lithiation of dibromobiaryls [107–110], iodination of aromatic compounds
[111, 112], and the reaction of a Grignard reagent with B(OMe) 3 [113] by virtue
of extremely fast micromixing using flow microreactor systems has been reported.
These successful results suggest the possibility of molecular weight and molecular
weight distribution control by using flow microreactor systems.
In flow-microreactor-system-controlled polymerization, extremely fast mixing of
an initiator solution and a monomer solution leads to selective initiation. Propagation
proceeds on the basis of the monomer/initiator ratio under homogeneous conditions,
and therefore polymers are obtained with good control of molecular weight and
molecular weight distribution. Minimization of local deviations in temperature by
fast heat transfer through the wall of a microreactor by virtue of the high surface-tovolume ratios is also important because polymerization processes are usually highly
exothermic.
“Cation Pool”-Initiated Polymerization of Vinyl Ethers Using Flow Microreactor
Systems
Flow-microreactor-system-controlled cationic polymerization requires extremely
reactive initiators, and highly reactive organic cations can serve as effective initiators
for this method. Usually, organic cations such as carbenium ions and onium ions
are generated by an acid-promoted reversible process from their precursors. Yoshida
and coworkers have developed the “cation pool” method [114, 115] in which
organic cations are generated irreversibly by low temperature electrolysis and
are accumulated in relatively high concentration in the absence of nucleophiles.
N-acyliminium ions [116], alkoxycarbenium ions [117–121], diarylcarbenium ions
[122, 123], and glycosyl triflate [124] have been generated and accumulated by this
method. Yoshida and coworkers reported that N-acyliminium ion pools serve as
extremely reactive initiators for cationic polymerization in flow microreactors [116].
A pool of an N-acyliminium ions is generated by low-temperature electrochemical
oxidation of its precursor, a a-silyl-substituted N-acylamine. Cationic polymerization
of NBVE using the N-acyliminium ion as an initiator in a conventional batch reactor
gives the polymer in a quantitative yield after quenching with i-Pr 2 NH/CH 2 Cl 2 ,
but the molecular weight distribution is broad (M n ¼ 5,700, M w /M n ¼ 2.56).
The reverse addition sequence (the N-acyliminium ion to the monomer) gives
rise to a similar molecular weight distribution (quantitative yield, M n ¼ 13,100,
M w /M n ¼ 2.25). The simultaneous addition of a monomer solution and a solution
of the N-acyliminium ion does not improve the molecular weight distribution control
(quantitative yield, M n ¼ 24,500, M w /M n ¼ 2.43). The molecular weight, however,
strongly depends upon the method of mixing because the rate of the polymerization is
so fast. In contrast, the use of a flow microreactor system consisting of two
micromixers (M1 and M2) and two microtube reactors (R1 and R2) shown in
Fig. 5 leads to excellent control of molecular weight and its distribution
(M n ¼ 6,700, M w /M n ¼ 1.14) [125]. Solutions of N-acyliminium ion (because the
yield of N-acyliminium ion from precursor is estimated to be about 80% based on the
8
A. Nagaki and J.-i. Yoshida
[111, 112], and the reaction of a Grignard reagent with B(OMe) 3 [113] by virtue
of extremely fast micromixing using flow microreactor systems has been reported.
These successful results suggest the possibility of molecular weight and molecular
weight distribution control by using flow microreactor systems.
In flow-microreactor-system-controlled polymerization, extremely fast mixing of
an initiator solution and a monomer solution leads to selective initiation. Propagation
proceeds on the basis of the monomer/initiator ratio under homogeneous conditions,
and therefore polymers are obtained with good control of molecular weight and
molecular weight distribution. Minimization of local deviations in temperature by
fast heat transfer through the wall of a microreactor by virtue of the high surface-tovolume ratios is also important because polymerization processes are usually highly
exothermic.
“Cation Pool”-Initiated Polymerization of Vinyl Ethers Using Flow Microreactor
Systems
Flow-microreactor-system-controlled cationic polymerization requires extremely
reactive initiators, and highly reactive organic cations can serve as effective initiators
for this method. Usually, organic cations such as carbenium ions and onium ions
are generated by an acid-promoted reversible process from their precursors. Yoshida
and coworkers have developed the “cation pool” method [114, 115] in which
organic cations are generated irreversibly by low temperature electrolysis and
are accumulated in relatively high concentration in the absence of nucleophiles.
N-acyliminium ions [116], alkoxycarbenium ions [117–121], diarylcarbenium ions
[122, 123], and glycosyl triflate [124] have been generated and accumulated by this
method. Yoshida and coworkers reported that N-acyliminium ion pools serve as
extremely reactive initiators for cationic polymerization in flow microreactors [116].
A pool of an N-acyliminium ions is generated by low-temperature electrochemical
oxidation of its precursor, a a-silyl-substituted N-acylamine. Cationic polymerization
of NBVE using the N-acyliminium ion as an initiator in a conventional batch reactor
gives the polymer in a quantitative yield after quenching with i-Pr 2 NH/CH 2 Cl 2 ,
but the molecular weight distribution is broad (M n ¼ 5,700, M w /M n ¼ 2.56).
The reverse addition sequence (the N-acyliminium ion to the monomer) gives
rise to a similar molecular weight distribution (quantitative yield, M n ¼ 13,100,
M w /M n ¼ 2.25). The simultaneous addition of a monomer solution and a solution
of the N-acyliminium ion does not improve the molecular weight distribution control
(quantitative yield, M n ¼ 24,500, M w /M n ¼ 2.43). The molecular weight, however,
strongly depends upon the method of mixing because the rate of the polymerization is
so fast. In contrast, the use of a flow microreactor system consisting of two
micromixers (M1 and M2) and two microtube reactors (R1 and R2) shown in
Fig. 5 leads to excellent control of molecular weight and its distribution
(M n ¼ 6,700, M w /M n ¼ 1.14) [125]. Solutions of N-acyliminium ion (because the
yield of N-acyliminium ion from precursor is estimated to be about 80% based on the
8
A. Nagaki and J.-i. Yoshida