The copolymerization of carbon dioxide and epoxypropane was investigated at 80°C in dioxane by using cobalt(II) acetate as catalyst in the presence of acetic acid. The addition of acetic acid hardly diminishes the copolymer yield, but the number average molecular weight of the copolymer decreases in proportion to the concentration of acetic acid. The 1H NMR spectrum of the copolymer displays a very strong peak due to the methyl protons attached to an ester carbonyl group (OCOCH3). From these results it has been concluded that acetic acid causes a transfer reaction which leads to acetoxy end groups (cf. Eq. (14) below). Some other carboxylic acids, such as C6H5COOH, ClH2CCOOH, Cl2HCCOOH etc., were found to cause similar transfer reactions, too.
Diethylzinc was allowed to react with γ‐alumina in n‐heptane at 50°C, and the copolymerization of propylene oxide and carbon dioxide was investigated in some detail at 30–90°C by using the reaction product as a catalyst. From an analysis of the catalyst it was found that diethylzinc reacted with the surface hydroxyl groups of γ‐alumina mainly to give the following A‐type species by evolving ethane:
magnified imageThe catalyst showed considerably high activity for the copolymerization. The polymer obtained was a white solid with a high molecular weight soluble in benzene, acetone, dioxane, and methylene chloride and insoluble in diethyl ether and water. It was confirmed as an alternate copolymer of propylene oxide and carbon dioxide.The copolymerization was also conducted with a reaction mixture of the catalyst and catechol in which the molar ratio of catechol to the A‐type species was varied. The copolymerization activity decreased linearly with an increase in the molar ratio and disappeared completely at the molar ratio of unity.On the basis of these results it has been concluded that the A‐type is the true active species for the copolymerization.
Diethylzinc was allowed to react with various metal oxides in n‐heptane at 60°C, and the copolymerization of propylene oxide and carbon dioxide was investigated at 60°C in solution in dioxane with reaction products as catalysts. An alternate copolymer was obtained with every catalyst, but the yield of copolymer and the number‐average molecular weight depended significantly on the supporting materials. In a kinetic study of the copolymerization we found that the catalytic efficiency (number of propagating species per number of zinc supported) was only a few percent with every catalyst. The copolymerization was also examined by using several kinds of silica, whose pore diameters are markedly different, as supports. The results obtained strongly suggested that only the active species existing in large pores act as the propagating species.
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