“inverse” organic-inorganic composite materials: high glass content non-shrinking sol-gel...

Materials Science and Engineering, A162 (1993) 257-264 257

"Inverse" organic-inorganic composite materials: high glass content non-shrinking sol-gel composites

Bruce M. Novak and Mark W. Ellsworth Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720 (USA)

Abstract

We, as well as others, have been interested in the sol-gel process for the synthesis of hybrid inorganic-organic composite materials. Since our first report on the application of tetraalkoxysilanes possessing polymerizable alkoxides for the production of non-shrinking sol-gel composites, we have extended our efforts towards increasing the glass content in these composite materials. The stoichiometry in the tetraalkoxysilanes limits the maximum glass content in the original non-shrinking composites to 10%-18%. In order to increase the glass content to greater than 50%, we focused our efforts on the use of silicic acid oligomers. Molecular weights of the poly(silicic acid) materials were varied from M~ = 5000 to M, = 2 000 000 by controlling reaction conditions. In addition, branching ratios (i.e. linear vs. spherical particles) can be controlled by changing the catalysts used. The properties of the resulting composite can range from a transparent flexible material to a transparent hard material simply by changing the organic polymer in the composite.

1. Introduction

Modern composites embody a general class of materials which is extremely broad, ranging from polymer-polymer blends and reinforced plastics to chopped-fiber and filled polymer composites [1]. The primary properties of structural materials are strength, stiffness and toughness. Secondary considerations include resistance to corrosion, creep, temperature and moisture. Both the strength and stiffness of a composite can be derived from the properties of the reinforcing fiber. Toughness results from the interac- tion between the matrix and the fibers, thus highlight- ing the importance of controlling the interfacial properties between the two phases. Occasionally a synergistic relationship exists between the components of a composite. The combination of a brittle fiber in a brittle matrix produces a material which is much tougher than either of the two single components. This synergism is achieved by a combination of mechanisms which tend to keep cracks small and isolated, and which dissipate energy [1].

Composite materials have evolved considerably over time, culminating in today's carbon-fiber reinforced resins [1], carbon-carbon composites [2], aramid fibers [3] (aromatic polyimides) and molecular composites [4]. In the most general sense, maximized mechanical properties should result from the combination of two or more very dissimilar components. A good example of dissimilar compounding is high- modulus glass-fiber reinforced organic matrix materi-

als [5]. These glass fiber composites have found a variety of uses ranging from fiberglass for pleasure boats to high-tech, aerospace "stealth" applications.

We [6-8], as well as others [9], have been interested in exploring the reverse side of this traditional glass- fiber composite approach by using organic polymers to reinforce an inorganic glass matrix. Although "inverted", the basic composite principles remain intact, but now the synergistic relationship results from the combination of a high-modulus organic polymer (for high tensile strength) with a three-dimensionally cross-linked, inorganic matrix (for high compressive strength). In order to achieve this goal, it becomes necessary to form the inorganic matrix under conditions in which organic polymers will survive. This can readily be accomplished using low-temperature sol-gel technology.

The sol-gel process for the preparation of glasses under mild conditions has received much attention with respect to the formation of highly homogeneous monolithic glasses and inorganic ceramic composites [10, 11]. The sol-gel process is based on the homogeneous hydrolysis and condensation of metal alkoxides in the presence of cosolvents to form highly cross-linked networks. Under controlled reaction conditions, large-scale, optically transparent monolithic samples can be obtained. The simplest sol-gel process is the formation of SiO2 from the hydrolysis of tetraethoxy orthosilicate (TEOS) (scheme 1 ).

