[acs symposium series] inorganic and organometallic polymers ii volume 572 (advanced materials and...

Chapter 13

Reaction of Boehmite with Carboxylic Acids New Synthetic Route to Alumoxanes

Christopher C. Landry, Nina Pappé, Mark R. Mason, Allen W. Apblett, and Andrew R. Barron

Department of Chemistry, Harvard University, Cambridge, MA 02138

Reaction of pseudo-boehmite, [Al(O)(OH)]n, with carboxylic acids (RCO2H) results in the formation of the carboxylate-alumoxanes, [Al(O)x(OH)y(O2CR)z]n

where 2x + y + z = 3 and R = C1 - C13. The physical properties of the alumoxanes are highly dependent on the identity of the alkyl substituents. The alumoxanes have been characterized by scanning electron microscopy, IR and NMR spectroscopy, and thermogravimetric analysis. A model structure of the alumoxanes is proposed, consisting of a boehmite-like core with the carboxylic acid substituents bound in a bridging mode. All of the alumoxanes decompose under mild thermolysis to yield γ-alumina.

The facile formation of ceramic materials from molecules has undoubtedly been one of the significant contributions made by chemistry to materials science (7). However, it is desirable not only to produce the ceramic per se but also to do so in a specific form, for example a fiber. Therefore, one of the key requirements for any ceramic precursor should be its processability. For this reason, there has been continued research effort aimed at the design of precursors with physical properties suitable for processing prior to pyrolysis. Two examples with significant commercial application are polyacrylonitrile and polyorganosilanes, both of which may be spun into fibers, and upon pyrolysis allow for the manufacture of carbon-graphite (2) and silicon carbide (3) fibers, respectively. Despite much effort, the extension of this polymer-type precursor strategy to other ceramic systems has only met with limited success.

In the case of alumina fibers a common synthetic route has involved the use of alumina gels, which are formed by the neutralization of a concentrated aluminum salt solution (4). However, the strong interactions of the freshly precipitated alumina gels with ions from the precursors' solutions makes it difficult to prepare the gels in pure form (5). Furthermore, the yield of alumina fibers from the gel is low because of the low processability of the precursor during spinning. To avoid these difficulties, alumina fibers have been prepared from alumoxanes (6). Alumoxane is the generic term given to aluminum oxide "polymers" (7) formed by the hydrolysis of aluminum compounds, AIX3 (Eq. 1) where X = alkyl, alkoxide, etc. (8).

0097-6156/94/0572-0149$08.00/0 © 1994 American Chemical Society

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In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

150 INORGANIC AND ORGANOMETALLIC POLYMERS II

AIX3 Η 2 0 ,Δ

* [Al(0)(X)]n (1) - 2 H X

While early examples of alumoxane precursors for alumina fiber synthesis had processing characteristics superior to alumina gels, they were found to be unstable and decomposed during spinning. In addition their structures were a complete mystery, making further developments difficult

The structure of alumoxanes have (despite contradicting spectroscopic data) traditionally been proposed to consist of linear or cyclic chains (I) (9).

However, recent work from this laboratory (70) has redefined the structural view of alumoxanes and shown that, while the core structure is dependent on the identity of the organic substituent, alumoxanes are not chains or rings but three-dimensional cage compounds. Thus, alkyl-alumoxanes, (RA10)n, adopt cage structures analogous to those observed for gallium sulfides (77) and iminoalanes (72), while the structure of the hydrolytically stable siloxy-alumoxanes, [Al(0)(OH)x(OSiR3)i-x]n, consists of an aluminum-oxygen core structure (Π) analogous to that found in die mineral boehmite, [Al(0)(OH)]n, with a siloxide-substituted periphery (ΙΠ) (10a).

