Silsesquioxane resin with high strength and fracture toughness and method for the preparation thereof

A cured silsesquioxane resin and method for its preparation are disclosed. By using a silyl-terminated hydrocarbon in the hydrosilylation reaction curable composition used to prepare the cured silsesquioxane resin, the cured silsesquioxane resin has improved strength and toughness without significant loss of modulus. A typical silyl-terminated hydrocarbon useful in this invention is silphenylene.

FIELD OF THE INVENTION
 This invention relates to cured silsesquioxane resins having high fracture
 toughness and strength without loss of modulus. This invention further
 relates to hydrosilylation reaction curable compositions used to prepare
 the cured silsesquioxane resins. This invention further relates to methods
 for preparing the cured silsesquioxane resins.
 BACKGROUND OF THE INVENTION
 Conventional thermoset networks of high cross link density, such as
 silsesquioxane resins, typically suffer from the drawback that when
 measures are taken to improve a mechanical property such as strength,
 fracture toughness, or modulus, one or more of the other properties
 suffers a detriment.
 For example, increasing the toughness of various silicone compositions has
 been previously carried out by adding a silicone fluid to a silicone
 resin. U.S. Pat. No. 5,034,061 discloses a silicone resin/fluid polymer
 adapted to form a transparent, shatter-resistant coating. The composition
 includes a silicone resin copolymer consisting essentially of R.sub.3
 SiO.sub.1/2 and SiO.sub.4/2 units with unsaturated olefinic functional R
 groups, a polydiorganosiloxane fluid with vinyl functionality, an
 organopolysiloxane crosslinker having hydrogen functionality and a
 catalyst. The composition is disclosed as being particularly adapted for
 use in coating incandescent glass lamps.
 Canadian Patent 691,206 (1964) discloses the use of silica-filled silicone
 resin/fluid combinations for damping vibrations. The ability of the
 disclosed silicone resin/fluid compositions to dampen vibrations is
 illustrated through the measurement of the ratio of G', the elastic shear
 modulus, to G", the loss shear modulus. The magnitude of this ratio is
 indicated as being inversely proportional to the ability of the material
 to absorb vibration. The ratio of G'/G" of the subject materials is
 compared to that of compositions prepared without a resin constituent.
 The above-described toughened silicone compositions are generally of the
 types having a fairly low modulus of elasticity. As used herein to
 describe silicone resins, the term "rigid" means that the resin material,
 in its unfilled condition, exhibits a certain "stiffness" characterized by
 having a Young's modulus of at least 0.69 GPa. As used herein, the term
 "unfilled" means that no reinforcing fillers, such as carbon or glass
 fibers or silica powders have been added to the resin.
 Another method for increasing toughness of a silicone resin is by modifying
 the silicone resin with a rubber compound. U.S. Pat. No. 5,747,608
 describes a rubber-modified resin and U.S. Pat. No. 5,830,950 describes a
 method of making the rubber-modified resin. The rubber modified-resin is
 prepared by reacting an uncured organosilicone resin and a silicone
 rubber. The resin and rubber can be reacted by addition reaction,
 condensation reaction, or free radical reaction. The resulting
 rubber-modified resin has a Young's modulus of at least 0.69 GPa in its
 unfilled condition. However, strength and toughness of the rubber-modified
 resin is generally inferior to tough organic polymers and still
 insufficient for some applications.
 Rigid silsesquioxane resins have been employed in applications that take
 advantage of their heat- and fire-resistant properties. These properties
 make the silsesquioxane resins attractive for use in fiber-reinforced
 composites for electrical laminates, structural use in automotive
 components, aircraft and naval vessels. Thus, there exists a need for
 rigid silsesquioxane resins having increased flexural strength, flexural
 strain, fracture toughness K.sub.Ic, and fracture energy G.sub.Ic, without
 significant loss of modulus or degradation of thermal stability. In
 addition, rigid silsesquioxane resins have low dielectric constants and
 are useful as interlayer dielectric materials. Rigid silsesquioxane resins
 are also useful as abrasion resistant coatings. These applications require
 that the silsesquioxane resins exhibit high strength and toughness.
 Therefore, it is an object of this invention to provide a curable
 composition that can be used to prepare a cured silsesquioxane resin
 having high strength and fracture toughness without loss of modulus. It is
 a further object of this invention to provide a method for preparing the
 cured silsesquioxane resin.