A solvent-swollen, three-dimensional SiO2 network is obtained at this point. Controlled drying of the

0921-5093/93/$6.00 © 1993 - Elsevier Sequoia. All rights reserved

258 B. M. Novak, M. IV. Ellsworth / Inverse organic-inorganic composites

Hydrolysis

Si(OR) 4 + H20

Condensation \ /

- - S i - O H + H O - S i - - / \

and/or

N / S i • O H + R O - S i - -

/ x

= (RO)3Si-OH + ROH

\ / - - S i . O - S i - - + H 2 0

/ x

\ / - - S i - O - S i - - + R O H / x

Net Reaction I

~ l - V . s . O" ,.,. kD I , si(OR), n o/I-r or OH L

(liq.) ' Cosolvent - - 's i o..u sfV l b .S~ 'O ' l " .... Si "0 ~*

Scheme 1

Solvent Swollen mm SiO2 Matrix

~ ~ . H20/H+ )

• _ _ ~ Si(OR)4

Scheme 2

solvent consisting of the excess water, added cosolvent and the alcohol hydrolysis product, under ambient conditions leads to crack-free, monolithic glass formation. One of the major obstacles to the widespread application of sol-gel techniques is the fact that this drying process is accompanied by extraordinary shrinkage of the glass (shrinkages of more than 50% are common)[12].

Sol-gel technology has recently been applied to the formation of composite materials [9]. The most basic approach involves dissolving a preformed organic oligomer or polymer in the sol-gel solution, and then allowing the hydrolysis and condensation of the inorganic network to occur. Under the appropriate conditions, the polymer remains homogeneously embedded in the inorganic gel throughout the synthesis and drying steps (scheme 2).

2 . R e s u l t s a n d d i s c u s s i o n

2.1. Preformed polymer composites During our preliminary studies in this area, we

identified a limited number of soluble polymers which, at the conclusion of the condensation and drying processes, remain homogeneously embedded within sol-gel derived SiO2 glasses. For example, we have found that polymers with basic functional groups such as amines and pyridines are soluble in the acid catalyzed,

pregelled, sol-gel solutions. Specifically, poly(2-vinyl- pyridine), poly(vinylpyrrolidone) and polyacrylonitrile can be dissolved in TEOS-H20 solution (tetramethoxy- silane (TMOS) can also be used), using organic acids as cosolvents. Under the proper conditions, subsequent hydrolysis and condensation of the TEOS produces optically clear gels containing the organic polymers. Slow ambient drying results in composite materials in which the organic polymer remains homogeneously embedded within the three-dimensional SiO2 network. These monolithic glassy materials display excellent optical clarity. Without further sintering, however, they remain brittle materials.

Further investigations led to the discovery that composites possessing superior mechanical properties could be obtained using cellulosics as the organic component. Using cellulose acetate, optically transparent composites with organic contents ranging from ca. 2% to 85% by weight have been prepared. Scanning electron microscopy (SEM) studies on composites possessing 30% cellulose acetate show a continuous phase of SiO 2 with the organic polymer dispersed in irregular domains averaging ca. over 1/~m in size [13]. Although the mechanical properties of these new composites have yet to be systematically measured, preliminary results indicate that composites with high cellulose contents are exceptionally tough and impact resistant (i. e. vigorous pounding in a mortar and pestle does not shatter these materials).

All of the above composites consist of linear polymers dispersed in the inorganic glass matrices without the benefit of planned, covalent links between the two phases. In order to increase further the mechanical properties of these composites, cellulose derivatives were synthesized possessing pendant trialkoxysilane groups (eqn. ( 1 )):

H° o~o oOd.), n ~Rohs~..~..sco

II Ib

Si(OR) 3

HO OH

o I1

o "~ ~ N . . . . ~ Si(OR) 3

.o. og+

0 NH~...~ .0 - Si-- Si. 0 ,% • .

~ S ' ' 0 Si--

(1)

Despite the preliminary success of some of the above materials, the formation of composites by the incorporation of preformed polymers into sol-gel glasses is severely limited by at least two factors. First, only a limited number of polymers are soluble in the tricomponent sol-gel solution. Secondly, the shrinkage associated with their drying introduces a considerable amount of stress within the dried glasses and preludes most molding applications. For these reasons, we began to explore alternative routes into these materials.

B. M. Novak, M. W. Ellsworth / Inverse organic-inorganic composites 259

Ultimately, our solutions to the first problem led to solutions to the latter.