We have reported that the physical properties of these siloxy-substituted alumoxanes are highly dependent on the relative organic content (10c). Low molecular weight clusters (M « 2400 g mol"1) with high siloxide content (Si:Al « 1.4) are soluble in hydrocarbon solvents. However, the alumoxanes formed from the equilibrium hydrolysis of aluminum compounds have a low siloxide content (Si:Al « 0.14), are insoluble in all solvents, and infusible; a similar trend had previously been observed for carboxylate-alumoxanes (13). Without the advantage of hindsight, Kimura proposed that the instability of the alumoxanes used for fiber formation was due to the coordinative unsaturation at aluminum, and suggested that the use of carboxylate ligands in an appropriate ratio with aluminum would allow for the latter to be "properly coordinated". Kimura and co-workers subsequently demonstrated that carboxylate-alumoxanes were excellent precursors to alumina ceramic fibers with properties superior to those formed from alumina gels (14). These preceramic carboxylate-alumoxanes were prepared via a multi-step synthesis requiring accurate control over the reaction conditions (Eq. 2).

(I)

Et3SiO QSiEt3

(Π) (ΠΙ)

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13. LANDRY ET AL. Reaction of Boehmite with Carboxylic Acids 151

+ HO2CR +H 2 0 AlEt 3 > AlEt 2(0 2CR) >

- EtH - 2 EtH

+ H0 2 CR A1(0)(Q2C»)- > A1(0)(Q2CR)(HQ2CR) (2)

Furthermore, while some of the carboxylate-alumoxanes formed gels in THF only those with long chain substituents (e.g., dodecanoic acid) and hence low ceramic yield were melt-processable. It would thus be desirable not only to prepare alumoxane preceramics in a one-pot bench-top synthesis from readily available starting materials, but also to determine if lower hydrocarbon substituents could yield better processability.

Synthetic Strategy

If we assume that all hydrolytically stable alumoxanes have the boehmite-like core structure (Π) it would seem logical that instead of synthesizing alumoxanes from a low molecular weight precursor they could be prepared directly from the mineral boehmite. In the siloxy-alumoxanes we have shown the "organic" unit surrounding the boehmite core itself contains aluminum (ΠΙ). Thus, in order to prepare the siloxy-alumoxane similar to those we have previously reported (10a) the anionic moiety the "ligand" [Al(OH)2(OSiR3)2]~ would be required as a bridging group; adding this unit would clearly present a significant synthetic challenge. However, the carboxylate-alumoxanes represent a more realistic synthetic target since the carboxylate anion, [RC0 2]-, is an isoelectronic and structural analog of the organic periphery found in our siloxy-alumoxanes, cf., ΠΙ and IV.

R

I

I . A I JAII JAIL

SIXI (IV)

In previous studies carboxylate-alumoxanes were prepared via a multi-step synthesis involving the reaction of a carboxylic acid with an alkoxy-alumoxane (75), Eq. 3.

[Al(0)(OR)]n +RCO2H > [Al(0)(02CR)]n + HOR (3)

In order to simplify discussion, the alkoxy-alumoxanes were represented as having the general formula [(RO)A10]n. However, data from our laboratory has demonstrated that the alkoxide-alumoxanes have a high hydroxide content, i.e., [ A ^ O ^ O H ^ O R ) ^ (16). Based on spectroscopic characterization, the carboxylate-alumoxanes resulting from the synthesis have a lower hydroxide content; therefore, the

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carboxylic acid must also react with the hydroxide groups as shown in Eq. 4. It is entirely reasonable therefore that the reaction of boehmite, [Al(0)(OH)]n, with a carboxylic acid, RCO2H should yield the appropriate carboxylate-alumoxane (Eq. 5).

OH 0 2 CR I r

A l + RC0 2 H > A l + H 2 0 / \ / \ (4)

[Al(0)(OH)]n +RCO2H > [Al(0)(02CR)]n + H 2 0 (5)

Despite the fact that boehmite is a naturally occurring mineral, the majority of commercial samples are man-made, often by the hydrolysis/thermolysis of aluminum salts. In addition, they commonly contain significant quantities of gibbsite, Al(OH)3, and are of variable porosity. Although we have shown die reactions discussed below to be applicable for samples from different commercial sources, to be self consistent we have chosen to discuss results obtained using a single source of low gibbsite (> 99 % boehmite) obtained from American Cyanamid.

