1 Introduction
Microtechnology is no longer the field of electronics, but is now moving into many
different areas of science and engineering, including mechanics, optics, and fluids,
because it provides better efficiencies while also answering the demands of society
for conservation of resources and energy. Following remarkable advances in
microfabrication technology, microtechnology has been used in the field of chemistry since the 1990s. Microdevices that are used for chemical reactions are called
microreactors and are reactors with micrometer-sized channels in which chemical
reactions are carried out.
Microreactors are normally set up as flow-type reactors (flow microreactors)
with a continuous flow of a solution through the reaction chamber. Nowadays,
chemical synthesis in flow microreactors is receiving significant research interest
from both academia and industry [1–24]. Recent investigations have revealed
significant features of flow microreactor systems involving fast mixing, stemming
from short diffusion paths and fast heat transfer by virtue of high surface-to-volume
ratios, which are advantageous for increasing the selectivity of chemical reactions
[25–28]. Short residence time in a microchannel is beneficial for controlling highly
reactive intermediates [29–45]. By taking advantages of such features of flow
microreactor systems, various chemical reactions for organic synthesis have been
developed [46–56].
Polymerization reactions that convert small molecules into macromolecules by
repeating chemical reactions are a fascinating field in the application of flow
microreactors. Extensive studies on cationic polymerization, anionic polymerization,
radical polymerization, coordination polymerization, polycondensation, and ringopening polymerization using flow microreactor systems have been carried out
so far. Reviews on various polymerization methods in microreactors have been
reported by Hessel et al., Wilms et al., Steinbacher and Mcquade, and Bally et al.
[57–61]. The main objective of this article is to provide, in a concise form, a current
overall picture of polymerization using flow microreactors, especially from the
viewpoint of industrial applications. Because of space limitations, polymer particle
synthesis will not be discussed here. Serra and Chang have published an excellent
review on synthesis of polymer particles with an improved control of their sizes, size
distributions, morphologies, and compositions using microreactors [62].
1.1 Characteristic Features of Flow Microreactors
Flow microreactors can influence the very essence of chemical reactions because of
the following characteristic features derived from their small size and flow nature:
1. Fast mixing [63, 64]: Many chemical reactions involve combining two
substances, and for this reason mixing to achieve homogeneity in solution is
very important, especially for fast reactions. Mixing occurs due to molecular
2
A. Nagaki and J.-i. Yoshida
Microtechnology is no longer the field of electronics, but is now moving into many
different areas of science and engineering, including mechanics, optics, and fluids,
because it provides better efficiencies while also answering the demands of society
for conservation of resources and energy. Following remarkable advances in
microfabrication technology, microtechnology has been used in the field of chemistry since the 1990s. Microdevices that are used for chemical reactions are called
microreactors and are reactors with micrometer-sized channels in which chemical
reactions are carried out.
Microreactors are normally set up as flow-type reactors (flow microreactors)
with a continuous flow of a solution through the reaction chamber. Nowadays,
chemical synthesis in flow microreactors is receiving significant research interest
from both academia and industry [1–24]. Recent investigations have revealed
significant features of flow microreactor systems involving fast mixing, stemming
from short diffusion paths and fast heat transfer by virtue of high surface-to-volume
ratios, which are advantageous for increasing the selectivity of chemical reactions
[25–28]. Short residence time in a microchannel is beneficial for controlling highly
reactive intermediates [29–45]. By taking advantages of such features of flow
microreactor systems, various chemical reactions for organic synthesis have been
developed [46–56].
Polymerization reactions that convert small molecules into macromolecules by
repeating chemical reactions are a fascinating field in the application of flow
microreactors. Extensive studies on cationic polymerization, anionic polymerization,
radical polymerization, coordination polymerization, polycondensation, and ringopening polymerization using flow microreactor systems have been carried out
so far. Reviews on various polymerization methods in microreactors have been
reported by Hessel et al., Wilms et al., Steinbacher and Mcquade, and Bally et al.
[57–61]. The main objective of this article is to provide, in a concise form, a current
overall picture of polymerization using flow microreactors, especially from the
viewpoint of industrial applications. Because of space limitations, polymer particle
synthesis will not be discussed here. Serra and Chang have published an excellent
review on synthesis of polymer particles with an improved control of their sizes, size
distributions, morphologies, and compositions using microreactors [62].
1.1 Characteristic Features of Flow Microreactors
Flow microreactors can influence the very essence of chemical reactions because of
the following characteristic features derived from their small size and flow nature:
1. Fast mixing [63, 64]: Many chemical reactions involve combining two
substances, and for this reason mixing to achieve homogeneity in solution is
very important, especially for fast reactions. Mixing occurs due to molecular
2
A. Nagaki and J.-i. Yoshida