2.2. Simultaneous interpenetrating organic-inorganic network composites

In order to circumvent the solubility problem associated with trying to incorporate preformed polymers and to provide better homogeneity between the two phases, we began investigations into the formation of simultaneous interpenetrating networks (SIPNs) [ 14] by the synchronous formation of both the organic polymer and the inorganic glass network [8]. We have identified two organic polymerization methods which are compatible with the restrictive conditions imposed by the sol-gel reaction (i.e. aqueous acidic or basic medium): vinyl, free radical polymerizations and aqueous ring-opening metathesis polymerizations (aqueous ROMP) catalyzed by a variety of Ru 3+ and Ru 2+ salts [15]. A ROMP example of this SIPN process is shown in scheme 3.

The utility of this technique is illustrated by the fact that poly-II (scheme 3) is an intractable, totally insoluble polymer and yet, using the SIPN approach, poly-II can be homogeneously embedded within the glass matrix in concentrations up to ca. 60% without under- going macrophase separation. Verification of polymer formation within these glasses can be accomplished by forming a soluble polymer, crushing the glass after its formation and extracting with a good solvent. For example, poly(5,6-dimethoxymethyl-7-oxanorborn-2- ene), poly-III, is formed under these in situ conditions in greater than 95% recovered yield. The molecular weight of poly-llI formed in this in situ process is quite high with Mn = 1.3 x 1 0 6, M w = 2.1 x 106 and PDI = 1.6.

In addition to utilizing two independent, non-inter- fering polymerization techniques, successful SIPN formation requires matching the polymerization rates of the two systems. Significant deviations from these matched rates result in systems which approach the homopolymerization limits: uncontrolled polymer precipitation when the ROMP or free radical rates are greater than the Si(OR)4 condensation rate, or

inorganic gells swollen with unreacted monomer when the condensation rate is much greater than the organic polymerization rates. Under ideal reaction conditions, a transparent glass-polymer composite is obtained.

SEM studies show that in comparison with the composites formed by incorporating preformed polymers, the SIPN composite materials show far greater homogeneity and small domain sizes (average polymer domain sizes are less than 1000 A).

The advantages of the SIPN approach over using preformed polymers to form sol-gel derived composites are three-fold. First, by incorporating difunc- tional monomers, the SIPN approach allows us to cross-link the organic polymer, thereby locking-in the interpenetrating phase morphology. Secondly, this approach allows for the in situ formation, and hence homogeneous incorporation, of polymers which would normally be completely insoluble. Lastly, these SIPN materials display greater homogeneity and smaller domain sizes than comparable preformed materials.

2.3. Non-shrinking sol-gel composites As with the preformed polymer composites, shrink-

age remains a problem in these SIPNs. In an effort to surmount this problem, we have synthesized a series of tetraalkoxysilane derivatives possessing polymerizable alkoxide groups in place of the standard ethoxide or methoxide groups (Table 1 ) [6, 7].

The hydrolysis and condensation of these siloxane derivatives liberates a polymerizable alcohol. In the presence of the appropriate catalyst (free radical or ROMP), and by using a stoichiometric amount of water and the corresponding alcohol as cosolvent, all components of these derivatives are polymerized. Since both the cosolvent and the liberated alcohol polymer- ize, gel drying is unnecessary and no gel shrinkage occurs. This overall process is illustrated in scheme 4.

2. 4. High glass, non -shrinking composites Since our first report on the application of these

tetraalkoxysilanes possessing polymerizable alkoxides for the production of non-shrinking sol-gel composites

~ , . ~ oH H20 ~

II

o r

~ = SiO 2

Scheme 3

260 B. M. Novak, M. W. Ellsworth / Inverse organic-inorganic composites

[6, 7], we have extended our efforts towards increasing the glass content in these composite materials. The stoichiometry in the tetraalkoxysilanes limits the maximum glass content in the non-shrinking composites to 10%-18%. In order to increase the glass content to greater than 50%, we focused our efforts toward the use of silicic acid oligomers and/or polymers substituted with polymerizable alkoxides [6].

Poly(silicic acid) can be generated in si tu by the hydrolysis and condensation of sodium metasilicate (NazSiO3 • 9H2 O) at low pH (3.6 M HCI) and extracted into organic solvent (THF) by the addition of salt to the organic layer [16]. Soluble silicic acid polymers of molecular weights ranging from 8000 to 7 000 000 can be prepared by increasing the reaction time from I h to ca. 72 h (eqn. (2)):

,1. .o ? '~ ,o.