 SUMMARY OF THE INVENTION
 This invention relates to cured silsesquioxane resins and methods for their
 preparation. The cured silsesquioxane resins have improved strength and
 toughness over known resins. The improvements in strength and toughness
 were made without significant loss of stiffness. The cured silsesquioxane
 resins are prepared by hydrosilylation reaction of a silsesquioxane
 copolymer with a silyl-terminated hydrocarbon as a crosslinker. When this
 crosslinker is used instead of a traditional hydridosilane or
 hydridosiloxane crosslinker, the resulting cured silsesquioxane resin has
 unexpectedly high mechanical properties.

DETAILED DESCRIPTION OF THE INVENTION
 This invention relates to a hydrosilylation reaction curable composition
 that is used to prepare a cured silsesquioxane resin. This curable
 composition comprises: (A) a silsesquioxane copolymer, (B)
 silyl-terminated hydrocarbon as a crosslinker, (C) a catalyst, (D) an
 optional catalyst inhibitor, (E) a first optional silicone rubber, (F) a
 second optional silicone rubber, and (G) an optional solvent.
 Component (A) is a silsesquioxane copolymer comprising units that have the
 empirical formula R.sup.1.sub.a R.sup.2.sub.b R.sup.3.sub.c
 SiO.sub.(4-a-b-c)/2, wherein: a is zero or a positive number, b is zero or
 a positive number, c is zero or a positive number, with the provisos that
 0.8.ltoreq.(a+b+c).ltoreq.3.0 and component (A) has an average of at least
 2 R.sup.1 groups per molecule, and each R.sup.1 is a functional group
 independently selected from the group consisting of hydrogen atoms and
 monovalent hydrocarbon groups having aliphatic unsaturation, and each
 R.sup.2 and each R.sup.3 are monovalent hydrocarbon groups independently
 selected from the group consisting of nonfunctional groups and R.sup.1.
 Preferably, R.sup.1 is an alkenyl group such as vinyl or allyl. Typically,
 R.sup.2 and R.sup.3 are nonfunctional groups selected from the group
 consisting of alkyl and aryl groups. Suitable alkyl groups include such as
 methyl, ethyl, isopropyl, n-butyl, and isobutyl. Suitable aryl groups are
 exemplified by phenyl. Suitable silsesquioxane copolymers for component
 (A) are exemplified by (PhSiO.sub.3/2).sub.75 (ViMe.sub.2
 SiO.sub.1/2).sub.25, where Ph is a phenyl group, Vi represents a vinyl
 group, and Me represents a methyl group.
 Component (B) is a silyl-terminated hydrocarbon having the general formula
 ##STR1##
 where R.sup.1 and R.sup.2 are as described above for component (A), with
 the provisos that when R.sup.1 in component (A) is a hydrogen atom,
 R.sup.1 in component (B) is an unsaturated monovalent hydrocarbon group
 and when R.sup.1 in component (A) is an unsaturated monovalent hydrocarbon
 group, R.sup.1 in component (B) is a hydrogen atom, and R.sup.4 is a
 divalent hydrocarbon group. R.sup.4 can have both arylene and alkylene
 segments.
 Component (B) can be prepared by a Grignard reaction process. For example,
 one method for making a silyl-terminated hydrocarbon for use in this
 invention comprises heating to a temperature of room temperature to
 200.degree. C., preferably 50 to 65.degree. C., a combination of magnesium
 and a solvent such as diethylether or tetrahydrofuran. A di-halogenated
 hydrocarbon, such as dibromobenzene is then added to the magnesium and
 solvent over a period of several hours.
 After complete addition of the di-halogenated hydrocarbon, a halogenated
 silane, such as dimethylhydrogenchlorosilane, is then added, and an
 optional organic solvent can also be added. The resulting mixture is then
 heated for a period of several hours at a temperature of 50 to 65.degree.
 C. Any excess halogenated silane is then removed by any convenient means,
 such as neutralization with a saturated aqueous solution of NH.sub.4 Cl.
 The resulting product can then be dried with a drying agent such as
 magnesium sulfate and then purified by distillation.