Synthesis and Characterization of Carboxylate-alumoxanes

Refluxing powdered pseudo-boehmite, in air, with an excess of a carboxylic acid, RCO2H, either neat (e.g., R = CH3, acetic acid) or as a xylene solution (e.g., R = C5H11, hexanoic acid), results in the formation of the corresponding carboxylate-alumoxane (see Table 1). The alumoxanes are isolated by either filtration of the cooled reaction mixture or removal of all volatiles under vacuum followed by washing with Et 2 0 to remove traces of free acid. During the course of the reaction in xylene, gel formation is observed for most of the carboxylates; however, no gelation is observed if boehmite is refluxed in xylene in absence of an acid. Despite gel formation during synthesis only the hexanoate and octanoate alumoxanes form gels in organic solvents once isolated. A list of the alumoxanes synthesized and their appropriate synthetic routes is given in Table 1.

The as-received boehmite is a free flowing (fluid) white powder with no aggregation. The physical appearance and solubilities of the materials resulting from reaction of the boehmite with carboxylic acids are highly dependent on the identity of the carboxylate substituent. Thus, for R = C n H 2 n + i (n = 1 - 3 and 13) and CH2C1 the alumoxanes are white microcrystalline powders, insoluble in common organic solvents, whereas for R = C5H11 and C7H15 the products are white solids, which readily form homogeneous gels in aromatic and other solvents. A summary of the physical appearance and solubility of each alumoxane is given in Table 1.

A sample of the boehmite examined by scanning electron microscopy (SEM), prior to reaction with the carboxylic acid, was found to consist of spherical particles varying in size from 10 -100 pm in diameter (average « 50 pm), see Figure 1. At higher magnifications the spheres may be seen to actually consist of small crystallites, packed together. From the SEI micrograph it is difficult to estimate a crystallite size; however, the largest distinct feature is ca. 0.1 pm in diameter, suggesting a crystallite size of < 0.1 pm. The average crystallite size as determined by XRD is in fact ca. 64 À (020 plane) (77).

In contrast to the near-perfect spheres observed by SEM for the boehmite starting material, the carboxylate-alumoxanes exist as large "fluffy" conglomerates, 50 - 200 pm in size (e.g., Figure 2a), with a particle size estimated from SEM to be less than 0.1 pm in diameter. At higher magnification these constituent particles of the conglomerates can be more readily seen. Figure 2b shows some of the individual needle-like particles of the hexanoate alumoxane.

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Tabl

e 1.

Syn

thet

ic R

oute

s an

d Se

lected

Phy

sical

and

Spec

trosc

opic

Dat

a fo

r Car

boxy

late

Sub

stitu

ted A

lum

oxan

es

Carb

oxyl

ic A

cid

Synt

hesi

sb So

lubi

lity

Cera

mic

yie

ldd

IR ν

(θ

2θ

, cm

-1 13

C N

MR

2? A

l NM

R

(RC

0 2H

), R

%

(°C)

an

tisym

sy

m

(ppm

) (p

pm)

acet

ic, C

H3

neat

no

ne

30 (3

90)

1586

14

66

179.

3 1.

0

chlo

roac

etic

, CH

2C

I xy

lene

none

27

(410

) 16

19

1464

17

5.4

-4.7

prop

ioni

c, C

2H5

neat

no

ne

28 (3

50)

1589

14

73

180.

0 e

buta

noic

, C3H

7 ne

at

none

23

(347

) 15

86

1466

18

1.7

-0.5

piva

lic, C

(CH

3)3

xyle

ne

none

25

(351

) 15

86

1466

18

3.7

e

hexa

noic

, C5H

11

xylen

e TH

F, p

y ar

omat

ics0

30 (3

52)

1587

14

65

180.

0 1.

5'

octa

noic

, C7H

i5a

xylen

e ar

omat

icsc

30 (3

32)

1586

14

66

180.