~Si Si HO ,,4.,, , ~ t j ~ OH

OH OH 0 0 ~ Q" Na2SiO 3 1. H20/IICI I I I I - - Si ~ S i . S i . S i . HO;, o<:_,

2. T .cl ( o. ( o . " o.)m o . . o . "Q, ,Q, Q3 ?'o.

These preformed polysiloxanes offer two adjustable parameters for controlling glass content: the number of Q3 and Q4 branch points, and the degree of alkoxide substitution. The weight per cent of glass in composites derived from these appropriately substituted poly-

TABLE 1. Candidate free radical and ROMP monomer for non-shrinking composites

]~OMP Monomers Free Radical Monomers

0 0

,, ), 0 0 0

0

siloxanes is given by eqn. (3):

x ~ 60.1 ]JJ

\FC/j] 1

+ p (3)

where MW is the molecular weight of the liberated alcohol, n, m and p are the percentages of Q2, Q3 and Q4 silicon centers respectively, and x and x' are the percentages of alkoxide substitutions on the Qz and Q3 moieties respectively. In addition, control over the Q ratio should allow for the formation of inorganic phases with morphologies ranging from spherical (high percentage of Q 4 ) t o more linear (high percentage of QZan d Q3 relative to Q4).

We have found that the Q4 content of these silicic acid polymers can be systematically changed from ca.

35% to greater than 70% (as measured by 298i nuclear magnetic resonance (NMR)) by adjusting the HCI concentration between 3 and 6 M. Although highly branched and/or cross-linked, these high Q4 polymers retain their solubility in polar organic solvents.

Alkoxide substitution on these poly(silicic acid) polymers can be effected by addition of the appropriate alcohol to the THF solution of polymer followed by an azeotropic distillation to remove the liberated water [16]. By varying the reaction time, the degree of substitution (DS) was controlled by the amount of THF]water azeotrope removed. DS values ranged from 25% to 75% as evaluated by endcapping of the unreacted silanols with trimethylsilylchloride (TMSCI) (scheme 5).

Composites were synthesized by allowing the silicic ester to condense in a solution of a polymerizable monomer and a free radical initiator (typically benzoyl peroxide) or aqueous ROMP catalyst. In most cases, transparent inorganic-organic composites are formed. A variety of cosolvent monomers can be used, includ- ing cross-linking agents, in order to achieve different properties in the final composite. For example, poly(hydroxyethylacrylate) (HEA) is a rubbery solid at room temperature (T~ = -15 °C) whereas poly(hydroxy-

o o o _ o .

o" b ~ - ~ o n

Scheme 4

[ ~ l = 2


OH Ho-si o~ n

OH O ÷

OH --OH

THF solution

RO--= .<

Scheme 5

+ROH -THF/H20

OH HO- Si' O~_

,.4., "11

OR O -(-- '.Si • O)-~ '.S,. O)- m

OR --OR

~ , ~ o ~ o ~ TMSCI

o ~ " ~ 0 % 0-~ OTMS

TMSO" Si O O -4.., n

~ . ~ OR O + Si'O):~ Si'O)- m

" o ~ 6R n 6R

ethylmethacrylate) (HEMA) is glassy at room temperature (Tg= 55 °C). A composite produced from a silicic ester of HEA is a rubbery material. Alterna- tively, using a H E M A silicic ester produces a stiff material. The glass-to-polymer ratio also influences the properties of the composite. Preliminary studies with HEMA/glass composites indicated that a higher glass composition improves the hardness and compression modulus of the composites.

3. Conclusion

SIPN technology provides a convenient method for incorporating insoluble polymers into sol-gel composites. This approach has been extended toward the synthesis of non-shrinking sol-gel composites. The use of silicic acid esters allows for the production of non- shrinking composites with a wide range of glass- to-polymer ratios. The physical and mechanical properties of these composites can be tailored by changing the glass content and using different monomers. Since long drying times are unnecessary, these composites may prove to be suitable for a wide range of applications.