 Component (B) is exemplified by compounds having the formulae:
 ##STR2##
 where R.sup.1 is as defined above and x is an integer from 1 to 6,
 preferably 1 to 4. Compounds suitable for use as component (B) are known
 in the art and are commercially available. For example,
 p-bis(dimethylsilyl)benzene is available from Gelest, Inc. of Tullytown,
 Pa.
 Components (A) and (B) are added to the composition in amounts such that
 the molar ratio of silicon bonded hydrogen atoms (SiH) to unsaturated
 groups (C.dbd.C) (SiH:C.dbd.C) ranges from 1.0:1.0 to 1.5:1.0. Preferably,
 the ratio is in the range of 1.1:1.0 to 1.5: 1.0. If the ratio is less
 than 1.0: 1.0, the properties of the cured silsesquioxane resin will be
 compromised because curing will be incomplete. The amounts of components
 (A) and (B) in the composition will depend on the number of C.dbd.C and
 Si--H groups per molecule. However, the amount of component (A) is
 typically 50 to 98 weight % of the composition, and the amount of
 component (B) is typically 2 to 50 weight % of the composition.
 Component (C) is a hydrosilylation reaction catalyst. Typically, component
 (C) is a platinum catalyst added to the composition in an amount
 sufficient to provide 1 to 10 ppm of platinum based on the weight of the
 composition. Component (C) is exemplified by platinum catalysts such as
 chloroplatinic acid, alcohol solutions of chloroplatinic acid,
 dichlorobis(triphenylphosphine)platinum(II), platinum chloride, platinum
 oxide, complexes of platinum compounds with unsaturated organic compounds
 such as olefins, complexes of platinum compounds with organosiloxanes
 containing unsaturated hydrocarbon groups, such as Karstedtfs catalyst
 (i.e. a complex of chloroplatinic acid with
 1,3-divinyl-1,1,3,3-tetramethyldisiloxane) and
 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane, and complexes of platinum
 compounds with organosiloxanes, wherein the complexes are embedded in
 organosiloxane resins. Suitable hydrosilylation reaction catalysts are
 described in U.S. Pat. No. 3,419,593 to Willing, Dec. 31, 1968, which is
 hereby incorporated by reference for the purpose of describing suitable
 catalysts.
 Components (A), (B), and (C) comprise 10 to 100 weight % of the
 composition. The composition may further comprise one or more optional
 components.
 Component (D) is an optional catalyst inhibitor, typically added when a one
 part composition is prepared. Suitable inhibitors are disclosed in U.S.
 Pat. No. 3,445,420 to Kookootsedes et al., May 20, 1969, which is hereby
 incorporated by reference for the purpose of describing catalyst
 inhibitors. Component (D) is preferably an acetylenic alcohol such as
 methylbutynol or ethynyl cyclohexanol. Component (D) is more preferably
 ethynyl cyclohexanol. Other examples of inhibitors include diethyl
 maleate, diethyl fumamate, bis (2-methoxy-1-methylethyl) maleate,
 1-ethynyl-1-cyclohexanol, 3,5-dimethyl-1-hexyn-3-ol,
 2-phenyl-3-butyn-2-ol, N, N, N', N'-tetramethylethylenediamine,
 ethylenediamine, diphenylphosphine, diphenylphosphite, trioctylphosphine,
 diethylphenylphosphonite, and methyldiphenylphosphinite.
 Component (D) is present at 0 to 0.05 weight % of the hydrosilylation
 reaction curable composition. Component (D) typically represents 0.0001 to
 0.05 weight % of the curable composition. Component (D) preferably
 represents 0.0005 to 0.01 weight percent of the total amount of the
 curable composition. Component (D) more preferably represents 0.001 to
 0.004 weight percent of the total amount of the curable composition.
