mixture of N mol% of a silanediol of formula and/or a derived precondensate thereof B HO-Si-OH B (I) and 0 0 (100-N) mol% of a diaryl silanediol of formula (III) and/or a derived precondensate Sthereof SAr HO-Si-OH I SAr (III) with one or more organically modified silanes of formula (IIa), (lib) or mixtures thereof OR" OR" g R'-Si-OR" R'-Si-R' SOR" OR" (Ia) (IIb) wherein each B is independently a sterically bulky non-aromatic group, each Ar is independently an aromatic group with up to 20 carbon atoms, and R' and R" are independently an alkyl, aralkyl or aryl group with up to 20 carbon atoms.
6. A polycondensate according to claim 5 wherein Nranges from 5 to
7. A polycondensate according to claim 5 or claim 6 wherein the B groups have sufficient steric bulk to prevent self-condensation of the silanediols of formula
8. A polycondensate according to any one of claims 5 to 7 wherein each B is independently a linear aliphatic, cycloaliphatic or branched aliphatic group.
9. A polycondensate according to any one of claims 5 to 8 wherein at least one of B, Ar, R' and R" bears a cross-linkable group. A polycondensate according to any one of claims 5 to 9 wherein up to 90% of the silanediols of formula (III) are replaced by a silanediol of formula (IV) B HO-Si-OH Ar (IV) 00 wherein Ar is an aromatic group with up to 20 carbon atoms. O S11. A method of preparing a polycondensate including the step of condensing one or more silanediols of formula and/or derived precondensates thereof 0B HO-Si-OH B e¢3 Swith one or more organically modified silanes of formula (IIa), (Ib) or mixtures thereof OR" OR" R'-Si-OR" R'-Si-R' OR" OR" (Ia) (Ib) wherein each B is independently a sterically bulky non-aromatic group, and R' and R" are independently an alkyl, aralkyl or aryl group with up to 20 carbon atoms.
12. A method according to claim 11 wherein up to 90% of the silanediols of formula can be replaced with one or more silanediols of formula (III) or (IV) Ar I HO-Si-OH Ar (III) B HO-Si-OH Ar (IV) wherein each Ar is independently an aromatic group with up to 20 carbon atoms.
13. A method according to claim 11 or claim 12 wherein the B groups have sufficient steric bulk to prevent self-condensation of the silanediols of formula or (IV). 00 O 0 1-(
14. A method according to any one of claims 11 to 13 wherein each B is independently a linear aliphatic, cycloaliphatic or branched aliphatic group. A method according to any one of claims 11 to 14 wherein at least one of B, R' and R" bears a cross-linkable group.
16. group. A method according to claim 12 wherein at least one Ar bears a cross-linkable
17. A polycondensate having general structure B OR" I I B R' m B R' or B R' m where m is at least 1, each B is independently a sterically bulky non-aromatic group, R' and R" are independently an alkyl, aralkyl or aryl group with up to 20 carbon atoms, and at least one of B, R' and R" bears a cross-linkable group.
18. A polycondensate according to claim 17 wherein each B is independently tert- butyl, cyclopentyl or cyclohexyl.
19. A polycondensate according to claim 17 or claim 18 wherein at least one of B, R' and R" bears at least one fluorine as a substituent. A polycondensate according to any one of claims 17 to 19 wherein the cross- linkable group is selected from alkene, epoxy, acrylate and methacrylate.