4. Experimental details

General All synthetic operations were carried out under dry nitrogen except where noted and in the case of glass polymerizations which were performed in air. Furan (Aldrich), triethylamine (Aldrich), 5-norbornene-2-methanol (Aldrich), and silicon(IV) chloride (Aldrich) were distilled immediately prior to use. Maleic anhydride (Aldrich), K2RuCI5 (AESAR), LiAIH 4 (Fluka), acryloyl chloride (Aldrich), and tri- methoxysilane (Petrarch) were used without further purification. THF, dimethoxyethane, toluene, and diethyl ether were distilled from sodium benzophenone

immediately prior to use. All other solvents were used without further purification. Melting points (Pyrex capillary) were determined using a Mel-Temp melting point apparatus and are uncorrected. 1H NMR spectra were recorded at 400 or 500 MHz on Bruker AM-400 or AM-500 spectrometers, t3C NMR spectra were recorded at 100.6 or 125.7 MHz on the same instru- ments. Chemical shifts are reported in d values with tetramethylsilane (TMS) as the internal reference. ~H NMR data are tabulated by chemical shift, multiplicity, number of protons, and coupling constants in hertz. All ~3C NMR spectra are proton decoupled. Mass spectra were obtained with Atlas MS-12, Consolidated 12-110B, or Kratos MS50 spectrometers. Mass spec- tral data are reported as m/z (intensity expressed as percentage of total ion current). IR spectra were recorded on a Nicolet DX Fourier transform IR spectrometer. Elemental analyses were performed by the Microanalytical Laboratory, College of Chemistry, University of California, Berkeley, CA.

Exo- 7-oxabicyclo[2.2. l]hept-5-ene-2, 3-dicarboxylic anhydride (I). To a solution of 59.0 g (0.60 mol) of maleic anhydride in 600 ml of dry ether was added 43.7 ml (0.60 mol) of furan. The reaction mixture was stirred for three days. The resulting white crystals were collected by filtration and washed with ether (3 x 50 ml). The crystals were dried under vacuum for 24 h to afford 74.2 g (74.5% yield) of white needles: m.p. 109-110°; IR (KBr)2959, 1825, 1753, 1231, 1159, 735; ]H NMR (400 MHz, CDCI3) 6 6.56 (s, 1H), 5.44 (s, 1H), 3.16 (s, 1H); ~3C NMR (100.6 MHz, CDC13) d 136.9, 82.19, 48.68; MS m/e 165 (0.54), 121 (1.25), 98 (1.84), 68 (100). Analysis calculated for C8H604: C, 57.85; H, 3.61. Found: C, 57.90; H, 3.41.

Exo- 7-oxabicyclo [ 2.2.1] hept- 5-ene-2, 3-dimethanol (II). To a suspension of 2.80 g (74.0 mmol) of LiAIH4 in 150 ml of THF was added dropwise a solution of 10.0 g (60.2 mmol) of I in 120 ml of THE The reaction mixture was then allowed to stir for 24 h. In sequence 3.0 ml of water, 3.0 ml of 15% NaOH, and 9.0 ml of water were added to the reaction solution. The resulting white precipitate was filtered and the filtrate concentrated. The precipitate was washed by Soxhlet extraction with CHzC12 for two days. The solution was concentrated and the combined residues were purified by Kugelrohr distillation to yield 8.20 g (86.3% yield) of a clear viscous oil: b.p. 100 ° at 0.003 mmHg; IR (neat) 3600-3220, 2853, 2820, 1313, 1151, 734; ]H NMR (400 MHz, CDC13) d~ 6.39 (s, 1H), 4.69 (s, 1H), 4.52 (t, 1H, Jan = 1.4 Hz), 3.73 (m, 2H), 1.92 (m, 1H); i3C NMR (100.6 MHz, CDC13) d 135.7, 81.12, 62.37, 42.35; MS m/e 157 (3.89), 155 (0.73), 121 (26.67), 68 (100.0), 57 (68.36). Analysis calculated for C8H]203: C, 61.56; H, 7.69. Found: C, 61.27; H, 7.89.