 Component (E) is an optional silicone rubber. The amount of component (E)
 in the hydrosilylation reaction curable composition is 0 to 30 weight %,
 preferably 5 to 20 weight %. Suitable silicone rubbers for component (E)
 and methods for their incorporation into a curable composition are
 disclosed in U.S. Pat. Nos. 5,747,608 and 5,830,950, both of which are
 hereby incorporated by reference. The silicone rubber has the empirical
 formula:
EQU (R.sup.5.sub.(3-p) R.sup.1.sub.p SiO.sub.1/2)(R.sup.5.sub.2
 SiO.sub.2/2).sub.z ((R.sup.5.sub.(2-q) R.sup.1.sub.q
 SiO.sub.2/2)R.sup.5.sub.2 SiO.sub.2/2).sub.z).sub.y (R.sup.5.sub.(3-p)
 R.sup.1.sub.p SiO.sub.1/2)
 wherein each R.sup.1 is as described above, p is 1, 2 or 3, q is 1 or 2, z
 is an integer greater than or equal to 6, and y is zero or an integer up
 to 10. Each R.sup.1 is a functional group which does participate in the
 curing reaction to form the cured silsesquioxane of the present invention,
 as discussed above. Each R.sup.5 group in component (E) is independently
 selected from the nonfunctional groups for R.sup.2, described above.
 In the empirical formula, z represents the average nonfunctional linear
 chain length of the silicone rubber, i.e. the average chain length between
 R.sup.1 groups. Hence, component (E) can be a mixture of silicone rubbers
 of various degrees of polymerization, all of which are represented by
 above empirical formula. Most silicone rubbers used in connection with the
 present invention have R.sup.1 groups only at the terminal groups of the
 chain. In such instances, the term "degree of polymerization" ("DP") as
 used herein is the same as the value of z. DP does not include the
 terminal functional siloxy groups.
 In the preferred embodiment of the invention, the R.sup.5 groups are methyl
 groups, phenyl groups, or a combination thereof. When a high percentage of
 the R.sup.2 groups of component (A) the silsesquioxane copolymer and the
 R.sup.5 groups of (E) the first silicone rubber are either predominantly
 methyl or predominantly phenyl, (A) the silsesquioxane copolymer and (E)
 the first silicone rubber are generally compatible, permitting the rubber
 to be dispersed throughout the cured silsesquioxane resin structure in a
 relatively homogeneous manner.
 Component (F) the second optional silicone rubber is a polydiorganosiloxane
 of the empirical formula R.sup.5.sub.2 R.sup.6 SiO(R.sup.5.sub.2
 SiO).sub.m (R.sup.5 R.sup.6 SiO).sub.n SiR.sup.6 R.sup.5.sub.2 wherein
 each R.sup.5 is as described above, each R.sup.6 is selected from the
 group consisting of R.sup.1 and R.sup.5, with the proviso that at least
 two R.sup.6 groups per molecule must be R.sup.1, m is 150 to 1,000,
 preferably 246 to 586, and n is 1 to 10. The amount of component (F) in
 the curable composition is generally 0 to 15 weight %, preferably 2-8
 weight %.
 The hydrosilylation reaction curable composition comprising components (A),
 (B), and (C), and any optional components can be dissolved in component
 (G), an optional solvent. Typically, the amount of solvent is 0 to 90
 weight %, preferably 0 to 50 weight % of the curable composition. The
 solvent can be an alcohol such as methyl, ethyl, isopropyl, and t-butyl
 alcohol; a ketone such as acetone, methylethyl ketone, and methyl isobutyl
 ketone; an aromatic hydrocarbon such as benzene, toluene, and xylene; an
 aliphatic hydrocarbon such as heptane, hexane, and octane; a glycol ether
 such as propylene glycol methyl ether, dipropylene glycol, methyl ether,
 propylene glycol n-butyl ether, propylene glycol n-propyl ether, and
 ethylene glycol n-butyl ether; a halogenated hydrocarbon such as
 dichloromethane, 1,1,1-trichloroethane and methylene chloride; chloroform;
 dimethyl sulfoxide; dimethyl formarnide; acetonitrile and tetrahydrofuran.
 Toluene is preferred.
 This invention further relates to a method for preparing the
 hydrosilylation reaction curable composition described above. The method
 comprises mixing the composition comprising components (A) to (G)
 described above. Mixing can be carried out by any suitable means. The
 curable composition can be made either as a one part or as multiple part
 composition, such as a two part composition.
 When the curable composition is formulated as a one part composition, the
 method for preparing the one part composition generally comprises: (I)
 premixing (C) the catalyst and (D) the inhibitor, thereby forming a
 complex, and (II) mixing the complex with components (A), (B), and any
 desired optional components (E) to (G).
