Catalysts for Polymerisation of Epoxides

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]. 

The first enantioasymmetric polymerisation of (R, S)-propylene oxide, reported in 1962, was carried out in the presence of the diethylzinc (+)-borneol catalyst [23,24]. Other optically active catalysts for propylene oxide stereoelective polymerisation, e.g. diethylzinc (R)-( )-3,3-dimethyl-1,2-butanediol were described later on [25]. 

Special interest has been devoted to such systems as diethylzinc methanol (7 8) [26 28] and diethylzinc l-methoxy-2-propanol (2 3) [29], since well-defined catalysts were isolated in the form of single crystals in these systems, namely [Zn(OMe)2] [EtZnOMe]6 and [Zn(OCH(Me)CH2OMe)2]2[EtZnOCH 

The formation of epoxide polymers with a very high molecular weight by the discussed catalysts containing associated multinuclear species (— Zn-0— Zn O ) indicates that only a small fraction of the metal species in the catalyst is effective for the polymerisation. The broad molecular weight distribution of polymers yielded by these catalysts corresponds to the existence of various active sites [30]. 

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