262 B. M. Novak, M. W. Ellsworth / Inverse organic-inorganic composites

Exo-5, 6-dimethoxyrnethyl- 7-oxabicyclo[2.2.1]hept-2- ene (III). Diol II (5.0 g, 32.03 mmol) in 10 ml of THF was added dropwise to a stirring suspension of 1.92 g (80.10 mmol) of Nail in 25 ml of THR. The reaction was stirred for an additional 30 min after which 8.0 ml (18.2 g, 128 mmol) of CH3I in 10 ml of THF was added dropwise to the solution. After complete addition, water was added dropwise until no further bub- bling was observed (ca. 10 ml). The reaction solution was poured into 500 ml of ether and filtered. The filtrate was concentrated and the residue purified by vacuum distillation to yield 11.33 g (71.4% yield) of a clear liquid: b.p. 49-51 ° at 0.006 mmHg; IR (neat) 2828, 2790, 1320, 1146, 733; 1H NMR (500 MHz, CDCI3) 6 6.17 (s, 1H), 4.65 (s, 1H), 3.30 (m, 1H), 3.19 (s, 3H), 3.13 (m, 1H), 1.73 (m, 1H); ~3C NMR (125 MHz, CDC13) 6 137.0, 80.14, 71.66, 58.40, 39.45; MS m/e 153 (0.23), 122 (0.93), 116 (4.51), 94 (2.74), 68 (100). Analysis calculated for C 10H1603: C, 65.21; H, 8.75. Found: C, 65.23; H, 8.85.

7-Oxabicyclo[2.2.1]hept-5-ene-2-carboxyl chloride (IV). In a round-bottomed flask flushed with dry air were placed 20.0 ml (18.72 g, 274.0 rnmol) of furan and 11.2 ml (12.40 g, 137.0 mmol) of acryloyl chloride. The flask was protected from light with aluminum foil and the reaction mixture was stirred for five days. The yield and product composition were determined from the ~H NMR spectrum for the crude reaction mixture. Comparison of the furan resonance at 6 8.46 and the acrylate resonance at 6 6.61 with the bridgehead protons of the Diels-Alder adduct at 6 5.35 showed 74.5% conversion for an overall product yield of 16.23 g (102.0 mmol): ~H NMR (500 MHz, CDCI3) 6 6.42 (m, 1H), 6.35 (m, 1H), 5.30 (s, 1H), 5.07 (dd, 1H, JnH=4.35 Hz; Jnn=9.73 HZ), 3.59 (quintet, Ji4n=4.26 Hz, C(2)H__, endo isomer), 2.89 (% Jnn = 3.95 Hz, C(2)//, exo isomer), 2.12 (m, 1H), 1.63 (m, 1H); ~3C NMR (125 MHz, CDCI3) ~ 174.0, 172.1, 137.9, 131.8, 80.72, 79.46, 79.22, 78.16, 55.31, 55.16, 29.84, 29.10.

2-Carbomethoxy- 7-oxabicyclo[2.2.1]hept-5-ene (V). The crude reaction mixture of IV was added dropwise to a solution of 5.60 ml (4.49 g, 140 mmol) of methanol and 19.5 ml (140 mmol) of triethylamine in 250 ml of THF at 0 °C. After complete addition, the reaction mixture was stirred overnight to ensure complete reaction. The reaction mixture was filtered and the filtrate was concentrated. The resulting red-brown residue was distilled under vacuum to yield 15.77 g (100% yield) of a clear oil (lit. bp. 66-69 °C at 0.12 mmHg), b.p. 38-40 ° at 0.05 mmHg; IR (neat) 2931, 2825, 1734, 1491, 1351, 1136, 945, 733; ~H NMR (500 MHz, CDC13) ~ 6.44 (d, Jnn = 5.82 Hz, C(6)H, endo

