(654c) Model-Based Synthesis of Ferrocene Microgels

Authors: 
Jung, F., RWTH Aachen University
Mitsos, A., RWTH Aachen University
Plamper, F., TU Bergakademie Freiberg
Mhamdi, A., RWTH Aachen University
Schneider, S., RWTH Aachen University
Microgels are functional cross-linked polymer networks, which adjust their size and shape in response to external triggers [1]: e.g., temperature [2], pH value [3], or an electrochemical stimulus [4]. Therefore, they are attractive for a wide range of applications including drug delivery [5], liquid-liquid separation [6], or switchable membranes [7]. Each application requires microgels with different properties. Especially the localization of functional monomer groups within the microgel is of interest.

In a typical batch synthesis, the localization of functional monomer groups depends on the reactivities of the different monomers [8], i.e. monomers with faster kinetics react first and therefore concentrate in the core of the microgel. A fed-batch synthesis allows a precise localization of functional monomer groups in the microgel. However, due to the large number of degrees of freedom commonly many trial experiments are used to find a suitable fed-batch recipe [9-12].

We demonstrate the use of model-based methods for synthesis design to avoid these trial experiments [13]. First, we estimate reactivity ratios of the considered monomers based on available data from batch experiments [14]. We employ the determined reactivity ratios in a mechanistic model of the microgel synthesis [8,16-19]. Second, we use model-based synthesis design to determine synthesis procedures for microgels with different localizations of the functional monomer: (1) functional monomer in the core, (2) functional monomer in the shell, and (3) functional monomer distributed homogeneously throughout the microgel. Third, we run the determined synthesis procedures experimentally and analyze the results using cryogenic transmission electron microscopy, dynamic light scattering, and elementary analysis.

We investigate the N-Isopropylacrylamide (NIPAM) – Vinylferrocene (VFc) system. VFc is the functional monomer to be localized in the microgel made of the main monomer NIPAM. The NIPAM – VFc system is an especially challenging candidate for synthesis design, as the addition of small amounts of VFc to the synthesis of a microgel based on NIPAM prolongs the synthesis duration by two orders of magnitude [14]. Hence, the addition of VFc has an impact on the reaction kinetics of the main monomer NIPAM. Model-based methods enable the consideration of the impact on the synthesis kinetics.

After parameter estimation, the model predictions of the monomer conversion agree with the available experimental data from batch experiments [14]. Reactivity ratios indicate that the synthesis duration increases due the addition of VFc, because active polymer chains with a VFc end group react several orders of magnitude slower than active polymer chains with a NIPAM end group. Additionally, active polymer chains with a VFc end group favor the reaction with VFc monomer to the NIPAM monomer.

Batch experiments have shown that microgels with a VFc rich core can be synthesized by adding the VFc to the reactor initially [14]. We test various fed-batch synthesis conditions in simulation and reveal that microgels with a VFc-rich shell can be synthesized by addition of the entire amount of VFc after a critical conversion of the main monomer NIPAM is reached. We also show that microgels with a homogeneous distribution of VFc can be synthesized by adding VFc to the reactor initially and feeding the additional VFc at separate time points during the synthesis.

We validate the results of the model-based synthesis design experimentally. Images of the synthesized microgels recorded using cryogenic transmission electron microscopy show that the functional monomer is localized at different positions in the microgel as desired. For the batch synthesis, the VFc is located in the core of the microgel. The determined fed-batch synthesis procedures allow for localization of the VFc in the shell of the microgel or a homogenous distribution of the VFc.

We show that model-based methods enable the synthesis of microgel with a specific localization of functional monomer without additional trial experiments. The determined model with the respective kinetic parameters improves the understanding of the reactions taking place during the synthesis and would allow for the model-based control of other microgel properties, such as microgel size, in the future.

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