 In an alternative embodiment of the invention, a one part composition can
 be prepared by (i) premixing (C) the catalyst and (D) the inhibitor,
 thereby forming a complex, (ii) mixing components (A), (B), (E), (F), and
 (G), (iii) removing (G) the solvent from the product of step (ii) thereby
 forming a fluid low viscosity composition, and thereafter (iv) mixing the
 complex with the product of step (iii).
 A two part composition can be prepared by
 (1) preparing a first part comprising component (A), and
 (2) preparing a second part comprising component (B),
 wherein component (C) is mixed with either the first or second part, and
 thereafter keeping the first and second parts separate. The first and
 second parts are mixed immediately before use.
 Preferably, the two part composition is prepared by (1') mixing components
 (A), (E), (F) and (G) to form a first part, (2') mixing component (C) with
 a part selected from the group consisting of the first part and a second
 part comprising component (B). The first and second parts are thereafter
 kept separate until mixing immediately before use.
 Each of the above methods may further comprise the step of degassing the
 composition before curing. Degassing is typically carried out by
 subjecting the composition to a mild vacuum.
 This invention further relates to a method for preparing the cured
 silsesquioxane resin. This method comprises heating the hydrosilylation
 reaction curable composition described above at a temperature for a time
 sufficient to cure the hydrosilylation reaction curable composition
 described above. The curable composition may be degassed before curing,
 and any solvent may be removed before or during curing. The solvent may be
 removed by any convenient means such as by exposing the curable
 composition to mild heat or vacuum.
 In an example of a hydrosilylation cure process, after the hydrosilylation
 reaction curable composition was degassed, it was then heated in an air
 circulating oven at 60.degree. C. After half an hour in the oven the
 mixture gelled. After continuing to heat at 60.degree. C. for an
 additional 24 hours to allow the residual solvent to escape, the
 temperature was then raised to 100.degree. C. for 6 hours, 160.degree. C.
 for 6 hours, 200.degree. C. for 4 hours, and 260.degree. C. for 8 hours.
 The oven was then turned off and the cast plate allowed to cool inside the
 oven. This curing cycle can be substantially shortened by removing the
 solvent more completely before cure.
 The cured silsesquioxane resin prepared by curing the hydrosilylation
 reaction curable composition of this invention has superior mechanical
 properties over those of conventional silsesquioxane resins. Typically,
 the cured silsesquioxane resin has: flexural strain up to 14%, flexural
 strength up to 7,000 psi, flexural modulus up to 220 ksi, K.sub.Ic up to
 1.08 MPam.sup.1/2, and G.sub.Ic up to 788 N/m.
 EXAMPLES
 These examples are intended to illustrate the invention to those skilled in
 the art and should not be interpreted as limiting the scope of the
 invention set forth in the claims.
 Reference Example 1
 Preparation of p-bis(dimethylsilyl)benzene
 ##STR3##
 A 5 L three necked, round bottomed flask was charged with 84g of magnesium
 (Mg) and 406 g tetrahydrofuran (THF). The flask was equipped with a
 stirrer, a condenser, two addition funnels, a thermometer, and heated with
 a heating mantle and purged with dry nitrogen. 10 g of BrCH.sub.2 CH.sub.2
 Br was added to activate the Mg. The solution of 270 g of dibromobenzene
 in 526g THF was added to one of the addition funnels, and 400 g of THF was
 added to the other addition funnel.
 The flask was heated to 50 to 60.degree. C., then 200 mL of THF was added
 and the dibromobenzene solution was added slowly. About 20 minutes later a
 strong exotherm was observed and heating was turned off. THF was added to
 control the exotherm. The addition of dibromobenzene was stopped to help
 control the exotherm.
 When the exotherm was under control, the addition rate of dibromobenzene
 was adjusted so that a good amount of reflux was maintained. It took about
 one and a half hours to finish adding dibromobenzene.
 After that 500 ml THF was added and the flask was heated at 65.degree. C.
 for 5 hours, then the heater was turned off and the flask was cooled
 overnight under nitrogen while being stirred. When it was cooled down to
 room temperature, 500 mL more THF was added and 440 g of dimethyl
 chlorosilane was added slowly while the flask was cooled by a ice water
 bath. A dry ice condenser was mounted on top of the condenser to help
 minimize the loss of dimethyl chlorosilane. The addition of dimethyl
 chlorosilane was adjusted so that a good reflux was maintained. After the
 addition of dimethyl chlorosilane the flask was heated at 60.degree. C.
 overnight.
