Copolymerization of carbon dioxide and propylene oxide using various zinc glutarate derivatives as catalysts

Various zinc dicarboxylate catalysts were synthesized by the reaction of zinc oxide with eleven different glutaric acid derivatives, and their coordination characteristics were investigated by infrared spectroscopy. The electronic nature and steric hinderance of substituents influenced the coordination of the carboxylate and the zinc metal ion. The coordination characteristics were classified into three categories: i) compounds exhibiting bridging bidentate coordinating bonding modes, such as syn-anti and syn-syn bridging; ii) compounds with only unidentate coordination; and iii) compounds with mixed coordinations of unidentate and bridging bidentate. All the zinc carboxylate catalysts produced poly(propylene carbonate)s (PPCs) by the copolymerization of carbon dioxide and propylene oxide. The first category of catalysts produced relatively higher yields than the other categories. Zinc glutarate without any substituent, which is a catalyst in the first category, produced PPC with the highest yield and the highest molecular weight. The catalytic activity of zinc glutarate was suppressed by incorporation of substituents. The suppression of the catalytic activity might be due to the variation in the Lewis acidity of the zinc site as well as changes in the morphological structure caused by the substituents. Methylaluminoxane was also evaluated as a catalyst for the copolymerization, but it produced copolymers containing a large amount of ether linkages.