Topics in Current Chemistry (2018) 376:44
1 3
4 Polymerization in Continuous‑Flow Microreactors
In this section, polymerizations conducted in continuous-flow microreactors are discussed. The chapter is divided into three sub-chapters according to the polymerization mode.
4.1 Homogeneous Polymerization
Homogeneous polymerization can be realized in microreactors to prepare polymers
without the phase separation procedure of solutions in the post-treatment [104]. Typically, the monomers and the initiators are dissolved in two miscible solutions, which
are delivered by pumps and introduced into a micromixer. Then the reaction mixture
enters into the microchannel and the polymerization proceeds. With the development
of microreactor technology, homogeneous polymerization processes, including free
radical polymerization, living polymerization, enzyme-catalyzed polymerization, and
condensation polymerization, have been conducted successfully in microreactors.
4.1.1 Free Radical Polymerization
Free radical polymerization (FRP) is a common approach for the production of polymers in industry due to its facile operating conditions and simple mechanisms [1,
34, 105]. Precise control over the temperature is essential for the free radical polymerization because most FRP processes are highly exothermic [3, 106]. Inefficient
heat removal and inevitable temperature gradient in conventional batch reactors lead
to uncontrollable molecular weight distribution [1]. Microreactors, with good heat
and mass transfer performance, was regarded as a substitute for the conventional
batch reactors in FRP processes.
Iwasaki et al. investigated free radical polymerization processes of five different monomers (butyl acrylate, benzyl methacrylate, methyl methacrylate, vinyl
benzoate, and styrene) using stainless-steel microreactors with inner diameters of
250–1000 μm [105]. It was found that a significant decrease of PDI could be realized
in microreactors for these highly exothermic polymerization processes compared to
the corresponding batch reactors. Moreover, these authors developed a numbered-up
device with eight parallel microchannels for a larger-scale production of poly(methyl
methacrylate) (PMMA) with controllable molecular weight and PDI by free radical
polymerization method [107]. This numbered-up microreactor system is presented
in Fig. 6, and it could run for 6 days, producing up to 4 kg of PMMA without any
problem under continuous operations. These results demonstrated that the microreactor system was suitable for the relatively large-scale production of polymers and
had great potential on industrial applications.
4.1.2 Living Polymerization
Living polymerization was reported for the first time by Szwarc in 1956, which can
be divided into anionic and cationic living polymerization according to the initiator
164
Reprinted from the journal
1 3
4 Polymerization in Continuous‑Flow Microreactors
In this section, polymerizations conducted in continuous-flow microreactors are discussed. The chapter is divided into three sub-chapters according to the polymerization mode.
4.1 Homogeneous Polymerization
Homogeneous polymerization can be realized in microreactors to prepare polymers
without the phase separation procedure of solutions in the post-treatment [104]. Typically, the monomers and the initiators are dissolved in two miscible solutions, which
are delivered by pumps and introduced into a micromixer. Then the reaction mixture
enters into the microchannel and the polymerization proceeds. With the development
of microreactor technology, homogeneous polymerization processes, including free
radical polymerization, living polymerization, enzyme-catalyzed polymerization, and
condensation polymerization, have been conducted successfully in microreactors.
4.1.1 Free Radical Polymerization
Free radical polymerization (FRP) is a common approach for the production of polymers in industry due to its facile operating conditions and simple mechanisms [1,
34, 105]. Precise control over the temperature is essential for the free radical polymerization because most FRP processes are highly exothermic [3, 106]. Inefficient
heat removal and inevitable temperature gradient in conventional batch reactors lead
to uncontrollable molecular weight distribution [1]. Microreactors, with good heat
and mass transfer performance, was regarded as a substitute for the conventional
batch reactors in FRP processes.
Iwasaki et al. investigated free radical polymerization processes of five different monomers (butyl acrylate, benzyl methacrylate, methyl methacrylate, vinyl
benzoate, and styrene) using stainless-steel microreactors with inner diameters of
250–1000 μm [105]. It was found that a significant decrease of PDI could be realized
in microreactors for these highly exothermic polymerization processes compared to
the corresponding batch reactors. Moreover, these authors developed a numbered-up
device with eight parallel microchannels for a larger-scale production of poly(methyl
methacrylate) (PMMA) with controllable molecular weight and PDI by free radical
polymerization method [107]. This numbered-up microreactor system is presented
in Fig. 6, and it could run for 6 days, producing up to 4 kg of PMMA without any
problem under continuous operations. These results demonstrated that the microreactor system was suitable for the relatively large-scale production of polymers and
had great potential on industrial applications.
4.1.2 Living Polymerization
Living polymerization was reported for the first time by Szwarc in 1956, which can
be divided into anionic and cationic living polymerization according to the initiator
164
Reprinted from the journal