isomer), 6.42 (d, JHH=5.81 Hz, C(6)/4, exo isomer), 6.34 (d, JHH = 5.81 HZ, C(5)t/, exo isomer), 6.22 (d, JnH = 5.83 Hz, C(5)H, endo isomer), 5.18 (bs, C(1)H, exo isomer), 5.15 (d, JHH=4.49 Hz, C(4)H, endo isomer), 5.06 (d, JnH= 4.47 Hz, C(1)H, exo isomer), 5.01 (d, JHH=4.46 HZ, C(4)H, exo isomer), 3.72 (s, CO2CH3, exo isomer), 3.64 (s, CO2CH3, endo isomer), 3.10 (quintet, Jm~ = 4.91 Hz, C(2)H, endo isomer), 2.43 (q, JHH=4.62 Hz, C(2)H, exo isomer), 2.11 (m, 1H), 1.56 (m, 1H); 13C NMR (125 MHz, CDC13) ~ 173.9, 172.3, 136.8, 134.4, 132.3, 80.60, 78.70, 78.42, 77.67, 51.79, 51.44, 42.39, 28.79, 28.23; MS m/e 154 (9.72), 94 (2.20), 86 (5.50), 85 (45.08), 68 (89.84), 55 (100.0). Analysis calculated for C8H1003: C, 62.36; H, 6.49. Found: C, 62.59; H, 6.58.

7-Oxabicyclo[2.2.1]hept-5-ene-2-methanol (VI). To a suspension of 2.80 g (74.0 mmol) of LiA1H 4 in 250 ml of THF was added dropwise 9.28 g (60.20 mmol) of ester V in 50 ml of THE The solution was allowed to stir overnight to ensure complete reaction. In sequence 3.0 ml of water, 3.0 ml of 15% NaOH, and 9.0 ml of water were added to the reaction solution. The resulting white precipitate was filtered and the filtrate concentrated. The precipitate was washed by Soxhlet extraction with CH2C12 for 24 h. The CHEC12 solution was concentrated and the combined residues were distilled under vacuum to afford 7.35 g (96.9% yield) of a clear liquid: b.p. 45-47 ° at 0.007 mmHg; IR (neat) 3400-3200, 2821, 2796, 1491, 946, 736; ~H NMR (400 MHz, CDC13) ~ 6.31 (d, JHH = 5.89 HZ, C(6)H, endo isomer), 6.24 (s, C(5) C(6)L-/, exo isomer), 6.20 (d, JHH=5.89 Hz, C(5)H, endo isomer), 4.94 (d, JHH=4.10 Hz, C(1)H, exo isomer), 4.85 (m, C(4)H), 4.78 (s, C(1)L-/, endo isomer), 3.60 (quintet, Jim = 4.55 Hz, CH2OH , exo isomer), 3.44 (m, CL-/2OH, endo isomer), 3.19 (bs, CH2OH, exo isomer), 3.05 (m, CH2OH, endo isomer), 1.85 (m, C(2)H, endo isomer), 1.69 (m, C(2)H, exo isomer), 1.25 (m, 2H); 13C NMR (100 MHz, CDCI3) ~ 136.2, 135.7, 134.6, 131.9, 79.43, 79.29, 78.14, 64.87, 64.63, 40.32, 39.66, 27.99, 27.58; MS m/e 127 (3.70), 126 (3.26), 108 (41.24), 95 (51.77), 68 (63.36), 58 (73.89). Analysis calculated for C7H1002: C, 66.69; H, 7.93. Found: C, 67.15; H, 8.15.

Tetrakis(5-norbornen-2-yl methoxy)silane. To a solution of 5.80 ml (5.96 g, 47.8 mmol) of 5-norbornene-2- methanol and 6.80 ml (48 mmol) of triethylamine in 150 ml of toluene was added dropwise over 1 h 1.35 ml (1.806 g, 10.63 mmol) of SiCI 4 in 30 ml of toluene. After complete addition, the mixture was heated at reflux for 3 h. The resulting white precipitate was filtered and washed with petroleum ether (2x25 ml). The filtrate was concentrated and the remaining solid was recrystallized from 50 ml of a 1:1


