DMC-Mediated Propoxylation in Semibatch with an External Loop:Insights into the Catalytic Action

The importance of mixing in a semibatch propoxylation for controlling the distribution width of poly(propylene glycol) (PPG) products using double metal cyanide (DMC) catalysis is demonstrated. A loop comprising a static mixer with Kenics elements was incorporated into a reactor setup, enabling propylene oxide to be added to the circulating reaction mixture shortly before the mixer. The analysis shows that when “catch-up kinetics” is operative, i.e., at higher catalyst particle concentrations and low molecular masses, improved mixing results in a PPG with a lower PDI and a higher steady-state PO conversion. Propoxylation of PPG with higher molecular masses and fewer catalyst particles per volume leads to products with a higher PDI or even bimodal distributions irrespective of improved mixing due to an imbalance between diffusion of the chain ends and propoxylation rate.