 The flask was cooled to room temperature and 1000 m1 of toluene was added.
 Saturated NH.sub.4 Cl aqueous solution was added slowly to hydrolyze and
 condense the excess dimethyl chlorosilane and the mixture was then washed
 with a large amount of water until a more or less clear bottom phase was
 obtained.
 The top organic phase was collected and dried with magnesium sulfate and
 most of the solvent was removed by distillation until a temperature of
 150.degree. C. in the flask was reached. The concentrated crude product
 was further purified by distillation under vacuum.
 The distillation yielded 140 g of &gt;96% pure p-bis(dimethylsilyl)
 benzene. Also obtained were 28 g of 80% pure, 25 g 41.5% pure and 15 g of
 16% product in a mixed solvent of THF and toluene. The total yield was
 .about.55%.
 Formation of the product HMe.sub.2 SiC.sub.6 H.sub.4 SiMe.sub.2 H was
 confirmed by the observation of a major species of a molecular weight of
 194 from GC-MS. FT-IR and NMR were used to further confirm the structure
 of the product. The infra-red spectrum of the product showed a strong SiH
 absorption at .about.2125 cm.sup.-1, methyl CH stretching at 2800 to 3000
 cm.sup.-1 methyl umbrella deformation at 1260 and 1415 cm.sup.-1,and
 vibration of Si--Ph at .about.1120 and 1440 cm.sup.-1. Little siloxane was
 seen. The .sup.1 H NMR spectrum of the product showed three types of
 proton, and their ratio was 2:1:6, corresponding to hydrogen on the
 aromatic ring, SiH and CH.sub.3. The splitting patterns of the methyl
 proton and the SiH proton were consistent with the expected structure.
 The.sup.13 C NMR showed three types of carbon, roughly in a ratio of
 1:2:2, again consistent with the expected structure. The.sup.29 Si in
 CDCl.sub.3 showed a single peak at 7.9 ppm.
 Reference Example 2
 Three Point Flexural Testing
 The three point bending test was performed on an Instron 8562 per ASTM
 standard D 790. The cured resin specimens prepared in the Examples
 described below were polished until smooth, and visible scratch free
 surfaces were obtained. All samples were polished through the same
 procedure to ensure a similar surface condition. The polished samples were
 dried at 80.degree. C. overnight and conditioned at the testing
 temperature and humidity for at least 24 hours before testing. The test
 temperature was 21.degree. C. For each sample at least three specimens
 were tested.
 During testing, force-displacement curves were recorded. The toughness of
 the cured resin was obtained as the area under the stress-strain curves.
 The flexural strength was calculated using the peak force as:
EQU S=3PL/2bd.sup.2
 where S is the stress in the outer surface at the mid span, P the maximum
 load, L the support span, and b and d are the width and thickness of the
 beam. The maximum strain was calculated, using the maximum displacement,
 as:
EQU .epsilon.=6Dd/L.sup.2
 where .epsilon. is the strain at break and D is the maximum displacement.
 The slope of the steepest initial straight-line portion of the
 load-displacement curve was taken as the Young's modulus.
 Reference Example 3
 Fracture Toughness Testing
 The plane strain fracture toughness, K.sub.Ic, was obtained per ASTM D
 5045, and the critical strain energy release rate, G.sub.Ic, was
 calculated from K.sub.Ic based on Linear Elastic Fracture Mechanics (LEFM)
 assumptions. Six specimens of each sample were obtained. A notch was cut
 at the center of the specimen, and a natural crack extending from the root
 of the notch to about half of the width was produced by gently tapping a
 sharp razor blade into the notch. Samples were conditioned at room
 temperature for at least twenty four hours before testing to allow full
 relaxation of deformation. FIG. 1 represents a specimen used for fracture
 toughness testing. In FIG. 1, P represents the highest load, a represents
 pre-crack width, W represents a specimen width of 9.525 mm, L1 is 38 mm,
 and L2 is 51 mm. The displacement rate of the test was 10 mm/minute. For
 the geometry and loading conditions shown in FIG. 1, with a support to
 width ratio of 4,
EQU K.sub.Ic =(P/(BW.sup.1/2))f(x)
 where P is the highest load and:
EQU f(x)=6x.sup.1/2 (1.99-x(1-x)(2.15-3.93x+2.7x.sup.2))/((1+2x)(1-x).sup.3/2)
 where x is the pre-crack to specimen width ratio, a/W. After the test the
 pre-crack length was measured. Only those specimens with a value between
 0.45 to 0.55 were considered valid. The variation of x across the
 thickness should be less than 10%. The validity of the test was further
 ensured by comparing the sample dimensions with the estimated plastic zone
 size enlarged by approximately 50:
EQU B,a,(W-a)&gt;2.5(K.sub.Ic /.sigma..sub.y).sup.2
 where a .sigma..sub.y is the yield stress of the sample.
