In-line Monitoring of Microgel Synthesis: Flow versus Batch Reactor

In-line monitoring of polymerization via Raman spectroscopy is standard for batch reactors but yet to be manifested for flow reactors. When transferring the in-line Raman spectroscopy from batch to flow reactors, the aim is to maintain the measurement precision. We present a customized Raman measurement cell for in-line monitoring in a tubular flow reactor and a systematic accuracy analysis of the obtained measurement. The accuracy analysis involves comparing three calibration models and measurements of flowing solvent, monomer solution, and microgel solution. The evaluation of the measurements reveals that multimolecular analyte solutions most significantly influence the quantification accuracy. From these investigations, we derive a quality criterion for the Raman measurements based on the root mean square error, and apply the criterion to the monitoring of precipitation polymerization of poly­(N-iso-propylacrylamide) (PNIPAM) microgels. The results compare the predictions from the Raman measurements of the microgel synthesis in the flow and batch reactor. Applying the quality criterion enables high-quality measurements and allows the functional detection of outliers during the synthesis. The identification and exclusion of outliers eliminate several potential errors to cause the difference in measurement results from the flow and batch reactors. This contribution serves as a guideline for transferring in-line monitoring from batch to flow reactors as the example of precipitation polymerization.