8 2

.4

dode

cano

ic, C

11H

23

xy

lene

pyrid

ine,

DM

F 60

(465

) 15

87

1466

18

1.1

a wax

y so

lid,b se

e Ex

perim

enta

l Sec

tion,

c gel

form

ation

in a

rom

atic

hyd

roca

rbon

s,d fro

m T

GA

,e not

obt

aine

d,f s

olut

ion

spec

trum

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Figure 1. SEI micrographs of unreacted boehmite particles. Dow

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LANDRY ET AL. Reaction of Boehmite with Carboxylic Acids

Figure 2. SEI micrographs of hexanoate-alumoxane particles.

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Despite the particulate nature of the alumoxanes, homogenous continuous films and bodies may readily be prepared. Films of the hexanoate-alumoxane can be formed by dissolution of the alumoxane in either CH2CI2 or THF followed by spin coating. For example, evaporation of a CH2CI2 solution of the hexanoate-alumoxane on a glass slide yields a thin film, the SEM of which is shown in Figure 3. The homogenous nature of these films implies that they consist of an interpenetrating organic/inorganic matrix. While films of the alumoxanes are contiguous, many show significant shrinkage upon drying, resulting in cracking of the surface.

The surface area of the carboxylate-alumoxanes as determined by gas desorption was found to be significantly higher than the boehmite precursor (ca. 22 m2). While the exact value is dependent on the acid and treatment conditions, all the alumoxanes were found to have a surface area of 120 - 150 m 2 (18).

Attempts to obtain self-consistent elemental analysis for the carboxylate-alumoxanes proved futile. For any specific alumoxanes the carbon and hydrogen analyses varied slightly between samples within a reaction run, and significantly between runs. Similarly, the determination of the hydroxide content by sodium napthalide titration/EDX analysis (10a) gave variable results. This means that while it is possible to define a general formula for the alumoxanes, i.e., [Al(0)x(OH)y(02CR)z]n, an exact representation of the product, i.e., the values for x, y and z, is not possible. Additional complications were found in determining the composition of the alumoxanes if the samples were not washed adequately with Et20 to remove any excess carboxylic acid, which based on TGA data is adsorbed to the surface of the as-synthesized alumoxanes. However, all of the analytical techniques employed indicate that the value for z, the carboxylate content, was slightly greater than one (i.e., 1.0 < ζ < 1.3) in all samples. This is in full agreement with the results of Kimura et al (75).

The variation in composition may be expected, since the reaction of the carboxylic acid with the pseudo-boehmite will be highly dependent on the reaction conditions; in particular the particle size, surface morphology, and the identity of surface groups (i.e., Al-O-Al versus A1-OH-A1). Thus, while it may be possible to obtain an average value for the number of acid groups per aluminum atom (z) one would still expect a large variation in this average from one particle to the next, even under identical reaction conditions. The variation of the acid:aluminum ratio between individual particles is best demonstrated by the EDX analysis of the alumoxane formed from the reaction of pseudo-boehmite with 6-bromohexanoic acid. The presence of the bromide allows the ready determination of the Br:Al ratio, and hence the number of carboxylate groups per aluminum atom, without disrupting the reactivity of the acid and/or the identity of the resulting alumoxane. EDX analysis shows a sizable variation in the Br: Al ratio from particle to particle.

Based upon their method of synthesis, the carboxylate-alumoxanes prepared by the route shown in equation 2 were proposed to be of the general formula of [AKOXC^CRXHC^CR)]!! in which one of the acid groups exists as the deprotonated form, while the other is protonated (13a). However, from *H NMR and IR spectroscopy the two carboxylate groups could not be differentiated (13a). In the present case the presence of greater than one equivalent of carboxylic acid per aluminum would possibly be consistent with this formulation. However, 1 3 C CPMAS NMR spectroscopy (see Table 1) of the carboxylate-alumoxanes indicates only a single carboxylate environment, consisting of the appropriate resonances due to the aliphatic carbons and one peak (ca. δ = 180 ppm) due to the carboxylate α-carbon (02CR).

