2 Homometallic Alkoxides

Industrial Applications 675
calcium barium copper oxide. 59 Other heterometaloxides of interest are lithium and
sodium aluminates (ˇ-aluminia), 60 yttrium aluminium oxides (YAlO3, Y4Al2O9 and
Y3Al5O12-YAG), 61 and aluminium zirconium oxide. 62 Other applications involve the
preparation of refractory oxides such as aluminium silicates (e.g. mullite), 63 and
magnesium aluminium silicates (e.g. cordierite), 64 and fast ion conductors such as
sodium zirconium silicon phosphorus oxide (Na2Zr2Si2PO12-NASICON) 65 and lithium
silicon vanadium oxide. 66
Another developing area in which metal alkoxide precursors have been important
is the sol–gel processing of hybrid inorganic–organic polymers called ceramers 67 and
ormosils (organically modified silicates). 68 These hybrid materials appear to combine
the hardness of the metal oxide (or silica) with the characteristic toughness of the
organic polymer and are used in thick coatings which are abrasion resistant or corrosion
resistant.
Alternatively the organic component can be removed by thermolysis or chemical
treatment leaving a microporous metal oxide material. 69
4 METAL ALKOXIDES AS CATALYSTS
Metal alkoxides are used as catalysts in a wide range of homogenous reactions. In
some cases a remarkable degree of steroselectivity has been achieved. The following
sections are representative of developments in this field.
4.1 Catalytic Polymerization of Alkenes (Olefins)
The use of metal alkoxides, especially those of Ti and Mg, as active components
of multicomponent Ziegler–Natta type catalysts has an established place in industry.
Many patent applications have been filed over the years but they read rather like
cookery recipes and will not be documented here. Anionic polymerization of styrene,
butadiene, and isoprene has also received much investigation. The addition of alkali
metal alkoxides MOR to alkali metal amides MNH2 gives rise to a range of complex
base initiators whose properties may be modulated by the nature of the metal, alkoxide
group, and solvent. Similarly the effect of lithium alkoxides on the rate of polymerization
of styrene initiated by n-butyl lithium has also been reported. 70–72
The effect of metal alkoxides on the properties of initiators for the anionic polymerization
of vinyl monomers has also been reported for polymethylmethacrylate, 73
polymethacrylonitrile 74 and poly-2-vinyl pyridine. 75 The polymerization esterification
of terephthalic acid with glycols is another important industrial process which is catalysed
by metal alkoxides, mainly titanium or alkali metal derivatives. Numerous patent
applications have appeared over the years.
4.2 Asymmetric Epoxidation of Allylic Alcohols
One of the most important applications of metal alkoxides post-1978 was reported
by Katsuki and Sharpless in 1980. 76 They discovered that a mixture of titanium