are controlled by the L/M and H2O/M ratios. Heterometallic alkoxides or oxoalkoxides are useful precursors to multicomponent polymers analogous to random copolymers and block copolymers. Recently, attempts have been made to synthesize well-defined oligomeric oxoalkoxides such as Ti16O16(OEt)32 for use as "molecular building blocks" to assemble networks with intermediate range order.
Structural features of traditional organic polymers are married with those of nontraditional inorganic polymers in hybrid materials. Polycarbosilanes [–SiRR'–CH2–], polysilazanes [–SiRR'–NR"–], and polyborazines [–BR–NR'–], precursors to silicon carbide, silicon nitride, and boron nitride, also fall in this category. A common synthetic route to the formation of hybrid materials is the hydrolysis of organoalkoxysilanes R'xSi(OR)4x in a sol-gel process. If R' is a nonreactive group such as an alkyl, it will serve to modify the inorganic network. If R' is itself polymerizable (e.g., epoxy or vinyl), interpenetrating organic and inorganic networks can form. Depending on the choice of catalyst and the uniformity of the hydrolysis process, the reaction can be designed so that the organic and inorganic networks form simultaneously or sequentially. Other interesting hybrid materials result from the dispersion of an inorganic phase within an organic matrix or vice versa. For example, the swelling of polydimethylsiloxane with tetraethoxysilane followed by in situ hydrolysis yields a silicon-dioxide-filled composite. The addition of appropriate organic molecules or enzymes to sol-gel matrices results in optically active or bioactive materials.
With most of the elements of the periodic table available, the opportunities for chemists to synthesize new inorganic polymers and networks with unique properties are clearly unlimited. However, the greater chemical and structural diversity represented by this class of materials compared to traditional organic polymers provides daunting synthetic challenges. In general, the ability to tailor chain (or network) architecture (e.g., topology, sequence, molecular weight, and stereochemistry) has not been widely demonstrated, especially for heterometallic systems. Future directions of research should focus not only on new materials but also on strategies to control the architectures of existing polymers and networks. Along these lines, some chemists are looking to nature, where there exist numerous examples of low-temperature routes to high-strength, high-toughness inorganic materials such as abalone shells and novel magnetic and nonlinear optical materials and processes such as magnetotactic growth of Fe3O4 in bacteria and cadmium sulfide particles formed by yeast. Biological systems rely on the organic matrix to control morphology and crystal nucleation sites, on interactive