If, as we have previously proposed, the core structure of alumoxanes is analogous to that of boehmite then one would expect the aluminum to retain exclusively six-fold coordination. This is clearly demonstrated to be the case by the

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Figure 3. SEI micrograph of a hexanoate-alumoxane film spin coated on glass from methylene chloride solution. D

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2 7 A l MAS NMR spectra of the alumoxanes (Table 1), in which only a signal attributable to six-coordinate aluminum is detected (δ = 0 to -6 ppm, W1/2 = 1800 -2000 Hz), e.g., Figure 4. It is worth noting that the solution 2 7 A1 NMR spectra of previously reported carboxylate-alumoxanes consists of a peak at ca. 0 ppm (W1/2 = 80-100 Hz) (756).

I « 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 400 300 200 100 0 -100 -200 -300 -400

Figure 4 . 2 7 A l MAS NMR spectrum of chloroacetate-alumoxane.

The IR spectra of all the carboxylate-alumoxanes contain bands at 1596 - 1586 and 1473 - 1466 cm - 1 (Table 1), consistent with a bridging mode of coordination (IV). The carboxylate-alumoxanes previously reported have carboxylate bands at 1580 and 1470 cm - 1 suggesting a similarity in the structures. Some samples with the highest carboxylate content contain additional bands at 1680 - 1640 and 1610 - 1570 cm*1, indicative of unidentate coordination of a carboxylic acid group to aluminum (V) (79). In addition, all the IR spectra show a broad absorption bands between 3700 and 3400 cm - 1 , consistent with our previous assignment for an aluminum-bound hydroxide group (10a).

-4.70 ppm

R

(V)

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A Structural Model for Carboxylate-Alumoxanes

While it is clear from the previous discussion that the carboxylate-alumoxanes are not a single species, we are able to define a structural model for the products formed from die reaction of boehmite with a carboxylic acid.

The structure of the boehmite precursor (Π) may be considered as consisting of two parallel but staggered chains of six-coordinate aluminum atoms, linked by oxygens p3-capping alternate faces and p2-bridging the sides of the chain. The remaining coordination site on the aluminum centers are occupied by the p2-bridging oxygen atom of the neighboring chains. These chains pack in layers joined by hydrogen bonding, see Figure 5.

Figure 5. Structure of boehmite. Oxygen atoms are to be imagined at the vertices of each octahedron, which has an aluminum atom at the center. The double lines indicate hydrogen bonding.

Based on the 2 7 A l MAS NMR spectra of the carboxylate-alumoxanes we propose that the aluminum core retains the six-coordinate structure of the boehmite. Although direct crystallographic evidence to support this proposal is unavailable, it is entirely feasible based not only on the structure of the precursor but also on literature precedents for a wide range of main group and transition metal clusters; the best supporting evidence for our proposal are the recently reported clusters Alio(OH)i 6(OSiEt 3)i4 (10a), Ni 8Oi 4[(OSiPh) 6] 2 (20) and Ca 9(OCH 2CH20Me)i 8

(HOCH2CH20Me)2 (21), all of which have boehmite-like metal-oxygen cores surrounded by a supporting organic framework.

It should be obvious, however, that whereas the mineral boehmite essentially has an infinite structure, carboxylate-alumoxanes must have a finite size. Thus there must be either end or edge groups encapsulating the boehmite-like core. We have shown previously that low molecular weight alkoxide- and siloxide-alumoxanes contain a significant proportion of four-coordinate aluminum centers, and that these four-coordinate aluminum atoms comprise the end groups. In the case of the

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carboxylate-alumoxanes no such four-coordinate environment is observed. Based upon 1 3 C NMR and IR spectral data we propose that the organic periphery consists of carboxylate groups occupying bridging positions across two adjacent aluminum atoms, such as shown in VI.

Crystallographic precedent for such a structural unit exists for the aluminum dimer [Al2(OH)(02CMe)2{0=C(OMe)Et}6]3+ (22), in which two carboxylates bridge two six-coordinate aluminum atoms [Vu, L = 0=C(OMe)Et]; the carboxylate bands in the IR spectrum of the dialuminum cation (1590 and 1480 cm -1) are within experimental error of the values for the acetate-alumoxane, 1587 and 1477 cm - 1.

