Synthesis of optically active polymers by asymmetric catalysts. VI. Behavior of organozinc catalyst systems in the stereoselective polymerization of propylene oxide

Abstract

AbstractThe reaction of propylene oxide with zinc alkoxide was studied to clarify the mechanism of the propylene oxide polymerization with the diethylzinc/alcohol catalyst system. In the reaction with zinc dimethoxide, ring scission of propylene oxide was observed which occurs almost exclusively at the methylene‐oxygen bond. Neither diethylzinc nor ethylzinc methoxide reacted with propylene oxide. Zinc dialkoxide of 1‐methoxypropanol‐2 (primary product of propylene oxide zinc dimethoxide reaction) showed a much lower reactivity to propylene oxide than zinc dimethoxide. Zinc dialkoxide of CH3O[CH2CH(CH3)O]nH (n = 11∼︁12), on the other hand, exhibited a rather high reactivity to propylene oxide. On these bases a discussion is made about the initiation and propagation mechanisms of the zinc dimethoxide catalyzed propylene oxide polymerization. In the copolymerization of the D‐ and L‐propylene oxides by the diethylzinc/water or/diethylamine catalyst systems, asymmetric selection of the antipode monomer was observed and interpreted by the formation of an asymmetric structure in the active species during the initiation reaction.