The Polyaddition of Alkylene Oxides to Hydroxyl Groups

The polyaddition of alkylene oxides to hydroxyl groups is catalysed by the alkali hydroxides (KOH and NaOH) or low hindered tertiary amines or, to a much lesser extent, by acid catalysts [Lewis acids and Bronstedt superacids in order to generate short 

It is well known that with the tertiary amines as catalysts it is impossible to obtain high molecular weight polyether chains (for example polyethers for flexible foams) but with short chain polyethers, having 1-3 alkylene oxide units, it is perfectly possible [31, 32]. 

The mechanism of alkylene oxide anionic polyaddition to hydroxyl groups, catalysed by alkali hydroxides, is discussed in chapters 4.1-4.1.5, the real active centre being the alkaline alcoholate, and the propagation reaction being the repeated SN-2 attack of the alcoholate anion on the a-carbon atom of the oxirane rings. The rapid equilibrium of the alcohol - alcoholate assures that each hydroxyl group from the reaction system is a chain initiator. 

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