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Topics in Current Chemistry (2018) 376:44
4.7 Heterogeneous Polymerization
High dispersion of two immiscible liquid phases is usually required in heterogeneous polymerization. Moreover, higher monomer concentration can be applied and
sometimes tedious purification can be avoided in heterogeneous polymerization, in
comparison with homogeneous polymerization. In many polymerization processes,
excess reactants or byproducts should be removed as soon as possible from the reaction system, and liquid–liquid extraction, for example, would be one of the best
methods to realize the separation. In addition, polymerization processes can be limited inside segmented liquid slugs for liquid–liquid heterogeneous polymerization
conducted in microreactors. Internal recirculation inside liquid slugs in heterogeneous processes results in the convection, providing higher mixing efficiency compared with molecular diffusion in homogeneous polymerization. The mixing inside
liquid slugs can be further improved by the use of winding microchannels by breaking symmetrical recirculation flow [129]. In fact, the segmented slugs/droplets can
be considered as mini batch reactors with high mixing efficiency, narrow residence
time distribution, and fast heat and mass transfer rates. Besides, when polymerization processes involving highly viscous fluids or polymer particles are conducted in
microreactors, the heterogeneous operation will partly solve the channel clogging
issue. Due to the different wettability of the channel walls for two immiscible liquid phases, a liquid film can be formed between the polymerization phase and the
inner walls of the microreactor, prohibiting the fouling of polymer particles on the
microreactor walls and the potential channel clogging [83]. It is worth noting that a
few photopolymerization processes involving heterogeneous operations as well as
the polymerization processes with the use of solid catalysts are also reviewed in this
section.
4.7.1 Free‑Radical Polymerization Preparing Polymer Microspheres
with Controllable Morphologies
Fine polymer particles play an important role in improving product functionality
of plastic products (e.g., fibers, films, and molded manufactures) or liquid products
(e.g., ink, paint, and adhesive materials). This creates a strong requirement for new
methods to precisely control key properties such as diameters, size distributions,
and molecular weights of fine polymer particles. Okubo et al. polymerized styrene
or methyl methacrylate (MMA) within microemulsion droplets using a microreactor system containing a K-M micromixer and delaying tubing within slug flow. It
was demonstrated that the internal recirculation within the slugs contributed to the
removal of heat generated in the free-radical polymerization, thus achieving better control over polymer sizes and molecular weight distributions [130]. Similarly,
Luo et al. used a coaxial capillary microreactor to realize controllable suspension
polymerization of butyl acrylate (BA) within monodispersed droplet flow. The sizes
of the droplets were quite uniform with the coefficient of variation value below 2%.
The authors further suggested a simplified CFD model for qualitatively calculating the temperature distribution within a single BA droplet. The simulation results
171
Reprinted from the journal
Topics in Current Chemistry (2018) 376:44
4.7 Heterogeneous Polymerization
High dispersion of two immiscible liquid phases is usually required in heterogeneous polymerization. Moreover, higher monomer concentration can be applied and
sometimes tedious purification can be avoided in heterogeneous polymerization, in
comparison with homogeneous polymerization. In many polymerization processes,
excess reactants or byproducts should be removed as soon as possible from the reaction system, and liquid–liquid extraction, for example, would be one of the best
methods to realize the separation. In addition, polymerization processes can be limited inside segmented liquid slugs for liquid–liquid heterogeneous polymerization
conducted in microreactors. Internal recirculation inside liquid slugs in heterogeneous processes results in the convection, providing higher mixing efficiency compared with molecular diffusion in homogeneous polymerization. The mixing inside
liquid slugs can be further improved by the use of winding microchannels by breaking symmetrical recirculation flow [129]. In fact, the segmented slugs/droplets can
be considered as mini batch reactors with high mixing efficiency, narrow residence
time distribution, and fast heat and mass transfer rates. Besides, when polymerization processes involving highly viscous fluids or polymer particles are conducted in
microreactors, the heterogeneous operation will partly solve the channel clogging
issue. Due to the different wettability of the channel walls for two immiscible liquid phases, a liquid film can be formed between the polymerization phase and the
inner walls of the microreactor, prohibiting the fouling of polymer particles on the
microreactor walls and the potential channel clogging [83]. It is worth noting that a
few photopolymerization processes involving heterogeneous operations as well as
the polymerization processes with the use of solid catalysts are also reviewed in this
section.
4.7.1 Free‑Radical Polymerization Preparing Polymer Microspheres
with Controllable Morphologies
Fine polymer particles play an important role in improving product functionality
of plastic products (e.g., fibers, films, and molded manufactures) or liquid products
(e.g., ink, paint, and adhesive materials). This creates a strong requirement for new
methods to precisely control key properties such as diameters, size distributions,
and molecular weights of fine polymer particles. Okubo et al. polymerized styrene
or methyl methacrylate (MMA) within microemulsion droplets using a microreactor system containing a K-M micromixer and delaying tubing within slug flow. It
was demonstrated that the internal recirculation within the slugs contributed to the
removal of heat generated in the free-radical polymerization, thus achieving better control over polymer sizes and molecular weight distributions [130]. Similarly,
Luo et al. used a coaxial capillary microreactor to realize controllable suspension
polymerization of butyl acrylate (BA) within monodispersed droplet flow. The sizes
of the droplets were quite uniform with the coefficient of variation value below 2%.
The authors further suggested a simplified CFD model for qualitatively calculating the temperature distribution within a single BA droplet. The simulation results
171
Reprinted from the journal