acetonitrile:petroleum ether solution to afford 5.22 g (85.0% yield) of white flaky crystals: m.p. 82-83°; IR (CC14) 2950, 2720, 1413, 1339, 1250, 945, 736; ~H NMR (500 MHz, CDC13) ~ 6.12 (m, 1H), 5.97 (m, 1H), 3.53 (m, 1H), 3.35 (m, 1H), 2.94 (bs, 1H), 2.78 (bs, 1H), 2.34 (m, 1H), 1.80 (m, 1H), 1.41 (d, JHH = 1.7 Hz, 1H), 1.23 (m, 1H), 0.49 (d of t, JHH = 9.9, JHH = 1.7 Hz, 1H); 13C NMR (125 MHz, CDCI3) 6 137.0, 132.6, 66.83, 49.34, 43.67, 42.23, 41.29, 28.69, 25.61; MS m/e 520 (10.66), 413 (8.72), 199 (8.83), 107 (78.79). Analysis calculated for C32H44045i: C, 73.80; H, 8.45. Found: C, 73.75; H, 8.69.

SIPN composites using various monomers and TMOS. SIPN xerogels were prepared by weighing 5.0 mg of K2RuC15 and 10-200 mg of monomer in separate scintillation vials. The solids were dissolved in 1.0 ml of aqueous NaF followed by 1.0 ml of methanol or ethanol. The NaF concentration is dependent on the monomer concentration and was chosen such that the gelation time did not exceed the time needed for complete polymerization of the ROMP monomers (see Table 1). The cosolvent is dependent on the solubility of the ROMP polymer. 1.0 ml of TMOS was then added. The vial was sealed and placed in a 60 °C bath for the required amount of time. The gel was removed from the bath and allowed to set overnight. The gel was dried by stretching a thin sheet of parafilm over the mouth of the vial and allowing the solvent to diffuse slowly through the parafilm. After one week, the partially dried gels were submersed in water for 24 h to remove residual ruthenium. The gels were then dried in the same manner for an additional three weeks to yield clear, low density glass composites.

Representative non-shrinking composite synthesis from tetraalkeneyl orthosilicates. In a scintillation vial was combined 1.0g of tetrakis(7-oxanorbornene methoxy) silane, 0.5 g of 7-oxanorbornene methanol, 5 m of K2RuCIs'H20 , and 0.01 ml of 50 mM NaF. The solution was heated at 60 °C under N2 for 2 h with occasional swirling during the first 5 min to maintain homogeneity. After 1 h, a transparent, light orange, rubbery composite was obtained. Further heating at 100°C for 24 h produced a more rigid composite material. The same procedure was used for the free radical composite synthesis with benzoyl peroxide as the polymerization catalyst.

Representative non-shrinking composite synthesis from silicic esters. A 35 ml aliquot (1.7 g H E M A silicic ester) was taken from a solution containing 4.8 g of H E M A silicic ester in 100 ml of THF. To the aliquot solution was added 0.5 g of HEMA, 20 mg of ethylene diacrylate, and 20 mg of benzoyl peroxide. The solu-

tion was concentrated in vacuo until a viscous solution formed. The solution was poured into a scintillation vial and the vial was evacuated in order to outgas the sample before polymerization. After 2-3 h, the vial was purged with N2 and placed in a 60 °C bath for 3 h. The solidified sample was removed by crushing the vial. The composite sample was then placed in a vacuum oven at 80 °C for 24 h after which a 2.0 g transparent composite containing approximately 50% glass and 50% polymer was obtained. The same general procedure was used for the 7-oxanorbornene monomers was K2RuCIs.H20 as the polymerization catalyst. IR (thin film). Analysis: found C, 30.41%; H, 4.78%, SiO2 residue, 48.9%.

Acknowledgment

We gratefully acknowledge financial support for this work from the National Science Foundation Presiden- tial Young Investigator Award, the Alfred P. Sloan Foundation, the Office of Naval Research, The Center for Advanced Materials, Materials Science Division, Lawrence Berkeley Laboratory, E. I. du Pont Nemours and Company, the Exxon Foundation, Amoco, and Chevron Research. M.W.E. acknowledges the Depart- ment of Education for a Graduate Student Fellowship. B.M.N. is an Alfred P. Sloan, Fellow 1991-1993, and National Science Foundation, Presidential Young Investigator 1991-1996.

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