Oxiranes, cocatalysts

Two parallel routes for the elimination of glycol formate are suggested, involving either reaction with H2 or with cocatalyst water. The detection of formic acid in the reaction products suggests another mechanism, with initial production of formic acid from H2 and C02, followed by reaction with the oxirane. This mechanism is not favored however since the yields of glycol formates varied substantially when various substituted oxiranes were reacted. This would not have been expected in a mechanism with formic acid as an intermediate. A third mechanism, not considered by the authors, could proceed through initial production of propylene carbonate, followed by reduction to the mono- or di-formate. 

Early work identified Group II organometallic compounds such as diethylzinc and ethylmagnesium bromide as coordinate initiators of oxirane polymerization. Under carefully controlled conditions with anhydrous, high-purity monomer, the initiator activity was usually found to be weak. Bailey (80) found that with propylene oxide, distilled and maintained anhydrous with molecular sieves, the rate of polymerization of propylene oxide with dibutylzinc actually decreased as the concentration of dibutylzinc increased. This observation strongly suggested that a trace cocatalyst was needed to effect polymerization. 

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