 From the K.sub.Ic, G.sub.Ic was calculated by:
EQU G.sub.Ic =K.sup.2.sub.Ic (1-v.sup.2)/E
 where .upsilon., the Poisson's ratio of the resin, was neglected to
 simplify the experiment. For a glassy polymer with a Poisson's ratio of
 0.3, G.sub.Ic was exaggerated by about 9%. However, the relative ranking
 of G.sub.Ic values would not be obscured since the change of the square of
 the Poisson's ratio is usually small from one resin to another of similar
 stiffness.
 Example 1
 The p-bis(dimethylsilyl) benzene prepared in Reference Example 1 was
 reacted with a silsesquioxane copolymer having the formula
 (PhSiO.sub.3/2).sub.75 (ViMe.sub.2 SiO.sub.1/2).sub.25,
 where Ph is a phenyl group, Vi represents a vinyl group, and Me represents
 a methyl group, by addition reaction. The resin copolymer was in a toluene
 solution with a solid content of 81.54 wt. % as determined in the lab by
 drying the resin solution at 130.degree. C. for 4 hours and then
 140.degree. C. for 2 hours and monitoring the weight change. 67.5 grams of
 resin solution was mixed with 12.87 grams of the p-bis(dimethylsilyl)
 benzene so that the SiH:C.dbd.C ratio was 1.1:1.0, and 10 ppm of
 chloroplatinic acid was added. The concentration of Pt in the mixture was
 based on the total mass. The mixture was poured into an flat mold with one
 side open and degassed in a vacuum oven at 50.degree. C. for fifteen
 minutes. The mold was then transferred to an air circulating oven at
 60.degree. C. After half an hour in the oven the mixture gelled and the
 temperature was kept at 60.degree. C. for an additional 24 hours to allow
 the toluene to escape. Then the temperature was raised to 100.degree. C.
 for 6 hours, 160.degree. C. for 6 hours, 200.degree. C. for 4 hours, and
 260.degree. C. for 8 hours. The oven was then turned off and the cast
 plate allowed to cool inside the oven.
 The resulting cured silsesquioxane resin was evaluated by the test methods
 in Reference Examples 2-3. Young's Modulus, Strain at Break, Flexural
 Strength, K.sub.Ic and G.sub.Ic values are presented in Table 1.
 Comparative Example 1
 An addition reaction cured silsesquioxane resin was prepared by the method
 of Example 1, except that p-bis(dimethylsilyl) benzene was replaced with
 tetrakis(dimethylsiloxy) silane.
 The resulting cured silsesquioxane resin was evaluated by the test methods
 in Reference Examples 4-5. Young's Modulus, Strain at Break, Flexural
 Strength, K.sub.Ic and G.sub.Ic values are presented in Table 1.
 Comparative Example 2
 An addition reaction cured silsesquioxane resin was prepared by the method
 of Example 1, except that p-bis(dimethylsilyl) benzene was replaced with
 phenyltris(dimethylsiloxy) silane.
 The resulting cured silsesquioxane resin was evaluated by the test methods
 in Reference Examples 2-3. Young's Modulus, Strain at Break, Flexural
 Strength, K.sub.Ic and G.sub.Ic values are presented in Table 1.
 TABLE 1
 Test Results for Cured Silsesquioxane Resins
 Young's Flexural
 Modulus Strain at Strength KIc GIc
 Properties (ksi) Break (%) (psi) (MPam.sup.1/2) (J/m.sup.2)
 Example 1 216.5 12.14 6893.2 1.083 788.41
 Comparative 179.3 3.12 3925 0.286 66.8
 Example 1
 Comparative 156.4 3.39 3645 0.296 82.0
 Example 2