Double Metal Cyanide (DMC) Catalysts: Synthesis, Structure, and Action Mechanism (A Review)

Double metal cyanide (DMC) catalysts have no alternatives for use in the industrial process of propylene oxide (PO) polymerization to obtain polypropylene oxide (PPO) with the properties required for specialty applications: low degree of unsaturation, high molecular mass, and high hydroxyl value. The modern commercial samples show high performance and allow the process to be performed with extremely low catalyst amounts (down to 25 ppm). Such amounts do not require the catalyst regeneration and to not impair the polymer properties. The main drawbacks of these materials are relatively complex synthesis and moisture sensitivity. Despite the fact that DMC catalysts are known since the 1960s, their hybrid character and variable composition still complicate their study and elucidation of the relationship between the preparation procedure, composition, and properties of these materials. This review is aimed at systematizing and analyzing the information on the synthesis, structure, and action mechanism of DMC catalysts. Both traditional synthesis and nontraditional methods for preparing DMC catalysts are described in detail. Much attention is paid to the catalytic site structure, polymerization mechanism, and physicochemical properties of these materials as heterogeneous catalysts.