While we have no accurate data for the molecular weights of the carboxylate-alumoxanes, based upon the gas phase desorption measurements and the SEI micrographs, we can propose that the particle size of the alumoxanes is significantly smaller than the parent boehmite. Furthermore, the alumoxane particles are rod or sheet-like in shape, not linear polymers. This is due to the destruction of hydrogen-bonding within the mineral as hydroxide groups are removed and replaced with acid functionalities, as shown in Scheme 1.

R

(VI)

Me Me

(VII)

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Scheme 1. Pictorial representation of the reaction of boehmite with carboxylic acids. The rectangles represent a side view of the aluminum-oxygen fused octahedra shown in Figure 5, while the carboxylate groups are represented by a semicircle and bar.

Thermal Decomposition of Carboxylate-Alumoxanes

Al l of the alumoxanes prepared from boehmite decompose between 180 and 385 °C to give AI2O3 in essentially quantitative yield. XRD spectra of the residues are consistent with their identity as γ-alumina. Based on X-ray photoelectron spectroscopic (XPS) analysis, carbon incorporation is found to be very low if the pyrolysis is carried out in an oxidizing atmosphere.

The thermogravimetric/differential thermal analysis (TG/DTA) of the boehmite starting material is shown in Figure 6a. Two distinct regions of mass loss are observed. First, between room temperature and 160 °C the mass decreases by 3 - 7 %, depending on the sample, its storage and age. Second, between 350 °C and 440 °C a loss of a further 14 - 16 % occurs. Based on mass spectroscopic analysis both of these are due to the loss of water. The first mass loss is due to water absorbed and/or hydrogen boded to the surface of boehmite, while the second is more likely due to the dehydration of boehmite giving γ-alumina with a theoretical mass loss of 15 %. The TG/DTA of the alumoxanes are distinct from that of boehmite. In general two regions are observed; the relative mass loss and temperatures at which these regions occur is dependent on the identity of the carboxylic acid. A representative example (the propanoate-alumoxane) is shown in Figure 6b.

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The DTA shows a small endotherm at 140 °C consistent with the boiling point of propanoic acid (141 °C); however, the insignificant mass loss suggests that only traces of free acid remained in the sample. A second endotherm at 245 °C is accompanied by a sharp mass loss of 24 %. A third larger endothermic mass loss occurs at 330 °C. The carboxylate-alumoxanes prepared by Kimura et al (13) showed similar endothermic mass losses (220 - 260 and 300 - 500°C). However, in contrast to the carboxylate-alumoxanes prepared from boehmite the second endotherm occurred over a larger temperature range. The reasons for this difference are at present unclear.

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Conclusions

Alumoxanes of the general formula [Al(0)x(OH)y(02CR)z]n have been prepared by refluxing boehmite with carboxylic acids. These carboxylate-alumoxanes have been shown by SEM to consist of conglomerates of tiny particles (less than 0.1 pm in diameter), while 2 7 A l NMR spectroscopy suggests that the boehmite core structure is retained. From 1 3 C NMR and IR spectroscopy, bridging carboxylate groups are proposed to encapsulate this boehmite-like core. The carboxylate-alumoxanes reported herein are spectroscopically similar to analogs prepared from small molecule precursors. In addition, several alumoxanes with moderate-length carbon substituents (e.g., hexanoate) form solutions or gels in various solvents, which may be readily spin-coated into films. Pyrolysis of the carboxylate-alumoxanes leads to die formation of γ-alumina with low carbon contamination. The boehmite-derived carboxylate-alumoxanes are spectroscopically similar, and have comparable ceramic yields, to analogs prepared from small molecule precursors.

Acknowledgments

This work was funded by a grant from the Office of Naval Research. We acknowledge NSERC (Canada) for a post-doctoral fellowship (A. W. Α.), the University of Paris XI Orsay for a masters scholarship (N. P.), and NASA Lewis Research Center under the Graduate Student Researchers Program (C. C. L.). We are indebted to Dr. J. D. Carruthers at American Cyanamid for the generous gift of boehmite and Prof. Y. Kimura for useful discussions and hospitality.

