Carboxylate-alumoxanes

Vogelson, C.T. et al.. Inorganic-organic hybrid and composite resin materials using carboxylate-alumoxanes as functionalized cross-Hnking agents, Chem. Mater., 12,795,2000. 

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. 

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. 

In order to simplify discussion, the alkoxy-alumoxanes were represented as having the general formula [(R0)A10]n. However, data from our laboratory has demonstrated that the alkoxide-alumoxanes have a high hydroxide content, i.e., [Al(0)x(0H)y(0R)z]n (76). Based on spectroscopic characterization, the carboxylate-alumoxanes resulting from the synthesis have a lower hydroxide content therefore, the... 

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

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 (18). 

The IR spectra of all the carboxylate-alumoxanes contain bands at 1596 - 1586 and 1473 -1466 cm l (Table 1), consistent with a bridging mode of coordination (TV). The carboxylate-alumoxanes previously reported have carboxylate bands at 1580 and 1470 cm"l suggesting a similarity in die structures. Some samples with the highest carboxylate content contain additional bands at 1680 - 1640 and 1610 - 1570 cm l, 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 consistent with our previous assignment for an aluminum-bound hydroxide group (10a). 

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. 

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 oxide- 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... 

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 hydrogenbonding within the mineral as hydroxide groups are removed and replaced with acid functionalities, as shown in Scheme 1. 

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 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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