Bimetallic oxoalkoxide catalyst

Besides direct hydrolysis, heterometaHic oxoalkoxides may be produced by ester elimination from a mixture of a metal alkoxide and the acetate of another metal. In addition to their use in the preparation of ceramic materials, bimetallic oxoalkoxides having the general formula (RO) MOM OM(OR) where M is Ti or Al, is a bivalent metal (such as Mn, Co, Ni, and Zn), is 3 or 4, and R is Pr or Bu, are being evaluated as catalysts for polymerization of heterocychc monomers, such as lactones, oxiranes, and epoxides. An excellent review of metal oxoalkoxides has been pubUshed (571). 

Aluminium alkoxides (especially aluminium isopropoxide), dialkylalumi-nium alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic -oxoalkoxides, aluminium porphyrins and aluminium Schiff s base complexes are the most representative coordination catalysts, containing multi-nuclear or mononuclear species, for lactone polymerisations (Table 9.5). 

When catalysed by bimetallic /(-oxoalkoxides, the polymerisation of / -, 3-and e-lactones involves the alkoxide group as the attacking nucleophile. Thus, the mean degree of association of the catalyst exerts a further decisive effect on the kinetics of the polymerisation [122,125], Bimetallic / -oxoalkoxides and aluminium alkoxides [106] have the great advantage of polymerising /f-propio-lactone and e-caprolactone in a living manner. 

The coordination catalysts for these reactions are diverse. They can be compounds of alkaline earth metals, like calcium amide, or calcium amide-alkoxide. They can also be Ziegler-Natta-type catalysts. These can be alkoxides of aluminum, magnesium, or zinc combined with ferric chloride. Others are reaction products of dialkylzinc with water or alcohol. They can also be bimetallic //-oxoalkoxides, such as [(RO)2A102]Zn. Other catalysts are aluminum or zinc metalloporphyrin derivatives (see Fig. 4.1). 

Bimetallic oxoalkoxides are excellent catalysts for the ring-opening-polymerization... 

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

Apart from the coordination polymerisation of lactones, the polymerisation of lactide (3,6-dimethyl-l,4-dioxacyclohexane-2,5-dione) with various coordination catalysts is of growing interest. This is connected with some advantageous properties of polyesters obtained from rac-D,L-lactide and mexo-D,L-lactide as potentially useful materials for biomedical and pharmaceutical applications in terms of their low toxicity and favourable biodegradability. A number of catalysts containing multinuclear species, such as aluminium isopropoxide [108,138,139], triethylaluminium-neopentyl (1 1) [139], triethylaluminium-(+)-menthol (1 1) [139], methylaluminoxane [139] and bimetallic /i-oxoalkoxide [140], as well as catalysts containing mononuclear species, such as (tpp) AlOMe [141] and (sal)AlOMe [130], have been used for the polymerisation of lactide. 

Catalytic activity and organic synthesis Bimetallic-p-oxoalkoxides are used as highly active catalysts for the polymerization of oxiranes, thiiranes, and lactones. Their reactivity can... 

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