References

1 See for example: (a) Better Ceramics Through Chemistry IV, Zelinski, B. J. J.; Brinker, C. J.; Clark, D. E.; Ulrich, D. R., eds., Mater. Res. Soc. Proc. 180, Pittsburgh, PA 1990. (b) Chemical Perspectives of Microelectronic Materials II, Interrante, L. V.; Jensen, K. F.; Dubois, L. H.; Gross, M. E., eds., Mater. Res. Soc. Proc. 204, Pittsburgh, PA 1991.

2 Ezekiel, H. M.; Spain, R. G. J. Polym Sci., C. 1967, 19, 249. 3 Yajima, S.; Hasegawa, Y.; Okamura, K.; Matsuzawa, T. Nature 1978, 273, 525. 4 See for example a) Serna, C. J.; White, J. L.; Hem, S. L. Soil. Sci. 1977, 41, 1009.

b) Hsu, P. H.; Bates, T. F. Mineral Mag. 1964, 33, 749. c) Willstätter, R.; Kraut, H.; Erbacher, O. Ber. 1925, 588, 2448.

5 Green, R. H.; Hem, S. L. J. Pharm Sci. 1974, 63, 635. 6 Ichiki, E. Kagaku Kogyo 1978, 31, 706. 7 While alumoxanes have often been classed as metalloxane polymers, this is

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8 The first report of an alumoxane was that formed from the hydrolysis of Al(OSiR3)3. Andrianov, Κ. Α.; Zhadanov, A. A. J. Polym. Sci. 1958, 30, 513.

9 (a) Storr, Α.; Jones, K.; Laubengayer, A. W. J. Am. Chem. Soc. 1968, 90, 3173. (b) Boleslawski, M.; Pasynkiewicz, S.; Kunicki, Α.; Serwatoswki, J. J. Organomet. Chem 1976, 116, 285. (c) Siergiejczyk, L.; Boleslawski, M.; Synoradzki, L. Polimery-Twoizywa Wielkoczasteczkowe 1986, 397. (c) Pasynkiewicz, S. Polyhedron 1990, 9, 429. (d) Bradley, D. C.; Lorimar, J. W.; Prevedorov-Demas, C. Can. J. Chem. 1971, 49, 2310.

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14 Rees, Jr., W. S.; Hesse, W. Mat. Res. Soc. Symp. Proc. 1991, 204, 563. 15 (a) Kimura, Y.; Nishimura, Α.; Shimooka, T.; Taniguchi, I. Makromol. Chem.,

Rapid Commun. 1985, 6, 247. (b) Kimura, Y.; Tanimoto, S.; Yamane, H.; Kitao, T. Polyhedron 1990, 9, 371.

16 Apblett, A. W.; Barron, A. R. unpublished results. 17 Carruthers, J. D., personal communication. 18 Minnick, R. B.; Perrota, A. J., personal communication. 19 Gurian, P. L.; Cheatham, L. K.; Ziller, J. W.; Barron, A. R. J. Chem Soc., Dalton

Trans. 1991, 1449. 20 Levitsky, M. M.; Schegolikhina, Ο. I.; Zhadanov, Α. Α.; Igonin, V. Α.;

Ouchinnikov, Yu. E.; Shklover, V. E.; Struchkov, Yu. T. J. Organomet. Chem. 1991, 401, 199.

21 Goel, S. C.; Matchett, Μ. Α.; Chiang, M. Y.; Buhro, W. E. J. Am. Chem. Soc. 1991, 113, 1844.

22 Sobota, P.; Mustafa, M. O.; Utko, J.; Lis, T. J. Chem. Soc., Dalton Trans. 1990, 1809.

R E C E I V E D June 29, 1994

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[acs symposium series] inorganic and organometallic polymers ii volume 572 (advanced materials and...

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