Selective copolymerization of carbon dioxide with propylene oxide catalyzed by a nanolamellar double metal cyanide complex catalyst at low polymerization temperatures

Abstract

Traditional cobalt-zinc double metal cyanide complex [Zn–Co(III)DMCC] catalysts could catalyze the copolymerization of carbon dioxide (CO 2) with propylene oxide (PO) to afford poly (propylene carbonate) (PPC) with high productivity. But the molecular weight (MW) of PPC and the polycarbonate selectivity were not satisfied. In this work, by using a nanolamellar Zn–Co(III) DMCC catalyst, the CO 2–PO copolymerization was successfully performed to yield PPC with high molecular weight ( M n : 36.5 kg/mol) and high molar fraction of CO 2 in the copolymer ( F CO 2 : 74.2%) at low polymerization temperatures (40∼80 °C). Improved selectivity ( F CO 2 : 72.6%) and productivity of the catalyst (6050 g polymer/g Zn) could be achieved at 60 °C within 10 h. The influences of water content on CO 2–PO copolymerization were quantitatively investigated for the first time. It was proposed that trace water in the reaction system not only acted as an efficient chain transfer agent, which decreased MW of the resultant copolymer, but also strongly interacted with zinc site of the catalyst, which led to low productivity of PPC and more amounts of cyclic propylene carbonate (cPC). These conclusions were also supported by the apparent kinetics of CO 2–PO copolymerization. ESI-MS results showed that all polymers have two end alkylhydroxyl groups. It was thus proposed that the alkylhydroxyl groups came from the initiation reaction of Zn–OH in the catalyst and the chain transfer reaction by H 2O. The proposed mechanism of chain initiation, propagation and chain transfer reaction were proved by the experimental results.