The mechanism of the initiation step of β-lactones polymerization with potassium anions of alkalide is verified, as had been suggested by Szwarc [Szwarc, M. Ionic Polymerization Fundamentals; Carl Hanser Verlag: Munich, 1996; p 69]. Contrary to previous papers, the carbon-carbon bond scission is not observed. It is shown that indeed this kind of bond rupture does not occur, at least in β-butyrolactone. In fact, the acyloxygen bond is mainly cleaved in this lactone by potassium anions. The reaction proceeds by lactone dianion as an intermediate succeeding formation of the radical anion. The latter decomposes by the alkyl-oxygen bond cleavage in a side reaction. Finally, potassium crotonate and potassium 3-hydroxybutyrate are mainly formed, and potassium butyrate is a side product. They become the real initiators of the polymerization. The same results are observed with alkalide K -, K + (15-crown-5) 2 , and K -,K + (18-crown-6).

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