Multisite Model of Polyol Preparation in Continuous Processes Using Heterogeneous Double Metal Cyanide Catalysts

Polyols are typically produced using batch processes and KOH as the dominant homogeneous catalyst. In this case, the main reaction is an ionic polymerization reaction, and products with narrow molecular weight distributions (MWDs) are typically obtained. As observed in living polymerization systems, polyols of varying molecular weights serve as starting points for manufacture of polyols of higher molecular weights. The transition from batch to continuous processes should lead to polydispersities approaching the theoretical limit of 2 for living polymerization systems. However, with the recent introduction of heterogeneous Double Metal Cyanide (DMC) catalysts, the expected increase in polydispersity is not observed. Polyols produced in continuous processes that use DMC catalysts still have polydispersities approaching 1. This unusual result is the result of a chain-length dependent effect that favors short chains over long chains and thus keeps the molecular weight distribution as narrow as possible, even in continuous reactors. Reaction mechanisms are published that attempt to describe the behavior of DMC catalysts during ringopening polymerizations. However, these do not go much further than providing a qualitative explanation of the observed results. They also do not address the generation of small fractions of high molecular weight chains observed in the production of polyols with DMC catalysts. In this paper, we present a model of a backmixed continuous reactor for polyol production using a DMC catalyst. This reactor could be, for example, a mixed flow reactor or a loop reactor with a high recycle ratio. The model considers the propagation reaction to be diffusion-controlled, thus allowing short chains to grow faster in each catalytic site compared to the longer chains and achieving a narrow molecular weight distribution in a single reactor. Furthermore, this model considers that polymerization occurs in two types of active sites with the irreversible conversion of site 1 to site 2, thus allowing the build-up of a distinct population of long chain molecules. The model provides new insight into the system and can be utilized to determine the most appropriate operating conditions for the optimization of final products. 1 Yamada, K., Takeyasu, H., Ikemura, M., , “Method for continuous preparation of polyethers”, Japanese Unexamined Patent Application 6[1994]-16806 2 Kim, I., Ahn, J., Sik Ha, C., Sik Yang, C., and I. Park, “Polymerization of propylene oxide by using double metal cyanide catalysts and the application to polyurethane elastomer”, Polymer 44, 3417–3428, 2003 3 Brons, J. F. J., Eleveld, M. B., “Process for preparing polyoxyalkylene polyether products”,EP 1,295,902, 2003 4 Zacca, J. J., Ray, W. H., “Modeling of the liquid phase polymerization of olefins in loop reactors”, Chem. Eng. Sci. 48(22), pp 3743-65, 1993